A numerical simulation is presented that combines the flow simulation during injection molding with an efficient algorithm for predicting the orientation of short fibers in thin composite parts. Fiber-orientation state is represented in terms of a second-order orientation tensor. Fiber-fiber interactions are modeled by means of an isotropic rotary diffusion. The simulation predicts flow-aligned fiber orientation (shell region) near the surface with transversely aligned (core region) fibers in the vicinity of the mid-plane. The effects of part thickness and injection speed on fiber orientation are analyzed. Experimental measurements of fiber orientation in plaque-shaped parts for three different combinations of cavity thickness and injection speed are reported. It is found that gapwise-converging flow due to the growing layer of solidified polymer near the walls tends to flow-align the fibers near the entrance, whereas near the melt front, gapwise-diverging flow due to the diminishing solid layer tends to align the fibers transverse to the flow. The effect of this gapwise-converging-diverging flow is found to be especially significant for thin parts molded at slower injection speeds, which have a proportionately thicker layer of solidified polymer during the filling process. If the fiber orientation is known, predictions of the anisotropic tensile moduli and thermal-expansion coefficients of the composite are obtained by using the equations for unidirectional composites and taking an orientation average. These predictions are found to agree reasonably well with corresponding experimental measurements.

This study employs a unified theoretical model to simulate the filling and postfilling stages of the injection-molding process. Implementation of such a model is based on a hybrid finite-element/finite-difference numerical solution of the generalized Hele-Shaw flow of a compressible viscous fluid under nonisothermal conditions. The shear viscosity of the polymeric material is represented by a Cross model for the shear-rate dependence and a WLF-type functional form for the temperature and pressure dependence, whereas the specific volume is modeled in terms of a double-domain Tait equation. The analysis also handles variable specific heat and thermal conductivity of the polymer as a function of temperature. Complex thin parts of variable thickness can be modeled and discretized by flat, triangular finite elements which may have arbitrary orientation in three-dimensional space, whereas runners and possible round pins or bosses in the part are represented as one-dimensional circular-tube elements. A control-volume scheme is employed that leads to automatic melt-front advanctment during the cavity-filling stage.

An experimental study of the heating and bonding mechanisms in ultrasonic welding is described. Polystyrene specimens were joined under a variety of welding conditions while the temperatures at the interface and within the interior of these specimens were measured. The power input, amplitude of vibrations, and amount of deformation during welding were measured concurrently. In general, the rate of heating at the interface is greatest at the beginning of the weld cycle, but slows markedly after the interface temperature reaches approximately 250°C. The interface temperature peaks well before the weld is completed. Temperatures within the body increase most rapidly at temperatures near the glass transition temperature. Welded specimens were broken on a special testing apparatus under combined torsional and compressional loads to determine the weld strength. The results show that weld strength is dependent on the amount of energy input and the degree to which material flows out of the interface region. Possible mechanisms for heating and bonding during ultrasonic welding are discussed in light of the observed behavior.

In recent years, increased attention has been paid to the design of cooling systems in injection molding, as it became clear that cooling affects both productivity and part quality. In order to systematically improve the performance of a cooling system in terms of rapid, uniform, and even cooling, the designer needs a CAE analysis tool. For this, a computer simulation has been developed for three-dimensional mold heat transfer during the cooling stage of an injection molding process. In this simulation, mold heat transfer is considered as cyclic-steady, three-dimensional conduction; heat transfer within the melt region is treated as transient, one-dimensional conduction; heat exchange between the cooling channel surfaces and coolant is treated as steady, as is heat exchange with the ambient air and mold exterior surfaces. Numerical implementation includes the application of a hybrid scheme consisting of a modified three-dimensional, boundary-element method for the mold region and a finite-difference method with a variable mesh for the melt region. These two analyses are iteratively coupled so as to match the temperature and heat flux at the mold-melt interface. Using an example, the usefulness of the simulation developed here in the design of a cooling system for an injection molding process is amply demonstrated.

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.