1. German Patent (filed June 10, 1881) 18,797 (1882).
2. Twin Screw Extrusion: Technology and Principles, Hanser, Munich (1990).
3. Anonymous, German Patent (filed Jan. 31, 1941) 750,509 (1945).
4. Schneckenmaschinen in der Verfahrenstechnik, Springer, Berlin (1972).
5. German Patent (filed July 4, 1951) 879,164 (1953).