等流体在注塑上的应用.doc

附件11外文资料翻译学生姓名： 陈卫国 专业班级： 材料成型1102 指导教师： 李丽 河北工程大学机电学院 年 月 在超临界流体和压模的帮助下对注射成型塑料齿轮精度的研究Jae D. Yoon 机械工程学系，延世大学，汉城，韩国Sung W. Cha 机械工程学院，延世大学，汉城，韩国Tae H. Chong 机械工程学院，汉阳大学，汉城，韩国Young W. Ha 机械设计工程系，汉阳大学，汉城，韩国塑料齿轮相对于钢齿轮更轻、噪音更少，而且很容易通过注射成型过程加工成各种形式。出于这个原因，塑料齿轮被广泛应用于工业。对于齿轮材料和塑料齿轮耐久度及更广泛应用的加工方法进行了大量研究。在这篇文章中，重点是加工方式结合注射成型工艺制造塑料齿轮。影响塑料齿轮耐久度的最大因素是其精度。不像钢齿轮，塑料齿轮的尺寸在成型时会有相当大的变化，这是由于注射成型过程中和模具结构的某些条件。这里的尺寸变量的主要原因是塑料的高粘度和树脂的收缩率。在我们的研究中，超临界流体被用来降低塑料的粘度，压模被用来控制树脂的收缩率。因此，生产的提高，更高精度的塑胶齿轮已经实现。关键词： 注射成型塑料齿轮；压模；超临界流体；粘度1. 简介相比钢齿轮，塑料齿轮有许多优点，因为塑料齿轮更轻、噪音更少、更容易批量生产。因此，塑料齿轮广泛应用于工业。塑料齿轮使用的材料是POM（聚甲醛），PC（聚碳酸酯）和PA（聚酰胺）。PA因为其高强度是应用最多的材料。然而，因为PA相对于聚甲醛相当昂贵且更难成型，在PA齿轮的水平上需要很大的努力提高注射成型聚甲醛齿轮的性能。此外，制造精确注塑件的努力和研究正在进行。根据赵等理论，当加入2%PC(聚碳酸酯),LCP（液晶聚合物）显示了更好的流动性和精确度。黄展示了聚合物中加入纳米陶瓷从而改善精度和耐磨损。微孔发泡工艺是由麻省理工学院研究人员在20世纪80年代开发。这是通过移除塑料矩阵中的微孔降低产品质量的一种塑料发泡方法。虽然它最初设计批量处理，现在可用于注射成型过程。许多研究者指出当流动气体如二氧化碳或氮气在超临界阶段注射到注塑机筒时塑料的粘度降低。传统上，作准确的塑料齿轮是一个艰巨的任务，因为根据塑料在模具中冷却的压力，塑料在熔融状态有很高粘度和不规则收缩，约2%。一般来说，高收缩发生在焊地区，低收缩发生在浇口附近。这问题通过加入超临界流体增加流动能力已经解决，如在微孔成型工艺中。然而，用这方法形成塑料齿轮是不合适的，因为微孔在齿轮里形成降低了抗拉强度。因此，为了制造精确塑料齿轮，需要一种新技术既能抑制微孔的核又能降低粘度。图1. 浓度关于温度和压力的函数曲线一种新的注射成型过程，被设计抑制微孔核的压模会在文章中提出。当塑料或者气体解决方案热力学不稳定时微孔核会产生。在微孔注射成型工艺中，超临界流体（SCF）注入进汽缸中的熔融聚合物，数量小于或者等于其溶解度限制，然后通过注射成型机螺杆搅拌直到溶解均匀做好注射准备。这种单相溶液然后注入到模具并冷却成形。在注射过程中，由于从气缸到模具的压力变化塔经历了热力学变化。当注射进入模具时溶解的超临界流体成核并且在模具压力足够溶解超临界流体时被抑制。传统的模具内的压力通常等于大气压力，但在文章中展现的压模改变了压力。通过证监会和压模成型工艺，制造的齿轮尺寸精度已调查且在这篇文章中呗描述。2. 微孔发泡工艺与压模2.1. 微孔发泡工艺麻省理工学院研究者在20世纪80年代开发的微孔发泡工艺是一项在塑料中制造微孔的技术。不像现有的发泡技术，它在超临界流体状态使用如二氧化碳和氮气的惰性气体，发泡孔大小在10微米左右或更小。如图1所示，当压力减小或者当温度升高溶解度降低时，塑料内溶解的气体变成饱和状态，在塑料矩阵中成核并结束发泡过程。相同的现象发生在当盖子打开溶解在汽水中的二氧化碳跑出。结合微孔发泡和注射成型的方法如图2所示。惰性气体如二氧化碳或者氮气在超临界流体状态下注入机筒内的熔融聚合物中，然后通过在注射成型机筒内特别设计的螺杆将聚合物和超临界流体混合。一旦这些材料混合成一相溶液，它注入模具并冷却至模具的形状。微孔成核发生是因为当溶液注入时从高压到低压（大气压）有一个热力学变化。当聚合物冷却成核孔长大并填满整个型腔，从而结束了一个周期。微孔发泡零件从超临界流体到聚合物的注射中获得优势因为它作为增塑剂的作用：熔融聚合物的粘度降低，使平滑易流动并在齿轮的制造中得到更高精度。图2.注塑机微孔发泡原理图2.2. 压模中的微孔工艺微孔发泡技术有许多优点，但其应用有一些困难，耐久度是关键，如塑料齿轮的例子上。塑料中微孔的存在降低了拉伸强度；结果齿形轮廓很容易破坏。使用压模的微孔工艺能够使惰性气体作为增塑剂使用。图3 模具的压力示意图在加压模具中伴随着成核的消除，齿轮保持着预期的性能在成型过程中。图3为加压模具的微细胞提供了轮廓。在塑料或超临界溶液注射之前，气体供应系统充满模具型腔。然后溶液通过注射成型机注射到型腔内。在溶液凝固之后，充满模具型腔的气体被释放到大气中。尽管加压的气体扰乱了溶液的流动性，超临界溶液作为增塑剂产生的积极影响比任何缺点都占的比重多。3实验3.1 材料用于此次实验的可塑性材料是广泛应用于塑料齿轮产品制造的聚甲醛（韩国工程塑料F20-02，韩国工程塑料有限公司）。聚甲醛有1.8%的目录收缩率。用于微细胞成型过程和加压过程的气体是具有99.99%纯度的氮气。3.2 实验方法在下面部分三种类型的样品被用来讨论。1）一般齿轮（固体齿轮）：命名为SGear2）微孔发泡齿轮：命名为MGear3）微细胞过程和加压模具制造的齿轮：PGearSGear由于是运用常规的注射模具过程加工的，可作为标准的齿轮为了MGear 和PGear作比较。MGear因为可以减少注塑体积0,10,20和35%质量的减少而产生。PGear是通过氮气加压模具制造出来的。因为聚合物或气体溶液完全填充模具而没有质量的减少（0%减少）。然而，考虑到封闭气体释放的时间段与塑料固化之间相关性的事实，样品在注射之后被划分成释放气体时间为1,2,3,4秒。3.3 检查特征这三种齿轮的齿顶圆跳动、表面粗糙度和齿形误差被测量用来比较齿轮的精确度。1）齿顶圆跳动齿轮的齿顶圆跳动是从齿轮中心轴到齿顶圆的中心半径的附加值和齿顶圆的圆度。这个量和跳动的偏心率的方向一般用在一个倒退循环的测量值的圆形图来表达。计算每个样品5个测量值得到的跳动平均值在这篇文章中将会被讨论。图4显示的是能测量齿轮齿顶圆跳动准确性的跳动测量仪器。表格1 显示了生产出来用于测量齿顶圆跳动的样品的类型。正如图4右边的图片显示，将探针放在齿宽为5毫米的齿轮齿顶面的中间后，齿轮跳动值能使用一台电脑的数据采集系统测量出来。2）齿轮表面粗糙度表面粗糙度是指在短的定期的间隔内的齿轮表面的分钟曲线，表格1显示了用于测量表面粗糙度的不同种类的齿轮。3） 辊型的塑料齿轮制造塑料齿轮的常用的方法是使用3或4个浇口的注塑成型过程。