Computer Modelling of Polymer Processing

The gas assisted injection moulding (GAIM) process has benefits realised through material saving, improved product performance and lower capital cost of machinery. This important process is similar to conventional injection moulding in so far as polymer melt enters a cavity under pressure. However, before the melt completely fills the cavity, nitrogen gas is introduced into the melt at high pressure to complete the filling stage. Gas pressure is maintained to replace the packing pressure during cooling, normally supplied by the melt injection unit. Modelling of the GAIM cavity filling stage has used a variety of methods adapted from the widely used 2-D approach used for conventional injection moulding. Although these methods have been useful for rough predictions in mould filling without requiring excessive computational power, crucial information is missing regarding the final distribution of polymer and gas in any given cross section. Only true 3-D modelling techniques can provide the necessary information regarding the cross sectional form in a gas channel to determine residual wall thickness and bubble penetration. Presented here is a 3-D finite element implementation of a pseudo-concentration method. A fixed finite element grid is used to mesh the cavity. After each time step calculations of velocities in the x, y and z directions (u, v, w), pressure (p) and concentration (c) are made. The pseudo-concentration method uses a material label, or concentration (c), to distinguish between the polymer and gas; gas is cgas=0 and polymer is cpolymer=1. The concentration value at each node is rounded to cgas or cpolymer after each time step, except at the nodes bordering an element where the concentration contour of c=0.5 passes; this represents the polymer/gas interface. The exact location of the polymer/gas interface can therefore be interpolated and the residual wall thickness predicted from these modelling results. A typical simulation result is shown in figures 1 and 2, which only show the concentration contours representing the polymer and clearly show the formation of the gas bubble inside the specimen. Figure 1 shows a cross-section of our test specimen near the beginning of the 200mm uniform gauge length. Figure 2 shows contours along the centreline (lengthways) of the specimen. Reference Haagh G.A.A.V. and Van de Vosse F. N. Simulation of three dimensional polymer mould filling processes using a pseudo-concentration method. International Journal for Numerical Methods in Fluids, 28:1355-1369, 1998. Figure 1 Figure 2