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大连交通大学2017届本科生毕业设计(论文）外文翻译毕业设计外文翻译（27-34）为了更好地成型塑件的末端，要保持熔体的流道通畅。温度上的差异也有部分取决于距离浇口的距离。 当冷却剂流过模具(没有循环和分流道),并且只有一个温度控制器时，模具就会被均匀冷却。由于在整个注射和包装步骤，熔融材料都会持续流经浇口部分，以至于甚至在冷却过程中，浇口一直是温度最高的部分。塑件末端温度最低则是由于聚合物熔体热量会通过腔体流失。 不同位置上的温度不同会导致塑件不均匀收缩并且发生翘曲。差分冷却和增加注射速率这两种方法可以减少温差。 差分冷却是将冷却剂引向温度最高并且远离温度最低的末端部分的浇口。典型的方法是减少冷却剂流经塑件末端并且将冷却剂阀门设计至靠近浇口处。使冷却剂从较热的部分流至末端在某些情况下也可以。 综上所述，增加注射速率将缩短注射时间并且减少不同位置的温差。 因为冷却过程是整个注塑过程中周期最长的，所以一般推荐的模具的温度都很低。因为模具及部分设计的多样性，很难确定准确的模具温度。冷却液的温度在32-50(0-10)时，聚烯烃的典型的模具表面温度范围是70-125（20-50）。对于熔点较低的材料，如EVAS选用这个范围下限附近温度，而熔点较高的材料，如HDPE和PP选用这个范围上限附近温度。然而，冷模具会使塑件表面不那么光滑，并且会限制树脂在模具内的流动。冷却模具会增加塑件在模具内的压力。提高模具温度可以增加塑件表面光泽度并且通过缩小流道的截面面积有可能会减少树脂流动所需周期。 注塑机和模具通常会设计包括模具打开所需时间，零件弹出时间和再次关闭的固定的总时间。模具可以通过增加模具开模距离让塑件自由下落，减少塑件接触空气的时间和减少推杆活动来减少模具的打开时间。收缩 无定型树脂ABS、聚碳酸酯、聚苯乙烯的收缩率要远低于聚烯烃。聚烯烃的收缩率较高，其原因是聚烯烃树脂是半结晶的，因此，在熔融状态下，其收缩量比固态更多。当树脂凝固时，结晶区域中的分子链紧密包紧在一起，导致体积减少。一般来说，聚烯烃的收缩率排行为：HDPE LLDPE LDPE PP 对于特定的树脂，通过模具和加工方法在一定程度上可以控制收缩（Table5）。对于一个模板厚度和浇口横截面积可以变化的模板，表示如下： 随着塑件的厚度减小，收缩减少，厚度的影响HEPE 比PP更为显著。 随着浇口面积减小，收缩减少。 由于收缩受冷却影响，可以通过在较低温度下注射和注射进入较冷的模具中来减少收缩量。注入更多树脂也可以减少收缩。这是通过在中等温度下和高压下注射或者在相当高的温度下和中等压力下成型来完成的。但是温度和压力过高会发生断裂。 减少收缩的另一种方法是增加压力和保压时间。这种方法中，树脂会随着材料冷却和收缩而流入模具中，尽可能多的填充模具，但这样也会增加循环时间和模具内压力。 脱模前较长时间的冷却对于塑件的尺寸影响很大。随着塑件在型芯周围冷却和收缩，型芯会保证塑件关键的内部尺寸。增加排气或减少会使塑件更容易推出。增加冷却时间会增加循环周期，因此许多模具会增加冷却时间来减少收缩。 收缩时时间函数。一般来说，聚烃烯塑件会在48小时后打到其总收缩率的90。如果零件保压时温度很高或者储存在一个温暖的仓库中，收缩可以持续很多天。封装后的模具如果堆叠在一起通常会发生“嵌套”的问题。翘曲 翘曲是由不均匀冷却引起的塑件的不均匀收缩造成的。当塑件脱出后发生变形时，可以通过减少冷却时对模具施加的应力来使其保持原样。尽可能的减少在冷却至室温会“记住”兵缓解的“锁定”的压力，这往往是一个难以解决的问题。在塑件脱出后处于固定状态的塑件，如果暴露于温度较高的环境下会发生松弛和翘曲。由于较厚的部分残留的温度较高，所以横截面厚度不均的零件比厚度均匀的零件更容易发生弯曲。除了不均匀的冷却冷却外，过度的模制压力、长时间的填充、低背压和过低的熔融温度也会在模具中产生应力。现在对于翘曲没有一个明确的补救方法。调整模具所处环境、重新设计零件和模具、切换到具有狭窄的MWD材料或其混合物可能会降低内部应力。通常，当熔体温度设定为最大、模具温度高、注射压力小并且注射时间短时，发生的翘曲最小。在高温下成型需要在零件成型之前减少注射期间产生的应力。使用温暖模具也需要在融化之前减少压力。通常需要在半模之间，特别是具有较大表面积的塑件进行差分冷却，以产生无翘曲的塑件。注射和保压的压力应足以容易填充，但不应设置过高，以便在塑件固化之前进行模具的应力松弛。增加注射速率会减少注射时间，这会使塑件末端在过冷之前更快的填充。这样可以是整个塑件均匀的冷却，减少翘曲。这些补救措施中，如提高温度或降低注射压力可以增加循环时间。使用流动性更好的MFR/MI树脂可以减少时间。具有较高流动性的树脂可以使用较低的注射压力，这样可以缩短模塑周期。此外，较高流动性的树脂具有较少的流动记忆，这也可以较少翘曲。低密度树脂（对于PE）仅比较高密度的树脂不宜发生翘曲。流动和转移收缩之间的差异可能导致翘曲。已知HDPE在这两者之间具有很大的差异，而PP在流动和转移收缩之间更平衡。因为收缩和翘曲受模具冷却模式和部分几何(厚度的均匀性和流动模式)的影响很大,所以在模具和塑件设计的早期阶段考虑这些是非常重要的。表5.减少聚烯烃制注射成型品中收缩和翘曲的一些方法收缩翘曲成型条件降低气缸和模具温度提高气缸和模具温度降低浇口及浇口附近温度降低浇口及浇口附近温度减少塑件末端的冷却减少塑件末端的冷却使用适当的气缸温度并且提高注射压力减小注射压力提高气缸温度并使用使用适当的注射压力提高气缸温度提高注射压力并延长注射时间缩短注射时间延长保压时间延长保压时间模具设计使用适当的浇口和浇口位置使用适当的浇口和浇口位置树脂性能使用低密度树脂不需要低密度树脂使用高流动性的MI/MFR树脂使用高流动性的MI/MFR树脂色散和排气在螺杆尖端和喷嘴之间的筒体端部使用破碎板是改善分散和防止气泡进入熔体中的有效方法（图41）。在大多数情况下，熔体背部的压力将所有空气挤压在熔体颗粒之间并产生无气泡的塑件。破碎板的厚度可以为1/4英寸（6.5mm），并且要求足够大装入喷嘴。改板钻出20-40、直径（1-32英寸或0.8mm）的孔。另一种选择是增加螺钉上的背压，确保其没有设置的太高以至于螺钉在下一个循环中不能复位。增加背压或者加入一个破碎板也有助于色散和使熔融温度均匀。当在高温下注射时，应将背压保持在最低限度来减少树脂分解。零件推出和脱模脱模受很多因素影响。一些聚烯烃树脂比其他聚烯烃树脂更容易脱模。已发现，这些树脂具有想同的缺点，例如光泽度低。颗粒状或霜状表面的树脂比光滑、高广泽的模制品(例如高光泽度的MI或澄清聚丙烯制成的塑件）更容易脱模。然而，即使同一种MI聚丙烯在脱模性能方面也不同。树脂有时与脱模添加剂混合使用。改变模具设计或改变一个或多个成型条件而不影响最终成型的属性可以减轻脱模问题。如果模具为了减少收缩二包装的太紧，就不容易脱模。如果持续时间太长，塑件通常会粘在模具上，并且塑件收缩到型芯上。在这种情况下，缩短冷却时间，可以改善脱模。另一方面，如果循环时间太短而不能使塑件从腔体壁收缩，也可能会发生同样的问题。在这种情况下，延长徐怒汉时间可以改善脱模。在模具打开方向上的表面绘制石块也可以缓解这个问题。脱模很大程度上取决于模具内部的抛光程度。用于深冲塑件的模具内表面的光洁度决定了塑件能否正确脱模还是黏在腔体或型芯上。推杆可以在型腔或型芯中推出塑件。必须提供足够的通风，特别是在深冲压模制品中（如长容器）中。尽可能的避免反向牵引和平行的侧面。EVA模具的涉及草图应该留出更多空间，因为它们在熔体喷出的温度下更粘。通常3-5的预留斜度有助于脱模。在潜水拔模中，可能会需要脱模环和空气喷射。不推荐拔模斜度小于1的拔模斜度，除非零件要求规定，否则应避免。图41.分散助剂是可以放置在注射机喷嘴中的插入物，以限制熔体流动并改善树脂的缓和，以获得跟均匀的熔融温度和已加入材料的混合物，例如着色剂明晰窗体顶端需要窗体底端需要高熔融温度和低压才能消除模制件中的流痕。通过降低模具温度，特别是在使用高MI树脂时，可以提高成型塑件的的清晰度。这减小了所形成的的晶体的尺寸，进而降低了光衍射。聚丙烯无规共聚物具有比PP聚合物更高的透明度。通过添加澄清剂可以进一步提高无规共聚物树脂的透明度。 在约430F（220）和模具温度约50至80F（10至25）的熔融温度下获得标称0.050英寸厚度范围内的PP制品的最佳透明度。通常，提高注射速率也可以提高透明度。高度抛光的工具是最高清晰度所必需的。光泽度模塑的表面光泽度受树脂性能，模具状况和成型条件的影响。聚烯烃树脂的MI或MFR越高，模制品的光泽越大。此外，较高密度的聚乙烯比较低密度的树脂具有更高的光泽度。高度抛光的模具是获得高光部件的最重要因素之一。对于聚乙烯，温模具比冷模具更好的光泽。控制浇口也可能有助于获得高额部分。限制浇口会产生较高的光泽度，因为当熔体注入模腔时，它会保持温度高。聚丙烯树脂可以通过冷模和提高注射速率提高光泽度。聚丙烯整体铰链聚丙烯可以模制成铰链部件，可以在故障之前弯曲许多周期。对于正确设计和制造的铰接部件，通过测试已经可以超过一百万次的弯曲。铰链部分必须足够薄到适合弯曲但足够厚以防止撕裂。正常铰链厚度为0.008-0.015英寸（0.2-0.38毫米）。如果铰链需要强度或承载性能，则需要更大的厚度。整体铰链的典型横截面如图42所示。当铰链处于关闭位置时，必须提供浅厚度或间隙以防止收集和过度的应力。可以在此设计上进行变化以获得具体的结果。浇口入部件必须设计成允许聚合物以均匀且连续的方式流过整个铰链部分，流动前部垂直于铰链。这种布置确保了最佳的铰链强度，而不会发生分层。优选是，所有门都在铰链的一侧，以消除焊缝的可能性; 然而，一些设计由于过于复杂，使得这种布置是不可能的，并且必须放置门，使得在铰链中不会出现焊接线。 如果使用多个门，建议将一个比壁厚略厚的部分平行于铰链放置。这种“集流器”将促进横跨铰链的均匀的流动模式。最佳铰链性能的成型条件是高熔体温度（通常为500F至525F / 260C至275C），快速注射速度和温热模具（120F至150F / 50 C至65）。为了发展最佳性能，铰链应在从模具中取出后立即弯曲几次。在一些应用中，例如多腔铰链盖，不可能进行这种弯曲。然而，如果铰链设计合理，它仍然会对应用的要求充分发挥作用。所有的聚丙烯都可以用于活动铰链，但最合适的铰链是用均聚物PP，然后是无规共聚物和冲击PP来。可接受的铰链可以由PP抗冲共聚物形成，但是在铰链区域中有一些潜在的分层。也可以利用二次操作来生产铰链。在约425F（220）下的加热钢模，其在50-100psi（345-690kPa）压力下被迫进入模制部件。也可以使用滚动加热模具; 这个过程通常被称为“压印”铰链。如果聚合物厚度减少到0.005至0.015英寸（0.13-0.38毫米），则会产生令人满意的铰链。该技术可用于将铰链放置在非常大或复杂的部件中。 图42.聚丙烯整体铰链规格附录1抗氧化剂：用于帮助保护塑料不受诸如热，年龄，化学物质，压力等因素的降解的添加剂。抗静电剂：用于帮助消除或减少塑料部件表面静电的添加剂。纵横比：总流量长度与平均壁厚之比。背压：螺杆回收期间施加到塑料上的压力。通过增加背压，改善混合和塑化; 然而，螺杆回收率降低。支撑板：用作模腔，支柱，衬套等的支撑板。老板：在塑料部件上突出设计，以增加强度，方便对齐，提供紧固等。断路器板：见图41。腔体：注射材料的模具内的空间（图43）。耗材：在一个周期内填充模具所需的材料的测量或重量。夹紧板：安装在模具上并用于将模具固定到压板上的板（图43）。夹紧压力：施加到模具上的压力在一个循环期间保持关闭，通常以吨表示。闭环控制：用于监控温度，压力和时间的完整，注塑成型工艺条件的系统，以及自动进行任何更改，以使零件生产保持在预设公差范围内。共注射成型：一个特殊的超材料注射过程，其中一个模具腔首先部分填充一个塑料，然后注射注射以封装第一次注射.冷却通道：在模具主体内的通道，冷却介质通过该通道循环以控制模具表面温度。澄清器：用于添加剂聚丙烯无规共聚物以提高透明度。冷料井：周期中留下的额外的材料，以保证帽子的部分被包装在他的时间。循环：在完成一整套模具的过程中的完整操作顺序。当再次达到该点时，在操作结束的某一点进行循环。开模距离：夹紧单元在完全打开位置的固定和移动板之间的最大距离。分层：当成品零件的表面分离或分层时，由层组成。地层或鱼鳞型外观这里的层可能分开。隔膜门：用于对称腔体填充，以减少焊缝线形成并提高填充率。直接浇口：直接进入模腔的浇道。分散助剂：放置在塑化器喷嘴中的穿孔板在着色剂流过穿孔时有助于混合或分散着色剂（图41）。牵伸：模腔侧壁的锥度或间隙角设计为便于将零件从模具中取出。图43.显示了一些典型的注塑模具的示意图，其中指出了一些要点喷雾：在填充或拍摄模具时，从塑化器喷嘴或喷嘴冲洗区域挤出或泄漏熔融树脂。停留：在模具完全关闭之前，在注射循环期间暂停对模具施加的压力。 这种停留允许形成或存在的任何气体从成型材料中逸出。推杆：当模具打开时从后方推入模腔中的推杆将加工零件压出模具。也称为顶针。推出器返回销：当模具关闭时将推出器组件推回。 也称为表面引脚或返回引脚。喷射杆：当模具打开时启动喷射器组件的杆。家庭模具：多腔模具，其中每个空腔形成组装的成品部件的组成部件之一。风门：用于通过将开口扩展到更宽的区域来帮助减少门区的应力集中的门。 通常可以通过使用这种类型的门来预期零件翘曲变小。填充：根据需要填充模具的空腔或空腔，以提供无闪光的完整零件。翅片：残留在模制零件中的孔或开口中的材料网，必须移除才能进行最终组装。闪光灯：额外的塑料通常沿着模具分型线连接到成型机上。流程：在成型过程中定性描述塑料材料的流动性。 其成型性的量度通常表示为熔体流动速率或熔体指数。流动线：标记在成品上可见，表示熔体流入模具的方向。流动标记：由于熔体流入模具而引起的模制件上的波浪表面外观。浇口：熔体进入模腔的孔口。滚刀：淬硬钢的主模型。 炉具用于将模具的形状沉入软金属块中。均聚物：由单一单体聚合而产生的塑料。料斗干燥机：从树脂颗粒中除去水分的辅助设备。料斗装载机：将树脂颗粒自动装载到机器料斗中的辅助设备。热流道模具：模具，其中流道与冷冻腔隔绝并保持热。热流道模具制造的零件没有废料。注射压力：将注射材料注入模具时，注射螺杆或压头的表面上的压力通常以psi表示。绝缘转轮：参见热流道模具。艾佐德冲击试验：通过将样品棒保持在一端并通过冲击破碎来测试样品的冲击强度。 样品样品可以是缺口或无缺口的。喷射：由不正确的浇口引起的熔体中的湍流，或薄壁部分变薄。夹具：在制造过程中保持组件的零件的工具。编织线：见焊缝。敲击销：用于将成品从模具中敲出的杆或装置。L / D比率：用于帮助定义注射螺杆的术语。 这是螺杆的长径比。熔体流动速率：在压力和温度的规定条件下，通过孔口挤出的聚合物的重量，测定聚合物的熔融粘度。特定条件取决于被测聚合物的类型。 MFR通常以每10分钟克数报告。 熔体流动速率限定了聚丙烯树脂的流动。使用在446F（230）下的2160克的挤出重量。熔体指数：定义聚乙烯树脂熔体流动速率的术语。 使用在310F（190）下的2160克的挤出重量。模具更换机：一种用于从机器中取出一个模具并用另一个模具更换的自动化设备。模具框架：一系列包含模具部件的钢板，包括模腔，芯，流道系统，冷却系统，喷射系统等。模具温度控制单元：用于控制模具温度的辅助设备。 有些单元可以加热和冷却模具。其他的叫冷水机，只能冷却模具。移动压板：由液压油缸或机械扳手移动的注塑机的压板。（图21-22）多腔模具：具有两个或多个印模的模具，用于在一个机器周期中形成成品。多材料成型：在单个成型周期内，依次将两种或三种材料注入单个模具中。 注塑机配有两个或三个塑料机。（另见共注射）巢板：在模具中具有用于腔体块的凹陷区域的保持板。止回阀：允许材料沿一个方向流动并关闭的螺杆头，以防止回流并将材料注入模具。喷嘴：中空的金属鼻子拧入塑化器的注射端。 喷嘴匹配模具中的凹陷。 该喷嘴允许熔体从塑化器转移到流道系统和空腔。成核剂：与聚丙烯一起使用的添加剂，通过提供晶体生长的附加位点来提高结晶速率。橙皮：在粗糙和微小的模制部件上的表面光洁度。 通常由模具腔内的水分引起。包装：模腔或空腔尽可能填充而不会对模具造成不适当的压力或导致成品上出现闪光。零件拾取器：通常安装在固定压板上的辅助单元，该辅助单元在下一个成型周期之前到达打开的模具中抓取零件并将其移除。也称为机器人，当您不想在弹出时将零件从模具中丢弃时使用该设备。分型线：在完成的部分，这条线显示了两个半模在关闭时遇到的位置。精密门：直径0.030英寸或以下的限制门，该闸门在热流道模具上很常见。 活塞：看内存。塑化：通过加热和混合软化。塑化器：注塑机上完全熔化注塑单元。压板：安装有半模的压力机的安装板。析出:塑料添加剂在机械加工过程中溢出。柱塞：看内存。压力垫：硬化钢筋分布在模具表面的死区周围，以帮助土地吸收关闭的最终压力而不会塌陷。清洗：在成型新材料之前，用另一种材料将一种成型材料从塑化剂中挤出。 使用特殊的清洗化合物。RAM：在迫使熔体进入模腔中的塑化机筒螺杆的向前运动。恢复时间：螺丝旋转并创建镜头的时间长度。限制门：注射模具中流道和腔之间的孔很小。当零件被弹出时，这个门很容易脱离流道系统。通常，零件通过一个滑槽和流道系统通过另一个导向造粒机和废品回收系统。垫板：模塑时可安装可拆卸件（如模腔，顶针，导销和衬套）的板。可收缩芯：当模制零件在不垂直于零件从模具中排出的方向的空腔中时使用。 在模具打开之前，芯子从模具中自动拉出，并且当模具再次关闭并且在注入之前重新插入。肋：模制件的加强件。环形浇口：用于一些圆柱形的形状。 该浇口围绕芯部以允许熔体首先在填充空腔之前围绕芯部移动。机器人：自动装置，用于在从打开的模具中弹出时移除零件，而不是使零件掉落。 另见零件选择器。 机器人还可以执行次要功能，例如检查，脱胶，输送机上零件的精确放置等。RMS粗糙度：测量材料的表面粗糙度/平滑度。 使用Profilometer测定表面“峰谷”的均方根（RMS）平均值。 数字越小，表面越平滑：一个或两个的读数将是非常抛光和光滑的表面。洛氏硬度：材料的表面硬度的测量值。当钢压头的载荷从固定的最小值增加到较高的值时，从印模的深度增加得到的值，然后返回到最小值。 这些值引用与对应于与给定的负载和压头组合相关的标度的字母前缀。流道：将浇口与浇口连接以将熔体传送到腔体的通道。无流道成型：见热流道模具。螺丝移动：当填充模具腔时螺钉向前移动的距离。短镜头：未能完全填充模具的模具或腔体。喷射：在成型周期中注入的熔体的量，包括填充流道系统的熔体的数量。注射量：通常基于聚苯乙烯，这是通过单次注射冲程可以移位或注射的塑料的最大重量。 通常表示为聚苯乙烯盎司。收缩：模制件与实际模具尺寸之间的尺寸差异。侧杆：用于承载一个或多个成型销并从模具外部操作的松散件。侧拉销：用于在除模具闭合线以外的方向上钻出孔的突出部，并且在将部件从模具中排出之前必须将其取出。参见可伸缩内核。银条纹:看到倾斜的标志。单腔模具：一个模具只有一个模腔，每个循环只生产一个成品零件。缩痕：通过空腔收缩或低填充产生的成品表面上的浅凹陷或凹坑。润滑剂：用于在塑料加工过程中及之后立即进行润滑的添加剂。滑动平面：由于焊接不良或冷却时收缩，生成在成品零件中或其上。螺旋流动：通过将样品注塑成螺旋模具进行测试，并用于加工性能不同的树脂上。启动标志:看到倾斜的标志。喷射标记：在成品部件的表面上标记或突出显示可能是由于熔体通过浇口而进入其设置的冷腔引起的缺陷。分割模具：模具，其中分割腔体组装在通道中以允许在模制件中形成底切。 这些部件从模具中弹出，然后与该部件分离。浇口套：在模具中的硬化钢插件，其接受塑化器喷嘴并提供用于转移熔体的开口。直接浇口:一项针对从喷嘴的模具型腔熔体流动。浇注锁：通过底切保留在冷块中的树脂部分。该锁用于在模具打开时将浇口从衬套中拉出。 浇道锁本身由推杆从模具中推出。浇口：喷嘴与腔或流道系统之间注射成型的进料口。堆叠模具：两个或多个类似类型的模具，一个定位在另一个之后，以允许在一个循环期间制造额外的零件。固定式压板：模具前板固定的注塑机的大前板。 该压板在正常操作期间不移动。应力开裂：应力开裂有三种类型： 1.热应力开裂是由于部件长时间暴露于高温或阳光下引起的。 2.当部件处于内部或外部诱发应变时，部件的晶体和非晶部分之间发生物理应力开裂。 3.当液体或气体渗入部件表面时，会发生化学应力开裂。所有这些类型的应力开裂都具有相同的最终结果：模制件的分裂或压裂。条纹：在指示熔体流动方向或冲击的模制零件表面上显而易见。串联：当模具打开并且该区域中的熔体没有充分冷却时，在成品部件和浇道之间发生。脱模板：从芯柱或强力塞上剥下模制件的板。脱模板通过模具的打开而被操作。结构泡沫成型：制造具有固体外皮和发泡芯的零件的工艺。潜伏浇口:从喷嘴打开到模具型腔位于分型线以下。也称为隧道门。回吸：当螺杆返回时，浇口上的压力不能保持足够长的时间以使熔体冷却。 空腔或流道系统中的一些熔体可能会扩展回到喷嘴中并导致成品部件上的凹陷。护耳型浇口：一个与模制品相同厚度的小型可拆卸片，但通常垂直于零件，便于拆卸。拉杆间距：注射成型机上水平连杆之间的间距。 基本上，这种测量限制了可以放置在连杆之间并进入成型机的模具的尺寸。切换：一种通过在膝关节上施加力来施加压力的夹紧机构。 一个开关用于关闭压力机上的模具并施加压力。隧道门:见潜伏浇口。底切：阻止从两件式刚性模具退出的突起或凹痕。 17大连交通大学2017届本科生毕业设计调研报告 大连交通大学2017届本科生毕业设计调研报告毕业设计（论文）题目： 快餐勺子注塑模具设计 学 院：机械工程学院专业班级：机械131班 姓 名：崔达 学 号：1304010815 指导教师：朱建宁 2017 年3月 2一、课题的来源及意义 注塑模具是生产各种工业产品的重要工艺装备【1】，随着塑料工业的迅速发展，以及塑料制品在航空、航天、电子、机械、船舶和汽车等工业部门的推广应用，产品对模具的要求也越来越高，传统的模具设计方法已无法适应当今的要求. 与传统的模具设计相比，计算机辅助工程（CAE）技术无论是在提高生产率、保证产品质量方面，还是在降低成本、减轻劳动强度方面，都具有极大的优越性。模具应用广泛，现代制造业中的产品构件成形加工，几乎都需要使用模具来完成【2】。所以，模具产业是国家高新技术产业的重要组成部分，是重要的、宝贵的技术资源。优化模具系统结构设计和型件的CAD/CAE/CAM，并使之趋于智能化，提高型件成形加工工艺和模具标准化水平，提高模具制造精度与质量，降低型件表面研磨、抛光作业量和制造周期；研究、应用针对各种类模具型件所采用的高性能、易切削的专用材料，以提高模具使用性能；为适应市场多样化和新产品试制，应用快速原型制造技术和快速制模技术，以快速制造成型冲模、塑料注射模或压铸模等，应当是未来520年的模具生产技术的发展趋势.这也是我们进行本次设计的目的【3】。本次设计以快餐勺子为设计课题，并应熟练地使用ProE、AutoCAD等软件来完成课题。在设计过程中，应对我们设计方法、软件绘图、资料查询、论文写作、外文翻译等方面进行全方位的训练，培养我们初步的设计能力，并加强我们对模具行业的理解和认知。二、国内外发展状况2.1国内发展状况历经半个多世纪,我国的模具工业水平有了飞跃的发展,高效、复杂、大型、精密、长寿命的模具在整个模具产量中所占的比重越来越大,模具水平有了较大提高【4】。虽然中国模具工业发展迅速,但与需求相比,显然供不应求,其主要缺口集中于精密、大型、复杂、长寿命模具领域。由于在模具精度、寿命、制造周期及生产能力等方面。中国与国际平均水平和发达国家仍有较大差距，因此每年需要大量进口模具。近年来，我国塑料模具水平已有较大提高【5】。大型塑料模具已能生产单套重量达50t以上的注塑模，精密塑料模的精度已可达到3m，制件精度为0.5m的小模数齿轮模具及达到高光学要求的车灯模具等也已能生产，多腔塑料模已能生产一模7800腔的塑封模，高速模具方面已能生产4m/min以上挤出速度的高速塑料异型材挤出模及主型材双腔共挤、双色共挤、软硬共挤、后共挤、再生料共挤出和低发泡钢塑共挤等各种模具。在生产手段上，模具企业设备数控化率已有较大提高，CAD/CAE/CAM技术的应用面已大为扩大，高速加工及RP/RT等先进技术的采用已越来越多【6】。模具标准件使用覆盖率及模具商品化率都已有较大幅度的提高，热流道模具的比例也有较大提高。三资企业蓬勃发展进一步促进了塑料模具设计制造水平及企业管理水平的提高。但相比于不足，国内生产的小模数塑料齿轮等精密塑料模具已达到国外同类产品水平。在齿轮模具设计中采用最新的齿轮设计软件，纠正了由于成型压缩造成的齿形误差，达到了标准渐开线造型要求【7】。显示管隔离器注塑模、高效多色注射塑料模、纯平彩电塑壳注塑模等精密、复杂、大型模具的设计制造水平也已达到或接近国际水平。使用CAD三维设计、计算机模拟注塑成形、抽芯脱模机构设计新颖等对精密、复杂模具的制造水平提高起到了很大作用。20吨以上的大型塑料模具的设计制造也已达到相当高的水平。34英寸彩电塑壳和48英寸背投电视机壳模具，汽车保险杠和仪表盘的注塑模等大型模具，国内都已可生产。2.2国外发展状况 国外注塑模具制造行业的最基本特征是高度集成化、智能化、柔性化和网络化。追求的目标是提高产品质量及生产效率【8】。国外发达国家模具标准化程度达到70%80%，实现部分资源共享，大大缩短设计周期及制造周期，降低生产成本.最大限度地提高模具制造业的应变能力 满足用户需求。模具企业在技术上实现了专业化，在模具企业的生产管理方面，也有越来越多的采用以设计为龙头、按工艺流程安排加工的专业化生产方式，降低了对模具工人技术全面性的要求，强调专业化。国外注塑成型技术在也向多工位、高效率、自动化、连续化、低成本方向发展。因此，模具向高精度复杂、多功能的方向发展【9】。例如：组合模、即钣金和注塑一体注塑铰链一体注塑、活动周转箱一体注塑；多色注塑等；向高效率、高自动化和节约能源，降低成本的方向发展。例如：叠模的大量制造和应用，水路设计的复杂化、装夹的自动化、取件全部自动化。 目前在欧美，CAD/CAE/CAM已成为模具企业普遍应用的技术。在CAD的应用方面，已经超越了甩掉图板、二维绘图的初级阶段，目前3D设计已达到了70%89%。PRO/E、UG、CIMATRON等软件的应用很普遍。应用这些软件不仅可完成2D设计，同时可获得3D模型，为NC编程和CAD/CAM的集成提供了保证。应用3D设计，还可以在设计时进行装配干涉的检查，保证设计和工艺的合理性。数控机床的普遍应用，保证了模具零件的加工精度和质量。3050人的模具企业，一般拥有数控机床十多台。经过数控机床加工的零件可直接进行装配，使装配钳工的人数大大减少【10】。CAE技术在欧美已经逐渐成熟。在注射模设计中应用CAE分析软件，模拟塑料的冲模过程，分析冷却过程，预测成型过程中可能发生的缺陷。在冲模设计中应用CAE软件，模拟金属变形过程，分析应力应变的分布，预测破裂、起皱和回弹等缺陷。CAE技术在模具设计中的作用越来越大，意大利COMAU公司应用CAE技术后，试模时间减少了50%以上。三、主要研究内容1.塑件建模2.塑料成型工艺分析3.拟定模具结构成型零件设计计算4.浇注系统设计5.成型零件设计计算6.模架选型7.排气槽设计8.脱模推出机构设计9.冷却系统设计10.导向和定位结构设计11.总装图和零件图的绘制12.编写设计说明书四、本课题的研究目标及方法1.首先从产品出发，首先对塑件建模，然后对塑件的尺寸，形状，材料等方面通过查阅手册进行分析，从而得出产品的可行性。2.塑件成型工艺性分析：从外形尺寸、精度等级、脱模斜度、材料性能、材料的注射成型过程及工艺参数等方面做出合理的工艺分析。3.模具结构分析：根据要求，通过计算确定模具的分型面，型腔数量和排列方式，并选出合适的注射机。4.浇注系统的设计：根据塑件，通过Proe软件输出的数据，设计出主流道，并确定出主流道的具体尺寸、凝料体积、当量半径、浇口套等数据，同时计算出分流道和浇口的各个参数，如有冷料穴，需设计出其尺寸。5.成型零件设计计算：选取钢材并通过计算设计出凹凸模的尺寸。6.脱模推出机构：通过查手册，确定推出方式，计算脱模力并且进行校核。7.图纸的绘制和说明书的编写：用 Pro/E、AutoCAD 软件绘制总装图和零件图；将设计过程中相应的数据整理后完成计算说明书的编写。预期达到的目标：在老师指导下，独立完成适用于产品的注射模的设计，并完成英文文献翻译，绘制好图纸及编写好计算说明书。五、进度安排第1 周：写调研报告。第2 周：翻译外文资料。第3 周：确定该塑件零件尺寸，进行工艺分析，制作零件模型；设计分型面、型腔。第4-6 周：设计模体、浇注系统、冷却系统，导向与定位机构，并进行相关计算。第7-9 周：用 Pro/E、AutoCAD 绘制模具零件三维图、零件二维图及实体装配图。第10-11 周：绘制模具装配图，标注尺寸。第12 周：编写计算说明书。第13 周：修改图纸，整理资料。第14 周：准备答辩。六、实验方案的可行性分析经过深入的调研和各方面的查阅资料，本课题的研究十分迎合当代的需求，注塑模具在现代工业中占有很重要的地位，需求量也很大。本课题在制定详细进度表的前提下，再加上老师的悉心指导，我觉得本课题具有较强的可行性。七、参考文献【1】叶久新, 王群. 塑料成型工艺及模具设计M. 机械工业出版社, 2008.【2】孙锡红. 我国塑料模具发展现状及发展建议J. 电加工与模具, 2010(S1):31-33.【3】李良福. 国内外模具加工现状和今后发展J. 装备机械, 1994(2):6-8.【4】董金狮. 我国可降解塑料快餐餐具的研究现状与发展方向J. 铁路节能环保与安全卫生, 1995(3):218-219.【5】工程塑料网标准频道. 聚苯乙烯(PS)的应用范围和注塑特性J. 【6】吴清文, 王莉, 马建旭. 塑料注塑模具CAD概略J. 光学精密工程, 1995, 3(6):11-17.【7】汪佑思. 透明塑料勺注射模优化设计J. 模具制造, 2013, 13(1):55-59.【8】贾长明, 张广兴. 基于Pro/E的快餐碗热流道模具设计J. 天津理工大学学报, 2010, 26(2):79-81.【9】王树勋. 注塑模具设计M. 华南理工大学出版社, 2005.【10】Guo H Y. Plastic Mould Design Optimization Method Research Based on the Reverse Engineering TechnologyJ. Applied Mechanics & Materials, 2013, 278-280:2261-2264.7One Source. More Resourceful.A Guide to Polyolefin Injection MoldingEquistar is one of the largest producers of ethylene, propylene andpolyethylene in the world today. One of the largest, yet we pay atten-tion to even the smallest needs of our customers.Were a leading producer of polypropylene, oxygenated chemicals, performance polymers and resins and compounds for wire and cable.Were an industry leader with an unwavering commitment to beingthe premier petrochemicals and polymers company.Our commitment starts with each of our more than 5,000 employees. It stretches out from our headquarters in Houston across 16 manufac-turing facilities along the U.S. Gulf Coast and in the Midwest. It continues through our 1,400-mile ethylene/propylene distribution system that spans the Gulf Coast.We are the product of many minds coming together with the singlefocus of providing the right product for every customer. Thats whatdrives us to maintain an extended product line, enhanced operatingefficiencies, greater geographic diversity and strong research anddevelopment capabilities. Thats what drives us to provide theresources that help us lead today and rise to the challenges of achanging industry tomorrow.One Source. More Resourceful.1A Guide to Polyolefin Injection MoldingTable of ContentsIntroduction 2Polyolefins are derived from petrochemicals2Molecular structure and composition affect properties and processability2Chain branching3Density 3Molecular weight 3Molecular weight distribution4Copolymers5Modifiers and additives 5Working closely with molders 5How polyolefins are made5Low density polyethylene (LDPE)6High density polyethylene (HDPE)6Linear low density polyethylene (LLDPE)7Polypropylene7Shipping and handling polyolefin resins7Material handling8How to solve material handling problems9Other material handling practices10The injection molding process10Injection units10Plasticator specifications13Screw designs13Nozzles 14Clamp mechanisms 14Clamp specifications15Injection molds16Types of mold16Sprues and runners 17Mold venting18Gating19Mold cooling20Ejection devices20Spiral flow measurement20General injection molding operating procedures21General safety21Heat22Electricity22Machinery motion22The injection molding process and its effect on part performance22The molding cycle 22Shrinkage27Warpage 28Color dispersion and air entrapment 29Part ejection and mold release29Clarity30Gloss30Polypropylene integral hinges30Appendices1: Injection Molding Terms 312: Metric Conversion Guide353: Abbreviations 374: ASTM test methods applicable to polyolefins 385: Injection molding problems, causes and solutions396: ASTM and ISO sample preparation and test procedures 437: Compression and injection molded sample preparation for HDPE44IntroductionPolyolefins are the most widely used plastics for injection molding.This manual, A Guide to PolyolefinInjection Molding, contains generalinformation concerning materials,methods and equipment for producing high quality, injectionmolded, polyolefin products at optimum production rates.Polyolefins that can be injectionmolded include: Low density polyethylene (LDPE) Linear low density polyethylene(LLDPE) High density polyethylene (HDPE) Ethylene copolymers, such asethylene vinyl acetate (EVA) Polypropylene and propylene copolymers (PP) Thermoplastic olefins (TPO)In general, the advantages of injection molded polyolefins com-pared with other plastics are: Lightweight Outstanding chemical resistance Good toughness at lower