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附件2：外文原文 Gas-Assisted Injection MoldingInjection molding is a very popular operation for production of commercial plastic parts with its sophisticated control and superior surface details. However, it has limitations, such as long cycle time for parts with thick sections due to slow cooling. Also packing of thick sections can produce sink marks on the part surface. Large thin parts can have warpage because the residual stress and strain induced during filling and packing. Thus traditional injection molding can be modified to solve these kinds of problems, also to improve the quality of the part and lower the cost of production.Currently, gas-assisted injection molding is in use and being developed worldwide. In the US, the process is known as Gas-Assisted Injection Molding (GAIM); it is also called Gas Injection Technique (GIT) in Europe (see Fig.4.3.1). This process is developed for the production of hollow plastic parts with separate internal channels. It is unique because it combines the advantages of conventional injection molding and blow molding while differing from both. GAIM offers a cost effective means of producing large, smooth surfaced and rigid parts using lower clamping pressure with little or no finishing. By introducing the gas before complete filling, numerous problems such as warpage, sink marks, and high filling pressure are mostly overcome. Moreover, the process gives great benefits in terms of higher stiffness-to-weight ratio than the solid parts with the same overall dimensions due to the elimination of material placed inefficiently near the neutral axis of the cross section, thus increasing the freedom of part design.In comparison with conventional injection molding, the gas-assisted process is more critical in terms of process control, especially for multi-cavity applications. The quality of the part is determined by both tool and process variables such as degree of under-fill, gas injection conditions, and mold temperature, thus indicating the importance of process control. The process is attracting many molders due to the demand for highly automated production of gas-assisted injection molded parts.The gas-assisted injection molding process is the most rapidly growing field with considerable work going on in the field of controls and the process development. Research interest is drawn towards the development of new gas injection units, the study of the process variable, the efficiency of the production process, and advantages offered by the new process. Many different companies are offering gas injection-molding units with the various options, which are mainly pressure controlled or volume controlled processes.In gas injection molding, the mold is partially filled with molten thermoplastic, and an inert gas, usually nitrogen, is injected into the plastic. Gas is injected into the molten thermoplastic material using either of two procedures. In one method, a measured volume of gas is pressurized in a container. A valve is opened to allow the gas to flow into the polymer, and a piston is activated to force all gas from the container into the mold. As the gas expands in the mold, its pressure drops. A second method holds gas pressure, rather than gas volume, constant. The gas rapidly travels down the thickest-and therefore the hottest-section of the part, advancing the melt front and filling and packing the mold. Additional plastic volume may be displaced by the pressurized gas as the material shrinks. After the plastic cools, the gas is allowed to escape, leaving a molded plastic part containing internal voids.The standard GAIM process can be divided into four partial steps. The first step is a stage of melts injection Fig.4.3.2 (a). The cavity is partially filled with a defined amount of melt. The required volume is empirically determined by performing filling studies in order to avoid blowing the gas through at the flow front and to ensure an ideal blowhole volume. Typically the polymer fills the cavity between 75%95% before the melt and gas transition.The gas inlet phase is the second stage, which is shown in Fig. 4. 