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     

Effects of film and substrate dimensions on warpage of film insert molded parts

Three‐dimensional flow and structural analyses were carried out for film insert injection molding to investigate warpage of film insert molded (FIM) parts with respect to variation of film and substrate thickness. Asymmetry of temperature distribution in the thickness direction was increased with increasing film thickness but decreased with increasing substrate thickness. Asymmetry of the in‐mold residual stress distribution in the FIM specimen was generated by the nonuniform temperature distribution, and it was increased with increasing film thickness but reduced with increasing substrate thickness. Warpage of the ejected FIM specimen was determined by relaxation of the asymmetric in‐mold residual stress distribution, and it was increased with increasing film thickness but reduced with increasing substrate thickness. Warpage of FIM specimens annealed at 80°C for 30 min showed complex behavior, and the behavior was understood by using factors such as degree of warpage of the ejected part, thermal shrinkage of the inserted film, and retardation of heat transfer. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Thermoviscoelastic Behavior of Film‐Insert‐Molded Parts Prepared under Various Processing Conditions

Film‐insert‐molded (FIM) tensile specimens were prepared under various molding conditions to investigate the effects of wall temperature and packing pressure on the residual stress distribution and thermoviscoelastic deformation. The warpage of the specimen increased with increasing mold‐wall temperature difference and decreased with increasing packing pressure. The FIM specimens produced with unannealed films showed the warpage reversal phenomenon (WRP) during annealing and the degree of WRP was affected significantly by the molding conditions and thermal shrinkage of the film. The warpage of the specimen was predicted by three‐dimensional flow and stress analyses and the prediction was in good agreement with the experimental results.

     

Relationship between the crystallization behavior and the warpage of film‐insert‐molded parts

The dimensional variation of an injection‐molded, semicrystalline polymer part is larger than the variation of an amorphous polymer part because the shrinkage of a crystalline polymer is generally greater than the shrinkage of an amorphous one. We investigated the warpage of film‐insert‐molded (FIM) specimens to determine the effect of the crystallization behavior on the deformation of FIM parts. More perfect crystalline structures and higher crystallinity developed in the core region of the FIM specimens versus other regions. Relatively imperfect crystalline structures and low crystallinity developed in the adjacent regions of the inserted films, whereas a thin, amorphous skin layer developed in the adjacent regions of the metallic mold wall. The crystallizable substrate in the FIM specimens caused very large warpage because nonuniform shrinkage occurred in the thickness direction of the specimens. Therefore, the warpage of an experimentally prepared FIM poly(butylene terephthalate) specimen was greater than that predicted numerically because of its complex crystallization behavior. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Residual stress and viscoelastic deformation of film insert molded automotive parts

Complex automotive parts were produced by film insert molding and the ejected parts were annealed to investigate the viscoelastic deformation. Warpage of the part was predicted by numerical simulation of mold filling, packing, and cooling stages with non‐isothermal three‐dimensional flow analysis. The flow analysis results were transported to a finite element stress analysis program and the stress analysis was performed by using time‐temperature superposition principle to investigate viscoelastic deformation. Predicted residual stresses, viscoelastic deformation, and warpage showed good agreement with experimental results. Thermal shrinkage of the inserted film and relaxation of the residual stress affected the viscoelastic deformation of the part significantly during annealing. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Flow‐induced warpage of injection‐molded TLCP fiber‐reinforced polypropylene composites

The most common belief is that warpage in injection‐molded fiber‐reinforced thermoplastics is primarily attributed to residual thermal stresses associated with shrinkage and thermal contraction of the parts. Therefore, it is assumed that flow‐induced stresses generated during mold filling do not play a significant role. Injection‐molded plaques of polypropylene (PP) reinforced with pregenerated thermotropic liquid crystalline polymer (TLCP) microfibrils were generated in order to investigate the role of residual flow‐induced stresses relative to that of thermal stresses on the warpage. In an effort to relate the material parameters to warpage, the rheological behavior of these fiber‐filled systems was investigated. The shrinkage and the thermal expansion of the TLCP/PP composites, and hence, the thermally induced stresses decreased with an increase in fiber loading while the flow‐induced stresses increased. The increase in the flow‐induced stresses was attributed to increased relaxation times (this is not the only cause, but is a significant factor) with an increase in fiber loading. Therefore, it was found that in order to accurately predict the warpage of fiber‐reinforced thermoplastics, the flow‐induced residual stresses must be accounted for. It is expected that the results reported here can be extended to glass‐reinforced PP composites as well. POLYM. COMPOS., 27:239–248, 2006. © 2006 Society of Plastics Engineers     

