Many methods and apparatus for manufacturing thermoplastic film are known in the art.
A commercial apparatus, commonly referred to as a spider die, requires an outer tubular body within which is mounted an inner cylindrical mandrel. The mandrel is fixed relative to the body by a plurality of supports or spiders. In use, a molten thermoplastic material is fed through the annular space between the body and the mandrel and is changed in form from a cylindrical rod to a tube or film of the desired diameter. The film produced by the above process and apparatus, however, exhibits spider marks or irregularities thereon and therein due to the flow interruptions caused by the spiders. This is especially true when polystyrene foam is the thermoplastic material. Another problem associated with the spider dies is that since high internal pressure is required for the extrusion of the highly viscous heat plasticized thermoplastic material, the mandrel in the die is subject to movement, vibration and distortion causing undesirable nonuniformity in the thermoplastic material thickness. More particularly, as the flow rate or viscosity of the thermoplastic material varies, the mandrel can be tilted or moved horizontally and/or vertically. Yet another problem associated with spider dies is that the supports or spiders interrupt the flow of the thermoplastic film not only in proximity to where the thermoplastic material passes the spiders, but also throughout the film due to a "rippling affect" caused by the thermoplastic material bouncing off the spiders. In order to maintain uniformity of thickness and acceptable profiles, it is essential that frequent die adjustments be made and very precise process conditions be maintained, both of which are difficult feats to accomplish in practice.
In an attempt to solve the so-called "spider" or "weld" line problem, the above described spider die was modified with a "scooper design." The scooper design requires a slightly wider passageway between the body and the mandrel in precise axial alignment with and downstream of the spiders, which is accomplished by an indentation along the mandrel. The purpose of the scooper is to fill in the portion of the thermoplastic material that is displaced, diverted or obstructed by the spider section. However, many thermoplastic materials including polystyrene have a structural memory, which does not allow the material to be remended or reknit in this way.
An apparatus eliminating spiders has been used which requires an outer annular body within which is mounted an inner cylindrical mandrel fixed relative to the body by an L-shaped annular support flange in alignment with the mandrel. The mandrel requires a series of openings perpendicular to the mandrel and body, around the L-shaped support flange. The series of openings allows the plastic to pass through the annular space between the body and the mandrel. The series of openings are perpendicular to the flow passageway between the body and mandrel, thereby requiring added pressure therethrough. Further, the machining of the modified mandrel incorporating the L-shaped support flange with a series of openings is extremely difficult, costly and time-consuming.
It is therefore desirable to provide an improved extrusion die apparatus and process for producing tubular film from thermoplastic molten material which overcomes most, if not all, of the aforementioned problems.
An object of the present invention is to provide a film of thermoplastic material such as polystyrene, which has excellent characteristics, including superior surface appearance and smoothness.
Another object of the instant invention is to provide a tubular film of thermoplastic material which has a uniform thickness and profile despite velocity, throughput or output rates, viscosity, temperature, and pressure interruptions or fluctuations in the flow of the thermoplastic material through the extrusion die apparatus.
Another object of the instant invention is to produce a thermoplastic material exhibiting satisfactory gauge while eliminating weld or spider lines in the material.