3. 2(b). Gas may be added at any point in time either during or shortly after melts injection. The gas can enter only if the gas pressure exceeds the melt pressured. In the interior of the molded part, the gas expels the melt from the plastic nucleus until the remainder of the cavity is completely filled. Gas injection pressures range from 0.530Mpa (704500psi).At the gas holding pressure phase, Fig. 4.3.2(c) the gas continues to push the polymer melt into the extremities of the cavity of the molded article acts as a holding pressure to compensate for path of least resistance as it pushes through the polymer. The final stage is a gas return for recycling or a gas release to atmosphere Fig. 4. 3. 2 (d). After the gas holding phase, the gas pressure in the molded article is released to the outside by suitable gas return and/ or by pressure release.A. Advantages of the GAIM processGas injection provides a solution to a number of problems that occurs in conventional injection molding.(1) Reducing stress and warpage With gas, the pressure is equal everywhere throughout the continuous network of hollow channels. When designed properly, these provide an internal runner system within the part, enabling the applied pressure, and therefore the internal stress gradients, to be reduced markedly. This reduces a parts tendency to warp.(2) Elimination of sink marks Sink marks resulting from ribs or bosses on the backside of a part have long been a problem. These surface marks result from the volume contraction of the melt during cooling. Sink marks can be minimized or eliminated if a hollow gas channel can be directed between the front surface of the part and the backside detail. With gas injection, the base of the rib made somewhat thicker to help direct the gas channel. With a gas channel at the base of a rib, material shrinks are away from the inside surface of the channel as the molded part cools because the material is the hottest at the center. Therefore, no sink mark occurs on the outside surface as the part shrinks during cooling.(3) Smooth surfaceUnlike structural foam, gas injection permits lighter weight and saves material in a structurally rigid part. With gas holding, a good surface quality can be achieved. (4) Reduced clamp tonnageIn conventional injection, the highest pressure occurs during the packing phase. The maximum injection pressure is significantly lower in GAIM and a controlled gas pressure through a network of hollow channels is used to fill out the mold. This means that clamp tonnage requirements can be reduced by as much as 90%.(5) Elimination of external runnersOne of the best features of gas injection is that flow runners can be built right into the part. Frequently, all external runners (both hot and cold) can be eliminated, even on a larger and complex part. These benefits include the reduced tooling costs, the lower quantities of regrind from runners, and the improvement of temperature control over the plastic melt. Often the internal runners can improve the flow pattern in the mold and eliminate or control knit-line location resulting from multiple injections from multiple injection gates. In addition to serving as flow channels, the ribs and thick sections can provide structural rigidity when required.