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
Experimental validation of numerical simulations of the kneading disc section in a twin screw extruder
A comparison of the results of a 3D model for the simulation of flow in the kneading disc region of a fully intermeshing twin screw extruder has been done against particle image velocimetry (PIV) experimental data. Experimental results using a geometry with an extended first and last kneading disc to capture the simplified inlet and outlet boundary conditions, are in good qualitative agreement for low flow rates. The results showed good agreement at flow rates less than 5 mL/s and rotation speeds of 60 rpm, while above that there was poor agreement. These results highlight the importance of the use of appropriate boundary conditions for the accurate prediction of the flow field. The flow field within the kneading discs with forward conveying elements placed before and after the kneading discs was also measured. For a flow rate of 8.5 mL/s and 60 rpm, results showed good agreement when conveying elements were deployed before and after the kneading blocks whereas for lower flow rates there was poor agreement.
A study of rib geometry for gas‐assisted injection molding
Large, thin, plate-shaped parts usually are strengthened with structural ribs. Ribs also serve as gas channels with gas-assisted technology. The layout and geometry of these gas-channel ribs are critical to the gas-assisted injection molding (GAIM) process. In this study, the effects of rib geometry, including aspect ratio and fillet geometry, on the GAIM process are investigated. Experimental results indicate that increasing the rib width widens the allowable operation range and thus improves the moldability. Adding fillets to the rib corner significantly enhances the moldability. Adding fillets also reduces the loss of rigidity due to void formation in the rib. A curved fillet improves moldability and rigidity more than one that is straight.