temperatures Excellent dielectric properties Non hygroscopicThe basic properties of polyolefinscan be modified with a broad range of fillers, reinforcements andchemical modifiers. Furthermore,polyolefins are considered to be relatively easy to injection mold.Major application areas for poly-olefin injection molding are: Appliances Automotive products Consumer products Furniture Housewares Industrial containers Materials handling equipment Packaging Sporting goods Toys and noveltiesThis manual contains extensiveinformation on the injection mold-ing of polyolefins; however, itmakes no specific recommendationsfor the processing of Equistar resinsfor specific applications. For moredetailed information please contactyour Equistar polyolefins sales ortechnical service representative.Polyolefins arederived frompetrochemicalsPolyolefins are plastic resins poly-merized from petroleum-basedgases. The two principal gases areethylene and propylene. Ethylene isthe principal raw material for mak-ing polyethylene (PE) and ethylenecopolymer resins; propylene is themain ingredient for makingpolypropylene (PP) and propylenecopolymer resins.Polyolefin resins are classified asthermoplastics, which means thatthey can be melted, solidified andmelted again. This contrasts withthermoset resins, such as phenolics,which, once solidified, can not bereprocessed.Most polyolefin resins for injectionmolding are used in pellet form.The pellets are about 1/8 inch longand 1/8 inch in diameter and usual-ly somewhat translucent to white incolor. Many polyolefin resins con-tain additives, such as thermal stabi-lizers. They also can be compound-ed with colorants, flame retardants,blowing agents, fillers, reinforce-ments, and other functional addi-tives such as antistatic agents andlubricants.Molecularstructure andcompositionaffect propertiesand processabilityFour basic molecular propertiesaffect most of the resin characteris-tics essential to injection moldinghigh quality polyolefin parts. Thesemolecular properties are: Chain branching Crystallinity or density Average molecular weight Molecular weight distribution The materials and processes used toproduce the polyolefins determinethese molecular properties.The basic building blocks for thegases from which polyolefins arederived are hydrogen and carbonatoms. For polyethylene, theseatoms are combined to form theethylene monomer, C2H4. HH|C = C|HHIn the polymerization process, thedouble bond connecting the carbonatoms is broken. Under the rightconditions, these bonds reform withother ethylene molecules to formlong molecular chains. H H H H H H H H H H| | | C C C C C C C C C C | | |H H H H H H H H H HThe resulting product is polyethyl-ene resin.2A Guide to Polyolefin Injection MoldingFor polypropylene, the hydrogenand carbon atoms are combined toform the propylene monomer,CH3CH:CH2. HH|H C C = C|HHHThe third carbon atom forms a sidebranch which causes the backbonechain to take on a spiral shape.HHHHHH| C C C C C C |H HCH H HCH H HCH|HHHEthylene copolymers, such as ethyl-ene vinyl acetate (EVA), are madeby the polymerization of ethyleneunits with randomly distributedcomonomer groups, such as vinylacetate (VA).Chain branchingPolymer chains may be fairly linear,as in high density polyethylene, orhighly branched as in low densitypolyethylene. For every 100-ethyleneunits in the polyethylene molecularchain, there can be one to ten shortor long branches that radiate three-dimensionally (Figure 1). The degreeand type of branching are con-trolled by the process (reactor), cat-alyst, and/or any comonomers used.Chain branching affects many ofthe properties of polyethylenesincluding density, hardness, flexibili-ty and transparency, to name a few.Chain branches also become pointsin the molecular structure whereoxidation may occur. If excessivelyhigh temperatures are reached during processing, oxidation canoccur which may adversely affectthe polymers properties. This oxida-tion or degradation may causecross-linking in polyethylenes andchain scission in polypropylenes.Polypropylene, on the other hand,can be described as being linear (no branching) or very highlybranched. Although the suspendedcarbon forms a short branch onevery repeat unit, it is also responsi-ble for the unique spiral and linearconfiguration of the polypropylenemolecule.DensityPolyolefins are semi-crystalline poly-mers which means they are com-posed of molecules which arearranged in a very orderly (crystalline)structure and molecules which arerandomly oriented (amorphous). Thismixture of crystalline and amorphousregions (Figure 2) is essential in providing the desired properties toinjection molded parts. A totallyamorphous polyolefin would begrease-like and have poor physicalproperties. A totally crystalline poly-olefin would be very hard and brittle.HDPE resins have linear molecularchains with comparatively few sidechain branches. Therefore, thechains are packed more closelytogether (Figure 3). The result iscrystallinity up to 95 percent. LDPEresins generally have crystallinityfrom 60 percent to 75 percent.LLDPE resins have crystallinity from60 percent to 85 percent. PP resinsare highly crystalline, but they arenot very dense. PP resins have anominal specific gravity range of0.895 to 0.905 g/cm3, which is thelowest for a commodity thermo-plastic and does not vary appreciablyfrom manufacturer to manufacturer.For polyethylene, the density andcrystallinity are directly related, thehigher the degree of crystallinity,the higher the resin density. Higherdensity, in turn, influences numer-ous properties. As density increases,heat softening point, resistance togas and moisture vapor permeationand stiffness increase. However,increased density generally results in a reduction of stress crackingresistance and low temperaturetoughness. LDPE resins have densities rang-ing from 0.910 to 0.930 gramsper cubic centimeter (g/cm3) LLDPE resins range from 0.915 to0.940 g/cm3 HDPE resins range from 0.940to 0.960 g/cm3As can be seen, all natural poly-olefin resins, i.e, those without anyfillers or reinforcements, have densities less than 1.00 g/cm3. Thislight weight is one of the keyadvantages for parts injection mold-ed from polyolefins. A generalguide to the effects of density onthe properties for various types ofpolyethylene resins is shown inTable 1.Molecular weightAtoms of different elements, such ascarbon, hydrogen, etc., have differ-ent atomic weights. For carbon, theatomic weight is 12 and for hydro-gen it is one. Thus, the molecularweight of the ethylene unit is thesum of the weight of its six atoms(two carbon atoms x 12 + fourhydrogen x 1) or 28.3Figure 1. Polyethylene chain withlong side branchesC C C C C C C C C C C C C C C C CCCCCCCCCCCCCCCCC CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCFigure 2. Crystalline (A) and amor-phous (B) regions in polyolefinFigure 3. Linear polyethylenechain with short side branchesC C C C C C C C C C C C C C C C CCCCCUnlike simple compounds, like ethylene or propylene, every poly-olefin resin consists of a mixture oflarge and small chains, i.e., chainsof high and low molecular weights.The molecular weight of the polymer chain generally is in thethousands and may go up to overone million. The average of these is called, quite appropriately, theaverage molecular weight.As average molecular weightincreases, resin toughness increases.The same holds true for tensilestrength and environmental stresscrack resistance (ESCR) crackingbrought on when molded parts aresubjected to stresses in the pres-ence of materials such as solvents,oils, detergents, etc. However, high-er molecular weight results in anincrease in melt viscosity andgreater resistance to flow makinginjection molding more difficult asthe average molecular weightincreases.Melt flow rate (MFR) is a simplemeasure of a polymers melt vis-cosity under standard conditions of temperature and static load(pressure). For polyethylenes, it isoften referred to as melt index (MI).MFR is the weight in grams of amelted resin that flows through astandard-sized orifice in 10 minutes(g/10 min). Melt flow rate isinversely related to the resins average molecular weight: as theaverage molecular weight increases,MFR decreases and vice versa.Melt viscosity, or the resistance of a resin to flow, is an extremelyimportant property since it affectsthe flow of the molten polymer filling a mold cavity. Polyolefins with higher melt flow rates requirelower injection molding processingpressures, temperatures and shortermolding cycles (less time needed for part cooling prior to ejectionfrom the mold). Resins with highviscosities and, therefore, lowermelt indices, require the oppositeconditions for injection molding.It should be remembered that pressure influences flow properties.Two resins may have the same meltindex, but different high-pressureflow properties. Therefore, MFR or MI must be used in conjunctionwith other characteristics, such as molecular weight distribution, to measure the flow and otherproperties of resins. Generally, injection molding resins are char-acterized as having medium, highor very high flow.For injection molding grades, theMFR (MI) values for polyethylenesare generally determined at 190C(374F) using a static load of 2,160 g. MFR values for polypropy-lenes are determined at the sameload but at a higher temperature230C (446F). The MFR of otherthermoplastics may be determinedusing different combinations oftemperatures and static load. Forthis reason, the accurate predictionof the relative processability of different materials using MFR datais not possible.Molecular weightdistributionDuring polymerization, a mixture ofmolecular chains of widely varyinglengths is produced. Some may beshort; others may be extremely longcontaining several thousandmonomer units.The relative distribution of large,medium and small molecular chainsin the polyolefin resin is importantto its properties. When the distribu-tion is made up of chains close tothe average length, the resin is saidto have a “narrow molecularweight distribution.” Polyolefinswith “broad molecular weight distribution” are resins with a widervariety of chain lengths. In general,resins with narrow molecularweight distributions have good low-temperature impact strength andlow warpage. Resins with broadmolecular weight distributions generally have greater stress crack-ing resistance and greater ease ofprocessing (Figure 4).The type of catalyst and the polymerization process used to produce a polyolefin determines its molecular weight distribution.The molecular weight distribution(MWD) of PP resins can also bealtered during production by con-trolled rheology additives that selec-tively fracture long PP molecular4AS MELT INDEXAS DENSITYINCREASESINCREASESDurometer hardness (surface)remains the sameincreasesGlossimprovesimprovesHeat resistance (softening point)remains the sameimprovesStress crack resistancedecreasesdecreasesMechanical flex lifedecreasesdecreasesProcessability (less pressure to mold)improvesremains the sameMold shrinkagedecreasesincreasesMolding speed (faster solidification)remains the sameincreasesPermeability resistanceremains the sameimprovesStiffnessremains the sameincreasesToughnessdecreasesdecreasesTransparencyremains the samedecreasesWarpagedecreasesincreasesTable 1. General guide to the effects of polyethylene physical propertieson properties and processingchains. This results in a narrowermolecular weight distribution and ahigher melt flow rate.CopolymersPolyolefins made with one basictype of monomer are calledhomopolymers. There are, however,many polyolefins, called copoly-mers, that are made of two or moremonomers. Many injection moldinggrades of LLDPE, LDPE, HDPE andPP are made with comonomers thatare used to provide specific propertyimprovements.The comonomers most often usedwith LLDPE and HDPE are calledalpha olefins. They include butene,hexene and octene. Othercomonomers used with ethylene tomake injection molding grades areethyl acrylate to make the copoly-mer ethylene ethyl acrylate (EEA)and vinyl acetate to produce ethyl-ene vinyl acetate (EVA).Ethylene is used as a comonomerwith propylene to producepolypropylene random copolymers.Polypropylene can be made moreimpact resistant by producing ahigh ethylene-propylene copolymerin a second reactor forming a finelydispersed secondary phase of ethyl-ene-propylene rubber. Productsmade in this manner are commonlyreferred to as impact copolymers.Modifiers andadditivesNumerous chemical modifiers andadditives may be compounded withpolyolefin injection molding resins.In some grades, the chemical modi-fiers are added during resin manu-facture. Some of these additivesinclude: Antioxidants Acid scavengers Process stabilizers Anti-static agents Mold release additives Ultraviolet (UV) light stabilizers Nucleators Clarifiers LubricantsWorking closelywith moldersEquistar offers a wide range ofpolyolefin resins for injection mold-ing, including AlathonHDPE,AlathonLDPE, PetrotheneLDPEand LLDPE, EquistarPP, UltratheneEVA copolymers and FlexatheneTPOs. These resins are tailored tomeet the requirements of manyareas of application. Polyolefin resins with distinctly dif-ferent properties can be made bycontrolling the four basic molecularproperties during resin productionand by the use of modifiers andadditives. Injection molders canwork closely with their Equistarpolyolefins sales or technical servicerepresentative to determine theresin which best meets their needs.Equistar polyolefins technical servicerepresentatives are also available toassist injection molders and end-users by providing guidance for tooland part design and the develop-ment of specialty products to fulfillthe requirements of new, demand-ing applications.How polyolefinsare madeHigh-purity ethylene and propylenegases are the basic feedstocks formaking polyolefins (Figure 5). Thesegases can be petroleum refinery by-products or they can be extractedfrom an ethane/propane liquifiedgas mix coming through pipelines5Figure 4. Schematic representationof molecular weight distributionMOLECULAR WEIGHTNarrow MolecularWeight DistributionBroad MolecularWeight DistributionPERCENTAGE OF EACH MOLECULAR WEIGHTMIXEDFEEDSTOCKLPG, HYDROCARBONS,AND FUEL COMPONENTSFRACTIONATIONCOLUMNETHANE AND PROPANEFEED TO CRACKERETHYLENE ANDPROPYLENEETHYLENECRACKERSEPARATIONCOLUMNPROPYLENEETHYLENEPURIFIEDETHYLENETO PIPELINE ORPOLYMERIZATIONPURIFICATION COLUMNSPURIFIEDPROPYLENETO PIPELINE ORPOLYMERIZATION654321654321Figure 5. Olefin manufacturing processfrom a gas field. High efficiency inthe ethane/propane cracking andpurification results in very pure ethylene and propylene, which arecritical in the production of highquality polyolefins.Equistar can produce polyolefins bymore polymerization technologiesand with a greater range of catalysts than any other suppliercan. Two of Equistars plants arepictured in Figure 6.Low densitypolyethylene (LDPE)To make LDPE resins, Equistar useshigh pressure, high temperaturetubular and autoclave polymeriza-tion reactors (Figures 7 and 8).Ethylene is pumped into the reac-tors and combined with a catalystor initiator to make LDPE. The LDPEmelt formed flows to a separatorwhere unused gas is removed,recovered, and recycled back intothe process. The LDPE is then fed toan extruder for pelletization.Additives, if required for specificapplications, are incorporated atthis point.High densitypolyethylene (HDPE)There are a number of basicprocesses used by Equistar for mak-ing HDPE for injection moldingapplications including the solutionprocess and the slurry process. Inthe multi-reactor slurry process usedby Equistar (Figure 9), ethylene anda comonomer (if used), togetherwith an inert hydrocarbon carrier,are pumped into reactors wherethey are combined with a catalyst.However, in contrast to LDPE pro-duction, relatively low pressures andtemperatures are used to produceHDPE. The granular polymer leavesthe reactor system in a liquid slurryand is separated and dried. It isthen conveyed to an extruderwhere additives are incorporatedprior to pelletizing.Equistar also utilizes a multi-reactorsolution process for the production6Figure 6. Left, polypropylene unit at Morris, Illinois plant. Right, HDPE unitat Matagorda, Texas plantFigure 8. LDPE high temperature autoclave process diagramETHYLENEHIGH PRESSUREAUTOCLAVEREACTORADDITIVESUNREACTED MONOMERTO RECOVERYPOLYETHYLENE MELTFIRST STAGECOMPRESSORSECOND STAGECOMPRESSORSECOND STAGESEPARATORFIRST STAGESEPARATORPOLYETHYLENE