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...
This article is a chapter from a forthcoming book THE PLASTICS EXTRUSION HANDBOOKontinuous processes such as extrusion should C lend themselves to systems of continuous process control such as the ones used in other chemical unit operations. Polymer materials are somewhat more complex than the usual fluids processed and the historical development of the extrusion process was such that the typical control systems approach was not considered. With the development of equipment with better defined process actions on the materials, better understanding of the rheology of plastics melts, better data on the materials, and, most importantly, much more critical requirements for products there has been a great deal of activity concerning the development of close control systems for extrusion. EXTRUDER CONTROLThere have been a large number of approaches to the process control improvement. One parameter that has been subjected to study is temperature and temperature control. From simple on-off controls the machine sensing and control systems have been brought to the stage of read around temperature measurements which control power input units that deliver the needed heating to promote a stable melt condition in the machine. The most sophisticated of these systems also measures melt temperature conditions at one or more locations in the melt stream and determines the interactive effects of shear rate and temperature to produce closely controlled melt temperatures.Another control parameter that has been under study is melt pressure a t the extruder head end.Improved sensing systems have been developed and there has been significant information generated on the effects of flow restrictors such as screens and valves and on the effect of machine screw speeds on the head pressures. One of the problems is that the melt pressure is a resultant of several different factors such as the temperature, amount of shear history, and the degree of mixing in the machine so t h a t it is difficult to control directly.The important question to ask is what is desired in the way of output from the extruder. Another way of stating this is what are the requirements on the melt parameters to produce stable output from the extrusion die. It is necessary that the melt delivered to the die have the same pressure, temperature, and shear history a t all times in order for the die delivery to be constant and stable. Since the shear history is a function dependent on the back pressure in the machine and this, in turn, interacts with the melt temperature i t is evident that the melt parameters are interdependent and strongly dependent on the machine settings and how it is operated. Present control practice takes these interactions into consideration only by means of the skill and experience of the machine operators and set up men. It would be invaluable in the operation of complex extrusion systems if the start up and running strategies of the operators could be formalized and the operation be consistent in performance without relying so heavily on operator...
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