Extruder Design and Operation

Mathematical models have been developed for models of the three zones of the screw to predict temperature, pressure, and melt channel geometry of the materials being processed as a function of extruder design and operating conditions. 

The extrusion die remains the critical part of an extruder since its design and operational control deeply affect the output and quality of the extrudate. In rubber processing, dies used are generally short and consequently, the elastic character of the extruded compound plays a highly significant role, and it is still difficult to separate effects due to the die geometry and to the viscoelastic response of the material. 

There are quite a number of ancillary operations associated with the extruder in its simplest form. Failure to pay careful attention to these can lead to a product of variable properties, normally a quite unacceptable state of affairs. It can also cause there to be fluctuations in operating conditions which can the extruder to cease to operate. The extruder can even be mechanically damaged by the generation of severe internal pressures. The extrusion thus has to be viewed as an integrated system at both the design and operational stages. 

The formation of laminations caused by extrusion and their implications depend to a very large extent on the raw material properties, the preparation technology employed, the design and operating mode of the extruder. Consequently the following discourse can only represent some hints and a rough orientation. Purposeful tests carried out directly at the extruder or within the upstream technological processes are unavoidable in individual cases. 

Besides body-orientated and process engineering methods it is possible to minimise laminations, and therefore avoid defects caused by these, by optimising the design and operation of the extruder. In practice it is, however, proven time and again that bodies with a minimum tendency towards laminations are a prerequisite in the avoidance of laminations and the associated defects. Or expressed another way even the best extruder design cannot fully compensate the effects of an unfavourable body composition highly susceptible to laminations. 

The purpose of the extruder is to feed a die with a homogeneous material at constant temperature and pressure. This definition highlights three primary responsibilities that the extruder must accomplish while delivering material to a shaping die. First, it must homogenize, or satisfactorily mix, the material. Second, the material entering the die must have minimal temperature variation with respect to both time and position within the melt stream. Third, there must be minimal melt pressure variation with time. It is important that the design and operation of an extrusion system consider all three of these objectives to produce a quality product. 

Kruder, G. A., and R. E. Nunn, Applying Basic Solids Conveying Measurements to Design and Operation of Single-Screw Extruders, 38th Annual Technical Conference of the Society of Plastics Engineers, 1980, p. 62. 

The design and operation of most polycondensation reactors for devolatilization of volatile by-products (namely vented extruders) is clearly a very difficult problem because of the complexity of the flows (with or without foaming). Further progress is likely to require a heavy use of computational fluid dynamics, as simplified models seem to have arrived at their limits. 

The heart of the extruder is the screw, and the success of an extrusion operation is largely dependent on its design. Since the early 1950s, a large volume of literature has been published on the theoretical and practical aspects of extruder screw design and operation. A typical single-stage extruder... 

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