MELTHOT MELT EXTRUDERFigure 7. LDPE high temperature tubular process diagramETHYLENEHIGH PRESSURETUBULAR REACTORADDITIVESUNREACTED MONOMERTO RECOVERYPOLYETHYLENE MELTFIRST STAGECOMPRESSORSECOND STAGECOMPRESSORSECOND STAGESEPARATORFIRST STAGESEPARATORPOLYETHYLENE MELTHOT MELT EXTRUDERof HDPE (Figure 10). In this process,the HDPE formed is dissolved in thesolvent carrier and then precipitatedin a downstream process. An addi-tional adsorption step results in avery clean product with virtually nocatalyst residues.Because both of these processesutilize multiple reactors, Equistarhas the capability of tailoring and optimizing the molecularweight distribution of the variousproduct grades to provide a uniquerange of processability and physicalproperties.Linear low densitypolyethylene (LLDPE)Equistar uses a gas phase processfor making LLDPE (Figure 11). Thisprocess is quite different from theLDPE process, but somewhat similarto the HDPE process. The major differences from the LDPE processare that relatively low pressure andlow temperature polymerizationreactors are used. Another differ-ence is that the ethylene is copoly-merized with butene or hexenecomonomers in the reactor. UnlikeHDPE, the polymer exits the reactorin a dry granular form, which issubsequently compounded withadditives in an extruder.With changes in catalysts and operating conditions, HDPE resinsalso can be produced in some ofthese LLDPE reactors.PolypropyleneTo make PP, Equistar uses both avertical, stirred liquid-slurry process(Figure 12) and a vertical, stirred,fluidized-bed, gas-phase process(Figure 13). Equistar was the firstpolypropylene supplier in the UnitedStates to use gas-phase technologyto produce PP. Impact copolymersare produced using two, fluidizedbed, gas phase reactors operatingin series.Equistars polyolefin productionfacilities are described in Table 2.Shipping andhandlingpolyolefin resinsIt is of utmost importance to keeppolyolefin resins clean. Equistarships polyolefin resins to molders inhopper cars, hopper trucks, corru-gated boxes, and 50-pound plasticbags. Strict quality control through-out resin manufacture and subse-quent handling, right through delivery to the molder, ensures thecleanliness of the products.When bulk containers are delivered,the molder must use appropriateprocedures for unloading the resin.Maintenance of the in-plant materi-al handling system also is essential.When bags and boxes are used,7Figure 9. HDPE parallel reactors slurry processPOWDERDRYERADDITIVESUNREACTED MONOMERS TO RECOVERYADDITIVE BLENDERSTIRREDREACTORVESSELHOLD VESSELSPOWDER SLURRYEXTRUDERSTIRREDREACTOR VESSELSEPARATIONVESSELETHYLENEBUTENECATALYSTETHYLENEBUTENECATALYSTPOWDERFEEDFigure 10. HDPE solution processFIRST STAGE PARALLEL REACTORSSECOND STAGEREACTORADDITIVESHOT MELT EXTRUDERETHYLENEOCTENECATALYSTSOLVENTETHYLENESOLVENTTHREE STAGESEPARATOR SYSTEMUNREACTEDMONOMERSAND SOLVENTTO RECOVERYADSORPTION UNIT(CATALYST REMOVAL)TUBULAR REACTORspecial care is necessary in openingthe containers, as well as coveringthem, as they are unloaded.Reground resin, whether used as ablend or as is, should also be strin-gently protected to keep it free ofcontamination. Whenever possible,the regrind material should be usedas it is generated. When this is notpossible, the scrap should be col-lected in a closed system and recy-cled with the same precautionstaken for virgin resin. In all cases,the proportion of regrind usedshould be carefully controlled toassure consistency of processingand part performance.Material handlingEquistar utilizes material handlingsystems and inspection proceduresthat are designed to prevent exter-nal contamination and productcross-contamination during produc-tion, storage, loading and shipment. Since polyolefin resins are non-hygroscopic (do not absorb water)they do not require drying prior tobeing molded. However, under certain conditions, condensationmay form on the pellet surfaces.When cartons of resin are movedfrom a cold warehouse environmentto a warm molding area or whentransferring cold pellets from a siloto an indoor storage system, thetemperature of the material shouldbe allowed to equilibrate, for up toeight hours to drive off any conden-sation before molding. The best way to improve resin uti-lization is to eliminate contaminantsfrom transfer systems. If bulk han-dling systems are not dedicated toone material or are not adequatelypurged, there is always the possibili-ty of contamination resulting fromremnants of materials previouslytransferred. 8Figure 11. LLDPE fluidized bed processREACTOR POWDERADDITIVESUNREACTED MONOMERS TO RECOVERYADDITIVE BLENDERFLUIDIZEDBED REACTOREXTRUDERCATALYSTPOWDERFEEDBUTENE ORHEXENEETHYLENEBAYPORT, TXPolypropyleneLow Density PolyethyleneCHOCOLATE BAYOU, TXHigh Density PolyethyleneCLINTON, IALow Density PolyethyleneHigh Density PolyethyleneLAPORTE, TXLow Density PolyethyleneLinear Low Density PolyethyleneMATAGORDA, TXHigh Density PolyethyleneMORRIS, ILLow Density PolyethyleneLinear Low Density PolyethylenePolypropyleneVICTORIA, TXHigh Density PolyethyleneTable 2. Equistar polyolefin production facilitiesFigure 12. PP slurry processWET REACTOR POWDERADDITIVESADDITIVE BLENDEREXTRUDERPOWDERFEEDDILUENT AND UNREACTEDMONOMER TO RECOVERYSTIRRED REACTORVESSELPROPYLENECATALYSTDILUENTPOWDER DRYERSEPARATIONVESSELOccasionally, clumps of “angel hair”or “streamers” may accumulate in a silo and plug the exit port.Contaminants of this type can alsocause plugging of transfer systemfilters and/or problems that affectthe molding machine. All of theseproblems can result in moldingmachine downtime, excessive scrapand the time and costs of cleaningsilos, transfer lines and filters.Polyolefin dust, fines, streamers andangel-hair contamination may begenerated during the transfer ofpolymer through smoothbore piping. These transfer systems alsomay contain long radius bends toconvey the resin from a hopper carto the silo or holding bin. A poly-olefin pellet conveyed through atransfer line travels at a very highvelocity. As the pellet contacts thesmooth pipe wall, it slides and friction is generated. The friction, in turn, creates sufficient heat toraise the temperature of the pelletsurface to the resins softeningpoint. As this happens, a smallamount of molten polyolefin isdeposited on the pipe wall andfreezes almost instantly. Over time,this results in deposits described asangel hair or streamers.As the pellets meet the pipe wall,along the interior surface of a longradius bend, the deposits becomealmost continuous and streamersare formed. Eventually, the angelhair and streamers are dislodgedfrom the pipe wall and find theirway into the molding process, thestorage silo or the transfer filters.The amount of streamers formedincreases with increased transfer airtemperature and velocity.Other good practices of materialhandling include control (cooling) of the transfer air temperature tominimize softening and melting ofthe pellets. Proper design of thetransfer lines is also critical in termsof utilizing the optimum bend radii,blind tees, and proper angles.Consult your Equistar technical service engineer for guidance in this area.How to solve materialhandling problemsSince smooth piping is a leadingcontributor to angel hair andstreamers, one solution is to rough-en the interior wall of the piping.This causes the pellets to tumbleinstead of sliding along the pipe,minimizing streamer formation.However, as the rapidly movingpolyolefin pellets contact anextremely rough surface, small particles may be broken off the pellets creating fines or dust.Two pipe finishes, in particular, haveproven to be effective in minimizingbuildup and giving the longest lifein transfer systems. One is a sand-blasted finish of 600 to 700 RMSroughness. This finish is probablythe easiest to obtain. However, dueto its sharp edges, it will initiallycreate dust and fines until theedges become rounded.The other finish is achieved withshot blasting using a #55 shot with 55-60 Rockwell hardness toproduce a 900 RMS roughness.Variations of this finish are com-monly known as “hammer-finished”surfaces. The shot blasting allowsdeeper penetration and increaseshardness, which in turn leads tolonger surface life.The rounded edges obtained minimize the initial problemsencountered with dust and fines.They also reduce metal contamina-tion possibly associated with thesandblasted finish.Whenever a new transfer system isinstalled or when a portion of anexisting system is replaced, the interior surfaces should be treatedby either sand or shot blasting. Theinitial cost of having this done is faroutweighed by the prevention offuture problems.Elimination of long-radius bendswhere possible is also important asthey are probably the leading contrib-utor to streamer formation. Whenthis type of bend is used, it is criticalthat the interior surface should beeither sand- or shot-blasted.The use of self-cleaning, stainlesssteel “tees” in place of long bendsprevents the formation of streamersalong the curvature of the bend,causing the resin to tumble insteadof slide (Figure 14). However, thereis a loss of efficiency within thetransfer system when this method isused. Precautions should be taken9Figure 13. PP dual reactors gas-phase processREACTOR POWDERADDITIVESADDITIVE BLENDEREXTRUDERPOWDER FEEDUNREACTED MONOMERSTO RECOVERYSTIRRED SECONDARYREACTOR VESSELETHYLENESEPARATIONVESSELSTIRRED PRIMARYREACTOR VESSELPROPYLENECATALYSTto ensure that sufficient blowercapacity is available to prevent clog-ging of the transfer lines and main-tain the required transfer rate. To extend the life of the transferpiping, it should be rotated 90 atperiodic intervals. Resin pellets tendto wear grooves in the bottom ofthe piping as they are transferredwhich not only contributes to fines and streamer formation butalso accelerated wear due to non-uniform abrasion. Regardless of the type of equip-ment used or the materials trans-ferred, a transfer system should bemaintained and kept clean in thesame manner as any other piece ofproduction equipment. Periodicwashing and drying of silos andholding bins reduces the problem of fines and dust build-up due tostatic charges.Other steps to eliminate contamina-tion include: Inspect the entire transfer systemon a regular basis Clean all filters in the transfersystem periodically Ensure that the suction line is notlying on the ground during stor-age or when the system is start-ed to prevent debris from enter-ing the system Place air filters over hopper carhatches and bottom valves dur-ing unloading to prevent debrisor moisture from contaminatingthe material Purge the lines with air and thenwith a small amount of productprior to filling storage silos or bins Allow blowers to run for severalminutes after unloading to clear the lines and reduce thechance of cross-contamination of product.Information regarding transfer sys-tems and types of interior finishesavailable can be obtained frommost suppliers of materials handlingequipment or by consulting yourEquistar technical service engineer.Complete systems can be suppliedwhich, when properly maintained,efficiently convey contamination-free product.A general reference manual,“Handling and Storage of EquistarPolyolefins” is also available.Other materialhandling practices Beside-the-press vacuum loaders areused to feed many injection mold-ing machines. These units drawresin pellets from drums or cartonsplaced beside the machine. In someset-ups, the vacuum loaders drawfrom multiple sources and directlyfeed the hopper with resin, regrind, colorants and other concentrateadditives. Good housekeeping procedures are particularly impor-tant when working with beside-the-press loaders since contaminantscan easily get into the material containers. Blending with colorants, additivesand other materials is done usingon-the-machine blending units con-sisting of multiple hoppers feedingdifferent resin compound ingredi-ents. Colorants, additives, regrindand base resin are combined usingeither volumetric or, the more accurate, weight-loss feeding (gravi-metric) techniques. Microprocessorcontrols monitor and control theamount of material fed into a mixing chamber below the hoppers.Recipe data can be stored in thecontrol unit for instant retrieval.Central blending units can also beused especially when much higheroverall volumes are required. A central vacuum loading systemtransfers the finished blend to theindividual molding machines.The injectionmolding processThe injection molding processbegins with the gravity feeding ofpolyolefin pellets from a hopper intothe plasticating/injection unit of themolding machine. Heat and pressureare applied to the polyolefin resin,causing it to melt and flow. Themelt is injected under high pressureinto the mold. Pressure is main-tained on the material in the cavityuntil it cools and solidifies. Whenthe part temperatures have beenreduced sufficiently below the mate-rials distortion temperature, themold opens and the part is ejected.The complete process is called amolding cycle. The period betweenthe start of the injection of the meltinto the mold cavity and the open-ing of the mold is called the clampclose time. The total injection cycletime consists of the clamp closetime plus the time required to openthe mold, eject the part, and closethe mold again.There are four basic components toan injection molding machine:1. Injection unit/plasticator2. Clamp unit 3. Injection mold 4. Control systemInjection unitsPlunger injection units (Figure 15)were the first types used for injec-tion molding, but their use today isquite limited. The reciprocating screw injectionmolder is the most common mold-ing machine in use today for mold-ing polyolefins. The injection unit(Figure 16) mixes, plasticates andinjects a thermoplastic melt into aclosed mold. The reciprocatingscrew accomplishes this in the following manner:10Figure 14. Eliminate long-radiusbends where possible. The use ofstainless steel “tees” prevents theformation of streamers along thecurvature of the bend.1. The injection cycle starts with thescrew in the forward position.2. Initially, the screw begins torotate in the heated barrel. Resinpellets are forced by this actionto move forward through thechannels of the screw.3. As the pellets move forward,they are tumbled, mixed andgradually compressed together as the screw channels becomeshallower. The section of thescrew nearest the hopper iscalled the feed section, in whichno compression takes place.4. As the pellets travel down thebarrel, they are heated by frictionand the heat conducted from theexternal electric heater bands.The friction is caused by the pellets sliding against themselvesand the inner wall of the barreland the screw surface. The heatfrom the friction and conductioncause the pellets to melt. Themajority of the melting occurs inthe transition section of thescrew, where compression of thepolymer is taking place as theroot diameter of the screw isincreased.5. Next, the melted polymer is further mixed and homogenizedin the metering section of thescrew. In the metering section ofthe screw, the root diameter hasreached its maximum, and nofurther compression takes place.6. The polymer melt flows in frontof the screw tip and the pressureproduced by the build-up ofpolymer in front of the screwcauses the screw to be pushedbackward in the barrel as it continues to rotate.7. The screw stops turning whenthe volume of melt producedahead of the screw tip is sufficient to completely fill themold cavity and runner system(the channels leading to themold cavity). This amount ofmaterial is called the shot sizeand the period during which thescrew rotates is called the screwrecovery time. 8. The screw is then forced forward,injecting the melt into the mold.This is called the injection stage.In order to compensate for materialshrinkage in the cavity due to cool-ing, an excess amount of material isgenerally held in front of the screwat the end of the injection stroke.This extra material is called thecushion and, during the packingphase, some of the cushion materialcontinues to be slowly injected intothe cavity to compensate for thevolume lost due to the shrinkage ofthe material in the mold and thecompressibility of the plastic.Backpressure is the amount ofhydraulic pressure applied to theback of the screw as it rotates.Varying the amount of backpressurealters the pressure exerted on thepolymer in front of the screw.Increasing backpressure alsochanges the amount of internalenergy