METHOD OF MAKING LIGHTWEIGHT, RIGID THERMOPLASTIC PIPE

摘要

Disclosed is a method of making a lightweight, rigid thermoplastic pipe having solid interior and exterior surfaces and a foamed interior, comprising heating a first thermoplastic material in a first extruder, heating a second thermoplastic material containing a heat decomposable chemical foaming agent in a second extruder to a temperature of about at least 400.degree.F, feeding said first thermoplastic material and said second thermoplastic material simultaneously and respectively to multiple and separate passages of a pipe shaping tool, dividing said first thermoplastic material into two streams to form the interior and exterior surfaces of the pipe, the chemical foaming agent of said second thermoplastic material decomposing as a result of said heating step but said second thermoplastic material remaining in a foamable but unfoamed state due to pressures within said second extruder and the pipe shaping tool until the pressure is reduced as the second thermoplastic material leaves said tool, whereby the solid interior and exterior surfaces of the pipe and the foamed interior thereof are formed simultaneously. Exemplified is the manufacture of graft ABS pipe having solid interior and exterior surfaces and a foamed interior.