This invention relates to apparatus for melting thermoplastic material and for delivering the melt under pressure to a forming operation.
Historically, solid thermoplastic material, usually in the form of pellets, has been transformed into a molten or otherwise flowable and homogeneous state for further reforming into sheets or film by use of a machine known as a screw extruder. A screw extruder will generally include a hollow barrel or tube having a screw-like member revolving inside the barrel or tube. Pellets and other solid matter are fed into one end of the barrel and are then carried spirally along the barrel by rotation of the screw member in the barrel. As the screw member rotates the pellets and additives are carried along inside the barrel and subjected to increasing pressure due to an increased tapering of the screw base and decreasing clearance between the screw base and the inside wall of the barrel. Due to the increasing pressure and the heat produced by the mechanical work of the screw, the pellets are reduced to a molten or semi-molten form and intermingled with any additives added with the solid pellets. After the pellets are reduced to a molten or semi-molten form, they are expelled out of a suitable aperture or diehead in the opposite end of the barrel. At the diehead the thermoplastic melt is discharged from the extruder and reformed and processed into finished film or sheet stock. This finished film or sheet stock is formed at the discharge end of the die by one or more pairs of opposing rollers which receive the extrudate and compress the extrudate between the rollers to impart the sheet-like configuration sought to be achieved.
Another method of achieving sheet material from the extruder is through the use of a sheet die at the discharge end of the extruder. A sheet die comprises a broad slot-like aperture through which the thermoplastic melt discharging from the extruder is forced. A problem arises, however, in that a certain portion of the extrudate issuing from the extruder must travel in a radial direction to the end of the slot die while melt issuing from the center portion of the die aperture will immediately be pushed through. The resulting travel differences create temperature differentials and stress differentials on the plastic material, degrading the quality of sheet material produced thereby.
A more detailed history of the problems associated with the screw extruder and a solution to some of these problems was disclosed in U.S. Pat. No. 3,880,564 by Beck, et al, incorporated herein by reference, wherein a roller was rotably mounted within a recessed area of a housing forming a substantially cylindrical space. The inner recess of the housing was eccentrically shaped such that by positioning the roller inside the recess a number of arc-like gaps, sometimes referred to as accumulator gaps, were created between the roller and the inner wall of the recess area. The plastic pellets were therefore deposited directly on the roller and thereafter effectively liquified as they were conducted through the first arc-like gap, sometimes referred to as the feed section, and about the roller approximately 180.degree. to the second arc-like gap, sometimes referred to as the compression section, which terminated at the die port. The thermoplastic melt was fed uniformly into the diehead along the length of the roller resulting in a sheet stock having a minimum of internal stresses. The uniform feed resulted because the roller was subject to equal and opposite pressures at the opposite gap locations and thus the roller revolved with little deformation along its length. These opposite pressures were created when quantities of plastic material collected in the opposite gaps creating equal but opposite balancing forces.
In spite of the novelty of concept of the rotary extruder, numerous problems have resulted in its operation. Due to the large internal pressures generated inside the rotary extruder by extrusion through certain types of dies, a significant leakage of thermoplastic melt resulted along the rotor shaft around the housing end plates. The installation of end seals although reducing some of the leakage, was not sufficient to overcome the leakage due to the high pressures generated within the apparatus. Due to the inability of the end seals to control the leakage, the loss of thermoplastic melt has been accepted as a operating cost of doing business. A rotor utilizing a herringbone grooved pattern at opposite ends of the rotor to help retain the melt inside the rotor housing was disclosed in the May 1979 issue of Plastics World magazine. The herringbone grooves were unsuccessful in retaining the thermoplastic material inside the housing however because they failed to re-deposit the thermoplastic material into the low pressure area of the extruder. Additional problems have arisen in the feeding of the solid thermoplastic material into the extruder. Historically, a flood feeding method was used, whereby the extruder was gravity fed from a filled hopper and as the rotor displaced the feed inside the feed section of the housing, it was immediately replaced from the hopper by an equivalent quantity of feed. The flood feeding method, although relatively successful in a screw-type extruder, has caused heat penetration of feed stocks prior to being drawn into the extruder, uneven feedage, and even blockage of the rotary extruder due to partial melting of the feed causing it to stick in the feed chute.
Further problems have resulted in attempting to adjust the accumulator gaps to accomodate the various types of feeds encountered. As is known in the art, different types of feeds, for example polystyrene or ABS, require different amounts of clearance between the body of the screw and the inside surface of the housing in both the feed and compression sections respectively in order to accomodate different polymers having different melt characteristics. In some rotary extruders, the size and shape of these arc-like gaps between the rotor and the housing are changed by repositioning the housing relative to the rotor. Because the housing is often made up of two or more sections, adjusting these gaps by changing the position of the housing has been unsuccessful because the housing sections are subsequently forced out of position due to the high internal pressures generated in the extruder. In addition, adjusting the individual housing sections was complicated, time consuming, and imprecise.
In addition, when using certain high pressure dies, such as a strand die, the discharge pressure from the rotary extruder was not sufficiently high to extrude through the die thereby severely limiting the usefulness of the rotary extruder.
Additional problems have been caused by the transfer of heat from the rotary extruder to the thermoplastic feed held in the feed chute. It has been found that this preheating of the thermoplastic feed results in its partial melting and agglomeration thereby blocking the feed opening to the extruder preventing a smooth delivery of feed to the rotor.
The present invention, however, has overcome the deficiencies associated with other rotary extruders to provide an apparatus which produces high quality stock without the inefficiencies and limitations of other apparatuses.