变模温与型腔气体反压辅助微孔发泡注塑技术及其产品内外泡孔结构演变

建立了一种变模温和型腔气体反压协同控制的微孔发泡注塑技术,研制了相应的变模温控制系统与型腔气体反压控制系统,构建了变模温与型腔气体反压辅助微孔发泡注塑试验线,并对变模温与型腔气体反压作用下的产品内外泡孔结构演变进行了研究。结果表明,变模温与型腔气体反压辅助工艺单独施加于微孔发泡注塑技术时,对其产品内外泡孔结构均具有双重影响:变模温可以改善产品大部分的表面形貌,但其对填充过程中的熔体发泡影响不大;型腔气体反压可以基本抑制填充过程中的熔体发泡,但却对产品内部泡孔密度有比较明显的降低影响。通过变模温与型腔气体反压的协同控制,可以实现微孔发泡注塑产品表面气泡形貌和内部泡孔结构的良好调控。     

The Effects of Gate Size in Powder Injection Molding

This paper studies the effects of gate size on the quality of powder injected molded (green) parts and the cavity filling pattern. A total of five rectangular gates with different shape factors (ratio between the width and the depth of the gate) were used. A weight and density analysis was performed on the samples molded for each of the five gates. It was found that the gate with a shape factor of 3.8 gives the highest weight and density. The result also indicates that an increase in the width or the depth of the gate will give different results even though the total cross sectional area of the gate is kept constant. Samples molded with gates of larger depth tend to have higher weight and density. The cavity filling pattern and microstructure were observed for each gate size.     

Prediction of shrinkage and warpage in consideration of residual stress in integrated simulation of injection molding

The accurate prediction of shrinkage and warpage of injection molded parts is important to achieve successful mold design with high precision. In this study, the numerical analysis of shrinkage and warpage of injection molded parts made of amorphous polymers was carried out in consideration of the residual stresses produced during the packing and cooling stages of injection molding. The temperature and pressure fields were obtained from the coupled analysis of the filling and post-filling stages. For residual stress analysis, a thermo-rheologically simple viscoelastic material model was introduced to consider the stress relaxation effect and to describe the mechanical behavior according to the temperature change. The effect of the additional material supply during the packing stage was modeled by assigning the reference strain. The deformation of injection molded parts after ejection induced by the residual stress and temperature change was analyzed using a linear elastic three-dimensional finite element approach. In order to verify the numerical predictions obtained from the developed program, the simulation results were compared with the available experimental data in the literature. In the case of residual stress, it was found that the present simulation results overpredicted the tensile residual stresses at the surface of injection molded parts. However, the predicted shrinkage was found to be reasonable to describe the effects of processing conditions well. Finally, an analysis of the shrinkage and warpage was successfully extended for a part with a more complex curved shape.     

Application of calcium carbonate in resin transfer molding process: An experimental investigation

The resin transfer molding (RTM) process is used to manufacture advanced composite materials made of continuous glass or carbon fibers embedded in a thermoset polymer matrix. In this process, a fabric preform is prepared, and is then placed into a mold cavity. After the preform is compacted between the mold parts, thermoset polymer is transferred from an injection machine to the mold cavity through injection gate(s). Resin flows through the porous fabric, and eventually flows out through the ventilation port(s). After the resin cure process (cross‐linking of the polymer), the mold is opened and the part is removed. The objective of this study is to verify the application of calcium carbonate mixed in resin in the RTM process. Several rectilinear infiltration experiments were conducted using glass fiber mat molded in a RTM system with cavity dimensions of 320 × 150 × 3.6 mm, room temperature, maximum injection pressure 0.202 bar and different content of CaCO₃ (10 and 40%) and particle size (mesh opening 38 and 75 µm). The results show that the use of filled resin with CaCO₃ influences the preform impregnation during the RTM molding, changing the filling time and flow front position, however it is possible to make composite with a good quality and low cost.     

