Volume 6, Issue 1

This paper compares the differences developed in rubber properties between compression and transfer molding methods for various types of polymers. It has been shown that the two molding methods produce distinctly different rubber properties for the same elastomer. Acrylonitrile butadiene rubber (NBR) shows little difference in durability between molding methods. Transfer molding (TM) reduces modulus and hardness of NBR, but improves compression set. The crosslink density of rubbers made with TM is higher than those made with compression molding (CM). Highly saturated acrylonitrile butadiene rubber (HNBR) properties are significantly different between the two molding methods. As for NBR rubbers, the TM method reduces modulus and hardness, but improves compression set. The crosslink density of rubbers made by TM is higher that those made with CM. Silicone rubber (VMQ) that does not require post cure shows the same rubber properties change as NBR. Transfer molding yields low tensile and modulus but better compression set. Polyacrylic rubber (ACM) molded with the TM method shows reduced tensile, elongation, modulus and hardness; but after post cure, modulus and hardness did not show substantial differences. The compression set was lower with TM both before and after post cure. Crosslink density is significantly higher with TM, but the difference diminishes after post cure. Ethylene acrylic rubber (EAM) and fluoroelastomer rubber (FKM) exhibit property changes similar to acrylic rubber when molded either with compression or transfer molding methods. Rubbers molded with TM methods showed a higher degree of anisotropy in rubber properties than with CM. With enlarged extension ratio, the coefficient of anisotropy increased, with the exception of ACM.

A 2D finite element model is presented for the prediction of the advancing front in the thickness of a part during mold filling in the polymer injection molding process. The full Navier-Stokes equations are considered for the determination of the velocity and the pressure. The computation of the flow front uses the Arbitrary Lagrangian-Eulerian approach and is coupled with an automatic remeshing technique triggered only when the mesh becomes distorted. Three series of numerical tests with a Newtonian fluid are presented. The first one deals with the fountain-flow in a flat gap and a tube and allows for validation of the code. The second example in a 90° bend illustrates the capability of the proposed method to deal with multiple fronts, and the third one deals with jetting.

A three dimensional finite element methodology is presented for the modelling of complex injection mold filling, using an extension of the Flow Analysis Network method coupled to a three dimensional solution of the thermal equations with phase change. The method allows the computation of the propagation of the polymer melt front with time as well as the prediction of the frozen skin which occurs during filling. Illustrations are presented in the context of an industrial mold, and simulation results are validated against experimental data showing a reasonable agreement.

To manufacture anisotropic plastic magnets with higher magnetic power, the compounding of Strontium Ferrite powder / plastics binder and injection molding processes should be optimized. In this study, we have carried out the optimization of injection molding machine design, and the mechanism analysis of the compounding operation. As a result, to generate effective strong magnetic fields during injection molding, the coiling system, mold structure, and pole shape were very important elements. Better Ferrite powder dispersion accelerated the particle orientation, which lead to higher magnetic power of the molded product when strong magnetic fields were applied. And then, particle dispersion depended on specific energy of the compounding machine unless excessive shearing force degraded Ferrite and plastics binder.

The development of skin/core fibre orientations in injection molded fibre reinforced compounds is considered, and the applicability of the Tadmor fountain-flow mold filling mechanism to long fibre reinforced is investigated. A disc and plaque mold cavity are used to consider the influence of each injection molding cycle parameter upon the skin/core structure. The influence of the flow path length is also considered. The independent orientation controlling mechanisms identified (the injection speed, melt temperature and the holding pressure cycle all of which influence the fibre orientation favourably) can be used in combination to achieve a optimised microstructure for the cavities under study.

Visualization of the fountain flow in a piston driven non-Newtonian flow is studied by means of experiments and numerical computation. The result will be applied to the multilayer injection molding technique, where an accumulator is used, to store the polymer melt. The discontinuity at the contactline of the piston and the wall of the cylinder appears to have considerable influence on the total deformation history.

The work describes an approach to simulation of a blow molding process. The development originated from the need to reduce the amount of experimentation through a screening of processing conditions and preform configurations. The simulation uses a standard finite element software capable of handling large deformations and an interaction with rigid surfaces, which are encountered in blow molding. The developed constitutive equation for a general purpose polystyrene in the rubbery state was incorporated into the model. Temperature and rate dependencies, typical for plastic materials, are accounted for in this model. The application of the developed model for analysis of the preform's deformations is illustrated here on a number of examples.

This paper describes analytical and experimental work which has been carried out to identify optimum process conditions for rotomolded products. The mechanical performance of the moldings has been assessed using tensile impact tests. It has been found that changes in the oven temperature or oven time cause a very significant shift in the ductile-brittle transition for the SCLAIR 8504 grade of polyethylene used in the work. Optimum combinations of process variables can be clearly defined and used to establish a processing window for the material. The ROTOSIM computer simulation for the rotomolding process has been used to establish the best (most economic) combinations of process conditions to obtain the optimum mechanical properties. A single equation is presented to relate pool depletion time to some of the process variables.

A series of experiments and a numerical simulation were carried out to investigate the flow of silly putty and sheet molding compound (SMC) at various processing conditions in compression molding. The flow resistance and the interface friction of both materials were characterized by using a squeeze flow rheometer and the ring compression test, respectively. Molding experiments were carried out on an instrumented mold and the finite element code ALPID (Analysis of Large Plastic Incremental Deformation) was used for numerical simulation. The flow front shape, temperature profile, and pressure change obtained by numerical simulation compared well with experimental results.

The flow of reacting monomers into thin planar cavities of irregular geometry is analyzed. The Hele-Shaw approximations are used to simplify the momentum and mass balances. At each time step, the flow domain is mapped onto a rectangular domain using elliptic expressions. The transformed governing equations are solved using finite-differences. The transient three-dimensional energy and species balances are solved using an explicit marching scheme coupled to the flow. Viscosity is modeled as a function of temperature and extent of reaction. Castro's model is used to compare several fountain flow approximations. A simplified algorithm was found to capture the essence of the fountain flow kinematics. The fountain flow approximations described here can also be used in thermoplastic injection molding modeling. Results for a rectangular end gated mold and for an irregular mold geometry are presented.