1. Field of Invention
The invention relates to extrusion apparatus and more particularly to extrusion heads through which extrudate may be supplied with constant shear history and under constant pressure to all points in the entrance to a die.
2. Prior Art
It has long been recognized in the extruding art that it is necessary to supply to all points behind a rectangular die extrudate having the same shear history and the same pressure if the extruded product is to have the same shape as the die. In conventional designs the extruder will feed a rod of material through an extrusion head in which the material is squeezed and spread out into, for instance a flat sheet which is much wider than the diameter of the rod. In this case, the material which reaches the die entrance varies in pressure from the center of the die to its extremities. This pressure variation results from a variation in pressure drop experienced due to differences in the distance of flow to each die entry point. Accordingly, to equalize the extrudate pressure at all points in the die entry region restrictions are introduced into the extruder cavity as required. Equal pressure may thereby be approximated in the extrudate in the die entry region by this complex and somewhat trial and error procedure, but to achieve this non-equal shear history is involved. In the extreme, this may cause problems due to excessive heat generation in certain portions of the extrudate. In general, it introduces variations in the degree of swelling experienced as the extrudate leaves the die. Non-uniform die swell does not permit the extrudate to retain the uniform sheet thickness produced by the die.
Optimally, the flow of extrudate should be guided within the extrusion head so that it is non-divergent. All elements of extrudate will then experience the same total flow path, the same shear history and the same pressure drop as they pass through the head. Those requirements are uniquely provided almost precisely in extruder heads designed to produce wide, flat sheets by the teachings of this invention.
In U.S. Pat. No. 3,384,925 there is disclosed a sheet extruding head for extruding thermoplastic synthetic resins which attempts to completely equalize the flow of extruded material in the extruding head. The extruding head is designed specifically with an infinite number of flow lines, each flow line of equal length, so that each particle in the extrusion process traveling along its own flow line will travel the same distance as every other particle from the point of entering the extrusion head to the point of exiting the extrusion head. This is accomplished essentially through an arched equalizing zone between the entry and exit points. This design however does not permit symmetrical flow and does not provide a constant flow path through the head. The design disclosed therein includes a fan shaped entry region in which the flow of material is divergent. In addition the design requires rather sharp changes in the direction of flow at the entrance to and the exit from the arched equalizer zone, especially at its central area, which will affect the extrudate.
The extrusion heads which have heretofore been designed along the above principles, do not meet the basic requirements for permitting total symmetrical flow and a constant flow path from the end of an extruder screw chamber through the extrusion head to a die.
It it towards elimination of these and other problems in the prior art that the present invention is directed.