Gilbu A., Method for producing tubes of reinforced setting plastic material, US3770541 A, 6 November 1973, Retrieved from http://www.google.is/patents/US3770541
Poulsen, Peder Ulrik "Working procedure of and apparatus for the manufacture of reinforced plastic tubes" U.S. Patent No. 3,431,158. 4 March 1969. Retrieved from https://patents.google.com/patent/US3431158 (on 8 February 2016)
Poulsen, P.U. "Driven mandrel rotatable about its longitudinal axis for continuous production of tubing in running lengths, preferably glass fibre reinforced plastic tubing." U.S. Patent No. 3,464,879. 2 September 1969.Retrieved from https://patents.google.com/patent/US3431158 (on 8 February 2016)