字典由于比起焊接区要求的更高的压紧力被用于靠近浇口处，靠近浇口处齿轮齿部的收缩小于其在焊缝区收缩。因此，塑料齿轮齿距可由一个正弦图代表。图5显示了轧辊轮廓示意图。一个更高的值在靠近浇口区得到和最低值在远离焊缝的区域（图5）具有更好准确性的齿轮有一个低维的区别于特征尺寸的最大值和最小值。精确度上的降低会导致总的组成错误的增加，在齿轮旋转时造成实质性的振动和噪声。表2显示了用于测量齿形误差的样本的类别。4） 聚甲醛的收缩率作为压力释放时间的功能为了理解为什么在实验中制造齿轮导致更高的精度，收缩率被当做压力释放时间的功能来检测。两种类型的模具被用于这个测试：一个是为标准化收缩试验，另一个是用于实际收缩率。用于标准化收缩的模具是60803毫米。用于实际收缩率的模具是和随着精度的提高生产实验用于齿轮制造模具一样的。由于取决于浇口位置和浇口数量的收缩率不同，在注射的塑料齿轮的焊缝区对收缩率进行了测量。样品在注射后释放加压气体1.5、2.0、2.5、3.0、3.5、4.0和5.0秒。表1用于测量齿顶圆跳动和表面粗糙度的模压齿轮类型齿轮类型注射条件SGear常规注射模具MGear重量减少0%/10%/20%/35%PGear计算压力释放时间1.5s/2.0s/2.5s/3.0s/3.5s/4.0sSGear：标准齿轮MGear：微孔发泡齿轮PGear：微细胞过程和加压模具制造的齿轮 图4 提示环路测量机4.结果和讨论1）齿顶圆跳动图6说明在测量生产不同质量减少的微孔发泡齿轮的跳动值时的变化，并显示了测量微细胞过程和加压模具制造的齿轮的跳动值比起通过压力释放而得到的齿轮的区别。图6显示随着质量的减少，微孔发泡齿轮的跳动值增加。产生这个的原因是由于质量的减少，聚合物或气体溶液的压紧力减少。最后，在焊缝线附近的收缩比起注射浇口更加明显。这意味着收缩率在两个区域的区别变得更大，在这种情况下齿顶圆的跳动会增加。对于图6中的PGear ，齿顶圆的误差值会随着气体释放时间的增加而得到改善，原因是，当加压到模具上时，施加到模具中注射高聚物溶液或溶解气上的压紧力相对较高，所以齿顶圆的误差值得以降低。换句话说，当压力释放时间为1.5秒时，需要0.5秒的时间填充内部型腔，1秒的时间用来对模具施压，这个时间不足以用来提高压紧力。当压力释放时间超过2,5秒时，实际的加压时间超过2秒，这可以获得高度精确的误差值。压力释放时间与固化速度的关系也被加以考虑。如果用来加压模具的气体在注射成型零件完全固化之前被释放，就会有收缩的可能。然而，若气体在固化过程之后释放，那么成型齿轮的轮廓尺寸就不会产生变化。当高聚物的温度低于结晶温度或玻璃态温度时，固化过程才会结束。对于POM来说，人们认为固化过程在注射后的2.5秒后完成。总结来说，MPear的齿顶圆由于不规则收缩，其精度与PGear相比要差。然而对于PGear，可以总结说，其误差偏值要好于SGear，并且随着压力释放时间的增加会到的更大的改善。图5. 辊型的塑料齿轮图6.在不同的发泡程度跳动测量数据。（上X轴）和压力释放时间（低的X轴）。三角标记显示了SGear运行超时值。2）轮齿的表面粗糙度图7表示了Ra值的变化，这是一个粗糙度的算术平均值。对于MGear,它是重量减的函数；对于PGear，它是压力释放时间的函数。结果表明，随着重量减的增加，粗糙度增加到了15倍。在注射模塑中，为了减轻产品的重量，高聚物溶液或溶解气被改变较小，因此高的重量减意味着低的压紧力，故当重量减增加时，MGear的粗糙度也会增加。另一方面，PGear的表面粗糙度在0.24到0.37微米之间，远远低于MGear并几乎和SGear一样。MGear的粗糙度值在1.2微米到3.4微米范围之间，这是由于压紧力的加剧。因此，可以看出，加压模的使用的确可以提高塑料齿轮的表面质量。 3）齿轮的齿廓偏差图8展示了测量的单齿距齿廓偏差和径向综合偏差值。理论上，没有螺距偏差和牙侧偏差的一对齿轮的综合偏差的形状是一条直线。但是，若被测齿轮有偏心率，随着齿轮旋转循环综合偏差就变成了正弦曲线。并且如果偏心率是e，那么径向综合偏差就是2e 。在注射成型齿轮中，即使没有偏心率，当节圆根据齿轮间隙位置形成一个三角形时，综合偏差呈现正弦波，三条曲线看起来像山形，如图8（a）所示。换句话说，在表明齿轮隙位置的弓形处，齿轮齿顶圆的直径看起来较大而齿轮隙之间处齿顶圆直径看起来较小。最后，正弦波的三点是拐点所在图8（b）展示的是3.5秒释放时间的PGear的测量数据。尽管单齿距齿廓偏差几乎和SGear一样，但综合偏差不是一条正弦波，而几乎是一条直线。这是因为齿轮间隙区域和焊头接头区的尺寸差别很小。焊头接头区的压紧力被认为和间隙区的几乎一致由于改善的流动性和SCF的塑化效应。我们可以做出结论，即压力释放时间超过3.5秒的PGear相对于SGear 和MGear 在齿顶圆误差、表面粗糙度和齿廓偏差上有更高的精度。这表明当用本文所提出的这种新的方法来加工出的塑料齿轮的持久性和耐用度会增加。 表2测量齿廓偏差所用的注射方法类型样本类型 注射条件SGear 传统注射模塑方法MGear 重量减超过10%无法测量PGear 背压释放时间 1.5s/2.0s/2.5s/3.0/3.5/4.0s4）作为压力释放时间函数的POM的收缩率图9表明了作为气体释放时间的函数的POM材料的收缩率。它与上面提到的精确结果有相似的趋势。起初，在标准收缩样本中，POM的收缩率值为1.8%，但是随着气体释放时间的增加，收缩率也发生改变。4秒时，收缩率呈现最低值，比最初的收缩率低了9%。在实际收缩试验中也得到了相同的结果。起初齿轮样本（焊头接头区）的收缩率值为2.61%，但是4秒时为2.45%。这意味着SCF和背压存在时注射成型零件会有精确的尺寸。4结论在本文中，提出了一种制造更高精度塑料齿轮的新工艺，并从齿顶圆误差、表面粗糙度和齿廓偏差等三个方面研究了用这种新工艺制造的塑料齿轮的精度问题。很明显，用微蜂窝工艺和加压模方法制造的塑料齿轮被证明精度得到了改善。因为在SCF状态下流动气体的塑化效应，微蜂窝工艺可以降低熔融塑料的粘度，而加压模可以抑制模具中熔融塑料的起泡并增加压紧力。我们有理由相信，这种改善后的齿轮同用传统方法制造的齿轮相比将表现出更好的性能，因为它拥有更高的精度。