Developments after Expiration of the Primary Patents

Once the co-rotating extruder had become the standard element for plastics conditioning and processing, compounding in other words, and the patents referred to in Section 2.2.2 had expired (approximately 1970), the co-rotating principle was adopted by numerous mechanical engineering companies. Today, approx. 50 companies around the world can supply small or medium-sized (approx. 120 mm) screws. However, just five are able to supply large screws of 120 mm to 380 mm (estimated values). 

The patent vacuum has also prompted new applications from several areas. Apparatus patents are among the foremost of these, i.e., numerous variants of innovative screw, mixing, and kneading elements from mechanical engineering companies, while process patents employing co-rotating extruders are proposed by the chemical industry. 

We are also witnessing a considerable increase in new specialized literature. The most important of these is the standard work Twin Screw Extrusion by J. L. White [44]. It is a rich source of information, both about the historical development and the state-of-the-art technology, as well as about patent literature and specialized publications. A few examples [45,46] from the wide-ranging specialized literature on the subject of co-rotating extruders should be mentioned here. 

50-year development in the economic viability of screw processes 

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