Circular extrusion dies with inner and outer lips are usually adjusted for concentricity by a circle of bolts located radially around the outer die lip. Concentricity adjustment is necessary to achieve a consistent transverse thickness of the material leaving the die. In most cases, the adjusting bolts move the complete outer lip to obtain concentricity with the inner lip. A compromise is needed due to temperature and machining variations around the two lips. Where a higher degree of control is desired, a flexible outer lip may be used. The adjusting bolts push on segmented shoes which flex a small sector of the outer lip. This forces the lip into the desired shape to achieve uniform thickness of material. Heated expansion bolts are a further sophistication allowing a finer, remote controlled adjustment. Flexible lip dies suffer from a number of deficiencies, such as undesired distortion of areas adjacent to the sector being flexed by an adjusting bolt, requirement for large forces to flex the lips making repeatability and fine adjustment difficult, necessity for construction of a very thin flex lip which is then susceptible to machining distortion and damage, and complications in heat transfer to and from the outer die lip due to thin construction and intimate contact with the segmented shoes around the periphery.
Hahn, et al., U.S. Pat. No. 4,548,570, describes an extrusion apparatus for producing thermoplastic pipe. Adjustment of the thickness of the pipe is achieved by varying the heat to heater cartridges which causes thermoplastic material in proximity to the heated section of sleeve to become less viscous and to flow at a faster rate. This increases the amount of thermoplastic material at that location, compared with lesser heated sections of the extrusion passageway. Additionally, radial repositioning of the sleeve due to expansion of the heater bolts will increase the width of the arcuate portion of the melt passageway in proximity to the heated section of the sleeve, causing increased thickness of the thermoplastic material. This heating operation does not appear to take place at the die lips. Phipps, U.S. Pat. No. 4,201,534 describes a foam extrusion die assembly which is adjusted by means of screws in the area of the die lips. Yazaki, et al., U.S. Pat. No. 4,057,385, describes cooling a deckle so as to increase the viscosity of the molten resin around the deckle, to prevent leakage. Solop, U.S. Pat. No. 3,984,508, also describes a method of controlling an extrudate using a deckle bar having heating and cooling means. Porter, et al., U.S. Pat. No. 3,973,890, describes adjusting relative positions of the die plates to maintain a constant pressure of heat plastified material. The temperature of the pellet cutting head can be heated or cooled by circulating fluids.