Screwless extruders

Normal Stress (Weissenberg Effect). Many viscoelastic fluids flow in a direction normal (perpendicular) to the direction of shear stress in steady-state shear (21,90). Examples of the effect include flour dough climbing up a beater, polymer solutions climbing up the inner cylinder in a concentric cylinder viscometer, and paints forcing apart the cone and plate of a cone—plate viscometer. The normal stress effect has been put to practical use in certain screwless extruders designed in a cone—plate or plate—plate configuration, where the polymer enters at the periphery and exits at the axis. 

Fig. 6.21 A rotary-sector slider pad extruder. [Reprinted by permission from R. F. Westover, A Hydrodynamic Screwless Extruder Soc. Plastics Eng. J., 1473 (1962).]...

There are a number of machines that do not utilize Archimedean screw for transport of the material, but still fall within the category of continuous extruders. Sometimes these machines are referred to as screwless extruders or elastic melt extruder. These machines employ disks or drums to melt, mix, pump, and extrude the material and are based on viscous drag transport principle. They are not widely used in industry as forming machines, and will not be discussed here. 

There are several extmders that do not use an Archimedean screw for transportation of materials, but still belong to the category of continuous extruders. These machines are sometimes referred to as screwless extruders. Usually they employ a disk or a drum. Most disk extruders are based on a viscous drag transport principle. To this category belong stepped disk extruders (Raleigh 1879 Westover 1962), drum extmders, spiral extmders (Ingen Housz 1975), disk pack extmder (Tadmor 1979, 1980 Tadmor et al. 1979,1983 Hold et al. 1979 Valsamis 1983), and many others. 

There are also several forms of screwless extruders that are less used. One form depends on friction between a rotating and a stationary plate to develop the shear action. A second form uses a ram feeding a manifold to result in continuous extrusion. 

Elastic melt extruders Screwless extruder Screw or disk type melt extruder Melt fed extruder... 

Disk extruders. There are several types of screwless extruders. These machines employ a disk or a drum to plasticate, mix, and extrude. Most designs are based on viscous drag flow. They include Maxwell s elastic melt extruder, Westover s stepped disk and drum extruders, and Diskpack [39]. The Diskpack extruder has the capability of performing all the elementary steps of plastics processing by combination of differently shaped rotating... 

Similar reasoning leads to the conclusion that in cone-and-plate or disk-plate viscometers (Sections 16.7 and 16.8) there will be forces tending to push the cone and plate or the disk and plate apart. Indeed, there are, and the measurement of these forces provides a means of determining the functions Ni j) and Also, for the case of torsional flow, Maxwell and Scalora showed that if a hole is drilled through the plate along the axis of rotation, a screwless extruder is obtained, as the axial normal stress pushes a polymer melt through the hole. Unlike a screw extruder, however, this device works only with viscoelastic fluids. 

The fact is that, in the long history of polymer processing, engineering and design has always been ahead of theory. The development of screwless (or disc-type) extruders, innovative for their time, on the basis of the earlier-discovered normal stress effect (which received the name of the Weissenberg effect) was, apparently, one.of the few exclusions. However, this example has clearly demonstrated the potential and the role of theoretical research in the progress of technology. 

Practically all PE grades can be processed by extrusion, but in order for the emerging extrudates to maintain their shape until they solidify, extrusion grades tend to have relatively high molecular weights associated with high viscosity and melt strength. Very viscous UHMWPE is often processed by screwless (ram) extrusion. 

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