原文Polymer-Plastics Technology and Engineering, 46: 815820, 2007Copyright # Taylor & Francis Group, LLCISSN: 0360-2559 print/1525-6111 onlineDOI: 10.1080/03602550701278046Study on the Accuracy of Injection Molded Plastic Gearwith the Assistance of Supercritical Fluid and a Pressurized MoldJae D. YoonDepartment of Mechanical Engineering, Yonsei University, Seoul, KoreaSung W. ChaSchool of Mechanical Engineering, Yonsei University, Seoul, KoreaTae H. ChongSchool of Mechanical Engineering, Hanyang University, Seoul, KoreaYoung W. HaDepartment of Mechanical Design Engineering, Hanyang University, Seoul, KoreaPlastic gears are lighter and less noisy compared to steel gears, and they can be easily shaped into diverse forms by the injection molding process. For this reason, plastic gears are widely used in industry. An extensive amount of research has been conducted on gear materials and methods of shaping plastic gears for durability and for broader applications. In this article, the focus is on producing a plastic gear shaping method incorporating the injection molding process.The most important factor influencing the durability of a plastic gear is its accuracy. Unlike steel gears, the dimensions of plastic gears are subject to considerable change during formation, and this is due to certain conditions during the injection molding process and to the mold structure. The main causes of size variability here are the high viscosity of plastic and the shrinkage ratio of the resin. In our study, a supercritical fluid was used to reduce the viscosity of the plastic, and a pressurized mold was used to control the shrinkage ratio of the resin. Thus, production of an improved, more highly accurate plastic gear was achieved.Keywords Injection molded plastic gear; Pressurized mold;Supercritical fluid; Viscosity1. INTRODUCTIONPlastic gears have many advantages, compared to steel gears, since they are lighter and less noisy, and are easily mass produced. Consequently, plastic gears are widely used in industry. The materials used for plastic gears are POM (Polyacetal), PC (Polycarbonate), and PA (polyamide)1. PA is the most frequently used material because of its high strength. However, because PA is relatively expensive and more difficult to mold, compared to POM, there are great efforts to improve the performance of injection molded POM gears, to the level of PA gears.Furthermore, extensive effort and research is being conducted to produce accurate injected parts. According to Zhao et al. LCP (Liquid Crystalline Polymer) showed better flow-ability when 2% PC (Polycarbonate) was added, and the use of LCP led to more precision2. Huang demonstrated improved accuracy and resistance to wear as a result of the addition of nano ceramics to the polymer3.A micro-cellular foaming process was developed by MIT researchers in the 1980s. This was a plastic foaming method that reduced the weight of the product by placing micro-voids in the plastic matrices4. Although it was originally designed for batch processing, it is now available for the injection molding process. Many researchers state that the viscosity of plastic is reduced