(6) Permitting different wall thicknessA constant wall thickness is maintained in the plastic parts. With gas injection, this design rule is flexible. Different wall thicknesses are possible if gas channels are designed into the part at the transition points. This permits uniform material flows in the mold and avoids the high stresses and warpage that normally result from this sort of geometry.(7) Cycle time ReductionCompared with structural foam, gas-injection parts do not have the same inherent insulating characteristics, so that cycle times are faster-reportedly even faster than would be conventional injection of the same part with no hollow sections.(8) Resin savingGas assist plays a direct role in part-weight saving in the conversion of current tools. The main factor in reducing weight is that the part cavity is never completely filled. Another major contributor to resin saving is scrap reduction. With proper tool design, gas assisted allows scrap-free startups and production runs. B. Disadvantages of the GAIM process All processes have their disadvantages, but those of GAIM and GAIMIC (Gas-assisted injection molding with internal-water cooling) appear relatively minor compared with their significant advantages.(1) Large hollow sectionsGIAM is not well suited for thin-walled hollow parts such as bottles or tanks. However, the thin-wall part has also tried out for some specific applications.(2) Vent holeThe gas must be vented prior to opening the mold, leaving a hole somewhere on the part. Normally this can be placed in a non-visible location, but if appearance or function is affected or secondary operations are required, it may be necessary to seal the hole.(3) Mold temperature control Since wall thickness along the gas flow channel is a function of cooling rate, consistent wall thickness requires precise mold temperature control.(4) Surface blush The gas channel may leave surface blush, which arises from differences in surface gloss leaves. The tendency for blush is a function of processing conditions and types of plastics.(5) Unique design The unique part design and mold design required in most cases to fully utilize that GAIM might be considered by some to be a disadvantage. The gas part design takes a relatively longer time than with the conventional injection molding process.(6) Extra cost of controllerIn order to control the gas injection, the process requires extra equipment. Gas-assisted injection molding with internal cooling requires a system for controlling the gas and the water, an expense not required with traditional injection molding.C. Types of process defects in the GAIMFingering, gas bubbles, hesitation lines, burning of resin, witness line cold slug, and gas blowout are typical defects normally encountered in GAIM. Fingering, or gas permeation, is a common problem encountered in GAIM. In fingering, gas escapes from the gas channel and migrates into undesired areas of the part. Severe gas fingering can result in significant reduction n in part stiffness, impact strength and reliabitity of the final molded part. During the gas holding phase, the transitional region between the gas channel and the flat area is possible for fingers to form within the flat area. In this case, the main cause of the fingering effect is the higher its shrinkage potential, and hence the greater danger of the fingering effect. In order to largely exclude the fingering effect through design, it is necessary to implement the following criteria: a basic wall thickness of 4mm or greater should be avoided for flat areas, a material with favorable solidification behavior should be selected, and the lowest possible gas pressure should be applied.Gas bubbles are caused by fingering. When fingering occurs, gas sometimes gets trapped in