THE INFLUENCE OF EXTRUDER GEOMETRY IN THE PRODUCTION OF HOMOGENEOUS CELLULAR PLASTIC INSULATION,
BELL TELEPHONE LABS INC MURRAY HILL N J
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The experiment which is described in this paper involves expanded polypropylene insulation on 19-gauge wire. The wall thickness was 9 mils. The gas for the expansion of the polypropylene was supplied by the decomposition of a chemical blowing agent which was dry mixed with the plastic granules before being introduced into the extruder hopper. With suitable control of extrusion conditions and adequate nucleation, what appeared to be a uniform fine-cell structure was produced in the laboratory by this method. Difficulties were experienced, however, with the integrity of the insulation as evidenced by the large number of faults picked up by the spark tester on the extrusion line. Examination of the insulation revealed that there was a longitudinal seam of relatively solid material containing large air cells. In some instances the large cells were interconnected so that there was an air dielectric path from the conductor to the surface of the insulation. These points registered as faults on the spark tester and as high capacitance excursions on the capacitance monitor chart. It is the purpose of this paper to describe 1 an experiment which revealed the cause of this condition, and 2 the development of a crosshead flowpath which provided the solution to the problem.
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