when a flowing gas, such as CO2 or N2 in the supercritical phase is injected into the injection molding machine cylinder57. Traditionally, it has been a difficult task to make accurate plastic gears because plastic in the molten state has high viscosity and shrinks irregularly, by approximately 2%, according to the stress applied when it cools in the mold. Generally, high shrinkage occurs in the welded region and low shrinkage occurs near the gate. This problem has been solved by increasing the flow-ability with the addition of a supercritical fluid, as during the micro-cellular injection molding process. However, plastic gears formed by this process are not suitable because micro-cells form in the gear, reducing the tensile strength. Therefore, in order to produce accurate plastic gears, a novel technique was required to both suppress the nucleation of micro-voids and reduce the viscosity.FIG. 1. Concentration curve as a function of temperature and pressure.A new injection molding process, the pressurized mold, which is designed to suppress the nucleation of micro-voids, is presented in this article. The nucleation of micro-voids occurs when the plastic=gas solution is thermodynamically unstable. In the micro-cellular injection molding process, the supercritical fluid (SCF) is injected into the molten polymer in the cylinder in an amount less than or equal its solubility limit and then mixed by an injection molding machine screw until the solution is homogeneous and ready for injection. This single phase solution is then injected into the mold and frozen to shape. During the injection process, it experiences thermodynamic changes due to variation of pressure from the cylinder to the mold. This dissolved SCF nucleates when injected into the mold and can be suppressed when the pressure of the mold is sufficient to dissolve the SCF. The pressure inside the conventional mold is typically equal to atmospheric pressure, but the pressurized mold design presented in this article can alter it.By using an SFC and the pressurized mold process, the gears produced have been investigated for dimensional accuracy and described in this article.2. MICROCELLULAR FOAMING PROCESSAND THE PRESSURIZED MOLD2.1. Micro-cellular Foaming ProcessThe micro-cellular foaming process developed by MIT researchers in the 1980s is a technique for producing micro-cells inside the plastic. Unlike the existing foaming techniques, it uses inert gases like CO2 and N2 in the SCF state and the foamed cell size is about 10 mm or less. As shown in Fig. 1, when the pressure is reduced or when the solubility is lowered by increasing the temperature, dissolved gas inside the plastic is transformed into a supersaturated state, which causes nucleation in the plastic matrices and ends the foaming process. The same phenomenon occurs when CO2 that is dissolved in soda pop escapes when the lid is