Optimization and numerical simulation analysis for molded thin‐walled parts fabricated using wood‐filled polypropylene composites via plastic injection molding

Plastic injection molding is discontinuous and a complicated process involving the interaction of several variables for control the quality of the molded parts. The goal of this research was to investigate the optimal parameter selection, the significant parameters, and the effect of the injection‐molding parameters during the post‐filling stage (packing pressure, packing time, mold temperature, and cooling time) with respect to in‐cavity residual stresses, volumetric shrinkage and warpage properties. The PP + 60 wt% wood material is not suitable for molded thin‐walled parts. In contrast, the PP + 50 wt% material was found to be the preferred type of lignocellulosic polymer composite for molded thin‐walled parts. The results showed the lower residual stresses approximately at 20.10 MPa and have minimum overpacking in the ranges of ?0.709% to ?0.174% with the volumetric shrinkage spread better over the part surface. The research found that the packing pressure and mold temperature are important parameters for the reduction of residual stresses and volumetric shrinkage, while for the reduction of warpage, the important processing parameters are the packing pressure, packing time, and cooling time for molded thin‐walled parts that are fabricated using lignocellulosic polymer composites. POLYM. ENG. SCI., 55:1082–1095, 2015. © 2014 Society of Plastics Engineers     

注射成型大尺寸透明平板的应力、翘曲及收缩分析

根据透明平板注射成型的特点,建立了残余应力计算模型和翘曲计算模型,采用HsCAE软件的相应模块对大尺寸透明平板注射过程中的残余应力、翘曲变形及收缩进行了分析。结果表明,平板边缘的残余应力和厚向收缩率较大;而中间区域的残余应力和厚向收缩率小且比较均匀;平板中间区域的翘曲变形量最大,且沿平板平面向四周均匀递减。同时进行了平板的注射成型试验,通过实测平板的外形尺寸进一步验证了模拟结果。     

Prediction of residual stress and viscoelastic deformation of film insert molded parts

Film insert molding (FIM) has been modeled numerically to predict residual stress and viscoelastic deformation of the part. Nonisothermal three dimensional flow analysis for filling, packing, and cooling stages was carried out by using a commercial software. It was assumed that the inserted film was solid throughout the entire molding procedure although remelting could occur at the interface with the substrate. The flow analysis results, e.g., temperature, stress, and density distribution in the substrate domain, were transported to a finite element stress analysis program for viscoelastic stress analysis. Deflection of the FIM part was obtained as soon as the part was ejected from the mold by assuming isotropic elastic material. The residual stress distribution in the FIM part was acquired by removing the constraints along the boundary of the molded part. Viscoelastic deformation of the FIM part was predicted by performing viscoelastic stress analysis in order to understand long term behavior of the FIM part when exposed to room temperature. Durability of automotive and electronic parts produced by the film injection molding can be predicted by the procedure adopted in this study. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Residual thermal stresses in injection molded products

Nonisothermal flow of a polymer melt in a cold mold cavity introduces stresses that are partly frozen-in during solidification. Flow-induced stresses cause anisotropy of mechanical, thermal, and optical properties, while the residual thermal stresses induce warpage and stress-cracking. In this study, the influence of the holding stage on the residual thermal stress distribution is investigated. Calculations with a linear viscoelastic constitutive law are compared with experimental results obtained with the layer removal method for specimens of polystyrene (PS) and acrylonitrile butadiene-styrene (ABS). In contrast to slabs cooled at ambient pressures, which show the well-known tensile stresses in the core and compressive stresses at the surfaces, during the holding stage in injection molding, when extra molten polymer is added to the mold to compensate for the shrinkage, tensile stresses may develop at the surface, induced by the pressure during solidification.     

