1. Field of the Invention
This invention relates to the formation of a continuous extrudate of thermoplastic resin. More particularly, it is concerned with controlling and reducing thickness variation in such continuous extrudate.
2. Description of the Prior Art
The extrusion of thermoplastic materials thru an annular die orifice to form seamless tubular plastic, such as for the manufacture of film, tubing, pipes and the like, is well known. It has frequently been found desirable to contact the molten or semimolten material leaving the extrusion die with a stream of cooling fluid to regulate the rate of cooling and solidification, thereby controlling the properties of the final product.
To obtain a product having substantially uniform properties, and particularly one having uniform gauge profile, it has long been considered of prime importance that the rate of cooling be completely uniform around the entire circumference of the extruded tube. Hence, numerous devices have been designed and built with the intent of providing uniform cooling by controlling the uniformity of the rate of flow of the cooling fluid around the periphery of the extruded material. For example, U.S. Pat. No. 2,952,874 discloses an air ring having internal baffles, the object of which is to provide for a substantially uniform air velocity around the inner orifice of the air ring, while U.S. Pat. No. 3,548,042 discloses an apparatus utilizing fixed vanes inside the cooling ring to cause the cylindrical wall of cooling air surrounding the extruded tubing to rotate. Another device, described in U.S. Pat. No. 2,926,384, places moveable interfitting blocks within the annular air chamber of the cooling ring to effect close regulation of the flow of the cooling fluid immediately before it emerges from the ring to strike the extruded article.
However, good gauge profile uniformity also requires that the die and cooling ring be very clean and in good condition. In most actual plant operations, the gauge profile is less than optiumum because of errors in the equipment, and it is nearly impossible to maintain the equipment well enough to avoid these errors. It has heretofore been the common practice to compensate for such equipment errors, which inevitably show up down stream in terms of poor gauge uniformity in the extruded product, by making adjustments in the extrusion die. This method of compensation is, unfortunately, substantially less than ideal because die adjustments are made entirely by feel so that it is difficult for the operator to observe exactly what amount of mechanical change has been made and, since it may take a substantial amount of time for the line to stabilize after a die adjustment, the results of the change are not apparent for some time. Moreover, any die adjustment affects a wide area of the extruded tube, making it impossible to correct narrow errors in gauge uniformity.