1. Field of the Invention
The present invention relates to an extruder screw of the metering screw type whereby material is advanced through only a feed zone, conic transition zone, and metering zone. Particularly, the present invention is directed to the extrusion of thermally sensitive plastic materials, and rigid plastic materials in a manner obtaining lower temperatures at higher output rates.
2. Description of Related Art
As is known, for example from U.S. Pat. No. 4,243,629, polyvinyl chloride (PVC) is very sensitive thermally above 100.degree. C. (212.degree. F.), making extrusion possible only by adding of stabilizers and plasticizers, which may negatively influence the quality of the product Furthermore, despite the use of stabilizers and plasticizers, local overheatings can occur.
Overheating that produces thermal decomposition of the PVC results in separation of hydrochloric acid as well as discolorization and ultimately carbonization. Still further, the damaged mass can become attached to the surface of the metal of the extruder. The net result is that the extruder must be shut down and disassembled. Even if the screw has not become damaged, all parts of it and the extruder must be carefully cleaned. After cleaning and/or repair, the extruder must then be reassembled and the overall process results in a considerable loss of time and production capacity, as well as the direct and indirect costs associated therewith. In order to avoid these problems, U.S. Pat. No. 4,243,629 discloses a complicated extruder screw construction which creates seven different zones and utilizes brake flow rings with orifices therethrough.
Another problem associated with the extrusion of thermoplastic materials, such as polyethylene and PVC, is obtaining a high degree of thermal uniformity and mixing. Illustrated and described in U.S. Pat. No. 3,762,693 as prior art is a "slotted ring" which is formed on an extruder screw by a circular array of 36 mixing pins projecting radially from the root diameter of the screw about the entire circumference thereof. In such an arrangement, the height of the pins is less than the depth of the flight, and the flight is interrupted to permit the ring of pins to be continuous. However, this design is described as being subject to various disadvantages due to the presence of the broken flight, especially when using PVC as the extrudate.
Thus, for purposes of overcoming the deficiencies of the described prior art design, U.S. Pat. No. 3,762,693 proposes a screw design for a screw of the compression relief type (having successive feed, compression, relief, and metering zones), wherein the helical flight is uninterrupted and the array of mixing pins is interrupted, instead, at the point at which the flight intersects the now only substantially annular array of 32 pins that are uniformly spaced through approximately 310.degree. of arc. Four such arrays of pins are used in the metering zone of the disclosed compression relief screw, one in proximity to each end of the metering zone, and two additional arrays uniformly spaced therebetween; although it is indicated that the number of pins, their location, diameter and spacing may vary according to a particular application of the extruder, the melt temperature, type of plastic shape extruded, type of materials, diameter of the screw, etc.
This continuous flight, interrupted array of mixing pins concept has been put into commercial use by others as well. For example, such have been utilized in the metering zone on 80 mm diameter metering screws produced by the Battenfeld-Fischer Blasformtechnik GmbH of Germany, and the 90 mm extrusion screws of the Germany company, Beckum Machinenfabriken GmbH. For example, a 90 mm screw produced by Beckum utilizes 24 pins at a 10.degree. diameter-to-diameter spacing.
Another approach which has been taken to obtaining a more intense mixing effect is described for the compression relief type of extruder screw shown in U.S. Pat. No. 4,367,190, wherein a mixing means is formed of three or four spaced-apart mixing elements, each of which comprises two or three closely spaced rings of pins. The pins of each row are in a staggered relationship with respect to those of the adjacent ring, in order to provide a tortuous path for polymer passing the mixing means.
However, it should be recognized that numerous different types of extruder screws exist in addition to the metering screw and compression relief types of screws described. Furthermore, it should be appreciated that the feature of any screw, such as the lengths of the various zones, screw flight depths, compression ratio, etc. cannot be viewed in isolation because each aspect has an effect on every other, thereby producing very different results in various combinations with each other, as well as with different materials or on different types and sizes of screws. Thus, despite the above-noted points, to the inventors' knowledge, up to this point, an extruder of the single flight metering type having an initial feed zone in which the screw flight has a constant depth, a distal metering zone in which the screw flight has a constant depth that is less than that of the feed zone, and a transition (compression) zone interconnecting the feed zone with the metering zone, and in which the depth of the screw flight progressively decreases from that in the feed zone to that in the metering zone, has not been achieved which will be truly effective for the extrusion of temperature sensitive thermoplastic materials, particularly for screws having an L/D ratio of the total length of said zones to the outer diameter of the screw in a range of 20-30:1, and particularly for use with rigid PVC or polyethylene.