Residual stresses and thermoviscoelastic deformation of laminated film prepared for film insert molding

Residual stresses and thermoviscoelastic deformation of a laminated film utilized for film insert molding was investigated through measurement of thermal expansion coefficient (CTE) and relaxation modulus. Thermoviscoelastic deformation of the film was also analyzed with numerical analysis by applying measured relaxation modulus, CTE, and residual stress to finite element method (FEM). Stress relaxation of the pristine film showed significantly different behavior from that of the unannealed film during annealing. Effects of the CTE and relaxation modulus on the thermoviscoelastic deformation were predicted by considering thermal shrinkage and structural relaxation. Moreover, numerical results on thermoviscoelastic deformation were in good agreement with experiments when initial stress distribution in the solid specimen was applied to the numerical analysis. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Effect of glass bead content and diameter on shrinkage and warpage of injection‐molded PA6

A simple technique has been introduced to evaluate the shrinkage and warpage behavior of injection molded products. Using the shrinkage values measured on specific locations of the specimen, three deformation factors have been defined to characterize the warpage behavior of the materials examined. Experiments were carried out to determine these properties of injection molded polyamide 6 (PA6) composites with solid glass bead (GB) contents of 10, 20, 30, 40 wt% and diameters of 11, 85, 156, 203 μm. It was concluded that the flow directional shrinkages can principally be described by the change in the bead content and diameter; it was proven that the increase in bead content and in bead diameter induces a reduction in flow directional shrinkage. The rising bead content and diameter increased the deformation factor, defined by the shrinkage differences caused by the pressure drop. It was pointed out that the ideal bead content can be determined in the function of bead diameter. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Process-induced residual stresses in compression molded UHMWPE

Non-isothermal cooling during processing causes the development of residual stresses, which are analyzed for compression molded UHMWPE, and affects the dimensional stability. The development of thermal residual stresses was predicted using an incremental stress analysis that included temperature-dependent material properties. Strain gauges were used to measure the residual stresses as layers were removed from a molded disk using a Process Simulated Laminate (PSL) approach. The PSL technique has not previously been applied to a compression molded neat polymer. For initial surface cooling rates of ～ 11°C/min, the model predicted a compressive stress at the bottom surface of 14 MPa and a tensile stress near the center of 2.5 MPa and matched the experimental distribution well. Because the compressive residual stress was 70% of the yield strength (～20 MPa), a lower cooling rate was also tested (2.6°C/min). The maximum tensile and compressive stresses for this cooling rate were, 0.91 MPa and 2.5 MPa, respectively. The model demonstrated its use for predicting thermal residual stresses in compression molded parts, instead of trial-and-error experimentation. UHMWPE is shown to develop residual stresses continually from ～ 120°C to 23°C.     

基于模糊集的车用升降器开关面板注射成型保压曲线优化研究

研究了车用升降器开关面板制件产生翘曲变形的原因;通过选择保压曲线模型及参数进行正交实验设计并在Moldflow软件中进行模拟分析,得到了制件的翘曲变形量;通过对翘曲变形量进行极差分析,得到了极差分析优化法的最优工艺参数组合;利用模糊集（Vague 集）对体积收缩率标准差（δ）和最大体积收缩率（V_max）进行多目标优化,求取了Vague集优化法的最优工艺参数组合;并将不同优化方法得到的翘曲变形量进行了对比。结果发现,收缩不均是引起该制件翘曲变形的主要因素;极差分析优化法对应的翘曲变形量较正交实验中最小翘曲变形量降低了1.5 %,优化效果不明显;Vague集优化法对应的翘曲变形量较正交实验方案中最小翘曲变形量降低了26.5 %,制件内部压力分布无较大差异,优化效果优于极差分析法,很大程度上降低了该制件的翘曲变形程度。     