the thin-wall sections of the part where the gas is unable to fully vent. These trapped gases can cause bubbles that will still be in the gas core after the mold is opened.Hesitation lines appear on the surface of a part produced by GAIM when the short shot of resin stops in the cavity, then starts moving again as the gas completes the fill.Burning of the resin can appear on either the outer surface of the part or within the gas channel itself. Burning of the part surface can be caused by gas pressure that is too high or by insufficient venting of the mold. Burning, the resin within the hollow sections of the part is also possible. Burning within the gas channel can cause gas injection pins to become plugged.On thin-walled parts molded in certain resins, a witness line, or gloss-level change, can occur over the gas channel. Excessive gas pressure can also cause witness lines over gas channels.When gas is injected through the molding machine nozzle, cold slugs of resin may occur on the part surface. A cold slug is caused when a small amount of unmelted resin is injected into the part.Gas blowout occurs when there is not enough resin in the cavity to hold the gas inside the part. If the part is short, gas will migrate to the non-filled area of the cavity and blow through. When blowout occurs, the part will sometimes look like a short shot.Most cases of defects are produced by the interface of the gas and the melt. These problems can be overcome by internal water-cooling between the interface of the gas and the melt. 气辅注射成型注射成型是一种很普通的生产方法，用于加工那种生产时难以控制和有复杂表面的商业塑件。但是这种成型方法也有一些局限性，如因为壁厚太厚，而要很长的冷却时间，使工件的生产周期变长。还有在局部厚壁处成型时，其表面会产生凹陷现象。因为在保压和补缩时会产生残余应力和应变，所以大的薄壁件会发生翘曲变形。因此可通过改良传统的注射成型工艺来解决这些问题，提高产品的性能和降低产品的成本。当前气辅成型工艺已投入使用，并在全球迅速发展。在美国，这种工艺被称为气体辅助成型（GAIM）；在欧洲这种工艺过程又被称为气体注射技术（GIT），见图4.3.1。这种工艺的发展是为了生产那种有内部通道的中空塑料产品。这是一种非常独特的工艺，因为它集合了传统的注塑成型和中空成型的优点，但跟两者又不一样。GAIM是一种高效率的生产方式，因为它能在比较小的锁模力，很小的或不需要保压力的情况下生产出表面光滑且坚硬的大型塑件。在塑料熔体完全充满型腔前，注入气体能解决许多问题，例如翘曲变形、表面凹陷和注塑压力的高需求量。这种工艺过程提供的许多便利是依据刚度-重量比比模具与制件尺寸比高（模具与制件尺寸比为一），因为这会减少塑件在横截面中心轴附近的物料，因此能扩大结构设计的自由空间。 与传统的注塑成型工艺相比，气辅成型在成型的控制方面有比较多的优点，特别是多型腔的应用。制品的质量由工具和生产过程中的工艺变量决定，这些工艺变量包括注塑温度，注塑气体的状态和模具温度，即过程控制是非常重要的。这种工艺过程吸引了许多模具设计师投入其中，因为社会需要高自动化的气体辅助注塑模具。随着控制过程和工艺方面的发展，在复杂工艺发展过程中，气辅成型工艺发展最快。这方面的研究方向是新型气体注射装置的发展、工艺变量的确定、产品的生产效率，新工艺过程的一些优势。多数公司能出售一系列可供选择的气体辅助模具的标准模架，这些模架一般是由压力控制或体积控制工艺过程。气辅注射成型时，先用热塑性塑料部分充填模具形腔，然后再注入一种惰性气体，通常是氮气。可用两种工序中的一种将气体注入到热固性塑料内部。一种方法是将等体积的气体注入到容器里。将阀门打开使气体流入聚合物中，并且用活塞来推动所有气体从容器流入到模具型腔中。当气体在模具中膨胀时，其压力将下降。第二种方法是保持气体的压力（不是体积）为恒量。气体迅速沿着最厚也是最热的部位向料流前锋推进，然后充满模具型腔并保压。另外在塑件收缩时，压力气体可以充填收缩的那部分塑料所占的空间。塑件冷却后，将气体排出塑料件，留下内部中空的且与模具形状相同的塑件。GAIM的标准工作过程分四步。第一步是熔体注射，见图4. 3.（2）。在型腔中注入定量的熔体（没有充满型腔）。所需的体积量是通过模拟实验来计量和决定的，以防止气体流穿塑件并且能保证理想的充气空间。通常在塑料熔融和注入气体前注入75%95%型腔体积的聚合物。第二步气体注射，见图4. 3. 2（b）。可在熔体注射时或其后短暂一段时间进行气体注射。只有在气体压力超过熔体压力时气体才能流进熔体。在模具内部，气体迫使塑料熔体从聚集物团状流到其完全充满行型腔。气体注射压力的范围为0.530Mpa（704500pis）。第三步气体保压，见图4. 3. 2（c）。气体继续使聚合物熔体完全充满型腔。在这个阶段，气体推着聚合物流动时，它总是沿着抵抗力最小的路径前进。第四步气体泄压，见图4. 3. 2（ d）。在气体保压后，塑件中的气压可通过适当的气体转换或卸压方式来释放。A. GAIM工艺的优势气体辅助注射成型能解决许多发生在传统注射成型阶段的问题。（1）减少应力和翘曲变形因为气体通过连通着的各个通道，使塑件各处的压力相等。经过很合理的设计，塑料制品内部可以提供合理的气流通道，以保证塑件的使用压力，因此塑件内部应力能平稳的下降。这样就可以减少塑件翘曲变形的趋势。（2）消除收痕现象 塑件背面的肋和凸台所引起的收痕现象会导致塑件长期的使用问题。这些表面缺陷是塑件在冷却时因体积收缩而引起的。如果在塑件的前、后表面间设计合理的气体通道，收痕现象将会减少或消除。气体辅助注射时，设计比较厚的肋骨，可以方便地在塑件内部形成气体通道。因为塑件内部温度最高，所以在肋处设有气体通道，这样塑件冷却时材料的收缩变形将远离塑件内部的气体通道。因此当塑件冷却时其表面就不会因收缩而发生收痕现象。（3）增加表面光泽度不像泡末成型，气体注射成型不但能节省材料，而且能生产出结构坚固且轻质的塑件。在保压状态下，塑件能自动完善其表面光泽度。（4）减小锁模力在传统注射成型过程中，保压阶段需要很高的压力。在GAIM生产过程中的最高注射压力显然低于因气体流经气体通道而需要的控制压力。也就是GAIM所需的锁模力将减少到原来的90%。（5）消除表面熔接痕气体辅助注射成型的一个最大特点是在塑件中设计合理的气流通道。所以塑件表面的熔接痕（冷的和热的）经常可以消除掉，即使在大而复杂的塑件上。这有许多优点，包括模具设计、制造费用降低，研磨熔接痕的次数减少和塑料熔融时的温度控制的发展。通常在模具中设计合理的气流通道可以改变熔体的流动方向，和通过开几个注射通口来减少或控制熔接痕。另外为了很好地保证气流通道的顺畅，在需要时可在肋和厚壁处进行结构的加强。（6）满足不同的壁厚要求在用一般注塑机注射时，塑件的壁厚必须等尺