Early Developments of Co-rotating Twin Screw Extruder Part I
The single screw extruder as an “extrusion apparatus” was developed in the second half of the 19th century and was used intensively in industrial and heavy engineering applications. It was used in three major industries:
▲ Pottery industry: ceramic compounds
- extrusion, shaping
▲ Rubber industry: natural rubber, gum
- plastification, extrusion,
- profile production
▲ Food industry: oily fruits, oil seeds
- extracting biological oils,
- separation of material using “strainer screws”
- meat processing by meat grinder
Product feed in a single screw initially appears somewhat strange. While every metal molecule remains in the same cross-sectional plane, the product is nevertheless conveyed axially. Following is an attempt at an explanation: when considering the screw and the product, the screw rotates without changing position, although the product does not rotate but slides axially, in other words, its is axially conveyed. This so-called “theoretical” conveying does not exist in practice, however, since the product is not a solid body but a highly viscous fluid with a rheological character.
The adhesion and friction characteristics of the plastic material determine the intensity of the flow. In the case of Newtonian fluids this is half of the theoretical conveying (at constant pressure) and even less with counterpressure (extrusion), even down to zero. In the latter case, the product rotates with the shaft and throughput ceases.
This weakness of the single screw, particularly the fact that there is no cleaning of the shaft and the strong dependence of conveying on rheological properties, motivated inventors to seek solutions to these problems. The co-rotating extruder was there for initially proposed as a self-cleaning mechanism. Six patent citations over a 70-year period (1869-1939) show that the co-rotating extruder remained very much at the forefront of engineers’ minds. J.L.White provides an extensive, thorough description of the developments and patent situation in this field.