transmitted to the melt bythe shearing action of the rotatingscrew. An increase in backpressureraises the melt temperature withoutrequiring an increase in heatingcylinder temperatures and improvesmixing and plasticating.Unfortunately, increasing backpres-sure also reduces screw recoveryrates and can add unnecessaryshear (heat) to the polymer whichmay lead to polymer degradation.Typically, backpressure is set at aminimum unless additional mixing is required.Two-stage systems, also calledscrew preplasticators, are available(Figure 17) in which the plasticatingunit feeds a separate injection cylinder called an accumulator. Meltis injected into the mold using aram in the accumulator. Machinesequipped with accumulators can beused for molding parts requiringvery large shot sizes, for the high-speed injection needed to fill longand narrow mold cavities, and for molding parts requiring bettercontrol of shot size and injectionpressure.11Figure 15. Schematic cross-section of a typical plunger (or ram or piston) injection molding systemCLAMP PRESSURECLAMPCYLINDERMOLDRESTRICTIVE GATINGHEATING ZONESHOPPERRESINRAM PRESSURECOOLING ZONEINJECTIONCHAMBERBACK PRESSUREPLATEHYDRAULICCYLINDERPLUNGER(OR RAMOR PISTON)HEATINGCYLINDER(OR BARREL)TORPEDO(OR SPREADER)NOZZLERUNNERMOLDCAVITY(REAR)(FRONT)12Figure 16. Schematic cross-section of a typical screw injection molding machine, showing the screw in the retract-ed (A) and the forward (B) positionHOPPERHEATINGCYLINDERINJECTIONCHAMBERHEATERSROTATING ANDRECIPROCATING SCREWSPRUEHOPPERROTATING ANDRECIPROCATING SCREWABFigure 17. In this 2-stage injection molding machine, the screw-type preplasticizer is atop and parallel to the hori-zontal plunger injection cylinder and chamber.ROTARY SHUT-OFF VALVEHEATING BANDSPREPLASTICIZEREXTRUDER SCREWHOPPERINJECTIONCYLINDERINJECTION PLUNGERINJECTIONCHAMBERPlasticatorspecificationsInjection capacity is defined as themaximum shot size in ounces (oz.)of general-purpose polystyrene (PS).In equating this to polyolefins, useapproximately 90% 95% of thecapacity stated for PS. The plasti-cating rate is usually given inpounds/hour or ounces/second forPS. Because of differences in melt-ing characteristics and different sen-sitivities to screw design variables, it is not possible to easily convert or apply this value to polyolefins.Injection rate is the maximum rateat which the plasticized materialcan be injected through the nozzlein cubic inches/minute at a statedpressure. Injection pressure is generallyexpressed as the hydraulic pressurein psi (pounds/square inch) appliedto the screw during injection. Themaximum injection pressure avail-able varies and the actual pressurerequired depends on the resin, melttemperature, mold cooling, partdesign and mold design. Most plasticating units have a chartwhich relates the hydraulic pressureto the pressure actually applied tothe polymer.Screw designsNumerous plasticating screwdesigns are available for injectionmolding polyolefins (Figure 18).However, since it is impossible tohave a screw designed for everymolding job, general-purposescrews are most commonly used.The shallower the screw channels,the smaller the resin volume con-veyed to the tip of the screw. Onthe other hand, while deep screwchannels accommodate larger shotsizes more quickly, they do not heatand plasticate the melt as efficientlyas a screw with shallower channels.The three basic screw sections aredescribed in Table 3. There are a number of barrier screwdesigns available which offer somebenefits not provided by general-purpose screws. Barrier screws pro-vide more efficient mixing withoutincreased backpressure and, insome cases, recovery times may bedecreased. These advantages areoffset by the increased risk of blackspeck formation. The deep flights ina barrier screw may have stagnantareas in which there is a reductionin the flow of the material. Themolten plastic tends to stay in theseareas and degrade, ultimately caus-ing black specks in the parts as thedegraded material flakes off thescrew. When purchasing barrierscrews, it is recommended that themolder work closely with the screwdesigner to ensure that stagnantareas are avoided and that thescrew is properly designed for thematerial being used.Plasticating screws for thermoplas-tics generally have interchangeabletips. The two most commonly usedtips in the injection molding ofpolyolefins are sliding check ringand ball-check non-return valves. Inthe molding cycle, as the screwmoves forward to inject materialinto the mold, the non-return valvesclose to prevent material from flow-ing back over the flights of thescrew. Typical sliding check ring andball check valves are shown inFigures 19 and 20.Because of their tendency to wear,it is critical to periodically inspectthe condition of sliding ring shut-offtips. Excessive wear will result ininconsistencies in shot size and melttemperature.13Figure 18. Screw type configurations used in injection molding machinesTRANS.TRANS.FEEDCONSTANT PITCHCONSTANT PITCHMETERING AND MIXING2-STAGEMETERINGFEEDMETERINGMET.DECOMPRESSIONTRANS.SECTIONCHANNEL FUNCTIONSDEPTHFeedDeepCool resin pellets are moved forward into (Constant)hotter barrel zones and begin melting.TransitionDecreasingResin is compressed, melted and mixed. Air (Tapered)carried along slips back to the feed section tobe vented out the hopper.MeteringShallowSufficient back pressure is created to make (Constant)the melt homogenous (uniform), make its temperature uniform and meter it into the injection chamber.Table 3. Functions of the three sections of an extrusion screwThe typical length-to-diameter (L/D)ratio for polyolefin reciprocatingscrews is about 20-30:1, with acompression ratio of 2-3:1. Longer screw lengths are generallypreferred as they provide betterhomogeneity of temperature andmelt quality.NozzlesThe injection-unit nozzle is connect-ed to the barrel and directs the flowof the melt into the mold. The nozzle extends into the fixed platenand mates to an indentation in thefront of the mold called the spruebushing.The nozzle may have a positiveshut-off device or it may be openand rely on the freezing-off of themelt in the gate areas of the moldto keep the resin from flowing back into the injection unit. Somenozzles may be connected to atemperature control device to con-trol the melt temperature. Clamp mechanismsThere are three basic types of injection-molding-machine clamps:mechanical, also called toggle units,hydraulic and a combination ofthese called hydromechanicalclamps.Toggle clamps, which are lessexpensive to build, are most widelyused on small tonnage machines(typically, less than 500 tons). Thetoggle action can best be under-stood by looking at your arm whenit is bent at the elbow and thenwhen it is fully extended. In thetoggle clamp, a hydraulic cylindermoves the units crosshead forward,extending the toggle links andpushing the platen forward. Themechanical advantage is low as theclamp opens or closes, which per-mits rapid clamp movement. Thisaction slows and the mechanicaladvantage increases as the platenreaches the mold-close position.The slow speed is important formold protection.14Figure 19. Typical sliding check ring showing injection stage (top) andretraction stage (bottom)MATERIAL FLOWFigure 20. Typical ball check assemly showing injection stage (top) andfront discharge-retraction stage (bottom)BALL SHUT-OFFCLOSEDINLETPASSAGEINSERTRETAININGPINMATERIAL FLOWNOZZLEADAPTERBALL SHUT-OFFOPENBODYSEATFull clamp pressure is reached whenthe linkage is fully extended. Toadjust the toggle clamp to differentmold heights, the entire togglemechanism and moving platenassembly are moved along tie rods.The position of the toggle mecha-nism depends on where the moldcloses when the toggle is at fullextension. The toggle opens whenhydraulic pressure is applied to theopposite side of the clamp cylinder.See Figure 21.Hydraulic clamps generally are usedon injection molding machines inthe 150-ton to 1,000+-ton clamptonnage range. In this type ofclamp, hydraulic oil is used to movethe platen through the full closingand opening strokes. The fluid ismetered into a reservoir behind themain ram. At first quite rapid, theoil flow is slowed as the ram reach-es the mold-close position in orderto protect the mold. An oil fill valvecloses when the mold is closed. Thearea behind the ram is then pressur-ized to build full clamp tonnage. Toopen the mold, the oil valve is firstpartially opened to smoothly openthe mold. Once the mold halves areseparated, the clamp accelerates toa fast open speed (Figure 22).Mold set-up is much easier with ahydraulic clamp than with a toggleclamp since hydraulic clamp ton-nage can be reached anywherealong the clamp stroke. Mold set-up is accomplished by setting theclamp position from the machinescontrol center.Hydromechanical clamps are com-monly used on very large injectionmolding machines, i.e., over 1000tons. In the hydromechanical clamp,a hydraulically actuated togglemechanism pushes the moving plat-en at high speed to a point wherethe mold halves are nearly closed. Amechanical locking plate or linksprevent rearward movement duringfinal build-up to full clamp tonnage.Short-stroke hydraulic cylinders areused to move the platen the finalshort closing distance and developfull clamp tonnage. See Figure 23.Clamp specificationsKey clamp specifications to considerin choosing an injection moldingmachine are: Clamp stroke Minimum mold thickness Clearance between tie bars Maximum daylight opening Platen size Clamp tonnageClamp stroke is the maximum dis-tance (inches) the moving platencan travel. Clamp stroke is a majorfactor in determining the minimummold thickness that can be usedwith the machine. Generally, clampspecifications also state the mini-mum mold thickness for which theclamp can develop its full tonnage.Maximum daylight opening is thedistance (inches) between the twoplatens when the clamp is com-pletely open. This measurement is amajor factor in determining theeffective maximum mold thicknesswhich takes into account the moldopening required for part ejectionor removal. Complicated molds mayrequire more opening space andrigid mounting surfaces, since highplaten deflection under load coulddamage the mold. An allowabledeflection of 0.001 in/ft of spanwith full clamp load on the centerof the platen is, generally, consid-ered acceptable.Platen size is given in horizontal andvertical measurements (inches) forthe full platen. Since there are tie-bars running through the corners ofthe platens, mold-size limits are lessthan full platen size. A mold canextend between the tie-bars ineither the vertical or horizontaldirection but, generally, should notextend outside of the platens.15Figure 21. Toggle clamping systemFigure 22. Hydraulic clamping systemClearance between tie-bars is givenfor the distance (inches) betweenthe top tie-bars (horizontal) andsidebars (vertical). Since the tie-barsare fixed on most injection moldingclamps, the distance between themdictates the maximum size of amold that can be placed in theclamp.Clamp tonnage is the maximumforce which the clamp can develop.A clamping pressure of five-tons-per-square-inch of the projectedarea of the molding (including therunner system) is more than ade-quate for polyolefins. However,where packing is not a major factor,this pressure may be as low as 2tons/in2. An industry rule-of-thumbis that a clamp force of 2 to 3 tonsper in2of the projected area of thepart(s) and cold runner system isadequate for reciprocating-screw-type, injection-molding machines.Some thin-wall stack molds mayrequire 5 tons/in2for optimum per-formance.Injection moldsThere are many types of injectionmolds and tooling in use today,such as two-plate, three-plate andstack molds. Two and three- platemolds are more commonly used forheavy wall and non-packagingproducts. Both cold and hot-runnersystems are used for two and three-plate molds. All stack molds use ahot manifold to convey the melt tothe cavities. Each mold componentmust be machined and finished toexact dimensions with very tight tol-erances and must be heat-treatedto be able to withstand very highinjection and clamp pressures.Injection molds are the most expen-sive molds used in plastics process-ing with very long lead timesrequired for their design and fabri-cation. Every mold must be testedand debugged to prove-out theejection system, cooling and/orheating system and operating com-ponents before it is placed in pro-duction.Types of moldsA two-plate mold (Figure 24) hasonly one parting line. If a runner isused, it is connected to the moldedproduct and requires manualremoval and separation after thepart is ejected. This type of mold isthe least expensive to make and iscommonly used for parts with rela-tively simple geometries.16Figure 24. Two-plate moldTOP CLAMPINGPLATEWATER COOLINGLINESCAVITY PLATECAVITY ANDMOLDED PARTMOLD PARTING LINERUNNERCORE PLATECORESUPPORT PLATEKNOCKOUT PINSEJECTOR HOUSINGEJECTOR RETAINERPLATEFigure 23. Hydromechanical clamping system. Top view shows clamp openposition of piston and ram. Bottom view shows clamp closed, toggled andram pressurized.Three-plate molds (Figure 25) havetwo parting lines, one for the run-ner system and one for the moldedproduct. When the mold opens, therunner is automatically separatedfrom the product to allow separatehandling. This eliminates the needfor manual separation and removaland the sprue and runner systemmay be fed directly to a recyclingsystem. This type of mold is moreexpensive than the two-plate mold. Stack molds (Figures 26 and 27) canbe two, three or four levels. Theadvantage of the stack mold is thatit can, generally, produce a largernumber of products versus a two orthree-plate mold utilizing the samemachine clamp tonnage. The disad-vantage is that it requires a moldingpress with much greater daylightopening to accommodate the moldheight. This type of mold is muchmore expensive and takes longer tobuild. Three level stack molds arevery new and four level stack moldshave been around for less than fiveyears. The dairy container and lidindustries commonly use stackmolds. The four-level is common forlids, and the two-level is commonfor containers. Sprues and runnersThe sprue and runner system is theconduit that connects the machinenozzle to the cavities. During theinjection phase of the moldingcycle, the molten material flowsthrough the sprue and runner tothe cavities.The sprue connects the machinenozzle to the runner and may beeither a cold or a hot- sprue. In thehot-sprue design, the sprue hasheating elements that maintain theplastic in a molten state eliminatingthe need for separation and recla-mation. Ideally, the sprue should beas short as possible to minimize thepressure loss during injection. Acold sprue is tapered for easyrelease from the mold.17Figure 26. 2x1 wash basin stack moldPhoto courtesy of Tradesco Mold, Ltd.Figure 27. 