注塑工艺参数对制品残余应力和收缩的影响

注塑成型工艺参数对制品的最终残余应力和收缩有着直接的影响。基于线性黏弹性模型模拟计算了注塑成型过程中由温度和压力引起的残余应力和收缩。以无定型材料PS和ABS为例，系统地研究了不同成型工艺条件下平板制件的最终残余应力和收缩，并和实验结果进行对比验证。结果表明：在流动方向上无定型材料的收缩基本保持不变，残余应力沿壁厚分布的形状也基本相同，但流动末端处的应力值稍大于流动入口处；保压压力是影响制品收缩的关键因素，提高保压压力和注射温度可以降低制品的最终收缩，而模具温度对收缩的影响较小。     

In-mold shrinkage and stress prediction in injection molding

In-mold shrinkage may occur for product parts that solidify under low holding pressure and are not restricted by ribs or flanges. It not only affects the final product dimensions but in addition may have a large effect on the residual stress distribution. A simple elastic model is used to study the effect of in-mold shrinkage on final product dimensions and residual stress distributions. Friction between polymer surface and mold wall as well as deformation of the mold cavity are taken into account. The model uses local values for temperature, pressure, and crystallization, which belong to the standard output of most simulation codes.     

基于制品体收缩率的多型腔注塑模工艺参数优化

选用L27(313)正交表设计实验,研究模具温度、熔体温度、充填时间、保压压力和保压时间以及三个两因素交互作用对多型腔模具PP塑料制品每个型腔体收缩率的影响程度。对于所选参数,模具温度和保压时间对体收缩率的影响是高度显著的,熔体温度和保压压力对体收缩率的影响是显著的。在优化工艺组合下体收缩率降低了21.46％。     

基于MPI的汽车内置储物盒注塑模具优化设计

以某汽车内置储物盒为研究对象,结合聚合物流变学的基本理论,采用Moldflow软件对汽车内置储物盒注射成型进行模流分析。针对储物盒制件在注射过程中出现的表面缩痕严重、达到顶出温度的时间长和翘曲量大等诸多缺陷,将实体模型的浇注系统和冷却系统等进行重新设计。通过对各个质量评价指标进一步分析发现,优化后储物盒制件的各种缺陷消除或减小,缩痕指数由0.638 2~4.009降低为零、零件达到顶出温度的时间由37.92~92.68 s降低到19.87~21.40 s、总翘曲量由1.166~1.782 mm降低到0.024 0~0.416 5 mm,优化效果明显。在此基础上进行汽车内置储物盒注塑模具的设计,并试模加工出了合格的制件。     

As-molded shrinkage measurements on polystyrene injection molded products

An experimental study of shrinkage in injection molded products is presented in this paper and documented with all information needed for any further analysis. In particular, the effect of holding pressure, holding time and mold geometry on product shrinkage was investigated for amorphous polystyrene. It turned out with increasing holding pressure the in-plane shrinkages varied from 0.6% to 0.1%, while the product thickness increased from about 1% to 10%. The holding time only affected product shrinkage for settings less than gate freeze-off time. Shrinkage usually increases along the flow path unless back flow sets in at the end of the holding stage. It was also found that if a constraint prevents in-mold shrinkage to take place, final shrinkage may decrease if holding pressure and time are small. The effects of all the variables analyzed were described by a thermomechanical model recently proposed.     

DILATOMETRIC STUDY OF THE EFFECTS OF CURE PROFILES, FILLERS, AND MOLD CONSTRAINTS IN A MODEL EPOXY

A linear dilatometer was used to investigate the effects of cure conditions, mold types, and the presence of filler in a model epoxy. These studies revealed shrinkage in the cured epoxy when heating it through the glass transition region. The magnitude of the shrinkage, related to stress buildup in the epoxy during cure, was influenced by the epoxy processing conditions, filler content, and the nature of the mold used to contain the resin. Curing at a lower temperature, prior to a postcure, decreased the magnitude of observed shrinkage. Annealing samples outside the mold allowed stresses that developed during the cure process to be reduced and led to less shrinkage observed than for samples annealed in the mold. Samples cured in a silicone mold exhibit less shrinkage in the dilatometer than samples cured in an aluminum mold. Soaking samples in water prior to dilatometric analysis led to stress relaxation in the sample and decreased the observed shrinkage. Specimens containing titanium dioxide filler exhibited less shrinkage than unfilled samples. The filled samples also exhibited less expansion slightly above the glass transition temperature. The reduced expansion might arise from restricted chain mobility in the presence of the filler.     

Modeling the packing stage in injection molding of thermoplastics

A model, for the packing stage In injection molding of thermoplastics is proposed. It allows one to calculate the time evolution of pressure and temperature fields and mass variations in simple geometries. The model holds for amorphous as well as for crystalline polymers if the kinetics of crystallization are known for temperatures far from the melting point, It applies after filling stage; initial temperature and pressure conditions are available from filling-simulating software. The principles rest on finite difference schemes computing simultaneously temperature and velocity of compressible non-Newtonian fluid in a filled cavity. The pressure field is determined from an equation of state at any time, owing to a mass balance in each gridmesh. The meaningful results of the simulation are local shrinkage and a good approximation of final weights of finished products. The algorithm is essentially checked for the influence of physical, thermal, and processing parameters on the cavity pressure for the injection of a polystyrene (Gedex 1541) into a rectangular mold cavity.     

