Extrusion die design for thermoplastic materials comparison of extrusion ease with PVC materials

Abstract

he material most widely used to make profile parts T by extrusion is PVC, both rigid and flexible grades. These materials, as indicated in the reports on PVC profile extrusion, exhibit melt characteristics which make them easier to predict than most other polymer materials. There is a large and growing application for profiles made from other materials. In order to be able to effectively design tooling for these shapes, it is necessary to know how they differ from the PVC materials in their processing characteristics. The design information may also indicate that it may be impossible to accurately control shapes in some materials, indicating that such profile products cannot be made by extrusion in the conventional sense. Table I lists a series of plastics in general order of ease of extrudability. There are several criteria involved in the ranking, such things as thermal stability of the melt, thixotropic character of the melt, total energy content removal requirements to set the stage, crystallinity effects in the material, melt strength, and others. In the table the specific problems with each material are indicated. The discussion on how to design dies for these materials will be aimed at resolving the problems.One of the most important differences between PVC and the other materials is in the melt strength and thixotrophy characteristics. Beside PVC, the other resins extruded into profiles that exhibit this property the best are the acrylic compounds, the cellulosics (such as cellulose acetate butyrate), modified PPO material, and the ABS materials. In each of these materials, the basic die design that would work with PVC will work on the other materials with small modifications. The relative land length requirements are somewhat different because the different materials have different shear stresslshear rate curves, making the required amount of correction different from that of a PVC die. Figure 1 shows on two simple die shapes the differences in the amount of correction required for several materials as compared to PVC. Figure 2 shows the difference that would be anticipated in the output from a square shape die corrected for rigid PVC when several different materials are run through that die.One of the other major considerations in extruding some of the other materials is involved with the lack of melt strength. This requires much closer spacing of the support fixtures and cooling applied close to the die. This is not a requirement for the cellulosics or for the acrylic materials, both of which have fairly good melt strength. The modified PPO is also good with regard to holding shape without support fixturing close to the die. In the case of the ABS materials and the polystyrenes, the requirement is usually more serious. In both cases, the best sequence for handling and cooling is to use support units close to the die, with cooling using either blowers or air rings. After an initial set of the shape, water sprays can be used and tank immersion is also used. Figure 3 shows a typical first stage cooling ar...

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