4x24 stack moldPhoto courtesy of Tradesco Mold, Ltd.Figure 25. Three-plate moldRUNNER PLATECAVITY PLATEMOLD PARTING LINECORE PLATECORESUPPORT PLATEKNOCKOUT PINSEJECTOR HOUSINGEJECTOR PLATESMOLD PARTING LINERUNNERThere are three basic runner typesin use: Cold Runner Insulated Runner Hot RunnerCold runners are commonly used intwo and three-plate molds, but notin stack molds which require theuse of a hot runner. The most com-monly used runner designs are full-round, half-round and trapezoidal(Figure 28). The full-round is gener-ally preferred for ease of machiningand lower pressure loss. For fastcycles a full-round is not recom-mended since the greater mass ofmaterial takes longer to cool andmay control the cycle time. The run-ner should be polished for ease ofmold filling and part ejection. The insulated runner (Figure 29) isthe precursor to the hot runner. Therunner diameter is very large and athick skin is formed on the outsideof the runner. The molten plasticflows in the center and, due toexternal insulation and the lowthermal conductivity of the polymer,remains molten during the cycle.This design eliminates the need forremoving and/or recycling the run-ner. The problem with this design isthat when the machine is down forany extended period of time therunner solidifies and has to be phys-ically removed before beginning thenext molding cycle. As moldershave become more comfortablewith hot runner technology, insulat-ed runners are rapidly becomingobsolete and not many molds arebuilt today utilizing this technology.The externally heated hot-runnersystem (Figure 30) also maintainsthe plastic in a fluid state while themold is running with the pressure ateach gate approximately the same.Maintaining a uniform temperaturein the sprue bar and the hot-runnermanifold is very critical to processand product consistency. Start-upprocedures must be carefully fol-lowed according to the moldmakers specifications to preventdamage and material leakage in the manifold. Mold ventingWhen molten plastic is injected intothe mold, the air in the cavity hasto be displaced. To accomplish this,vents are machined into the partingline to evacuate the air and areextremely important to the consis-tent production of high qualityproducts. In many cases, this is anarea in mold design and construc-tion that is often overlooked. Vents should be located at theextremities of the part and at loca-tions where melt flow fronts cometogether. Venting is also easilyachieved around ejector pins andcore slides provided that there issufficient clearance between thepin/slide and the mold. Typical moldvents are channels cut from the cav-ity or runner straight to the edge ofthe mold. Closest to the part, theyare typically 0.0005-0.001 in.18Figure 28. Schematic showing typical runner designs found in injectionmolds.FULLROUNDHALFROUNDTRIANGLETRAPEZOIDWINGFigure 29. Insulated runner systemELECTRIC HEATERINSULATEDRUNNERMOLD PARTINGLINECORECAVITYINSULATIONTORPEDOFigure 30. Hot-runner systemHOT PROBEHEATEDMANIFOLDMOLD PARTINGLINECORECAVITY(0.013-0.025 mm) in depth and0.063-0.5 in. (1.6-12.7 mm) inwidth. The initial vent thicknessshould be maintained for about 0.5 in. (12.7 mm) and then thedepth can be increased to about0.003 in. (0.076 mm) to the edgeof the mold. The vents should bepolished towards the edge of themold to make them self-cleaning.Build-up in the vents will eventuallyaffect mold filling resulting in non-uniform fill and unbalanced cavities.For this reason, it is important thatvents be inspected between produc-tion runs to ensure that they areclean and within specification. Insome cases, reduction of the injec-tion rate prior to final filling of thecavity will prevent burning and alsoprevent the mold from opening. Continuous parting-line ventingmay be necessary in high speedmolding operations. Even thoughburning is not evident, the lack ofburn marks does not ensure thatthe molds are properly vented.Increasing vent areas may helpreduce cycle time. Proper ventingwill also aid mold filling by decreas-ing the resistance due to air pressure on the flow front. Mild sand blasting or vapor blastingof the mold cavity assists in ventingand part release. However, for high-gloss finishes, this blasting is notadvisable. Vapor honing may helpalleviate a venting problem area butcare must be taken that honing isnot too deep or wide to be notice-able on the finished part.GatingThe gate is the bridge between therunner and the cavity. Dependingon the specific material and partdesign (wall thickness, geometry,etc.) there are many different typesof gates which can be used (Figure 31). The type and size of gate are verycritical since they can affect manyfactors including mold-fill time,overall cycle, orientation, shrinkage,warpage, and part appearance. Because it acts as a restriction tothe polymer flow, a high shear rateis created at the gate often result-ing in a temperature increase. Thereis also a high pressure drop acrossthe gate which needs to be over-come by increased injection pres-sure or higher temperatures. Thepressure drop can be reduced, to acertain degree, by using shortergate land lengths. A large gate provides easy fillingwith relatively low shear rates andpressure drops. However, if it is toolarge, it will require an excessiveamount of time to cool, lengthen-ing the cycle. It is also possible thatinsufficient packing and subsequentsinks or voids will occur if eithersections of the part or the sprueand runner system freeze off beforethe gate. A gate which is too small willrequire higher pressures to injectthe material and may cause prob-lems in part filling. If the gatefreezes off before the part cools, it will not be possible to developadequate packing, resulting in voids or sinks. With extremely smallgates, jetting or melt fracture of the polymer flow will cause surfaceappearance defects, includingdelamination.To ensure uniform fill, it is criticalthat the feed system (sprue, runners, and gates) be balanced.This depends on the size and location of the gates and is oftendetermined by experience.Advances in mold filling simulationsoftware have provided an addition-al tool for analysis prior to the manufacture of the tool. Fine-tuning may be required and is gen-erally done by utilizing a series ofshort shots, observing the fill pattern, and making minor adjust-ments, as required. For multi-cavitytools utilizing single gates and a hotrunner system, adjustment of thetemperatures of the individual gatesmay be used to balance the overallfill pattern.Figure 31. Gating systems19A. GateB. RunnerC. PartD. MoldVALVE GATEDISC GATEDABCTAB GATECDABSUBMARINE GATECDABFAN GATEDBACEDGE GATECDABCENTER GATEDBACOPENPOSITIONCLOSEDPOSITIONIn high-speed, thin-wall molding, itis common to provide coolingaround the gate to remove the heatproduced by the high shear rates.This may be supplemented by theuse of inserts fabricated of highconductivity alloys, such as beryllium-copper, in these critical areas.Mold coolingAlthough mold cooling is extremelycritical to cycle time, warpage,molded-in stresses, mold-filling,etc., the sizing and layout of thecooling pattern are often over-looked and neglected aspects in the initial stages of tool design.The cooling layout should be con-sidered in relationship to the thick-ness profile of the part and thegeneral filling pattern in order toprovide adequate cooling in criticalsections and not overcool otherswhich may cause part warpage. Inareas where coolant flow may berestricted due to part geometry i.e.,bosses, the use of inserts fabricatedfrom high thermal conductivityalloys, such as beryllium-copper,should be considered. In all cases, cooling channels shouldbe sized in relation to the availablecoolant flow to ensure turbulentflow which is much more effectivefor heat removal than lowering thetemperature of the coolant. Routineinspection and acid-cleaning ofcooling channels are recommendedto maintain the coolant flow veloci-ty and minimize pressure drops.Ideally, the temperature differentialbetween coolant inlets and outletsshould be about 2F. Jumpersbetween cooling circuits should beavoided in order to reduce tempera-ture differentials in the coolant. The utilization of low pressure-dropmanifolds, valves, fittings, etc. andin-line flowmeters and temperatureindicators are also good practices to provide information regardingthe efficiency and condition of thecooling system.Ejection devicesThe ejection of injection moldedparts is most commonly accom-plished by air, vacuum, pins or strip-per plates. Depending on partdesign, combinations of these sys-tems are used for rapid positiveejection. Care should be taken inselecting ejection surfaces becauseof aesthetic and moldability require-ments. Wherever possible, the partshould be ejected off the core. Forsmall, thin-walled moldings thatmay shrink onto the core, air ejec-tion through the core is usually ade-quate for part removal. On someproducts with threaded or undercutfeatures, collapsible, retractable orunscrewing cores are used. Spiral flowmeasurementThe relative processability of aninjection molding resin is oftendetermined by its Melt Index (MI) orMelt Flow Rate (MFR). This involvesmeasuring the relative flow of themolten resin through a specifiedcapillary in a calibrated laboratoryinstrument, while maintaining themolten resin at 190C (374F) and43.5 psi for Polyethylenes or at230C (446F) for Polypropylenes.Melt index is a good measurementof a resins relative flow propertiesat low shear rates, but only forresins of the same molecular weightdistribution (MWD). Under actualinjection molding conditions, differ-ences in MWD will affect the resinsmelt viscosity (flow characteristics)at high shear rates. Temperatures,pressures and shear rates of actualmolding do not conform to thoseof the MI or MFR test methods.Equistar has a number of uniquemanufacturing processes availablewhich allow the control not only ofthe melt index and density, but alsoMWD. This capability results in abetter overall balance in resin prop-erties and processability. Becausemelt index and MWD play a keyrole in performance in actual end-use applications, Equistar has uti-lized “Spiral Flow” (SF) as a morepractical method of measuring andcomparing a resins performanceusing realistic processing conditions.Spiral flow measures the flow lengthwhen molten resin is injection20Figure 32. Broad MWD (left) andnarrow MWD (right) spiral flowSPIRAL FLOW (IN)20253035404550MWD (as measured by flow ratio)Narrow MWD Broad MWD11.010.510.09.59.08.58.0Figure 33. Effect of MWD on spiral flow of HDPE (all materials have MFR = 5 gms/10 min,)molded into a long, 0.0625 radius,half-round spiral channel (Figure32). The higher the spiral flow num-ber (SFN), the easier the resin is toprocess. The melt temperature ismonitored and maintained at 440F(227C) and injection molding isconducted using a constant pres-sure of 1,000 psi (7000 kPa). Spiralflow is a more realistic measure-ment than melt index because it isrun at a much higher shear rateallowing resins of similar MIs anddifferent MWD to be compared atrealistic conditions. The broaderMWDs resins exhibit lower melt vis-cosity (higher SFN) at higher shearrates than narrower MWD resinswith similar melt indices (Figure 33).Since it does not take into accountthe effects of MWD, relying only onthe melt index can be misleading.For example, ALATHON ETPH5057, a broad MWD, 57 melt-index resin for thin-wall HDPE appli-cations, exhibits flow propertiessimilar to many narrow MWD resinsin the 75 to 80 melt index range.Equistar has established the use ofspiral flow as a specification for allhigh-flow (30 melt index andabove) HDPE resins and has begunreporting the spiral flow number foreach lot on the Certificate ofAnalysis (COA). This allows themolder to compare the spiral flowof an incoming lot of resin with theSFN of the lot on-hand and readilyestimate how the new lot willprocess relative to current produc-tion. For example, if the current lotbeing run has a SFN of 20 in. andthe new lot has a reported SFN of22 in., the new lot can beprocessed at either lower tempera-tures and/or at a faster productionrate. Only minor adjustments ineither melt temperature and/orinjection pressure may be requiredto compensate for SFN variabilityfrom lot to lot.General injectionmolding operatingproceduresPrior to starting up the injection-molding machine, be sure to havethe following available: Safety glasses for all personnelassisting in the start-up. Loose fitting, heavy-duty insulat-ed work gloves. A large metal container or cardboard for collecting meltproduced during the start-upprocedure. Soft beryllium-copper, bronze, or aluminum tools for use inremoving any plastic from thenozzle area.Always refer to the manufacturersoperating manual for any specificstart-up and shutdown procedures. Refer to the Equistar suggested resinstartup conditions (Table 4) for gen-eral guidelines to use in starting upan injection molder on polyolefins.General safetyAs with any process involving ener-gy and mechanical motion, injectionmolding can be a hazardous opera-tion if appropriate safety proceduresarent well documented and fol-lowed. (Refer to the Manufacturersoperating manual.)Mechanical, electrical, and hydraulicinterlocks are critical to the safeoperation of any piece of process-ing equipment. In some cases, theseinterlocks may need to be bypassedwhile performing set-up and maintenance functions. Under nocircumstance should this be doneby non-qualified personnel. In orderto assure utmost safety during normal operation, interlocks shouldnever be bypassed. 21PRODUCTCYLINDER TEMPERATUREEVA LDPELLDPEHDPEPPRear320F325F350F450F400FCenter340F340F375F470F425FFront340F350F400F475F450FNozzle340F350F400F475F450FHigher temperature settings may be necessary for parts or cycles requiringmore plasticizing capacity. EVAs with higher VA incorporated may requireless heat.Mold Temperature40-70F45-75F45-75F45-65F60-80FMold temperature may be raised to improve flow and surface finish or lowered for faster cycles, lower shrink and better ejection.Mold CycleInjection15 sec.15 sec.15 sec.15 sec.15 sec.Booster0-5 sec.0-5 sec.0-5 sec.0-5 sec.0-5 sec.Cure15-30 sec.15-30 sec.15-30 sec.15-30 sec.15-30 sec.Injection time and cure will vary with part thickness. Reduce injection timeas much as possible. Follow by reducing cure time.Screw Rotation Speed should be adjusted to provide a 15-30 second retrac-tion time.Screw Back Pressure set slightly above minimum. Higher screw back pres-sure may be necessary for parts or cycles requiring more plasticizing capacity.Injection Speed Set at miximum. May need to be slower for thicker partsor smaller gates.Injection Pressure Minimum PSI without shortshot (adjust to fill cavity).Second stage may be lower for holding.Table 4. Suggested start-up conditions (based on general purpose meltindex/flow rate products)Keep all molding equipment andthe surrounding work areas cleanand well maintained.Hydraulic leaks should be repairedimmediately to eliminate safety haz-ards. Hydraulic lines, valves, fittingsand hoses should be checked peri-odically per the manufacturers rec-ommendations. Good housekeeping is essential.Loose pellets, tools, oil, etc. on andaround the molding machine cancause accidents, damage to theequipment, or contamination of the parts.HeatHigh temperatures are necessary inthe injection molding process.Always use heat-resistant gloves,safety glasses and protective cloth-ing. Modern injection moldingmachines have warning signs identi-fying specific hot areas on moldingmachines; do not ignore thesesigns. Keep the splashguard in placeduring purging and when themachine is operating.Polymer left in the barrel or hotrunner system may often be pres-surized. Care should be takenwhenever resin flow is interrupteddue to blockage or mechanicalproblems.If the molding machine will be shutdown for an extended period oftime (30 minutes or longer), lowerthe heats, purge the machine orcycle it until the lower temperatureis reached before shutting it down,leaving the screw full of resin and inthe retracted position. ElectricityMolding machines utilize high elec-trical voltage and have warningsigns pointing out electrical hazards;do not ignore these signs. Keepwater away from these areas. Aperiodic inspection of all electricaldevices and connections for wear,looseness, etc. is very important.Machinery motionThere is considerable mechanicalmotion during the injection moldingprocess. Neckties and loose fittingclothing should not be worn aroundmolding machines since these canbe caught by the equipment move-ment and lead to physical injury. Donot reach around, under, through,or over guards while the equipmentis operating.Be sure all people working near theinjection molding machine knowwhere the Emergency Shut Off but-ton is located. Never disengage anyof the safety mechanisms or inter-locks on the injection moldingmachine.Some machines store energy(hydraulic, pneumatic, electrical, orgravitational) which can be presenteven when the machine is turnedoff. Consult the manufacturersoperating guide for methods toproperly de-energize the equipment.As with any piece of potentiallyhazardous equipment, a suitablelock-out/tag-out procedure shouldbe implemented and enforced.The injectionmolding processand its effect onpart performanceThe molding cycleAs detailed in the section on theInjection Molding Process, there areseveral steps in the production ofinjection molded parts. In mostcases, the injection molding cyclebegins with the mold open, ejectorpins/slides retracted, and the screw/ram ready with the next shot ofmaterial. The cycle then proceeds as follows:1. MOLD CLOSE: Mold closes andclamp develops full closing pres-sure2. INJECTION: Material is injectedinto the mold cavity3. PACKING: Material is packed intothe mold to fill out the part4. PLASTICATION: Screw begins torotate (or ram retracts) to devel-op the next shot of material5. COOLING: Coinciding with thestart of plastication, the coolingcycle begins (Note: Since coolantcontinuously circulates throughthe mold, cooling