opened. The method of combining micro-cellular foaming and injection molding is shown in Fig. 2. Inert gas such as CO2 or N2 in the SCF phase is injected into the molten polymer that is in the cylinder and then the polymer and SCF are mixed together by a specially designed screw in the injection molding machine cylinder. Once these materials are mixed into a one phase solution, it is injected into the mold and it freezes to the shape of the mold. The nucleation of micro-voids occurs because there is a thermodynamic change from high pressure to low pressure (atmospheric pressure) while the solution is injected. The nucleated voids grow and fill the entire cavity while the polymer freezes, thus ending one cycle.Microcellular foamed parts gain advantage from the injection of SCF into the polymer because of its role as a plasticizer: the viscosity of the molten polymer can be reduced, providing a smooth and easy flow and leading to greater accuracy in the production of gears.2.2. Micro-cellular Process with the Pressurized MoldThe micro-cellular foaming technique has numerous advantages, but there are some difficulties in its application, in which durability is critical, such as in the case of plastic gears. The presence of micro-cells within the plastic reduces the tensile strength; the tooth profile could easily break as a result. The micro-cellular process using a pressurized moldFIG. 2. Schematic of injection molding machine for micro-cellular foaming.FIG. 3. Schematic of the pressurized mold.enables the use of an inert gas as a plasticizer. With the elimination of nucleation in the pressurized mold, the gear maintains the desired properties during the process. Figure 3 provides an outline for the micro-cellular process with the pressurized mold. Before injection of the plastic=SCF solution the gas supply system fills the mold cavity. Then the solution is injected by the injection molding machine. After the solution solidifies, the gas that filled the mold cavity is released to the atmosphere. Although the pressurized gas disturbs the fluidity of the solution, the positive effects produced by the SCF as a plasticizer outweigh any disadvantages.3. THE EXPERIMENT3.1. MaterialsThe plastic material used for this experiment is POM(KEPITAL F20-02; Korea Engineering Plastics Co.), which is widely used for the production of plastic gears. It has a catalog shrinkage ratio of 1.8%. The gas used for the micro-cellular process and for pressurizing the mold is nitrogen gas with 99.99% purity.3.2. Method of ExperimentThree types of samples were made as discussed in thefollowing section:1) Normal Gear (Solid Gear): named SGear2) Micro-cellular foamed Gear: named MGear3) Gear with microcellular process and the pressurized mold: PGearSGear is a standard gear that was manufactured by using the conventional injectionmolding process, in order to compare it with MGear and PGear. MGear was produced by reducing the shot volume by 0, 10, 20, and 35% weight reduction. PGear was manufactured by pressurizing the mold with N2. There was no weight reduction (0% reduction) because the polymer=gas solution filled the mold entirely. However, considering the fact that there is a correlation between the time period of the enclosed gas release and the solidification of the plastic, samples were made that released the gas in 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 seconds after injection.3.3. Examined FeaturesThe run-out of the tip circles, surface roughnesses, andprofile deviations of the 3 types of gears were measured in order to compare gear accuracies.1) The Run-out of Tip CircleThe run-out of the tip circle of the gear is the added value of the eccentric radius from the center axis of the gear to the tip circle and the roundness of the tip circle. The volume and the direction of eccentricity of the run-out are generally expressed as a circular graph with the measured values in a regressed circle. The average values of the run-out, calculated from five measurements of each sample, are discussed in this article. Figure 4 shows the run-out measuring machine that can measure the accuracy of the run-out of the tip circle of the gear. Table 1 shows the types of samples produced to measure the run out of the tip circle. As shown in the right hand side picture of Fig. 4, after placing the probe tip in the middle of the top surface of the gear tooth with a width of 5 mm, the run-out value of the gear was measured by the data acquisition system using a computer.2) The Surface Roughness of the Gear TeethSurface roughness refers to the minute curves in short regular intervals on the surface of a gear and Table 1 shows the types of gears that were used for measuring the surface roughness.3) Roll Profile of a Plastic GearThe usual method of making plastic gears is the injection molding process that uses 3 or 4 gates. Since a higher packing force is applied to the area near the gates than is required for the weld area, the shrinkage of the gear teeth near the gate area is less than its shrinkage in the weld area. Therefore, the circular pitch of the plastic gears can be represented by a sinusoidal graph. Figure 5 shows a schematic diagram of roll profiles. A higher value is obtained in the area near the gates and the lowest value is achievedFIG. 4. Tip circle measuring machine.in the area far from the weld (Fig. 5). The gears with better accuracy have a lower dimensional difference between the maximum and the minimum values of this feature size. A decrease in the accuracy leads to an increase in the total composition error, resulting in substantial vibration and noise as the gears rotate8. Table 2 shows the types of samples used to measure the profile deviation.4) Shrinkage Ratio of POM as a Function of Pressure