Extrusion extruder design

Extrusion and extruder design are apparently almost entirely empirical at present. This field might well defy rigorous theoretical analyses until the simpler problems of flow in annular spaces and rectangular ducts and of nonisothermal fluid flow are understood. 

The selection of the extruder design is based on the principal requirements of the extrudate and the nature of further processing. For the production of uniform granules to be dried in a fluid-bed drier, a low-compaction system, such as that provided by the various types of screen extruders may be suitable. Cylinder or gear-type extruders may be more appropriate when aiming for a densified extrudate, such as that required for spheronization. Ram-extrusion systems, which allow precision control of extrudate density, size, and shape, are ideal for the extrusion and forming of pharmaceutical polymers of the type used for sub-dermal implants. 

As stated above, extrusion die design is a complex task because the extrudate product dimensions depend not only on the die design (die shape), but also on the polymer properties and extrusion process parameters. The following are general recommendations for extrusion die design ... 

In high pressure axial screw extruders, dies and their orifice(s) can have different shapes and sizes. Their selection depends on the plasticity and flow properties of the materials to be extruded and, obviously, on the desired product shape and size. Two basic types of dies are used in extrusion. For the formation of small agglomerates, such as strands or pellets, extrusion plates are used. Such plates can be simple or more complex, the latter, so called sandwich types, consist of multiple plates that are made of different materials. Fig. 8.78 presents sketches of two extrusion plate designs. Parameters are based on manufacturer know-how as well as vendor experience from different applications and can not be discussed here. 

One further limitation is that the extrapolation not be taken too far. Capillary rheometric measurements (Fw = 10 up to 1(F [1/s]) have shown that the flow curves level off in that area. Thus, ceramic extrusion compounds count as structurally viscous across wide shear-rate ranges. Since the measured data in question stay within the extruder s operating range, the material-data yield provides a solid foundation for die- or extruder design as well as compound developping. 

Hot Extrusion. This term is best reserved for the process of extrusion in which clay that has already been prepared and has the correct water content is heated while passing through the extruder these features distinguish the process from steam tempering (q.v.). The Hot Extrusion process is typified in the extruders designed by the Bedeschi Company, Padova, Italy. 

R. Steiner, Extruder design, in I. Ghebre-Sellassie, and C. Martin, eds.. Pharmaceutical Extrusion Technology, Taylor Francis, pp. 17-35, 2003. 

Extruder Drives The higher specific heat requirements of SPS and lack of preheating through drying necessitates extruder drive with greater capability. On a theoretical basis, extrusion of SPS requires 2.5-2.75 times the drive horsepower typically needed to process a preheated polyethylene terephthalate (PET). For preheated PET extrusion, extruder drives are designed for approximately 12.01b/h/hp room temperature SPS would require approximately lOlb/h/hp [13]. 

More than 90% of PVC pipe is produced on twin-screw extruders. Two types of extrusion dies for pipe are shown in Figure 10.3. In the modern design (Figure 10.3a), a more uniform flow and less back pressure occurs. There are three important factors in extrusion die design the pressure drop, the shear rate, and the residence time. 

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