technicallystarts as soon as the melt con-tacts the cavity during injection)6. MOLD OPEN: Mold opens, slidesretract, ejector pins activate,part(s) are ejected.The length of each of these stepswill depend on the complexity ofthe mold, the size of the machine,and the geometry and end-userequirements of the part. A typicalcycle for a four cavity, 16 oz. stadi-um cup can be found in (Figure 34)while one for a single cavity, 12 lb.bumper fascia can be found in(Figure 35).Regardless of part size, weight, ormold complexity, nearly half ofevery injection cycle is spent coolingthe part(s) to a temperature suffi-cient to allow ejection without post-mold distortion. Factors that affectthe cooling rate of the part(s) willbe covered later.In almost all cases, part quality isthe result of steps 2-5. The highestquality parts begin with a homoge-neous plastic melt in terms of tem-perature and composition.Therefore, the quality of the nextpart to be produced is the result ofthe development of the shot duringthe current cycle. Shot size shouldbe sufficient to produce a cushionof material at the end of the step 3of 0.1-0.5 in. (2-13 mm). This cush-ion will keep the screw from bot-toming out and help maintain plas-tic pressure within the cavity. Achieving a homogeneous melt iscontrolled by many factors includ-ing: screw design, screw speed,screw & barrel wear, back pressure,shot to barrel capacity ratio, soaktime, and heater band settings. Screw design for polyolefin process-ing was covered in a previous sec-tion. A worn barrel and/or screw22creates an increased gap betweenthe two resulting in resin movingbackward or staying in placeinstead of being conveyed forwardby the screw. A worn screw and/orbarrel can lead to poor melt consis-tency, degradation of the resin, andshot inconsistency. Screw speedshould be set so that the screwconsistently recovers 1-2 secondsbefore the mold opens.Backpressure is typically set at theminimum level that delivers ahomogeneous melt (no unmeltedpellets leaving the nozzle). However,backpressure may need to beincreased to improve temperatureconsistency in the melt and to mini-mize or eliminate streaks due topoor dispersion of colorant. Duringplastication, most of the energyprovided for melting the resin pel-lets comes from shear heating dueto friction between the pellets,screw and barrel. As the back pres-sure is increased, the screw worksthe material more in order to con-vey it forward, thereby raising thetemperature of the melt morequickly. The increased work by thescrew also increases the mixing ofthe molten plastic resulting in bettertemperature and homogeneity ofthe melt. However, too much backpressure can result in degradationof the plastic, an increase in thescrew recovery time, increased ener-gy costs, and more wear on thescrew and barrel. Shear heating isalso dependent on the viscosity ofthe plastic, screw design, screwspeed, and back pressure. The lattertwo can be varied to some extentby the processor to control theshear heating and melt tempera-ture. The shot-to-barrel capacity ratio(SBCR) can also have an effect onmelt, and therefore part quality. Theideal range for the shot to barrelcapacity ratio is 30-60%. If theSBCR is less than 30%, too manyshots of material reside in the barrelunder the influence of heat fromthe heater bands and shear fromthe screw. This may lead to over-heating and degradation of theresin. If the SBCR is greater than60%, less than 2 shots of materialare in the barrel, which typicallydoes not allow the melt tempera-ture to equilibrate. A high SBCR willalso mean that the screw mayrecover (develop the next shot) justbefore the mold opens which canlead to cycle alarms due to inade-quate shot size. A SBCR range of30-60% will provide adequate time for the melt temperature toequilibrate.In order to achieve proper melthomogeneity, all of the pelletsshould be melted by the time theyreach the middle of the transitionzone on the screw. Figure 36depicts the amount of energy needed to process a polypropyleneimpact copolymer. In order for thepellets to be fully melted halfwaythrough the transition zone, 71% of the energy to reach thedesired melt temperature (in thiscase, 450F) must be transferred tothe polymer.The heater bands on the barrel pro-vide only a small amount of theenergy needed to melt the plastic.Most of the energy from the heaterbands maintains the barrel tempera-ture during processing and raisesthe temperature of the solid pelletsin the feed zone. There are fourtypical temperature-setting patternsfor injection molding on a barrelwith five heater zones (Figure 37):23Figure 34. Injection molding cycle for a 16 oz. stadium cup (4 cavity, HDPE,31g each, 7.8 sec. cycle)MOLD CLOSEINJECTIONPACKINGCOOLINGPLASTICATIONMOLD OPEN02468TIME (SECONDS)STAGEMOLD CLOSEINJECTIONPACKINGCOOLINGPLASTICATIONMOLD OPEN020406080100120TIME (SECONDS)STAGEFigure 35. Injection molding cycle for automotive fascia (12 lb. shot, 0.125in. thickness, 107 sec. cycle)1. Increasing: This pattern has thelowest temperature setting at thefeed throat and the highest atthe front of the screw with asteady increase of the tempera-ture settings in between. Thenozzle is typically set at the sametemperature as the front zone.This pattern is the one mostcommonly used and is particular-ly recommended for lower melt-ing point materials (such as EVAor EMA) to prevent bridging atthe feed throat of the extruder. Itis also recommended when theSBCR is low, typically 50% and screwrecovery and residence times(time from resin entering theextruder to leaving the nozzle)are short. Sufficient feed throatcooling must be provided to pre-vent bridging. Otherwise, a lowtemperature set point should beused at the feed throat. In addi-tion, this profile can increase thechance of air being entrapped inthe melt instead of venting backthrough the hopper.3. Hump: This pattern has the high-est temperature settings (typically20+ degrees higher than desiredmelt temperature) in the middleof the screw to correspond withthe transition section where themajority of the melting takesplace. Settings near the feedthroat are typically at the soften-ing point of the resin while thesettings at the front of the barreland the nozzle should be at thedesired melt temperature. Thisprofile is recommended whenSBCR is 25-50% and overall resi-dence time is 2 to 4 minutes.4. Flat: This profile uses the desiredmelt temperature as the settingsfor all of the barrel zones exceptfor the feed throat, which shouldbe set at or below the softeningpoint of the resin. This profile istypical of processes where theSBCR is 20 to 40%.In actual practice, the specific screwdesign also plays an important partin obtaining the desired melt. It ispossible that different screw designsmay require different profiles toachieve similar melt characteristicseven if they have the same or anequivalent SBCR. By varying the screw speed andback pressure, shear heating is, forthe most part, an easily controlledsource of heat to the material. Thebest parts are typically producedwhen there is a balance of shearheating and heat from the heaterbands. Once a temperature profileis chosen, it is recommended thatthe processor monitor current flowinto the heater bands. The properbalance of shear heating and ther-mal heat is achieved when the cur-rent cycles regularly (typically severaltimes a minute). This is of particularimportance in the transition zone ofthe barrel; if the barrel is dividedinto quarters along the length, thetransition zone is typically the mid-dle two quarters of the barrel.Typically the heater bands in thefeed zone will cycle regularly or beon nearly all the time. The heaterbands in the metering zone andnozzle should cycle regularly but24Figure 36. Energy needed to bring PP to melt temperatureRAISING TEMP. OFSOLID RESINMELTING OFPELLETSRAISING MOLTENRESIN TO MELT TEMP.55045035025015050RESIN TEMPERATURE (F)020000400006000080000ENERGY (CALORIES/LB.)FEEDREARZONE 2ZONE 3FRONTNOZZLEBARREL LOCATIONTEMPERATUREINCREASINGDECREASINGHUMPFLATFigure 37. Various barrel temperature profilesless frequently than the feed zones.Because accurate temperature con-trol is so important, it is always agood practice to routinely check thecalibration of the controllers.If a heater band is on all the time,either the set point is too high tobe reached or there is a problemwith the thermocouple or heaterband (it is working but not readingthe actual temperature). If the ther-mocouple is fine and the desiredsetting for the zone is not out ofline, the processor can increase thetemperature settings upstream(closer to the feed throat) of thezone in question. Should this notreduce the time that the heaterband is on, the temperature settingon the zone should be lowered untilthe band cycles regularly. This willprolong the life of the heater bandand reduce the energy usage.If a heater band does not draw cur-rent, or does so infrequently, thereare two possible problems. Eithermost of the heat going into theplastic at this barrel zone is viauncontrolled shear heating or thethermocouple is broken, both ofwhich should be corrected. A bro-ken thermocouple will typically readout the maximum permissible tem-perature. If all of the cavities are fill-ing with acceptable cycle times, theheater band set points in the zonesupstream of the zone in questionshould be reduced. If this fails toget the heater band cycling, resetthe upstream zone(s) to their origi-nal temperature(s) and increase thetemperature set point.It may appear that the proceduresabove are only serving to increasethe overall melt temperature. Whilethis is true to a small extent, thebenefit is that they aid in providinga more homogeneous and control-lable melt temperature that willimprove the molded parts.Now that we have a homogeneousmelt stream in the barrel or accu-mulator, we need to examine theintroduction of the plastic into themold. The viscosity, or resistance toflow, of the resin is affected by tem-perature and shear rate. Increasingthe melt temperature reduces theviscosity of the resin making it easi-er to fill the mold. Increasing thepressure or injection rate increasesthe shear rate, which decreases theviscosity making it easier to fill themold. Therefore, given a resin,machine and mold, there are threevariables that can be used to fill outthe mold: Injection/packing pressures Injection rate Melt temperatureThe curves in Figure 38 indicate therelative temperature-pressure rela-tionships for PE resins of varyingmelt indices. The higher the MFR orMI, the lower the injection pressureand/or the temperature required tofill a mold. Assuming the samemold filling characteristics (fill speedand fill time), cycle time and injec-tion temperature, a high flow resin:1. Will allow pressures to bereduced about 25% when theresin MI or MFR is doubled.2. Will allow a decrease in melttemperature of about 70F(40C) when the resin MI or MFRis doubled.The effect of a higher flow PE resinon temperature and pressure can beseen in Figure 39. Note that as theMI or MFR of the resin increases thepossible reduction in temperatureand/or pressure will become less.However, the switch to a polyolefinwith higher flow characteristics usually results in a loss of otherproperties such as resistance tostress cracking and impact strength,especially at lower temperatures.Injection and/or packing pressuresare typically the first settings adjust-ed by the processor because theyhave a quick response on mold fill.Increasing the pressures will help fillout the mold correcting for shortshots and reducing or eliminatingsurface defects such as sink marksand ripples near the gate. Thedownside of increasing pressure isthe chance of trapping air in the25Figure 38. Temperature-pressure relationships for polyethylene resins ofseveral melt indicesMELT INDEX 3.0MELT INDEX 5.0MELT INDEX 8.0MELT INDEX 22300350400450500550600650700INJECTION TEMPERATURE, FINJECTION TEMPERATURE, C150200250300350INCREASING SCREW OR RAM PRESSUREABCcavity resulting in burn marks or ofincreasing the flash on the partingline due to the mold opening.Increasing injection pressures alsopack resin more tightly into the cav-ity, which may reduce shrinkage,increase the gate temperature(s),and increase molded-in stress. Thereduced shrinkage can lead to apart sticking in the cavity and alsopost-mold dimensional differences.Increasing the injection rate(s)reduces the viscosity of the resin,which may reduce the amount ofmolded-in stress in the part. In addition, an increased injection ratemay also yield a more uniform parttemperature (due to faster intro-duction of material into the mold)which can reduce differentialshrinkage (i.e. warpage) due totemperature variation. Increasingthe injection rate(s) without adecrease in injection pressure canlead to flashing of the part.Changing the injection rate(s) alsohas a fast effect on part qualityalthough it may take time to fine-tune the rate(s) for optimum quality.Excessive molded-in stress can leadto an increase in warpage and adecrease in impact strength andenvironmental stress cracking resistance.Sometimes the injection rate andinjection pressure are not independ-ent variables; i.e. the machine is setwith a maximum pressure and runson injection rate settings which areset on screw position. This setup willallow the machine to vary the injec-tion pressure based on the pressureneeded to meet the rate set points.Conversely, the injection pressurecan be specified based on screwposition and the rate is allowed tovary. Some processors are now utilizing pressure sensors within thecavity to control the operation ofthe machine via cavity pressure. Thisis a new approach that is gainingacceptance for molding parts withcritical tolerances. It is also applica-ble to molds (such as syringes)where core shift is of concern.The final way to control the viscosity of the resin is to adjust themelt temperature. An increase intemperature will decrease viscosity.Changing temperature settingsyields a slower response than pres-sure or injection rate. High resintemperature can lead to degrada-tion and require longer cooling timewhile low temperatures can lead toshot inconsistency, higher injectionpressures, and excessive wear/dam-age to the screw and barrel. When setting an injection rate orinjection and packing pressure pro-file, the aim of the processor shouldbe to provide a smooth delivery ofmaterial into the mold. A momen-tary slowing of the screw due toeither the transfer from one step toanother or too large of a step canresult in a hesitation of the plasticflow front. Hesitation of the meltfront can cause surface defects suchas flow lines or tiger stripes, whichmay lead to poor weld and/or knitline strength. Therefore it is necessary to reduce the rates orpressures in a consistent manner toprevent flashing of the tool, poten-tial core shift(s), and bottoming outof the screw.In general, 95% of the partweight(s) should be delivered duringthe injection step. The final partweight is achieved via the packingand holding step. Packing pressuresare typically about half the level ofthe injection pressure and serve toachieve final part weight(s) and alsoto allow time for the gates to freezeoff before plastication can begin forthe next shot.During the injection and packingsteps, coolant (typically a mixture ofethylene glycol and deionized waterif 45F) is circulating through themold. The coolant takes heat out ofthe mold and therefore out of thepart via conduction. Optimum cool-ing is achieved when the water is inturbulent flow. In general, anincrease in coolant flow rate willremove more heat than a decreasein coolant temperature.As resin flows into the mold, thematerial in contact with the moldsurface solidifies very quickly form-ing a skin layer and an inner flowchannel through which materialcontinues to flow (Figure 40). Asmore heat is taken from the plasticby the mold, the flow channel isreduced to the point that no morematerial will flow. The optimumprocess parameters should be cho-sen to allow complete mold fill andpack before the flow channel solidi-fies completely. Keeping the melt26Figure 39. Effect of melt index of polyethylene resin on injection temperature61224MELT INDEXp0.75p0.56p0.47pPRESSUREtt-70t-140t-210INITIAL p,tTEMPERATUREPRESSUREchannel open allows for betterpacking of the extremities of thepart. There are also differences inpart temperature depending onproximity to the gate. Uniform cooling to the mold occurswhen the coolant makes one