Primary embodiment the diverting valves
According to one embodiment of the present invention a single screw extruder selectively feeds molten plastic material through a diverting valve to one of two accumulator-injectors arranged in parallel. While the extruder feeds molten plastic to one of the accumulator-injectors, the other accumulator-injector injects molten plastic through another diverting valve to an extruding die.
After the inject cycle has been completed the valves are switched connecting the other accumulator-injector to receive molten plastic from the sheet production extruder, and connecting the first accumulator-injector to deliver molten plastic in its inject phase to the die. By accurately controlling the speed of the accumulator-injectors during the inject phase, the volume per increment of length of the resulting extrudate is very accurately controlled. Therefore, by also accurately controlling the volume of air delivered through the die to the interior of the extrudate the tolerances of each of the interior diameter, the exterior diameter and wall thickness of the tubing are accurately controlled. Since each accumulator is capable of producing a complete section of tubing including, for example, a nominal diameter portion and an enlarged diameter portion, variations in the output of the extruder do not effect the extruded tubing. In a modified form of the primary embodiment the diverting valves are replaced with check valves that eliminate the necessity for synchronizing valve switching with the end of the accumulator-injector cycle.
Another embodiment of the present invention a sheet production extruder is provided connected to deliver molten plastic to an extruding die through a conduit having two or more serially arranged accumulator-injectors therein. The cycles of each of the accumulator-injectors remain fixed with the initiation of each of the actuators for the accumulator-injectors being programmed by a programming device to achieve a desired contour tubing. With each of the accumulator-injectors having a constant rate accumulate phase occurring simultaneously, a nominal size tubing results with two or more enlargements occurring at different intervals or one on top of the other, as desired. The actuators in this embodiment may also be phased so that while one or more accumulators are withdrawing molten plastic, one is injecting slowly to provide the nominal tubing diameter. Thereafter the actuator which was injecting slowly may switch to its withdrawal stroke while the other actuators continue in their withdrawal strokes, producing as a result a section of small diameter tubing.
single screw extruder feeds molten plastic to an extruding die through four parallel arranged feed branches each having one accumulator. This is basically the same functional arrangement as the third embodiment described above where the accumulators are serially arranged. No valves are provided between the accumulator-injectors and either the extruder or the die so that all may proceed in the withdrawal phase at the same time or in the inject phase at the same time, or with any desired phase relationship to provide a wide variety of tubing configurations.