Warpage Prediction Research Articles

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

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.

Use of the Fast-cool pVT Data for Shrinkage Analysis in Injection Molding

Abstract The prediction of shrinkage and warpage of crystalline polymers is quite difficult. This is because of the complications of the crystallization process and associated material property changes. Particularly, the pVT behavior is dependent on the cooling rate during injection molding. Traditionally, the pVT data were measured in a device where the cooling rate is very slow compared to that in the actual injection molding process. This cooling-rate dependent pVT data is considered to affect the shrinkage and warpage calculation significantly. In the present study, a method has been developed to obtain pVT behavior at the same cooling rate as in the actual injection molding process. Obtaining other material data such as viscosity, thermal conductivity and heat capacity at the cooling rate of the actual injection molding process has also been considered. Injection-molding experiments have been conducted to measure the shrinkage of a part using semi-crystalline polymers. The polymers used in this study are polyamide 66 (PA 66) and polybutylene terephthalate (PBT). Simulation has been done to calculate the shrinkage using the cooling-rate dependent material properties. Using the fast-cool pVT behavior, the agreement between the simulation and the experiment improved significantly compared to the traditional approach.

Comparison of First-Order Shear and Plane Strain Assumptions in Warpage Prediction of Simply Supported Printed Wiring Boards

The influence of transverse shear strain in the lamination theory modeling of Printed Wiring Board (PWB) deflection due to support conditions was examined. The in-plane mechanical properties of the core materials of a commercial PWB were measured as a function of temperature. Classical laminated plate theory and first-order shear deformation theory solutions for the out-of-plane deflection of a bare board configuration with two opposite edges simply supported and the remaining edges free were obtained. The weight of the board was approximated as a distributed transverse load. The effect of material property decrease with temperature and FR-4 layer thickness were examined to compare first-order shear and plane strain assumptions for the predicted warpage.

Warpage of Plastic IC Packages as a Function of Processing Conditions

Variation of processing conditions on warpage prediction of a plastic quad flat package (PQFP) is examined. Thermal mismatch between package constituent materials is the major cause of IC package warpage. To minimize the warpage problem, a thorough understanding of epoxy molding compound (EMC) properties with molding parameters is necessary as EMC is epoxy-based with time and temperature dependent viscoelastic properties. This paper first addressed the thermal characterization of encapsulating material. Degree-of-cure (DOC or β), coefficient of thermal expansion (CTE or α), glass transition temperature Tg, and shear modulus G′ and G of the molded specimens were measured by various thermal analysis techniques. The glass transition temperature was shown to be a good and direct measure of the degree-of-cure. The CTEs (α1 and α2),G′ and G″ were found to be decreasing functions of degree-of-cure. Viscoelastic EMC material models with DOC (i.e., Tg) dependent were formulated. Package warpage predictions against different processing conditions were performed via finite element analyses. Out-of-plane displacement measurements were performed on plastic quad flat package (PQFP) to validate the numerical results. Warpage prediction by the viscoelastic material model was found to agree with the measured data better than the thermoelastic one. For a given cured content, less warpage was found in packages molded at low temperature and longer molding time OR high temperature and shorter molding time.

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.

Thermoviscoelastic simulation of thermally and pressure-induced stresses in injection moulding for the prediction of shrinkage and warpage for fibre-reinforced thermoplastics

In this paper we present a detailed thermoviscoelastic formulation for the simulation of thermally and pressure induced residual stresses in injection moulded short-fibre-reinforced thermoplastics. The computed residual stresses enable us to predict shrinkage and warpage in the finished products. We also apply an anisotropic version of a rotary diffusion equation to calculate the flow-induced fibre orientation distribution. The predicted fibre orientation state, together with micromechanical theories, allows the incorporation of anisotropy in material properties into the thermoviscoelastic model. Finally we report three numerical examples to indicate the success of the present model.

Shrinkage Prediction for Slowly-Crystallizing Thermoplastic Polymers in Injection Molding

Abstract The prediction of shrinkage and warpage of crystalline polymers is quite difficult. This is because of the complications of the crystallization process and associated material property changes. In typical injection molding of semi-crystalline polymers, it is difficult to calculate crystallinity accurately because of its complicated dependence on temperature, time and stress. In the case of slowly-crystallizing polymers, typically the crystallinity of the polymer remains small during the process. In this case, therefore, the importance of accurate calculation of crystallinity is decreased. The difficulty in this case is the measurement of material properties under variable crystallinity because of the possible crystallization during measurement. In the present study, methods have been developed to obtain material properties which include the effect of crystallinity. The material properties measured include viscosity, thermal conductivity, heat capacity, PvT relation and crystallization kinetics. Injection-molding experiments have been conducted to measure the shrinkage of a part using a slowly-crystallizing polymer. Shrinkage has been measured by comparing the mold cavity dimension and the dimension of the molded sample. The polymer used in this study is PET (polyethylene terephthalate). A simulation program has been developed to analyze the injection-molding process. This program is based on quiescent crystallization kinetics. The predicted shrinkage and measured shrinkage are found to agree reasonably well.

The Effect on Warpage Prediction for Fiber Reinforced Material by Difference Consideration for Fiber Orientation

The Effect on Warpage Prediction for Fiber Reinforced Material by Difference Consideration for Fiber Orientation

Prediction of Shrinkage and Warpage of Fiber Reinforced Thermoset Composite Parts

One of the most challenging tasks in designing plastic parts, especially those that are fiber reinforced, is to predict shrinkage and warpage of the molded parts. Shrinkage and warpage result from material inhomogeneities caused by flow induced fiber orientation, curing, poor thermal mold lay-out, and processing conditions. Shrinkage and warpage are directly related to residual stresses which result from locally varying strain fields that occur during the curing or solidification stage of a manufacturing process. This paper presents research conducted in modeling, analysis and process simulation of the thermomechanical behavior of compression molded fiber reinforced composite parts. The theory behind shrinkage and warpage of fiber reinforced composite parts is described first, followed by a description of finite element/finite difference simulation of the thermo mechanical behavior of fiber reinforced composites during a part's manufacturing process. A coupled temperature and stress simulation program with a three-noded shell element formulation was developed to calculate the residual stress build-up during curing and solidification stages of a compression molding process. The effects of fiber content, part thickness, unsymmetric curing and flow-induced fiber orientation on the shrinkage and warpage of the molded parts are also investigated.

Prediction and Evaluation of Warpage of Injection Molded Parts Using MF/WARP

Prediction and Evaluation of Warpage of Injection Molded Parts Using MF/WARP

Shrinkage and Warpage Prediction for Injection Molded Components

Shrinkage and warpage analysis is now an important part of Computer Aided Engineering (CAE) for injection molded components. The theory behind this anal ysis (including effects of crystallinity, orientation and cooling stress relaxation) is pre sented, along with a number of case studies showing the successful application of this technology.

Integrated Simulation of Fluid Flow and Heat Transfer in Injection Molding for the Prediction of Shrinkage and Warpage

This work employs a coupled analysis of the fluid flow and heat transfer in the polymer melt during the filling and post-filling stages of the injection-molding process and of mold cooling/heating which occurs during the entire process. Polymer melt analysis (PMA) has been carried out through a unified theoretical model implemented using a hybrid finite-element/finite-difference/control-volume numerical solution of the generalized Hele-Shaw flow of a compressible viscous fluid under non-isothermal conditions. Further, mold-cooling analysis (MCA) has been carried out utilizing a periodic heat conduction model implemented using a modified three-dimensional boundary-element method. To faithfully accommodate the effects of mold cooling on the fluid flow and heat transfer in the polymer melt, PMA and MCA have been coupled for appropriate data exchange and iterations carried out until a convergent solution for mold temperatures and for flow, pressure and temperatures within the polymer melt is obtained. The results obtained from this integrated simulation for different test cases have been compared with experimental data and a favorable agreement has been noticed. Using an illustrative example, the results are discussed in detail.