基于DSC实验的液态硅橡胶反应注射成型模拟

采用差示扫描量热法测定了液态硅橡胶的反应固化曲线,得到了固化度和固化速率之间的对应关系,拟合得到了该材料反应固化模型的反应速率常数、反应级数和反应活化能,并通过Moldflow软件进行反应注射成型的模拟,得到了注射压力和固化度,注射压力模拟值与生产中的实际值相差约10 %。     

Novel approach to characterize the cross-linking effect of liquid silicone rubber via cavity pressure analysis during injection molding

The processing of liquid silicone rubber (LSR) in the injection molding process to molded parts offers high economic potential due to its great scalability. Unlike the well-researched and understood injection molding process of thermoplastics, LSR still lacks basic process knowledge. The cavity pressure curve shows a completely different pattern, as the cold LSR expands strongly once placed in the hot mold due to volume dilation. During processing of LSR exothermic and irreversible cross-linking, occurs. In this work, a method is introduced to display the cross-linking reaction in the cavity pressure curve. Using a self-developed testing device and conventional differential scanning calorimetry measurements, a correlation between cross-linking and the pressure signal can be demonstrated. A typical deflection is evaluated in the pressure curve by differentiating, which is attributed to the cross-linking. The relationships established are confirmed by pressure measurements in the mold during the processing of LSR in the injection molding process. The knowledge gained contributes to a better understanding of the process, helps to optimize existing processes and save resources. Existing cavity-pressure-measuring-systems are to be supplemented by an evaluation logic and thus, in addition to cycle time optimization, process variations are to be detected, energy consumption optimized and quality ensured and verified.     

Modeling processing of silicone rubber: Liquid versus hard silicone rubbers

The processing of hard and liquid silicone rubbers (LSR) are compared by means of modeling and simulation. The curing process for both, hard and liquid silicone, are modeled using the auto‐catalytic Kamal‐Sourour model and a nonlinear regression method is used to find the kinetic parameters. The fitted kinetic model is then combined with the heat balance equations to simulate real processing conditions. Both resins are compared in terms of process performance and consistency of the final part. The results show that even though hard silicone rubbers are less expensive resins, its processing conditions present several issues of consistency and quality control when compared with LSR. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119:1864–1871, 2011     

In-situ investigation of dielectric properties and reaction kinetics of a glass-fiber-reinforced epoxy composite material using dielectric analysis

Monitoring the curing behavior of a thermosetting material is a key issue for ensuring a stable manufacturing process (e.g., injection molding). Dielectric analysis (DEA), which is applicable for online-monitoring, is used to investigate the curing behavior of a glass-fiber-reinforced epoxy molding compound. At first, the influences of experimental settings (pressure, temperature, and frequency) on dielectric responses (dielectric loss and ion viscosity) are characterized in a fully crosslinked material. Results show a significant impact of temperature and frequency on dielectric responses. Furthermore, DEA is combined with differential scanning calorimetry (DSC) to investigate dielectric properties depending on crosslink density under non-isothermal and isothermal conditions. The results show that DEA can detect cure changes only for a crosslink density <80%. Finally, reaction kinetics, which can predict the crosslink density, is derived using DSC and validated through DEA for determining the best suitable kinetic expression for the investigated material. The crosslink density, estimated by reaction kinetics, can be correlated with the dielectric properties.     

PP注射成型中的压力及熔体流动过程

讨论了聚丙烯在注射成型中充模、增密、保压、冷却各个阶段的压力变化情况和熔体流动过程，以及二者对制品成功质量的影响。认为在聚丙烯注射成型过程中，要保证制品成型质量，不应以升温的办法来降低熔体的粘度，而应以提高注塑压力和剪切速率为主。     

Modeling/simulating the injection molding of isotactic polypropylene

A unified formulation is presented for modeling the injection molding of isotactic polypropylene (i‐PP). The crystallization kinetics are based upon the differential Nakamura equation in which the characteristic time is dependent upon temperature, pressure and flow‐induced shear stress, without any explicit need for an induction time. A Cross/WLF model is used to represent the shear viscosity in which η_O is dependent upon temperature, pressure and crystallinity. Use is made of a recent correlation for the PVT behavior of i‐PP with an explicit dependence upon crystallinity. A finite‐difference implementation of the modeling is applied to two independent molding experiments available from the literature, with notable results concerning the late‐time cavity pressure traces and time‐dependent gapwise shrinkage prior to ejection.     

In situ flow state characterization of molten resin at the inner mold in injection molding

Online viscosity information on processing lines can reflect the material flow resistance and offer valuable guidance for manufacturing across various industries. Considering the accuracy, devices, and processes involved in injection molding, characterizing the melt's flow state during material processing poses a significant challenge. To reduce investment in viscometers, avoid influencing the components' surface aesthetics due to the installation of sensors, and make the flow state detect online in mold, this study designs a rheometric mold with cylindrical runners for identifying the in situ viscosity of molten resin during injection molding. The detection conditions of injection speed and cavity pressure variations, the entrance effect, and the viscous dissipation for Polycarbonate are analyzed under various conditions. The in situ viscosity is identified and compared with the standard cross-WLF model. The result shows that the melt velocity and cavity pressure variations during the filling process create a stable environment for in situ rheological characterization and the detected viscosity is related to the shear rate, melt temperature, and channel dimension in injection molding. The designed mold with cylindrical runners for determining the in situ thermal-rheological behavior of polymer is distinguished successfully and exhibits prospects for the development of low-cost, nondestructive, and inner-mold measurement in manufacturing applications.     

INCREASING FLOW LENGTH IN THIN WALL INJECTION MOLDING USING A RAPIDLY HEATED MOLD

Injection molded parts are driven down in size and weight especially for portable electronic applications. While gains are achieved via cost reduction and increased portability, thinner parts encounter more difficulty in molding due to the frozen layer problem. To increase moldability in thin wall molding, a rapid thermal response (RTR) mold was investigated. The RTR mold is capable of rapidly raising the surface temperature to the polymer melt temperature prior to the injection stage and then rapidly cooling to the ejection temperature. The resulting filling process is done inside a hot mold cavity and formation of frozen layer is prohibited. Concepts of scalable filling and low-speed filling are discussed in the article to address the benefit of this molding method. Simulation results showed that significant reduction in injection pressure and speed can be achieved in RTR molding. In contrast to the filling behavior in conventional molding, the injection pressure in RTR molding decreases as the injection speed decreases, and therefore, extremely thin parts can be molded at lower injection speeds. Filling lengths of both RTR and conventionally molded polycarbonate samples, with two levels of thickness, under two levels of injection speed were experimentally studied. The experimental results demonstrated the advantage of the new molding method.     

聚碳酸酯在高注射速度下成型超薄导光板的仿真试验分析

基于传热学和流变学相关原理,建立了导光板的几何模型,对高速注射成型超薄导光板进行了仿真研究。利用Moldflow软件研究分析了聚碳酸酯(PC)在高剪切速率下的温度控制模型,探索了不同注射速度、塑化温度和模具温度下PC熔体在模具型腔中的温度变化规律。仿真结果表明,剪切速率在一定范围内,熔体温度的变化量随注射速度升高而增大,超过临界剪切速率后,由于存在壁面滑移,温度变化量减小;实际生产时可将注射速度增大到600~1000 mm/s,使塑化熔体温度降低10 ℃左右,注射压力降低50 MPa左右,从而获得更好的填充效果。     

Evaluation of injection‐molding simulation tools to model the cure kinetics of rubbers

Rubber injection molding is a process whereby a rubber mix is injected into a closed mold where the material is shaped to the desired geometry. Having completely filled the cavity rubber mix is vulcanized. Vulcanization is the process whereby a viscous and tacky uncured rubber is converted into an elastic material through the incorporation of chemical crosslinks between the polymer chains. The degree of cure achieved depends on the formulation recipe and the time–temperature history endured by the material during the curing process while in the mold. The aim of this study was to check the capability of commercial injection‐molding simulation tools, such as Moldflow and Cadmould, to predict the degree of cure achieved in spiral‐shaped parts when subjected to various cure cycles. To use the simulation tools, it was necessary to characterize the material in terms of their thermal properties and kinetic behavior during curing. The degrees of cure were determined with swelling techniques and by the measurement of the residual cure exotherms with differential scanning calorimetry. On comparing the experimental values of the degree of cure with those predicted by the simulation tools, we found that the initial simulations underestimated the degrees of cure. Consequently, the criteria used to calculate the cure model parameters were modified to invoke faster cures. In so doing, good agreement was achieved between the degrees of cure predicted by the simulations and those obtained experimentally.© 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Application of the rheology of monodisperse and polydisperse polystyrenes to the analysis of injection molding behavior

Monodisperse and polydisperse polystyrenes of equal weight average molecular weight (M _w) are evaluated for melt flow behavior in an Instron capillary rheometer and for injection molding behavior in a 12 ounce in-line reciprocationg screw injection molding machine. The influence of molecular weight distribution on the shape of the flow curves is deconstrated over a wide range of shear rate and temperature. The influence is also reflected in injection molding behavior as defined by pressure to fill or flash the mold at a given melt temperature. Studies of capillary rheometer data for correlation with injection moling beghavior indicate poor agreement when low shear rate viscosity data are used. Good agrement is foun using high shear rate viscosity data in the range 10³ to 10⁴ sec^?1 Striking crossover points on melt rheology and injection colding area diagram curvs are found with the monodisperse and polydisperse polystyrenes of the same M These crossovers shift with melt temperature and make possible the determination of a “controlling shear rate” for the injection molding process. This is found to be 3500 sec^?1 for short shot and 6200 sec^?1 for flash with the ASTM test specimen mold used in this study.     

仿真研究聚丙烯流变性能对气辅注塑的影响

应用计算机仿真手段研究了不同聚丙烯在充模速度相同条件下的压力及锁模力变化规律。结果表明，气辅注癃民传统注塑相比，所需压力，锁模力均有显著降低，且聚合物熔体流动速率越小，气体注射后产生的压力降赵大，表明在生产中应尽可能选用高ＭＦＲ树脂以利于气辅注塑工艺。     

Postfilling analysis of viscoelastic polymers with internal structure parameter

A new constitutive model with internal structure parameter is presented in this study for simulating orientation and deformation of polymer chain in the postfilling stages of injection molding process. Implementation of such a model is based on a hybrid finite-element/finite-difference numerical solution of the generalized Hele-Shaw flow. The simulation of a compressible viscoelastic fluid under nonisothermal conditions has not yet been discussed in most previous works. In addition to the distribution of pressure, temperature, density, and velocity, theoretical predictions also include shear and normal stresses, which can be used for subsequent calculation of residual stresses. An amorphous material, namely a commercial-grade polystyrene (PS), was used in the present work. Good agreement is obtained between the simulation and experimental pressure trace from this study.     

环氧复合绝缘子APG制造工艺分析

主要阐述了环氧复合绝缘子的自动压力凝胶(APG)工艺技术,重点分析了APG工艺对环氧材料和模具的要求、材料预处理及成型温度和压力设定,并提出了环氧复合绝缘子在制造中常见的环氧固化收缩和开裂常见缺陷的原因与解决对策。     

Moldability diagrams for the reaction injection molding of a polyurethane crosslinking system

A trial and error approach reflects the state of the art in reaction injection molding. Material and process parameters determine the “moldability” of a specific system in a particular application. The concept of “molding areas” on the critical parameters plane can be extended form thermoplastic injection molding (TIM) to reaction injection molding (RIM). In this work moldability diagrams for the filling and curing stages of a RIM process are obtained based on a simplified engineering approach. The key process parameters chosen for the filling stage are initial material temperature and filling time. In the curing stage, the critical parameters are considered to be mold wall temperature and demold time. Experimental results obtained on a laboratory-scale RIM machine on a Crosslinking polyurethane system are used to check the validity of the predicted molding areas. The agreement obtained is satisfactory considering the broad range of processing parameters used.     

注射成型中聚合物剪切诱导结晶行为的三维模拟

在考虑剪切导致分子链取向并升高其平衡熔点的基础上,建立了基于Nakamura方程的剪切诱导结晶动力学模型。在WLF-Cross黏度模型中引入结晶对黏度系数的影响,构建了考虑结晶的注射成型过程模型。采用改进的有限体积法对聚合物剪切诱导结晶行为进行了三维数值模拟,模拟中耦合了流动场、熔体压力、温度、诱导时间与结晶度。结果表明,本方法可清晰模拟出注射成型过程中聚合物的三维“喷泉”流动行为以及3层“皮-芯”结晶结构,同时,诱导结晶时间指数与相对结晶度的模拟结果与理论及实验结果吻合。     

A non‐isothermal finite element model for injection stretch‐blow molding of PET bottles with parametric studies

A non‐isothermal finite element (FE) model for the injection stretch‐blow molding (ISBM) process of polyethylene terephthalate (PET) bottles is presented in this paper. The constitutive behavior of PET is modeled by the physically based Buckley glass‐rubber model in form of UMAT in ABAQUS. The heat transfer between the stretch rod, the preform, and the mold is modeled. Particular attention is paid to thermal and contact modeling, material model, and selection of proper element types. Extensive FE simulations are carried out to model ISBM of a 20 g‐330 ml bottle made in plant tests. Comparisons of numerical results with the measurements demonstrate that the model can satisfactorily predict the bottle thickness and material distributions. Significant nonlinear differentials are found in strain, temperature, and temperature reduction rate in both bottle thickness and length direction during the process. A volume approach is therefore necessary for accurate predictions of final bottle properties because they are governed by orientation and crystallinity, which are highly temperature and strain dependent. Parametric studies on contact modeling and heat transfer coefficient are also conducted and the results are discussed. Polym. Eng. Sci. 44:1379–1390, 2004. © 2004 Society of Plastics Engineers.     

Experimental Results of Low Thermal Inertia Molding. I. Length of Filling

In injection molding, complete mold cavity filling is a design goal that has to be met 100% every time. Mold cavity filling is a complicated process which depends on many variables such as mold cavity surface temperature, injection pressure, injection speed, melt temperature, flow index of material being molded, etc. The aim of experimental investigation of the low thermal inertia molding (LTIM) [1] process is to demonstrate the feasibility of molding completely filled, thin parts at low injection pressure and injection speed without sacrificing part quality. The evaluation of the new molding concept consists of comparison of a conventionally molded thin rectangular part with an identical part molded by the LTIM process. The length of filling in the conventional cavity and in the LTIM cavity are compared at different injection pressures and injection speeds. The mold design, experimental procedure, and results of the molding are discussed in the following sections.     

UV光固化注射成型工艺实验研究

在传统注射成型工艺的基础上,对模内“光化学制造”工艺参数进行实验研究,分析得到UV材料温度、保压压力对制品精度的影响。其中,利用成型制品平均质量评价UV光固化制品整体成型质量精度。此外,采用横截面图形比较法对制品微结构形态质量进行评价。结果表明,UV光固化注射成型具有替代热塑性材料注射成型方面的可行性。     

Simulation and Experimental Investigations of Material Distribution in the Sandwich Injection Molding Process

In sandwich injection molding, two polymeric materials are sequentially injected into a mold to form a multilayer product with a skin and core structure. Different properties of these polymers and their distribution in the cavity greatly affect the applications of the moldings. In an ideal situation, the core material should be entirely encapsulated in the skin material. When the flow front of the core material overtakes that of the skin material, breakthrough occurs, resulting in a defective part. The focus of this study is to determine the effect of molding parameters on the skin/core material distribution. The commercial simulation package (Moldflow) has been extensively compared with experiments. Both simulated and measured results suggest that in order to obtain the optimum encapsulated skin/core structure in the sandwich injection molded parts, it is necessary to select a proper core volume fraction and suitable processing parameters. A good agreement between simulation and experimental results indicates that the Moldflow program can be used as a valuable tool for the prediction of melt-flow behavior during the sandwich injection process.