Measurement of residual stresses in injection‐molded polymer parts by time‐resolved fluorescence

Time‐resolved fluorescence properties of 9‐methylanthracene (9MAn) dispersed in film of polyvinylchloride (PVC) containing carbon black were studied under tensile loadings. The fluorescence lifetime of 9MAn decreased from 5.70 to 5.55 ns, whereas the stresses acting on the films increased from 0 to 3 MPa. The change in fluorescence lifetimes of 9MAn during the stress relaxation process showed that the fluorescence lifetimes were correlated with the stresses, not with the strains. The results suggest that 9MAn is a useful probe for monitoring stresses acting on the matrix. With the use of the fluorescence properties of 9MAn, the residual tensile stresses on the skin‐layer of PVC injection‐molded test pieces were estimated. The estimated residual stresses were about ～ 1 MPa. The residual stresses were relaxed to 0 MPa with annealing at 100°C. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2600–2603, 2002     

Review on the Effects of Process Parameters on Strength,Shrinkage, and Warpage of Injection Molding Plastic Component

Injection molding process causes differences between designed shape and real parts, all of them associated to shrinkage and warpage phenomena. Temperature, pressure distribution, and other injection parameters during molding process originate local shrinkage and the internal stresses depending on the relative stiffness of each part area. The aim of this work is to review and report the influence of injection molding process parameters on the postmolded strength, shrinkage, and warpage of injection molded parts. It is also to investigate the influence of injection molding process parameters on the postmolding shrinkage and warping of parts made of polypropylenes.     

Double yield points in poly(tetramethylene terephthalate) and its copolymers under tensile loading

Double yield points before necking were observed in injection molded specimens of poly(tetramethylene terephthalate) (PTMT) and its copolymers under tensile loading. The first yield is associated with deformation of the amorphous region and the second yield is caused by the alpha to beta transition of PTMT crystallites. The first yield point became less apparent with an increase of crystallinity of the specimens. The second yield point became more apparent with an increase of crystallinity. Annealing of injection molded specimens increased the crystallinity and increased the second yield point on stress-strain curves. Copolymerization decreased the crystallinity and made the first yield point more prominent. Effects of annealing on mechanical and thermal properties of the specimens were also measured. The specimens changed their properties from ductile to brittle during annealing. Their change during annealing was mainly attributed to the increase of crystallinity and not to thermal degradation and/or crosslinking.     

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.     

Effects of residual stress and orientation on the fatigue of injection molded polysulfone

The tensile fatigue behavior of unnotched injection molded polysulfone specimens has been investigated. The effects of orientation and residual stress were studied by comparing asmolded specimens with annealed or annealed and quenched specimens with a known residual stress pattern. The treatments are shown to have differing effects at high stresses, where failure is by shear yielding and necking, and at intermediate stresses, where failure is by fatigue crack propagation. The geometries of fatigue cracks are described for each case. An attempt is made to separate the effects of crack and craze initiation from crack propagation, and cyclic loading from cumulative time under load.     

Thermal residual stress development for semi‐crystalline polymers in rotational molding

In rotational molding, polymer powders are subjected to heating, melting, cooling, and subsequent crystallization processes. Because of the asymmetrical cooling condition, thermally induced residual stresses are created inside molded parts leading to part warpage. A detailed theoretical heat transfer model is presented for the entire rotational molding process including the consideration of endothermic and exothermic transitions. At the same time, the development of residual stress inside the molded parts is simulated with thermoelastic model. The warpage values are calculated under different processing cases, and the generated numerical results are in good agreement with data reported in the literature. Our results show that both crystallinity and temperature gradients developing within the polymer during the cooling process greatly affect the dimensional stability of ethylene copolymers typically processed in rotational molding. The latter is found to be the determining factor in evaluating the effect of cooling conditions on the warpage generated in a molded product. Our results also demonstrate the importance of the crystallization kinetics, the material stiffness, and its evolution during the solidification process on the dimensional stability of the molded products. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers