A numerical simulation of the warpage of injectioncompression molded optical media such as CD's and DVD's due to asymmetric cooling is developed. A thermal viscoelastic constitutive model is employed to calculate the thermally induced stress. The stress is not symmetric with respect to the mid-plane of the disk because of the differential mold temperatures employed. Before demolding, the out-of-plane deformation is constrained by the mold walls. After ejection, the disk is quenched in the air, and is free to deform, causing warpage to develop in the disk. A finite element analysis code is developed using axisymmetric plate elements to simulate the warpage of the disk after demolding. The complete finite element formulation is described. Simulation results of warpage for CD-R moldings are compared with experimental observations under different processing conditions such as melt temperature, mold temperature, and packing pressure using an optical grade of polycarbonate. Good agreement of the simulation results and experimental observations is obtained. The comparison of the simulation and experiment reveals that the processing conditions have complicated effects on the warpage, and mold temperature has the greatest effect on the warpage in the current study.
A numerical simulation of a coining type of injection‐compression molding is developed. A hybrid finite element/finite difference method is employed to model the temperature and pressure fields of the process using a non‐isothermal compressible flow model. Simulation results for CD‐R molding with respect to injection pressure and mold displacement are compared with experimental observations using an optical grade of polycarbonate. The simulation shows similar trends as experimental observations on the dependence of various processing parameters such as melt temperature, mold temperature, and packing pressure. However, the mold displacement measurement does not show the effect of punch delay time as does the simulation, and needs further investigation.
The maximum flow length of a polymer for a given set of processing conditions is important in injection molding to avoid incomplete mold filling. Experimental analysis, using various processing conditions, can generate the actual influence of processing conditions on the maximum flow length. However, the experimental determination of the flow length for all known industrial polymers would be time consuming and expensive. A non‐Newtonian, nonisothermal model of mold filling was developed to evaluate the flow length without requiring large amounts of computation time. The model implements the use of both a temperature and shear rate–dependent viscosity as well as viscous heating. This paper presents the model and its numerical implementation, followed by simulation results. The model is compared with other simulation programs and experimental results using both an amorphous Styron 484‐27 polystyrene and a semicrystalline 640I polyethylene in a spiral mold geometry. Good agreement between the three is observed.
In this research, a one-dimensional finite difference model has been developed to simulate the progression of material properties during the processing of metal-clad, multi-layered, fiber mat reinforced, thermoset resins. Using a micro-mechdcal model, the simulation is also capable of predicting the dimensional movement observed during processing and the through-thickness residual stress distribution within thin laminates that will lead to the development of warpage or curling. The ability to predict the overall movement is quite complex; however, the contributing factors that lead to warpage of epoxy, glass-fiber mat laminate composites have been experimentally and numerically identified. It has been found that the dominant factor that leads to warpage in asymmetric multi-layered laminates is the differences in the coefficient of thermal expansion of the individual plies. Thus, by selecting appropriate combinations of the degree of cure and resin content of the thermoset in the individual plies, it is possible to reduce the material property variability of the laminate through thickness. The planar movement of individual plies is a function of the glass-fiber mat tension during pre-processing operations. Variability in pre-processing mat tension can be compensated for after lamination via post-baking processes.
The development of the viscosity.of a thermoset material during processing is complicated because of the dependence of the initial material state and the kinetic rate of conversion from a liquid to a solid material. Uncured thermoset materials typically have a low enough viscosity such that the consumption of energy to generate flow is relatively low. However, as the curing process advances, the flow mechanisms become hindered by the development of a network gel during crosslinking. Once the resin has reached the appropriate degree of cure for gelation, the resin system is incapable of large fluid-like deformations. In this research, the rheological properties of an epoxy resin system used in laminate processing were measured and numerically fit with a modification to the dual Arrhenius model to predict the progression of the viscosity during cure. The numerical results were compared with the experimental measurements, and it was found that the model predicts the experimental observations quite well. It was found that the initial degree of cure of the prepreg is not as significant a factor as the temperature rate dependence on the processing time between the point of flow onset and gelation. However, the minimum viscosity during processing is strongly influenced by the initial degree of cure of the prepreg system. IlPTRODUCTIONoper characterization of the progression in the P viscosity of thermoset resins is important in the methodology of defining appropriate processing conditions for various processing methods. For example, the minimum viscosity and the point of gelation are important parameters in the determination of the applied pressure and temperature profiles during processing of glass-fiber reinforced, copper-clad laminates. If the minimum viscosity is too high or the gel-point is reached too soon during processing, the consequence may include improper fiber wetting and diminished adhesion characteristics between the prepreg and copper-foil. The determination of the time and the degree of cure at gelation have also been found to play an important role in the prediction of the mechanical property development as well as the residual stress developed during the cure of thermoset resins [1][2][3][4][5].This study investigates the advancement of the viscosity and the point of gelation as a function of the initial degree of cure of a DICY cured epoxy system prepreg. With a modification to the dual Arrhenius model, it is shown that the viscosity and the point of gelation of the epoxy prepreg system can be predicted. MATERIALSThe epoxy resin used in this investigation is a DICY cured epoxy resin formulation used in the manufacturing of copper-clad laminates. After the epoxy resin system is applied to the glass fiber mat, the dimethylformamide and acetone solvents are evaporated
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