气体辅助注射成型聚丙烯的多层次结构

用扫描电镜(SEM)观察了气体辅助注射成型(GAIM)和常规注射成型(CIM)等规聚丙烯(iPP)在不同部位的结晶形态。发现CIM试样的"皮-芯"结构不明显,而GAIM试样在不同部位则形成了包括球晶、串晶和取向片晶,进而表现出明显的多层次结构。在结晶形态分析的基础上,初步探讨了GAIM制品多层次结构的形成机理。     

Temperature and pressure evolution in fast heat cycle injection molding

An accurate mold temperature control during injection molding processes allows obtaining objects with better surface finishing, more accurate surface replication, and reduced frozen-in orientation. The control of these properties is particularly important when the thickness of the molded part is very small as in the case of microinjection. In this work, thin multilayer heaters are adopted to obtain a very fast mold surface temperature evolution during the process of an isotactic polypropylene (iPP). The effect of mold temperature history on the pressure developed inside the cavity is analyzed and correlated to both gate solidification and mold deformation. Results confirmed that, with a fast control of the cavity surface temperature, a reduction of the injection pressure is achieved, without affecting the cycle time. The understanding of the phenomena that occur during the fast temperature evolution on the cavity surface can allow the microstructural calibration of the molded parts.     

Replication of metal microstructures by micro powder injection molding

In this paper, a study on the production of 316L stainless steel microstructures by μPIM (powder injection molding) is presented. Two types of mold inserts were used and the molding was conducted on a conventional injection molding machine. Based on the characteristics of the mold inserts and the feedstock, suitable processing parameters were selected. Some requirements for the production of the microstructures are discussed. For example, a relatively high mold temperature, high injection pressure and holding pressure were required. The study showed that 316L stainless steel microstructures of φ100 × 200 μm can be injection molded, but there were incomplete filling and demolding problem in the case of smaller microstructures of φ60 × 191 μm. The molded parts were successfully debound and sintered.     

Fabric structure and mold curvature effects on preform permeability and mold filling in the RTM process. Part II. Predictions and comparisons with experiments

In Part I, an experimental study was completed in a series of five molds, each having corners of different radii (from 0.06 to 8.0 in.). The primary goal of this work has been to determine whether corners in LCM molds significantly affect the filling process, by altering the structure of the preform locally in such regions. Consistent trends were found for each series of experiments completed in the mold, for which the same preform type, and number of layers were used. For constant flow rate injection, the required injection pressures to fill the two molds with tighter radii was significantly increased as compared to the other molds. Composite parts were manufactured in these molds, measurements made on these parts revealing design flaws, the cavity thicknesses not being equal in all sections of the molds. Several numerical simulations are presented in this paper, the goal being to separate any effects due to the varying thicknesses from effects due to the corners present. Careful simulations have been completed, taking into account the actual thicknesses in each mold, and the resulting preform volume fraction. Experimentally measured permeability data was employed, and the predicted injection pressures match very well for all four molds studied, seeming to indicate that the corners present have not affected the filling. A simple model for preform compression in corners has been developed, which predicts local permeability modifications due to in-plane compression of the fabric layers. These predictions have been employed in conjunction with an existing tool to analytically predict the permeability components of a preform in a flat cavity. This code requires from the user a geometrical model of a preform unit cell, this data being measured from samples cut from the parts manufactured. The resulting predictions for injection pressure are good for an entirely predictive approach, underpredicting experimental values by only 30–60%. Sensitivity analyses have demonstrated the strong relationship between permeability and the details of the preform cell. Two numerical studies were completed to determine how sensitive the injection pressure curves are to reduced permeabilities in the corner regions. For the two injection schemes having two different gate locations, pressures were not significantly affected, while the permeabilities in this region were reduced up to 100 times. Though the molds used were not ideal for isolating effects on mold filling due to corner radii, the evidence presented does not show the existence of any strong behavior related to mold radii.     

A model for fiber length attrition in injection-molded long-fiber composites

Long-fiber thermoplastic (LFT) composites consist of an engineering thermoplastic matrix with glass or carbon reinforcing fibers that are initially 10–13 mm long. When an LFT is injection molded, flow during mold filling degrades the fiber length. Here we present a detailed quantitative model for fiber length attrition in a flowing fiber suspension. The model tracks a discrete fiber length distribution at each spatial node. A conservation equation for total fiber length is combined with a breakage rate that is based on buckling of fibers due to hydrodynamic forces. The model is combined with a mold filling simulation to predict spatial and temporal variations in fiber length distribution in a mold cavity during filling. The predictions compare well to experiments on a glass–fiber/PP LFT molding. Fiber length distributions predicted by the model are easily incorporated into micromechanics models to predict the stress–strain behavior of molded LFT materials.     

Branch and bound search to optimize injection gate locations in liquid composite molding processes

In liquid composite molding (LCM) processes the resin is injected into a mold cavity containing pre-placed reinforcement fabrics through openings known as gates, and the air leaves the mold through openings called as vents. The gate and vent locations play a crucial role as to whether the resin covers all empty spaces between the fibers in the mold, which dictates the quality and the properties of the final product. Optimization methods are used to find the gate and vent locations that will create a favorable flow that will prevent dry spots. In this paper, branch and bound search is adapted to mold filling in LCM processes and is used to find the optimal injection gate location that fulfill two different objectives. First, to find the gate locations that will yield the shortest fill time. Second, to find the auxiliary gate locations that will counteract a disturbance during filling and minimize the size of the dry spot. In each case, three geometries have been studied. The results are compared with exhaustive search and genetic algorithms results to illustrate the efficiency and accuracy of branch and bound search method.     

注塑成型工艺参数对制品体收缩率变化的影响及工艺参数优化

结合CAE及Taguchi DOE技术研究工艺参数对注塑制品体收缩率变化(制品中体收缩率的最大值与最小值的差值)的影响并获得优化的工艺参数以使制品的体收缩率变化最小。文中采用L9(3^4)正交矩阵进行实验,并研究了各个参数对制品体收缩率变化的影响程度,对于所选参数,保压压力和模具温度对注塑制品的体收缩率变化的影响较大。     

An experimental investigation of class A surface finish of composites made by the resin transfer molding process

Achievement of high class surface finish is important to the high volume automotive industry when using the resin transfer molding (RTM) process for exterior body panels. Chemical cure shrinkage of the polyester resins has a direct impact on the surface finish of RTM molded components. Therefore, resins with low profile additives (LPA) are used to reduce cure shrinkage and improve surface quality of the composite parts. However, little is known about the behaviour of low profile resins during RTM manufacturing and their ultimate effects on the surface quality of molded plaques. In this work, the effects of controlled material and processing parameters on the pressure variations, process cycle times and ultimately on the surface quality of RTM molded components were investigated. Taguchi experimental design techniques were employed to design test matrices and an optimization analysis was performed. Test panels were manufactured using a flat plate steel mold mounted on a press. Pressure sensors were inserted in the mold cavity to monitor pressure variations during different stages of cure and at various locations in the mold cavity. It was found that a critical amount of LPA (10%) was required to push the material against the mold cavity and to compensate for the resin cure shrinkage. A significant increase in pressure was observed during the later stages of resin cure due to the LPA expansion. The pressure increase had a significant effect on the surface roughness of the test samples with higher pressures resulting in better surface finish. A cure gradient was observed for low pressure injections which significantly reduced the maximum pressure levels.     

微结构塑件的注塑充填尺度效应

微尺度聚合物熔体流动具有明显的尺度效应,模具温度和注射速率是微注塑充填流动的关键影响因素。文中采用微细电火花铣削技术设计分别制造了一模八腔的带有200μm和300μm微孔的注塑模具。以聚丙烯(PP)进行单因素充模流动工艺实验,研究了模具温度和注射速率对直径为200μm和300μm微圆柱充填高度影响规律。结果表明,当模具温度为30℃和注射速率为60%时,直径200μm微圆柱孔的充填高度小于直径300μm微圆柱孔的充填高度,且Ⅰ型腔的微圆柱充填高度大于Ⅱ的充填高度。随着模具温度升高和注射速率增加,2种微孔充填高度差在减小,Ⅰ型腔和Ⅱ型腔之间的充填高度差值也在减小。可见,升高模具温度和增加注射速率可以减少微尺度效应对微圆柱孔充填高度的影响,同时,还可以减小流动不平衡程度。     

Plastic Injection Molding of High-Aspect Ratio Micro-Rods

Technological advancements have led to an increase in demand for fabrication of small and cheap miniature components or parts, especially in the medical and electronic fields. In this research work, micro-rods of varying lengths and diameters were molded using a specially designed tabletop injection molding machine. The main purpose was to investigate whether complete filling of the microcavities was possible and whether small cavity openings will restrict melt flow into the cavities.

Process parameters, such as injection pressure, mold temperature and melt temperature were varied. The micro-rods obtained were analyzed with Scanning Electron Micrographs (SEM). The results showed that injection pressure was the most important parameter for microinjection molding. The use of a vacuum system also yielded better results.     

Plastic Injection Molding of High-Aspect Ratio Micro-Rods

Technological advancements have led to an increase in demand for fabrication of small and cheap miniature components or parts, especially in the medical and electronic fields. In this research work, micro-rods of varying lengths and diameters were molded using a specially designed tabletop injection molding machine. The main purpose was to investigate whether complete filling of the microcavities was possible and whether small cavity openings will restrict melt flow into the cavities.

Process parameters, such as injection pressure, mold temperature and melt temperature were varied. The micro-rods obtained were analyzed with Scanning Electron Micrographs (SEM). The results showed that injection pressure was the most important parameter for microinjection molding. The use of a vacuum system also yielded better results.     

微发泡注塑成型PC制件的表面缺陷及形态分析

微发泡注塑成型制品常有漩涡状流痕和银纹两类表面缺陷。漩涡状流痕与均相溶液的流动形态关系密切,多在浇口附近;而银纹多在远离浇口处。文中利用扫描电子显微镜(SEM)对微发泡注塑成型聚碳酸酯(PC)制品的漩涡状流痕、银纹及泡孔形态进行观察,发现微孔形态和表面缺陷之间存在如下关系:漩涡状流痕对应的气泡剪切变形较小,而银纹相关的气泡剪切变形较大;根据气泡SEM的形态,银纹可细分为银带纹、银丝纹。剪切变形大、不破裂气泡表面形成银带纹,破裂则形成银丝纹。     

熔体温度对注射成型热塑性聚氨酯制件残余应力及外形精度的影响EI北大核心CSCD

采用光学双折射法研究注射成型过程中熔体温度对透明热塑性聚氨酯(TPU)弹性体平板制件残余应力的影响规律和作用机理。结果表明,TPU制件在近浇口区域的残余应力大于远浇口区域,整体残余应力随熔体温度上升而下降。通过对平行于熔体流动方向和垂直于熔体流动方向上的制件截面进行双折射测量发现,注射成型TPU制件具有明显的皮-芯结构和零应力层,且随熔体温度升高芯层厚度显著增加。残余应力分析结果表明在制件的芯层以流动残余应力为主,而皮层区域是流动残余应力和热残余应力叠加的结果。此外,制件的翘曲变形与残余应力的分布直接相关,TPU制件尺寸的改变主要是由于流动残余应力释放引起的。     

Effects of gate design and cavity thickness on filling,morphology and mechanical properties of microinjection mouldings

Miniaturized parts weighing up to tens of milligrams represent a large category of microinjection moulded products. Both miniaturization and extreme processing under microinjection moulding cause material to experience high shear rates and high cooling rates, and to have different morphology and final properties from conventional injection moulding. It also makes mould design quite challenging. This study investigates micro gate design (opening and thickness) and cavity thickness (100–500 μm) on filling, morphology and mechanical properties of Poly(ether-block-amide) miniaturized dumbbell parts. It is found that a reduction of gate size has two conflicting effects: increased shear heating increases flow length; increased cooling rate reduces flow length. Filling increases significantly with an increase of cavity thickness. In addition, skin ratio reduces from ∼70% to ∼10%, when part thickness increases from 100 μm to 500 μm. Such oriented skin layer determines molecular orientation and broadly influences Young’s modulus, elongation and yield stress. Natural aging at room temperature induces an increase of modulus and yield stress, and a decrease of strain at break. Mechanical properties of microinjection mouldings are significantly different from conventional injection mouldings and measurement at the microscale is required for successful miniaturized product design.     

振动场作用下注射成型制件力学性能的实验研究

将聚合物熔体在振动场中进行注射成型,对制件力学性能与振动参数之间的关系进行了实验研究。实验采用自行研制的振动实验装置,分析使用结晶型材料PP,HDPE和非晶型材料PS为原料,在振动场中进行注射成型,对成型试样进行了拉伸试验,对结果进行了分析,得出了结论。