Simulation of microstructure development in injection molding of engineering plastics

Mathematical models were developed to predict the various microstructural properties, including birefringece, residual stress, and density distributions, in the freely quenched compression molded samples as well as in the injection molded samples. To model the birefringence distribution in the injection molded samples, the BKZ type integral constitutive equation was employed to account for the nonisothermal stress relaxation, which takes place during the cooling stage of the molding cycle. The predicted birefringence agreed well with the experimental data near the mold walls. The residual stress distribution was modeled by the existing thermoelastic theory. The residual thermal stress distribution in the freely quenched samples was predicted very well by the model. However, the predicted residual thermal stresses in the injection molded samples were much larger than the measured ones. A phenomenological model to predict the density distribution in injection molded sample is proposed by including the effects of both cooling rate and the pressure on the density development. The predicted results agreed well with the experimental data.     

Residual stresses and density gradient in injection molded starch/synthetic polymer blends

Residual stress distribution in injection molded starch/synthetic polymer blends was evaluated using the layer removal technique. The synthetic polymers in the blend were either polybutylene succinate (PBS) or polycaprolactone (PCL). The starch content ranged from 0 to 70% by weight in the PBS blend and was held constant at 70% in the PCL blend. The effects of various molding conditions, aging and starch content were investigated. The residual stress profiles were found to be parabolic in nature with surface compressive stresses and interior tensile stresses. Increasing the injection pressure and mold temperature decreased the tensile stresses but had no significant effect on the surface compressive stresses. Decreasing the packing pressure produced a significant decrease in the magnitude of residual stresses. Varying melt temperature and packing time did not significantly affect the residual stress distribution for the range of values investigated. The residual stresses relaxed with time, decreasing over a period of 57 days. The magnitude of residual stresses increased as the starch content in the PBS blends was varied from 0 to 70%. Density gradient measurements were made in a 70% starch/PBS blend. The density was found to be higher in the interior than at the surface with a steep gradient close to the surface. Varying the molding conditions had a complex effect on the average density and the density distribution.     

Some aspects of orientation in injection molded objects

The birefringences of injection molded plates and the birefringence during steady, isothermal shear flow were compared for some amorphous polymers. The materials studied were a polystyrene, a “toughened” polystyrene and an acrylonitrile-butadiene-styrene copolymer. The birefringence of the plates, notably the maximum value for the average over the thickness was found to be related to the shear stress at the cavity wall that had occurred during the mold filling process. This relationship was independent of temperature. To a good approximation, it was also the same as the relationship between the flow birefringence and the shear stress at the wall in isothermal channel flow. It thus appears that the anisotropy of injection molded objects is dominated by the shear stresses during the mold filling process regardless of the temperature and of the macroscopic rate of deformation.     

Residual stresses and aging in injection molded polypropylene

The residual stresses in injection molded bars of polypropylene have been examined using a stress relaxation method and by the layer removal technique. A positive value for the internal stress parameter was obtained with newly molded specimens and was found to be retained by specimens stored at liquid nitrogen temperature. The stress relaxation parameter reduced to zero both for specimens aged at room temperature and also for those aged at ?40°C. Nevertheless the relaxation behavior of specimens aged at all three temperatures was quite distinct. The layer removal technique showed that the stresses near to the surface were compressive and those in the interior tensile, in apparent contradiction to the interpretation of Kubat and Rigdahl for the meaning of a positive internal stress parameter. A marked reduction in stress levels on aging at room temperature was confirmed, however. The relevance of the relaxation spectrum of polypropylene to these observations is discussed.     