passthrough the mold (no looping orconnecting of flow channels) andthere is only one temperature con-troller. During even cooling, thegate area is always the hottest areaof the part because throughout theinjection and packing steps, moltenmaterial continues to flow throughthe gate. The extremities of the parttend to have the lowest tempera-ture since the polymer melt hastransferred heat as it has flowedthrough the cavity.Differences in part temperature canlead to differential shrinkage andtherefore warpage. There are twoways to minimize temperature dif-ferentials either through differentialcooling or an increase in injectionrate.Differential cooling involves direct-ing coolant towards the gate area,which has the highest part temper-ature and away from the extremitieswhere the part temperature is low-est. The typical method is to reducethe coolant flow to the coolingchannels nearest the extremitiesand open up the valves to the chan-nels nearest the gate. Directing thecoolant flow from the hotter por-tions of the tool to the extremitiesmay also be effective in some cases.As covered previously, increasingthe injection rate will shorten theinjection times allowing for a moreuniform part temperature.Because the cooling step is usuallythe longest time period in the injec-tion molding cycle, a cold moldtemperature is generally recom-mended. Because of the wide rangeof mold and part designs, it is verydifficult to specify mold tempera-tures. A typical range of mold sur-face temperatures for polyolefins is70-125F (20-50C), which requirescoolant temperatures of 32-50F (0-10C). Materials with lower melt-ing temperatures, such as EVAs, willbe at the lower end of the rangewhile the higher melting materials,such as HDPE and PP will be at thehigher end. Cold molds, however,tend to give a less glossy surfacefinish and will restrict the flow ofresin within the mold. A cold moldcan also lead to a higher amount ofmolded-in stress within the part.Warmer mold temperatures willincrease the gloss level and mayalso improve resin flow by constrict-ing the melt channel less, at theexpense of increased cycle times.The injection molder and the molddesign typically fix the amount oftime needed for the mold to open,eject the part and then close again.Mold open time can be reduced byusing only enough daylight to allowthe part(s) to fall freely, reducingthe amount of time required for airassists and also the number oftimes the ejector pins activate. ShrinkageAmorphous resins such as ABS,Polycarbonate, and Polystyrene havemuch lower shrinkage values thanthe polyolefins. The higher shrink-age of polyolefins is due to the factthat, in their molten state, they takeup more volume than in the solidstate because polyolefin resins aresemi-crystalline. When the resinsolidifies, the chains in the crys-talline regions pack tightly togetherresulting in a reduction in volume.In general, the polyolefins can beranked for shrinkage:HDPE LLDPE LDPE PPOnce a resin has been selected,shrinkage can be controlled, tosome extent, through mold designand processing conditions (Table 5).Studies on a test mold in which thethickness and gate area of a flatplaque can be varied, indicate thefollowing: Shrinkage is reduced as partthickness decreases. Theresponse to a thickness change ismore pronounced with HDPEthan PP. Shrinkage is reduced when thegate area is reduced.Since the degree of shrinkage ispartly a result of cooling, it can bereduced by molding at lower injec-tion temperatures and running acolder mold. Packing the part morewill also minimize shrinkage. This isdone either by molding at moderatetemperatures and high pressures orby molding at fairly high tempera-tures and moderate pressures.However, excessive temperature orpressure can result in flash.Another means of reducing shrink-age is the use of higher pressureand longer packing time. Thisallows additional resin to flow intothe mold as the material in themold cools and shrinks, packing outthe mold as much as possible butmay also increase cycle time andhigher molded-in stress. Longer cooling time in the moldbefore ejection is especially usefulwhenever an inside dimension iscritical. As the molded article coolsand contracts around the core, thecore will maintain the critical insidedimension of the part. Generousdraft, or tapering, will allow easierpart ejection. A longer cooling timewill mean an increase in cycle time,therefore many molders willincrease the mold cooling to reduceshrinkage.Shrinkage is a time-dependentfunction. In general, a polyolefinpart has achieved about 90% of itstotal shrinkage after 48 hours.Shrinkage can continue for severalmore days if the parts are packed27Figure 40. GATEMOLDPOLYMERMELTSOLIDIFIEDPLASTICMOLDhot and/or are stored in a warmwarehouse. Parts that have shrunkafter packaging typically exhibitnesting problems if the parts arestacked inside each other.WarpageWarpage results from non-uniformshrinkage of the molded partcaused by non-uniform cooling.When a part warps after beingejected, it is assuming its naturalshape by relieving the stressesforced upon it while being cooled inthe mold. The problem, often a dif-ficult one to solve, is to minimizethe locked-in stresses, which thepart might later remember andrelieve during cooling to room tem-perature. In cases where parts arefixtured after ejection, subsequentexposure to higher temperaturesmay cause relaxation and warpage.Part designs incorporating signifi-cant differences in cross-sectionalthickness are more prone towarpage than those with a moreuniform thickness, due to higherresidual temperatures in the thickersections. In addition to non-uniform cooling,locked-in stresses are generated inthe mold by such operating condi-tions as excessive molding pressures,slow fill times, low backpressure, ortoo low a melt temperature.There is no single, clear-cut remedyfor warpage. Adjusting mold condi-tions, redesigning the part or themold, switching to a material with anarrower MWD, or a combinationof these may reduce the internalstresses. Generally, the leastwarpage occurs when the melttemperature is set at the maximum,the mold temperature is high, injec-tion pressure is a minimum and theinjection time is short (Table 5). Molding at high temperaturesallows the stresses induced duringinjection to be reduced before thepart sets. Running a warm moldalso allows the stresses to relaxbefore the melt sets. Differentialcooling between the mold halves isoften required to produce warp-freeparts especially those having large,flat sections.Injection and packing pressuresshould be adequate to permit easyfill but should not be set excessivelyhigh in order to allow some of themolded-in stresses to relax beforethe part sets.Increasing the injection rate willdecrease the injection time, whichwill allow the mold to fill fasterbefore the extremities can cool toomuch. This gives the entire moldinga chance to cool at a more uniformrate, which reduces the warpage.Some of these remedies, such ashigh melt temperature or low injec-tion pressure can increase the cycletime. Switching to a higher MFR/MIresin can offset the increase in time.Higher flow resins will allow lowerinjection pressure, which can short-en the molding cycle. In addition,higher flow resins typically exhibitless “elastic memory” which canalso reduce warpage. Lower densityresins (for PE) are only slightly lesssusceptible to warpage than higherdensity resins.Differences between flow andtransflow shrinkage can result inwarpage. HDPE is known to have alarge difference between these twowhile PP is more balanced betweenthe flow and transflow shrinkage.Because both shrinkage andwarpage are strongly influenced bythe mold cooling patterns and partgeometry (uniformity of thicknessesand flow patterns), it is very impor-tant that these be considered in theearly stages of part and mold design.28SHRINKAGEWARPAGEMolding conditionsReduce cylinder temperature and lowerUse high cylinder temperature and high mold mold temperaturetemperatureLower temperatures near mold gatingUse low temperatures near mold gating andand sprue and entrance to molded partsprue and entrance to molded partUse minimum cooling at mold extremitiesUse minimum cooling at mold extremitiesModerate cylinder temperature and useLower injection pressurevery high injection pressureUse fairly high cylinder temperature andUse high cylinder temperaturemoderate injection pressureUse high injection pressure and extendedShorten injection timeinjection timeUse longer dwell timeUse longer dwell timeMold designUse proper location of sprue and gatingUse proper location of sprue and gatingResin propertiesUse lower density resinLower density resin of little importanceUse higher MI or MFR resinUse higher MI or MFR resinTable 5. Some ways to reduce shrinkage and warpage in polyolefin injection moldingsColor dispersion andair entrapmentAn effective means of improvingdispersion and preventing air bub-bles from getting into the moldwith the melt is to use a breakerplate at the end of the barrelbetween the screw tip and the noz-zle (Figure 41). Backpressure on themelt, in most cases, squeezes all theair out between the melting pelletsand produces bubble-free moldings.The breaker plate may be 1/4 inch(6.5 mm) thick and must be largeenough to fit into the opening ofthe nozzle. The plate is drilled with20- to-40, small diameter (1/32 inchor 0.8 mm) holes. Another option isto increase the back pressure on thescrew making sure that it is not settoo high so that the screw cannotrecover in time for the next cycle.Increasing the backpressure and/oradding a breaker plate can also aidin color dispersion and melt temper-ature homogeneity. When runningat high processing temperatures,backpressure should be kept to aminimum to reduce degradation ofthe resin.Part ejection and moldreleaseMold release is affected by a number of factors. Some polyolefinmolding resins exhibit better moldrelease than others do. It has been found that these resins haveaccompanying disadvantages, suchas less gloss. Resins that develop agrainy or frosty surface release bet-ter than smooth, high-gloss mold-ings, such as those made from highMI grades or clarified polypropy-lene. However, even polyolefins ofthe same MI may vary in their mold-release properties. Resinssometimes are compounded with a mold-release additive.Changing the mold design or oneor more molding conditions withoutaffecting the end properties of themolding usually can alleviate moldrelease problems. Mold release maybe difficult if the mold is packedtoo tightly in an effort to reduceshrinkage. Frequently, a moldedarticle sticks to the mold if the cycletime is too long and the moldingshrinks to a core. In such cases,shortening the cooling time mayimprove mold release. On the otherhand, the same problem can occurif the cycle is too short to allow themolding to shrink away from thecavity walls. In such cases, length-ening the cycle time may improvemold release. Draw stoning of thesurfaces in the direction of moldopening may also alleviate thisproblem.Mold release greatly depends onthe degree of polish on the insideof the mold. Proper surface finishesinside a mold for a deep-draw itemdetermines whether the part can beejected easily or will stick to thecavity or the mold core. Ejectionpins may be used to move themolding from either the cavity orthe core first.Enough draft must always be pro-vided, especially in deep-drawmoldings, such as long containers.Reverse draft or even parallel sides29DISPERSIONAIDPLACED HEREVENTURI PLATESNOZZLEBREAKERPLATE TYPE0.020”-0.030”0.030” OR LESS0.06”30-450.030”-0.050”Figure 41. Dispersion aids are inserts that may be placed in the nozzle ofthe injection molding machine to restrict the melt flow and improve mix-ing of the resin for a more homogenous melt temperature or mixture ofmaterials that may have been added, such as colorantshould be avoided wherever possi-ble. The draft allowance for EVAmoldings should be generous, asthey tend to be stickier at part ejec-tion temperatures. Normally, a 3- to5-degree draft significantly assists inpart ejection. On shallow draftmolds, the use of stripper ringsand/or air ejection may be neces-sary. Draft angles of less than onedegree are not recommended andshould be avoided unless dictatedby part requirements. ClarityHigh melt temperature and lowpressure are necessary to eliminateflow marks in molded parts. Clarityof molded parts also can beimproved by lowering the moldtemperature, especially in thin-walled sections when higher MIresins are used. This reduces thesize of the crystals formed which, inturn, reduces the light diffraction.Polypropylene random copolymershave greater clarity than PP homo-polymers. The clarity of randomcopolymer resins can be furtherenhanced by the addition of clarify-ing agents. Optimum clarity for PParticles in the nominal 0.050 in.thickness range is obtained at melttemperatures of about 430F(220C) and mold temperaturesabout 50 to 80F (10 to 25C).Generally, high injection rates alsoenhance clarity. Highly polished toolsare necessary for highest clarity.GlossSurface gloss of the molding isaffected by resin properties, thecondition of the mold and moldingconditions. The higher the MI orMFR of the polyolefin resin thegreater the gloss of the molding.Further, higher density polyethyl-enes give higher gloss than lowerdensity resins.Highly polished molds are one ofthe most important factors forobtaining high-gloss parts. For poly-ethylenes, a warm mold gives bettergloss than a cold mold. Gating alsomay contribute to obtaining a high-gloss part. Restrictive gating pro-duces higher gloss because it keepsthe temperature high as the melt isinjected into the mold cavity.Highest gloss for polypropyleneresins is obtained with a cold moldand a fast injection rate.Polypropylene integralhingesPolypropylene can be molded into ahinged part that can be flexedmany cycles before failure. Lifetimesin excess of a million cycles havebeen measured for properlydesigned and manufactured hingedparts.The hinged section must be thinenough to flex properly but thickenough to prevent tearing. Normalhinge thickness is 0.008-0.015 in.(0.2-0.38 mm). The greater thick-ness is necessary if the hingerequires strength or load bearingproperties.A typical cross-section of an integralhinge is shown in Figure 42. A shallow relief or clearance must beprovided to prevent gathering andexcessive stress when the hinge is inthe closed position. Variations maybe made on this design to achievespecific results.The gating into the part must bedesigned to allow the polymer toflow through the entire hinged sec-tion in a uniform and continuousfashion with the flow front perpen-dicular to the hinge. This arrange-ment ensures optimum hingestrength without delamination. It ispreferable that all gates be on oneside of the hinge to eliminate thepossibility of weld lines; however,some designs are so complicatedthat this arrangement is not possi-ble and gates must be placed so aweld line does not occur in thehinge. If multiple gates are used, itis recommended that a section,slightly thicker than the wall thick-ness, be placed parallel to thehinge. This flow collector will pro-mote a uniform flow pattern acrossthe hinge.Molding conditions that lead tooptimum hinge properties are ahigh melt temperature (typically500F to 525F/260C to 275C),fast injection speed and a warmmold (120F to 150F/50C to65C). In order to develop optimumproperties, the hinge should beflexed several times immediatelyafter removal from the mold. Insome applications such as multi-cavity, hinged closures, it is not pos-sible to perform this flexing.However, if the hinge is properlydesigned, it will still perform ade-quately for the requirements of theapplication. All polypropylenes canbe used for living hinges but themost optimum hinges are achievedwith homopolymer PP followed byrandom copolymer and impact PP.Acceptable hinges may be formedfrom PP impact copolymers butthere is some potential for delami-nation in the hinge area.It is also possible to produce hingesutilizing secondary operations. Onepost-forming procedure involves aheated steel die at about 425F(220C) which is forced into themolded part at 50-100 psi (345-690kPa) of pressure. A rolling, heateddie can also be used; this process isoften referred to as coining ahinge. If the polymer thickness isreduced to 0.005 to 0.015 in.