Residual stresses and toughness of polycarbonate exposed to environmental conditions

The role of the size and type of residual stresses and their distribution in the interior of polycarbonate mold-injected test bars was studied, in view of the interrelationship between toughness and thermal or environmental history. The large thermal gradient during solidification of the polymer in the mold builds up compressive stresses near the wall and tensile stresses in the core. Annealing followed by slow cooling may reverse the type of stresses near the surface, while quenching augments the compressive stress. The latter stress near the wall is responsible for the extraordinarily high impact strength of polycarbonate. Exposure to the atmosphere and immersion in hot water may affect the distribution of residual stresses and thus contribute to the embrittlement of the originally tough polymer. The importance of molecular weight and polymer stabilization is elucidated.     

Residual stresses and cracking in large ceramic injection mouldings subjected to different solidification schedules

In ceramic injection moulding, the moulding dimensions and the residual stresses are related to the hold pressure history during solidification in the cavity. In conventional moulding, the residual stresses are generally compressive at the surface. In this work, an insulated sprue device was made. It allows prolonged pressure transmission to the moulding and this can result in residual tensile stresses at the surface of thick section mouldings. At very low holding pressures (<9 MPa) or under conditions of premature gate solidification, a reversal of the sign of surface stresses occurs with compressive stresses near the surface. The appearance of cracks during binder removal was related to these residual stresses. For 25 mm thick mouldings which were subjected to a low but persistent hold pressure of 5 MPa no defects developed during the binder removal stage.     

Residual stresses in injection-molded amorphous polymers

Nonisothermal flow of a polymer melt into a cold cavity and its rapid cooling give rise to the buildup of flow and thermal stresses in the molded article. In the present investigation the resultant residual stresses (RS) induced by these two sources were studied in two stages. First, the flow-induced stresses were relaxed by proper heat treatment followed by quenching, resulting in only thermal stresses. The experimentally determined RS profiles in polysulfone and amorphous polyamide showed a parabolic shape and were correlated with the initial and final quenching temperatures, the glass transition temperature, and Biot Number. In the second stage, the combined effect of thermal- and flow-induced stresses was studied using injection-molded specimens prepared under a wide speptrum of molding conditions including melt and mold temperatures and injection rate and pressure. Results here indicated that the basic thermal-induced parabolic RS profiles are altered by the flow-induced stresses resulting in complicated profiles including local maxima and unbalanced RS. Finally, the tensile mechanical properties obatained for plaques molded under the various injection-molding conditions were studied and correlated in part with the previously determined RS profiles. Results have shown that a property gradient exists as a function of distance from both the gate and surface of the molded plaque.     

Calculation of Temperature Distributions during the Solidification Stage in Ceramic Injection Molding

A finite difference procedure for calculating unsteady-state temperatures in a molded ceramic–polymer suspension during solidification is described. The method incorporates temperature-dependent thermal diffusivity and enthalpy of fusion of the polymer. An experimental method is used to assess the surface heat transfer coefficient h at the mold wall. It is found that at low injection pressures h varies with time as the molded body shrinks from the wall, but at high injection pressures h can be treated as constant throughout the solidification stage. Using an analytical method, graphical charts are produced for dimensionless temperature as a function of dimensionless time for values of Biot's modulus in the region relevant to ceramic injection molding.     

Solidification of thermoviscoelastic melts. Part 4: Effects of boundary conditions on shrinkage and residual stresses

The solidification of a molten layer of amorphous thermoplastic between cooled parallel plates is used to model the mechanics of part shrinkage and the buildup of residual stresses in the injection-molding process. Flow effects are neglected, and a thermorheologically simple thermoviscoelastic material model is assumed. The model allows material to be added to fill the space created by the pressure applied during solidification, so that this model can be used to assess packing-pressure effects in injection molding. The interactions between the mold surfaces and the solidifying material are accounted for by modeling different types of constraints through different model boundary conditions. For several sets of boundary conditions, parametric results are presented on the effects of the packing pressure—the pressure applied during solidification to counteract the effects of volumetric shrinkage of the thermoplastic—on the in-plane and through-thickness shrinkages, and on residual stresses in plaque-like geometries. Plaques that can shrink in the in-plane direction while in the mold are shown to shrink more and to have higher residual stresses than plaques that are fully constrained while in the mold. Although the results are presented in terms of normalized variables based on the properties of bisphenol-A polycarbonate, they can be interpreted for other amorphous thermoplastics such as modified polyphenylene oxide, polyetherimide, and acrylonitrile-butadiene-styrene.     

