Coefficient of static and dynamic friction measurements for plastics against thermoforming plug materials at elevated temperatures
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
Void shape in gas‐channel ribs in gas‐assisted injection‐molded plates
Large, thin, plate-shaped parts are usually strengthened with structural ribs. Ribs also serve as gas channels with gas-assisted technology. Gas penetration in the gas channels results in improved moldability and also eliminates sink marks and prevents warpage. However, gas leaves the rib core void, which degrades rigidity. This study examines the shape of the gas void and its relation to rib geometry and processing conditions. The effects of rib geometry, including aspect ratio and fillet geometry, along with processing conditions such as delay time, melt temperature, gas pressure and characteristics of the GAIM molded parts on void shapes are investigated. Numerical simulation of temperature distribution in the rib area is used to aid interpretations of gas void shapes. ©1999 John Wiley & Sons, Inc. Adv Polym Techn 18: 11–17, 1999
Flow field analysis of the kneading disc region in a co‐rotating twin screw extruder
Co-rotating, intermeshing twin screw extruders are widely used in polymer compounding and blending. Among the different modules of the co-rotating twin screw extruder, the kneading discs are the dominant ones in determining mixing efficiency. The major difficulty in solving the flow problem in the kneading disc region arises from the complex geometry and the time-dependent flow boundaries as the discs rotate. In this work, a fluid dynamics analysis package—FIDAP—using the finite element method was employed to simulate the flow patterns in the kneading disc region of a Werner & Pfleiderer ZSK-30 co-rotating twin screw extruder. The problem of time dependent flow boundaries was solved by selecting a number of sequential geometries to represent a complete mixing cycle. The flow field was characterized in terms of velocity profiles, pressure distributions, shear stresses generated and a parameter λ quantifying the elongational flow components. The last two parameters are the most important ones in analyzing mixing efficiency. The influence of design variables (stagger angle, right or left handed configuration) and processing conditions (rpm, axial pressure gradient) on the flow characteristics was analyzed.