DILATOMETRIC STUDY OF THE EFFECTS OF CURE PROFILES,FILLERS, AND MOLD CONSTRAINTS IN A MODEL EPOXY

A linear dilatometer was used to investigate the effects of cure conditions, mold types, and the presence of filler in a model epoxy. These studies revealed shrinkage in the cured epoxy when heating it through the glass transition region. The magnitude of the shrinkage, related to stress buildup in the epoxy during cure, was influenced by the epoxy processing conditions, filler content, and the nature of the mold used to contain the resin. Curing at a lower temperature, prior to a postcure, decreased the magnitude of observed shrinkage. Annealing samples outside the mold allowed stresses that developed during the cure process to be reduced and led to less shrinkage observed than for samples annealed in the mold. Samples cured in a silicone mold exhibit less shrinkage in the dilatometer than samples cured in an aluminum mold. Soaking samples in water prior to dilatometric analysis led to stress relaxation in the sample and decreased the observed shrinkage. Specimens containing titanium dioxide filler exhibited less shrinkage than unfilled samples. The filled samples also exhibited less expansion slightly above the glass transition temperature. The reduced expansion might arise from restricted chain mobility in the presence of the filler.     

双组分注射成型件体积收缩的数值模拟研究

以ABS为第一次成型的内嵌件材料,PP为第二次成型的外嵌件材料,通过对双组分注射成型的数值模拟,系统研究了熔体注射温度、模具温度、注射时间、保压时间和冷却时间等工艺参数对其平均体积收缩率的影响规律,并基于流变学理论,揭示了其影响机理。结果表明,随着熔体注射温度和模具温度的升高,双组分注射成型制品的体积收缩增加,而随着注射时间、保压时间和冷却时间的增加,其平均体积收缩率减小。     

Effect of molecular parameters on the shrinkage of injection-molded polypropylene

The effect of some molecular parameters on the mold shrinkage, annealing shrinkage, and melting shrinkage of injection-molded polypropylene was studied. The mold shrinkage in the machine direction (MD) became higher as the melt flow index (MFI) became lower and the molecular weight distribution became broader. On the other hand, the mold shrinkage in the transverse direction (TD) showed a similar value (about 1.6%), regardless of the kind of resin, except for a glass fiberreinforced polypropylene which showed a very low value. The annealing shrinkage and melting shrinkage showed similar tendencies as the modl shrinkage. The mold shrinkage and annealing shrinkage in the MD were in linear relationships with the thickness of the skin layer having positive slopes; the melting shrinkage in the MD was in a downward-curved relationship with the thickness of the skin layer having a positive slope, respectively, regardless of the cylinder temperature and the kind of resin.     

微孔塑料注射成型制品体积收缩与翘曲变形数值分析

采用Moldflow软件，通过微孔注射成型过程的数值模拟，系统研究了熔体注射温度、模具温度对其体积收缩、翘曲变形和残余应力的影响规律，并基于流变学理论，揭示了其影响机理。研究结果表明，随着熔体注射温度和模具温度增加，微孔注射制品翘曲变形和残余应力均增加，成型制品的体积收缩随着熔体注射温度升高而增加，而随着模具温度升高而减小。本研究为微孔注射成型工艺和模具的设计提供理论指导。     

水溶性芯模在VARI成型工艺中的应用研究

制备了一种低成本易成型的水溶性芯模,对其成型收缩率、压缩性能、水溶性及气密性进行了研究,结果表明采用石英砂制备的可溶性芯模成型前后具有良好的尺寸稳定性,在室温至120℃区间内压缩强度均高于3 MPa,浸泡在常温水溶液中1 min内能够快速溃散至砂粒状态,经表面封孔处理后能够达到VARI成型工艺对模具的气密性要求。通过在填充可溶性芯模的中空金属框表面进行VARI工艺验证试验,表明制备的水溶性芯模能够作为VARI工艺模具使用,且脱模简单方便,对制品表观质量基本没有影响,具有良好的应用前景。     

Estimates for material shrinkage in molded parts caused by time‐varying cavity pressures

The phenomenology of shrinkage is established through injection molding experiments in which shrinkage was measured at 25‐mm intervals along the length and width of rectangular plaques, molded in an instrumented mold. A simple solidification model, which assumes the solidified material to be elastic, is developed for the effect of time‐varying temperature and pressure histories on part shrinkage. This model predicts a linear dependence of shrinkage on an “effective pressure,” which combines the thermal diffusivity of the material, the wall thickness, and the time‐varying cavity pressure into a single parameter that is uniquely related to the shrinkage. The effective pressure is shown to effectively correlate in‐plane shrinkage data. The solidification model characterizes two material parameters, which can be estimated from the pressure‐volume‐temperature (PVT) diagram for the material, that describe the sensitivity of the shrinkage to the local cavity pressure history. The residual stresses predicted by this model are rather crude. POLYM. ENG. SCI., 59:1648–1656 2019. © 2019 Society of Plastics Engineers     

注塑模具随形冷却结构对制品成型的热响应分析

研究了模具随形冷却结构对制品成型的热响应变化,利用有限元软件Ansys对线性与随形2种冷却结构进行瞬态热响应分析与对比,得到了2种结构模式的冷却效率和型腔温度场分布规律,并借助Moldflow和计算流体动力学（CFD）软件对注塑制品在随形冷却结构模式下成型所得到的温度场分布及其可能产生的缺陷进行了分析研究。结果表明,随形冷却结构较传统水道具有更均匀的冷却效果,冷却时间缩短了50 %,体积收缩率降低了15 %,且能更迅速地调控模具温度,更快地进入稳定的工作状态。     

Thermally stimulated shrinkage forces in oriented polymers: 2. Time dependence

The four-state model proposed in a previous paper to describe thermal shrinkage in oriented polymers is solved to describe the time dependences of shrinkage forces under constant temperature conditions. The time dependences of shrinkage forces have been recorded for polycarbonate and polyethylene terephthalate at temperatures below the glass transition. It is shown that experimentally recorded time dependences of shrinkage forces can be satisfactorily fitted by dependences predicted by the model, and the comparison leads to reasonable values of activation energy and activation volume for the shrinkage process.     

An Analytical Study of Constrained Shrinkage of Injection Molded Semi-Crystalline Plastic Parts

This paper presents an analytical approach to predict in-mold constrained shrinkage of injection molded semi-crystalline plastic parts. Existence of mold constraints exerts a noticeable effect on the final dimensions of molded parts due to the viscoelastic characteristics of polymeric materials. Variation of in-mold cooling time introduces variations in final part dimensions in relation with the constraints. Although this inter-relationship is well applied, few investigations have been reported to analyze the effect. In the present study, an analytical approach is introduced to predict constrained shrinkage of semi-crystalline thermoplastics and experimentally examined using a case study. Essential to this analytical solution, innovative mathematical expressions of elastic modulus and relaxation time constant with respect to temperature have been introduced. A mold was constructed to produce a plate featured by holes and injected to produce experimental data. The results indicated that there is a good agreement between the prediction and the experimental data, especially at longer cooling times. It can also be shown that the material relaxation time has the most dominant influence on the final shrinkage.     

聚丙烯注射成型收缩研究进展

在介绍注射成型中浇日凝固前收缩、制品模内冷却讨程收缩及脱模后制品自由收缩的收缩讨程和热收缩、结晶收缩、取向收缩、负收缩及后收缩等注射成型收缩原因的笨础}一,详细回顾了聚丙烯(PP)分子结构、共混、填充及加入成核剂等因素对注射成型收缩率的影响。同时,还讨论了注射温度、成型压力、成型时间、模具温度、冷却时间、充模辣度等注塑工艺条件以及制品厚度对PP注射成型收缩率的影响,介绍了生产中注射成型收缩的控制方法     

基于Moldflow与正交试验的尾罩注塑工艺与模具设计

选取电连接器中的聚苯硫醚(PPS)尾罩为研究对象,利用Moldflow软件分析得出了最佳浇口位置,并结合正交试验完成了尾罩的最优注塑工艺参数设计,分析了充填时间、模具温度、熔体温度和保压压力对体积收缩率的影响,并在此基础上成功实现了模具设计与试模生产。结果表明,Moldflow软件和正交试验的结合运用能为电连接器尾罩注塑成型提供理论依据与技术支持。     

PA成型收缩率与注射工艺条件的关系

分析了注塑制品的收缩机理及收缩过程，并讨论了聚酰胺（PA）注射成型过程中模腔平均压力、熔体温度、模温、充模速率、成型时间等工艺条件对其收缩率的影响及制品后收缩率的因素，给出了减小制品收缩率，提高制品尺寸稳定性的方法。     

Thermoforming shrinkage prediction

Most thermoforming product development processes rely on costly and time‐consuming forming trials to determine the adequacy of the mold and process. In this paper, an analytical method is developed for shrinkage predictions on the basis of a visco‐elastic constitutive material model with initial conditions from a commercial thermoforming simulation. The theoretical analysis has been developed to accommodate different sets of materials, process conditions, and mold geometry. The shrinkage model consists of a transient thermal analysis for temperature solution; a stretching phase analysis for Inflation‐induced stress estimation; a post‐contact analysis for thermal stress and relaxation; and a post‐molding strain analysis based on stress solution. The shrinkage prediction analysis has been developed and validated with a complex geometry thermoforming application. The results indicate that the shrinkage estimates provided by the analysis were within the objective tolerances of 0.1%, as measured in terms of absolute prediction error of part dimensions.