(0.13-0.38 mm) a satisfactory hingeresults. This technique may be usedto put hinges in very large or com-plex parts.30Figure 42. Polypropylene integralhinge specifications0.030” RADIUS0 TO 0.005” RELIEF0.100” (TYP)0.008”- 0.015”Appendix 1Some terms pertainingto injection moldingAntioxidant: Additive used to helpprotect plastics from degradationthrough sources such as heat, age,chemicals, stress, etc.Antistatic Agent: Additive used tohelp eliminate or lessen static elec-tricity from the surface of the plasticpart.Aspect Ratio: Ratio of total flowlength to average wall thickness. Back Pressure: The pressureapplied to the plastic during screwrecovery. By increasing back pres-sure, mixing and plasticating areimproved; however, screw recoveryrates are reduced.Backing Plate: A plate used as asupport for the mold cavity block,guide pins, bushings, etc.Boss: Protuberance on a plastic partdesigned to add strength, facilitatealignment, provide fastening, etc. Breaker Plate: See Figure 41. Cavity: The space inside a moldinto which material is injected.(Figure 43). Charge: The measurement orweight of material necessary to fill amold during one cycle. Clamping Plate: A plate fitted to amold and used to fasten the moldto a platen (Figure 43). Clamping Pressure: The pressureapplied to the mold to keep itclosed during a cycle, usuallyexpressed in tons. Closed-loop Control: System formonitoring complete, injection-molding-process conditions of tem-perature, pressure and time, andautomatically making any changesrequired to keep part productionwithin preset tolerances. Co-injection Molding: A specialmultimaterial injection process inwhich a mold cavity is first partiallyfilled with one plastic and then asecond shot is injected to encapsu-late the first shot. Cooling Channels: Channels located within the body of a moldthrough which a cooling medium is circulated to control the mold surface temperature.Clarifiers: Additive used inpolypropylene random copolymersto improve clarity.Cushion: Extra material left in barrel during cycle to try and ensurethat the part is packed out duringthe hold time.Cycle: The complete sequence ofoperations in a process to completeone set of moldings. The cycle istaken at a point in the operationand ends when this point is againreached. Daylight Opening: The maximumdistance between the stationaryand moving platens of the clampunit in the fully open position. Delamination: When the surfaceof a finished part separates orappears to be composed of layers.Strata or fish-scale-type appearancewhere the layers may be separated. Diaphragm Gate: Used in symmetrical cavity filling to reduceweld-line formations and improvefilling rates. Direct Gate: The sprue that feedsdirectly into the mold cavity. Dispersion Aids: Perforated platesplaced in the plasticator nozzle toaid in mixing or dispersing colorantas it flows through the perforations(Figure 41).Draft: The degree of taper of amold-cavity sidewall or the angle ofclearance designed to facilitateremoval of parts from a mold.31Figure 43. Schematic showing a typical injection mold with some of thepoints identifiedRETURN PINPILLARLEADERSUPPORTPLATELEADERRETURN PINPILLARCLAMP PLATEMOLD PART(CAVITY)RUNNERLOCATING RINGSPRUE BUSHINGCAVITYRETAINERCORE RETAINERCLAMPPLATESPRUEPULLEREJECTORPLATEDrooling: The extrudation or leakage of molten resin from a plasticator nozzle or nozzle sprue-bushing area while filling or shoot-ing the mold.Dwell: A pause in the applied pres-sure to a mold during the injectioncycle just before the mold is com-pletely closed. This dwell allows anygases formed or present to escapefrom the molding material.Ejector Pins: Pins that are pushedinto a mold cavity from the rear asthe mold opens to force the fin-ished part out of the mold. Alsocalled knockout pins.Ejector Return Pins: Projectionsthat push the ejector assembly backas the mold closes. Also called sur-face pins or return pins.Ejector Rod: A bar that actuatesthe ejector assembly when the moldopens.Family Mold: A multi-cavity moldwhere each of the cavities formsone of the component parts of anassembled finished part.Fan Gate: A gate used to helpreduce stress concentrations in thegate area by spreading the openingover a wider area. Less warping ofparts can usually be expected bythe use of this type of gate.Fill: The packing of the cavity orcavities of the mold as required togive a complete part or parts thatare free of flash.Fin: The web of material remainingin holes or openings in a moldedpart which must be removed forfinal assembly.Flash: Extra plastic attached to amolding usually along the moldparting line.Flow: A qualitative description ofthe fluidity of a plastic material during the process of molding. Ameasure of its moldability generallyexpressed as melt flow rate or meltindex.Flow Line: Marks visible on the fin-ished items that indicate the direc-tion of the flow of the melt into themold.Flow Marks: Wavy surface appear-ances on a molded part caused byimproper flow of the melt into themold.Gate: An orifice through which themelt enters the mold cavity.Hob: A master model in hardenedsteel. The hob is used to sink theshape of a mold into a soft metalblock.Homopolymer: Plastic that resultsby the polymerization of a singlemonomer.Hopper Dryers: Auxiliary equip-ment that removes moisture fromresin pellets.Hopper Loader: Auxiliary equip-ment for automatically loading resinpellets into machine hopper.Hot-runner Mold: A mold inwhich the runners are insulatedfrom the chilled cavities and arekept hot. Hot-runner molds makeparts that have no scrap. Injection Pressure: The pressureon the face of the injection screwor ram when injecting material intothe mold, usually expressed in psi. Insulated Runner: See hot-runnermold.Izod Impact Test: Test to deter-mine impact strength of a sampleby holding a sample bar at one endand broken by striking. Samplespecimen can be either notched orunnotched.Jetting: A turbulent flow in themelt caused by an undersized gateor where a thin section rapidlybecomes thicker. Jig: A tool for holding parts of anassembly during the manufacturingprocess. Knit Lines: See weld lines. Knockout Pins: A rod or device forknocking a finished part out of amold. L/D Ratio: A term used to helpdefine an injection screw. This is thescrew length-to-diameter ratio. Melt Flow Rate: A measure of themolten viscosity of a polymer deter-mined by the weight of polymerextruded through an orifice underspecified conditions of pressure andtemperature. Particular conditionsare dependent upon the type ofpolymer being tested. MFR usuallyis reported in grams per 10 min-utes. Melt flow rate defines theflow of a polypropylene resin. Anextrusion weight of 2160 grams at446F (230C) is used. Melt Index: Term that defines themelt flow rate of a polyethyleneresin. An extrusion weight of 2160grams at 310F (190C) is used. Mold Changer: An automateddevice for removing one mold froma machine and replacing it withanother mold. Mold Frame: A series of steelplates which contain mold compo-nents, including cavities, cores, run-ner system, cooling system, ejectionsystem, etc. Mold-temperature-control Unit:Auxiliary equipment used to controlmold temperature. Some units canboth heat and cool the mold.Others, called chillers, only cool themold. Moving Platen: The platen of aninjection molding machine that ismoved by a hydraulic ram ormechanical toggle. (Figure 21-22)Multi-cavity Mold: A mold havingtwo or more impressions for form-ing finished items in one machinecycle. Multi-material Molding: Theinjection of two-or-three materials,in sequence, into a single mold dur-ing a single molding cycle. Theinjection molding machine isequipped with two-or-three plasti-cators. (See also co-injection) Nest plate: A retainer plate in themold with a depressed area for cavity blocks.32Non-return Valve: Screw tip thatallows for material to flow in onedirection and closes to prevent backflow and inject material into themold.Nozzle: The hollow-cored, metalnose screwed into the injection endof a plasticator. The nozzle matchesthe depression in the mold. Thisnozzle allows transfer of the meltfrom the plasticator to the runnersystem and cavities.Nucleating Agent: Additive usedwith polypropylene to increase crys-tallization rate by providing addi-tional sites for crystal growth.Orange Peel: A surface finish on amolded part that is rough andsplotchy. Usually caused by moisturein the mold cavity.Packing: The filling of the moldcavity or cavities as full as possiblewithout causing undue stress onthe molds or causing flash toappear on the finished parts.Part Picker: An auxiliary unit usual-ly mounted on fixed platen, whichreaches into the open mold to grabparts and remove them prior tonext molding cycle. Also called arobot, the device is used when youdo not want to drop parts frommold upon ejection.Parting Line: On a finished part,this line shows where the two moldhalves met when they were closed.Pinpoint Gate: A restricted gate of0.030 in or less in diameter, thisgate is common on hot-runnermolds.Piston: See ram.Plasticate: To soften by heatingand mixing.Plasticator: The complete meltingand injection unit on an injectionmolding machine.Platens: The mounting plates of apress on which the mold halves areattached.Plate-out: The blooming of addi-tives onto machinery during pro-cessing of plastics.Plunger: See ram.Pressure Pads: Reinforcements ofhardened steel distributed aroundthe dead areas in the faces of amold to help the land absorb thefinal pressure of closing withoutcollapsing.Purging: The forcing one moldingmaterial out of the plasticator withanother material prior to molding anew material. Special purging com-pounds are used.Ram: The forward motion of thescrew in the plasticator barrel thatforces the melt into the mold cavity.Recovery Time: The length of timefor the screw to rotate and create ashot.Restricted Gate: A very small ori-fice between runner and cavity inan injection mold. When the part isejected, this gate readily breaks freeof the runner system. Generally, thepart drops through one chute andthe runner system through anotherleading to a granulator and scrap-reclaim system.Retainer Plate: The plate on which demountable pieces, such asmold cavities, ejector pins, guidepins and bushings are mountedduring molding.Retractable Cores: Used whenmolding parts in cavities not per-pendicular to the direction in whichthe part is ejected from the mold.The cores are automatically pulledfrom the mold prior to the moldopening and reinserted when themold closes again and prior toinjection.Rib: A reinforcing member of amolded part.Ring Gate: Used on some cylindri-cal shapes. This gate encircles thecore to permit the melt to firstmove around the core before fillingthe cavity. Robot: Automated devices forremoving parts upon ejection froman open mold rather than lettingthe parts drop. Also see parts pick-er. Robots also can perform second-ary functions, such as inspection,degating, precise placement ofparts on a conveyor, etc. RMS Roughness: A measure of thesurface roughness/smoothness of amaterial. The root mean square(RMS) average of the “peaks andvalleys” of a surface is determinedusing a Profilometer. The lower thenumber, the smoother the surface:a reading of one or two would be avery polished and smooth surface. Rockwell Hardness: A measure ofthe surface hardness of a material.A value derived from the increase indepth of an impression as the loadof a steel indenter is increased froma fixed minimum value to a highervalue and then returned to the min-imum value. The values are quotedwith a letter prefix corresponding toa scale relating to a given combina-tion of load and indenter. Runner: The channel that connectsthe sprue with the gate for transfer-ring the melt to the cavities. Runnerless molding: See hot-run-ner mold. Screw Travel: The distance thescrew travels forward when fillingthe mold cavity. Short Shot: Failure to completelyfill the mold or cavities of the mold. 33PIN PT. CENTERPIN PT. MULTIPLERING OR DISCGATEGATEShot: The complete amount of meltinjected during a molding cycle,including that which fills the runnersystem. Shot Capacity: Generally based onpolystyrene, this is the maximumweight of plastic that can be dis-placed or injected by a single injec-tion stroke. Generally expressed asounces of polystyrene. Shrinkage: The dimensional differ-ences between a molded part andthe actual mold dimensions. Side Bars: Loose pieces used tocarry one or more molding pins andoperated from outside the mold. Side-draw Pins: Projections usedto core a hole in a direction otherthan the line of closing of a moldand which must be withdrawnbefore the part is ejected from themold. See also Retractable Cores. Silver Streaks: See splay marks. Single-cavity Mold: A mold hav-ing only one cavity and producingonly one finished part per cycle. Sink Mark: A shallow depression ordimple on the surface of a finishedpart created by shrinkage or low fillof the cavity.Slip Agent: Additive used to pro-vide lubrication during and immedi-ately following processing of plas-tics.Slip plane: Marks evident in or onfinished parts due to poor weldingor shrinking upon cooling.Spiral Flow: Test performed byinjection molding a sample into aspiral mold and used to comparethe processability of different resins.Splash Marks: See splay marks.Splay Marks: Marks or droplet-type imperfections on the surface ofthe finished parts that may becaused by the splaying of the meltthrough the gates and into the coolcavity where they set up.Split-ring Mold: A mold in which asplit cavity block is assembled in achannel to permit the forming ofundercuts in a molded piece. Theseparts are ejected from the mold andthen separated from the piece.Sprue Bushing: A hardened-steelinsert in the mold that accepts theplasticator nozzle and provides anopening for transferring the melt.Sprue Gate: A passagewaythrough which melt flows from thenozzle to the mold cavity.Sprue Lock: The portion of resinretained in the cold-slug well by anundercut. This lock is used to pullthe sprue out of the bushing as themold opens. The sprue lock itself ispushed out of the mold by an ejec-tor pin.Sprue: The feed opening providedin injection molding between thenozzle and cavity or runner system.Stack Molds: Two or more moldsof a similar type that are positionedone behind the other to allow foradditional parts to be manufacturedduring a cycle.Stationary Platen: The large frontplate of an injection molding pressto which the front plate of themold is secured. This platen doesnot move during normal operation.Stress Cracking: There are threetypes of stress cracking:1. Thermal stress cracking is causedby prolonged exposure of thepart to elevated temperatures orsunlight.2. Physical stress cracking occursbetween crystalline and amor-phous portions of the part whenthe part is under an internally orexternally induced strain.3. Chemical stress cracking occurswhen a liquid or gas permeatesthe parts surface.All of these types of stress crackinghave the same end result: the split-ting or fracturing of the molding.Striations: Marks evident on themolded-part surfaces that indicatemelt flow directions or impinge-ment.Stringing: Occurs between the fin-ished part and the sprue when themold opens and the melt in thisarea has not cooled sufficiently.Stripper Plate: A plate that strips amolded piece from core pins orforce plugs. The stripper plate is setinto operation by the opening ofthe mold.Structural Foam Molding: Processfor making parts that have solidouter skin and foamed core.Submarine Gate: A gate wherethe opening from the runner intothe mold cavity is located below theparting line. Also called a tunnelgate. Suck-back: When the pressure onthe sprue is not held long enoughfor the melt to cool before thescrew returns. Some of the melt inthe cavities or runner system mayexpand back into the nozzle andcause sinks marks on the finishedpart. Tab Gate: A small removable tababout the same thickness as themolded item, but usually perpendi-cular to the part for easy removal. Tie-bar Spacing: The spacebetween the horizontal tie-bars onan injection molding machine.Basically, this measurement limitsthe size of molds that can be placedbetween the tie-bars and into themolding machine. Toggle: A type of clamping mecha-nism that exerts pressure by apply-ing force on a knee joint. A toggleis used to close and exert pressureon a mold in a press. Tunnel Gate: See submarine gate. Undercut: A protuberance orindentation that impedes withdraw-al from a two-piece
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