Numerical prediction of residual stresses and birefringence in injection/compression molded center‐gated disk. Part I: Basic modeling and results for injection molding

The present study attempted to numerically predict both the flow‐induced and thermally‐induced residual stresses and birefringence in injection or injection/compression molded center‐gated disks. A numerical analysis system has been developed to simulate the entire process based on a physical modeling including a nonlinear viscoelastic fluid model, stress‐optical law, a linear viscoelastic solid model, free volume theory for density relaxation phenomena and a photoviscoelasticity and so on. Part I presents physical modeling and typical numerical analysis results of residual stresses and birefringence in the injection molded center‐gated disk. Typical distribution of thermal residual stresses indicates a tensile stress in the core and a compressive stress near the surface. However, depending on the processing condition and material properties, the residual stress sometimes becomes tensile on the surface, especially when fast cooling takes place near the mold surface, preventing the shrinkage from occurring. The birefringence distribution shows a double‐hump profile across the thickness with nonzero value at the center: the nonzero birefringence is found to be thermally induced, the outer peak due to the shear flow and subsequent stress relaxation during the filling stage and the inner peak due to the additional shear flow and stress relaxation during the packing stage. The combination of the flow‐induced and thermally‐induced birefringence makes the shape of predicted birefringence distribution quite similar to the experimental one.     

Investigation of thermal residual stresses during laser ablation of tantalum carbide coated graphite substrates using micro-Raman spectroscopy and COMSOL multiphysics

Laser ablation of high-temperature ceramic coatings results in thermal residual stresses due to which the coatings fail by cracking and debonding. Hence, the measurement of such residual stresses during laser ablation process holds utmost importance from the view of performance of coatings in extreme conditions. The present research aims at investigating the effect of laser parameters such as laser pulse energy, scanning speed and line spacing on thermal residual stresses induced in tantalum carbide-coated graphite substrates. Residual stresses were measured using micro-Raman spectroscopy and correlated with Raman peak shifts. Transient thermal analysis was performed using COMSOL Multiphysics to model the single ablated track and residual stresses were reported at low, moderate and high pulse energy regimes. The results showed that the initial laser conditions caused higher tensile residual stresses. Moderate pulse energy regime comprised higher compressive residual stresses due to off centre overlapping of the laser pulses. Higher pulse energy (250 μJ), higher scanning speed (1000 mm/s) and moderate line spacing (20 μm) caused accumulation of tensile residual stresses during the final stage of laser ablation. The deviation of experimental residual stresses from COMSOL numerical model was attributed to unaccounted additional stresses induced during thermal spraying process and deformation potentials in the numerical model.     

Analysis of the packing stage of a viscoelastic melt

The packing stage starts at the end of mold filling. During this stage, additional material is forced into the mold to compensate for the shrinkage during subse-quent cooling. Underpacking results in molded parts with dimensional variation. Overpacking causes flash at the parting lines, stick during ejection, and excess residual stresses resulting in warpage. The packing stage is thus extremely important in the determination of the final quality of the product. Despite its importance, analysis of the packing stage has been relatively ignored, particularly the viscoelastic effect. In this work, the analysis of the isothermal packing stage is presented for a Maxwell fluid. A set of governing equations is derived for a two-dimensional mold and solved using the Galerkin finite element method. In addition to the distribution of velocity and pressure, the model predicts the stresses in the planar direction, which could be used for subsequent calculation of the residual stresses.     

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.     

Nature of contact between polymer and mold in injection molding. Part I: Influence of a non‐perfect thermal contact

In injection molding, the pressure in the cavity usually reaches the atmospheric pressure before the ejection, therefore the thermal contact between polymer and mold is modified. This paper aims to evaluate the nature of the thermal contact between the polymer and the mold during the holding and cooling phase. An experimental plate mold has been designed to study this phenomenon. Thermal sensors facing each other and pressure sensors have been set in the mold. An inverse method is used to determine the heat flux density crossing the polymer mold interface, and the mold surface temperature. Then, a second inverse algorithm allows to determine the temperature profile at the end of the filling and the time evolution of the thermal contact resistance (TCR). Finally, the polymer temperature distribution in the thickness is determined between the thermal sensors. The results of this study show that the TCR between the polymer and the mold is not negligible and not constant with time. The polymer temperature at the surface can be 20°C higher than the mold surface temperature. Moreover, asymmetric air gaps have been observed when cavity pressure becomes equal to atmospheric pressure, therefore asymmetric temperature profile in the thickness are generated.     

Improvement of the surface quality of foam injection molded products from a material property perspective

Microcellular injection molding is an attractive method. However, their surface imperfections have been a major problem hindering wide industrial applications. Several methods have been proposed to improve the surface appearance of foams. In this study, we proposed a method to improve the surface appearance of polypropylene (PP) foams from the material property perspective, especially with regard to crystallization and viscosity. The basic idea of the surface improvement is to reduce the size of bubbles generated at the flow front, delay the solidification behavior of the polymer at the mold interface, squeeze the bubbles existing at the mold–polymer interface, and redissolve the bubbles into the polymer by holding pressure. Blending a low-modulus PP delays the crystallization of the polymers at the skin layer and solidification, taking enough time to squeeze the bubbles smaller. A sorbitol-based gelling agent, bis-O-([4 methylphenyl]methylene)-D-Glucitol, was used to increase the viscosity at a low strain rate to reduce the size of the bubbles generated at the flow front during the filling stage. The foam injection molding experiments demonstrated that the proposed method effectively improved the surface appearance of the foams. In particular, the surface appearance of the foams became almost equivalent to that of solid samples using low-modulus PP.     

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     

Evolution of internal cracks and residual stress during deposition of TBC

Thermal barrier coating (TBCs) are ceramic coatings that are deposited on metallic substrates to provide high thermal resistance. Residual stress is among the critical factors that affect the performance of TBCs. It evolves during the process of coating deposition and in-service loading. High residual stresses result in significant cracking and premature delamination of the TBC layer. In the present study, a hybrid computational approach is used to predict the evolution of internal cracks and residual stress in TBC. Smooth particle hydrodynamics (SPH) is first used to model the deposition of yttria-stabilized zirconia (YSZ) layer that contains various interfaces and micropores on a steel substrate. Then, three-dimensional (3D) finite element analysis is utilized to predict the evolution of internal cracks and residual stress in the ceramic coating layer. It is found that multiple cracks emerge during the solidification of the coating layer due to the development of high tensile (quenching) stresses. The cracking density is higher at regions near the coating interface. It is also found that compressive (residual) stresses are developed when the deposited coating is cooled to room temperature. The residual stress state is equibiaxial and nonlinear across the thickness/width of the TBC layer. The residual stress profile predicted compares well with that of hole drilling experiments.     

Reducing residual stresses in molded parts

Means of reducing the flow-induced residual stresses in injection molded parts through optimization of the thermal history of the process are presented. An approach through the use of a passive insulation layer with low thermal inertia on the cavity surface was investigated. The passive insulation layer prevents the polymer melt from freezing during mold filling and allows the flow-induced stresses to relax after the filling. The criteria for the optimal thermal properties and the required thickness of the layer are presented. A numerical simulation model of non-isothermal filling and cooling of viscoelastic materials was also used to understand the molding process and to evaluate this approach. This model predicts the stress development and relaxation in the molding cycle. Both simulation and experimental results show that the final stresses in the molded parts can be reduced significantly with the use of an insulation layer. This technique can also be applied to other molding or forming processes in order to decouple the material flow and cooling process for minimum residual stresses in the molded parts.     

Exfoliation of Kaolinite Nanolayers in Poly(methylmethacrylate) Using Redox Initiator System Involving Intercalating Component

Injection molding is one of the most widely employed methods for the fabri- cating of polymer articles, being characterized by high production rates and accurately dimensioned products. The process includes the flow of polymer melt through a runner system and gates followed by injection into a cold mold, packing under high pressure, and subsequent cooling to solidification. Accordingly, during the injection-molding process the polymer undergoes simultaneous mechanical and therma! influences while in fluid, rubbery, and glassy states. Such effects introduce residual stresses and strains into the final product [1,2], resulting in highly anisotropic mechanical behavior [3–9] and warpage and shrinkage [10–13]. Thus, understanding the factors governing the residual-stress development during molding is of great importance.     

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.     

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     

Modeling and simulation of thermally induced stress and warpage in injection molded thermoplastics

Thermally induced stress and the relevant warpage caused by inappropriate mold design and processing conditions are problems that confound the overall success of injection molding. A visco-elastic phase transformation model, using a standard linear solid for the solidified polymer and a viscous fluid model for the polymer melt, of 2-D finite element scheme with 8 noded overlay isoparametric elements was used to simulate and predict the residual stress and warpage within injection molded articles as induced during the cooling stage of the injection molding cycle. Computed results are in good agreement with published experimental data. The approach proposed here is to examine and simulate the injection molding solidification process with the intent of understanding and resolving more inclusive and realistic problems.     

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.