注射成型聚苯乙烯制品双折射的光弹测试与分析

分析了无定形聚合物注射成型双折射行为及其产生机理,利用光弹法测试了不同工艺条件下聚苯乙烯(PS)平面内双折射分布,考察了熔体温度和保压压力变化对制品双折射和流动残余应力的影响。结果表明,PS制品残余双折射值在流动方向上从浇口附近至流动末端逐渐降低,并且最大双折射值随保压压力增加和熔体温度降低而升高;保压压力对浇口附近双折射分布影响明显。根据双折射的光弹测试结果分析了P制品分子取向和流动残余应力的分布趋势。     

Residual thermal stresses in injection moldings of thermoplastics: A theoretical and experimental study

Internal stresses in injection molded components, a principal cause of shrinkage and warpage, are predicted using a three‐dimensional numerical simulation of the residual stress development in moldings of polystyrene and high‐density polyethylene. These residual stresses are mainly frozen‐in thermal stresses due to inhomogeneous cooling, when surface layers stiffen sooner than the core region as in free quenching. Additional factors in injection molding are the effects of melt pressure history and mechanical restraints of the mold. Transient temperature and pressure fields from simulation of the injection molding cycle are used for calculating the developing normal stress distributions. Theoretical predictions are compared with measurements performed on injection molded flat plates using the layer removal method on rectangular specimens. The thermal stress development in the thinwalled moldings is analyzed using models that assume linear thermo‐elastic and linear thermo‐viscoelastic compressible behavior of the polymeric materials. Polymer crystallization effects on stresses are examined. Stresses are obtained implicitly using displacement formulations, and the governing equations are solved numerically using a finite element method. Results show that residual stress behavior can be represented reasonably well for both the amorphous and the semicrystalline polymer. Similarities in behavior between theory and experiment indicate that both material models provide satisfactory results, but the best predictions of large stresses developed at the wall surface are obtained with the thermo‐viscoelastic analysis.     

薄壁塑件注射压缩成型工艺的模拟分析

基于正交分析法和单因素分析法,用Moldflow软件数值模拟注射压缩成型中不同工艺条件对薄壁制品残余应力的影响.计算的残余应力沿厚度方向的分布表明:薄壁制品残余应力主要为流动诱导残余应力.模具温度与压缩距离对制品残余应力影响显著,模具温度越高,压缩距离越大,制品残余应力越小;其他工艺参数对残余应力均有不同程度的影响.     

注塑制品内应力计算的非线性黏弹性本构方程

在聚合物黏弹性理论的基础上,构建了新的注塑制品内应力计算的四元件串联力学模型,并推导了其瞬态黏弹性响应的非线性本构方程,给出了聚合物材料参数弹性模量和黏壶系数的计算公式,并对PS平板注塑制件脱模前的内应力进行了模拟计算。计算结果与固体高聚物的结构和力学性能的相关研究结论相一致,所建计算模型合理可靠。     

Improvement of optical quality and vibration characteristics of injection molded disk with processing condition control

Increased application of optical disks has required a rotating disk with more dynamic stability and better optical quality. A new concept of controlling the processing condition of injection molded disks is developed to improve their optical quality and vibration characteristics. To assess the effect of process conditions on residual stresses, birefringence, and critical speed, an orthogonal array for design of experiments is used. Melt temperature, filling speed, and packing pressure were effective parameters, but mold temperature and interactions among process conditions were not. The birefringence and critical speed were affected by the residual stress distribution, which varied according to the distance from the gate and processing condition. Considering the effect of the processing conditions and distance from the gate, we calculated the weight factors on residual stresses along the radial direction. Choosing weighted stress to be the target value for optimization of residual stresses, processing conditions control was accomplished. Under the newly proposed conditions, optical quality and stability of injection molded disk were simultaneously improved. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3275–3285, 2006     

ELIMINATING FLOW INDUCED BIREFRINGENCE AND MINIMIZING THERMALLY INDUCED RESIDUAL STRESSES IN INJECTION MOLDED PARTS*

Eliminating flow-induced birefringence and stresses and reducing thermally induced stresses in the injection molded parts have been studied using rapid thermal response (RTR) molding technique. In the RTR molding, mold surface temperature can be rapidly raised above T _g in the filling stage, while the normal injection molding cycle time is still maintained. Therefore, the melt can fill the cavity at temperatures above T _g, which enables the flow-induced stresses to relax completely in a short time after filling and before vitrification. Residual stresses and birefringence in a RTR molded strip specimen are compared with the conventional molded parts by applying layer removal method and retardation measurement. For the material (Monsanto® Lustrex Polystyrene) and process conditions chosen, the birefringence level decreased as the RTR temperature approached and exceeded the glass transition temperature until it almost disappeared at a RTR temperature of 180°C. Reduction of magnitude and shift of peak location were observed in the gapwise stress profile for RTR molded specimen.

     

Birefringence in injection‐compression molding of amorphous polymers: Simulation and experiment

The influence of the processing variables on the residual birefringence was analyzed for polystyrene and polycarbonate disks obtained by injection‐compression molding under various processing conditions. The processing variables studied were melt and mold temperatures, compression stroke, and switchover time. The modeling of flow‐induced residual stresses and birefringence of amorphous polymers in injection‐compression molded center‐gated disks was carried out using a numerical scheme based on a hybrid finite element/finite difference/control volume method. A nonlinear viscoelastic constitutive equation and stress‐optical rule were used to model frozen‐in flow stresses in moldings. The filling, compression, packing, and cooling stages were considered. Thermally‐induced residual birefringence was calculated using the linear viscoelastic and photoviscoelastic constitutive equations combined with the first‐order rate equation for volume relaxation and the master curves for the Young's relaxation modulus and strain‐optical coefficient functions. The residual birefringence in injection‐compression moldings was measured. The effects of various processing conditions on the measured and simulated birefringence distribution Δn and average transverse birefringence <n_rr?n_θθ> were elucidated. Comparison of the birefringence in disks manufactured by the injection molding and injection‐compression molding was made. The predicted and measured birefringence is found to be in fair agreement. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers     

The measurement of thermal stress distributions along the flow path in injection-molded flat plates

Internal stresses in injection-molded parts are the result of thermal, flow, and pressure histories. Internal stresses can be roughly divided into thermal and flow-induced stresses. In this paper, a modified layer-removal method is presented to determine thermal stress distributions in injection-molded flat plates. With this method, the curvature of a rectangular specimen is determined after the removal of a layer from one surface. This curvature is converted into a stress via a mathematical relation, originally derived by Treuting and Read. By determining the local curvatures after successive layer removals, stress distributions along the flow path were obtained within a single specimen. Validation of this modified layer-removal method is described. A good reproductibility was obtained. The method can be regarded as semi-quantitative. Flat plates were injection-molded from three amorphous polymers: polystyrene, polycarbonate, and a polyphenylene ether/high-impact polystyrene blend. In general, the flat-plate cross-section shows a three-region stress distribution with a tensile stress region both at the surface and in the core of the flat plate and an intermediate region with compressive stresses. The modified layer-removal method was used to determine influences of mold temperature, annealing treatment, and pressure history on the thermal stress distributions. Increasing mold temperature results in a decreasing overall stress level, while the compressive stress region shifts to the surface. An annealing treatment significantly reduces the overall stress level, without affecting the stress pattern. Stress distributions along the flow path were influenced by the varying pressure histories from the entrance to the end of the mold cavity. The various features of the stress profiles are explained by the influence of the pressure decay rate in the injection-molding process.     

注塑件残余热应力的数值计算

注塑件残余应力主要有流动残余应力和残余热应力 ,文中主要考虑注塑件的残余热应力 .利用文中建立的二维积分型热黏弹本构方程的递推公式 ,推导了单元的有限元求解公式 .数值算例考察了冷却效率、熔体注射温度对注塑件残余应力的影响 ,得到的结论与文献中结论一致 .     

Development of warpage and residual stresses in film insert molded parts

Residual stresses, bending moments, and warpage of film insert molded (FIM) parts were investigated by experimental and numerical analyses. Thermally induced residual stresses in FIM parts were predicted by numerical simulations with both commercial and house codes. Bending moments and warpage of FIM tensile specimens were calculated numerically and compared with experimental results. Thermally induced residual stresses were predicted by utilizing a one‐dimensional thermoelastic model where constant material properties are assumed. The residual stress distribution depended remarkably on the Biot number and the heat was removed rapidly through the surface resulting in high residual stresses. Asymmetric residual stresses generated by nonuniform cooling of the part provoked nonuniform shrinkage and warpage of the molded tensile specimen. It was found that the numerically calculated bending moment is in good agreement with the experimental results. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

工艺条件对夹芯注射制品残余应力的影响

采用Moldflow公司MPI软件中的Co-injection分析模块，对ABS／PS夹芯注射成型过程进行动态模拟分析，以揭示工艺参数对夹芯注射成型过程中残余应力影响的规律。结果发现：在各种成型工艺参数中，保压条件对残余应力影响较为突出，模温条件对残余应力影响次之；最大残余应力主要分布在浇口位置和芯层最厚处，芯层前缘处残余应力次之，壳层最厚处残余应力最小。     

Comparison of residual stress predictions and measurements using excimer laser layer removal

Residual stresses in injection molded polymer parts can have a major effect on product performance and are therefore often undesirable. Two different models for residual stress formation in injection molded products exist in the literature. The first model assumes that stresses in molded parts are identical with free quench stresses. Deviations are attributed to molecular orientation effects. The second model assumes that no slip occurs and that stresses are related to the holding pressure. Careful measurements should be able to differentiate between these models. In this study the layer removal stress measurement technique was improved by using an excimer laser for the milling operation. In that way stress relaxation during measurements was avoided and thin layers of uniform thickness could be removed. Both resolution and measurement accuracy were greatly improved. Stress distributions in polycarbonate plates molded under low holding pressures turned out to be of the free quenching type, whereas those molded under higher pressures were of the pressure type. All stress distributions could be predicted by a recent theory that comprises the two models as limiting cases.     

注塑件残余应力影响因素的模拟实验研究

在分析注塑过程中残余应力产生机理和原因的基础上,应用软件模拟实验方法,研究了不同成型工艺条件对聚苯乙烯平板注塑件残余应力的影响,分别给出了沿厚度方向的残余应力和固化压力分布。研究发现：残余应力在厚向上呈拉-压-拉（表层-次表层-中间层）分布,并随熔体温度和保压压力升高而增大,固化压力是决定注塑件厚向残余应力分布的一个主要因素。     

Numerical Simulation for Effects of Injection Mold Cooling on Warpage and Residual Stresses

Abstract

The dimensions quality of the injection‐molded parts is the result of a complex combination of material, part, and mold designs and process conditions. In this article, warpage prediction relies on the calculation of residual stresses developed during the molding process. The solidification of a molten thermoplastic between cooled parallel plates is used to model the mechanics of part warp in the injection‐molding process. Flow effects are neglected, and a thermorheologically simple thermoviscoelastic material model is assumed. The warp and residual stresses numerical simulation with finite element method (FEM) is time dependent. At each time step, the material properties can be temperature and pressure dependent. Mold temperature or mold‐cooling rate effects on part warp have been numerically predicted and compared with experimental results. By showing the mold‐cooling effects, it was concluded that mold cooling has a significant effect on part warpage, and mold‐cooling parameters, such as mold temperature, resin temperature, cooling channels, etc., should be set carefully.     

Residual stresses,shrinkage, and warpage of complex injection molded products: Numerical simulation and experimental validation

A numerical simulation model for predicting residual stresses and residual deformations which arise during the injection molding of thermoplastic polymers in the post-packing stage has been developed. A thermoviscoelastic model with volume relaxation is used for the calculation of residual stresses. The finite element method employed is based on the theory of shells as an assembly of flat elements. This theory is well suited for thin injection molded products of complex shape. The approach allows the prediction of residual deformations and residual stresses layer by layer like a truly three-dimensional calculation, while reducing the computational cost significantly. The hole drilling technique is used to measure the residual stresses across the thickness of the product. A three-dimensional laser digitizing system, an image processing technique and a dual displacement transducer system are used to measure the warpage. Experiments are carried out on polycarbonate and high density polyethylene parts. Numerical results are in qualitative agreement with experimental observations, i.e., the skin of the box is surrounded by a compressive region while the core region is in traction. The trend of both the experimental and the predicted residual stress profiles is close. Different examples are presented to illustrate the influence of the geometrical complexity of the shape on the final deformations and residual stresses. The influence of the mold temperature on residual stresses and warpage is also analyzed.