Answer to Question 1
The nylon temperature profile is higher because nylon melts at a much higher temperature than polyethylene. However, when nylon reaches its melting temperature, it melts over a narrower range. With a general purpose screw, normally the feed section would be shorter than with the ideal nylon-type screw. Therefore, the nylon temperature in the feed zone would be higher to compensate for the reduction in the adiabatic heating and the time in the feed zone. The compression zone in the general purpose screw is longer than in the nylon-type screw and therefore the heat can be reduced in the compression zone. The net result is a declining heat profile for nylon when a general purpose screw is used. The opposite is true for polyethylene. When using a general purpose screw, the heat profile for polyethylene increases.
Answer to Question 2
The outside diameter is usually set by forcing the still-soft extrudate outward against forming rings in the cooling tank. To obtain dimensional control for the inside diameter, a mandrel needs to be extended into the cooling bath for sufficient length that the extrudate will cool while in contact with the mandrel. While this is conceptually simple, actual practice is quite difficult. Some factors that must be allowed for are the thermal contraction of the pipe, difficulty in start-up, and adherence of the material to the cold mandrel.
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