The cut lengths are collected on a dumping table or collection chute. The entire profile extrusion system plus the material's viscosity, extruder output, extrudate temperature, die temperature, calibration unit control, speed of the take up unit, and cutter must work collaboratively to produce a consistent product with uniform properties. In many cases, production set-ups require three to five hours to bring all the system's speed and temperature on line. Tubing Dies Tubing can be a round drink straw, pipe, or square shaped. A typical tubing extrusion line is illustrated in Figure 5-31.
Small tubing requires careful adjustment for concentricity.
Concentricity occurs when both the outside and inside radius of the tube share the same center point. Variation in wall thickness can be corrected by adjusting die centering bolts. A special tubing die, with spider plate adjustment, is illustrated in Figure 5-33. The flow of material in this die is around a streamline torpedo with diverging and then converging flow prior to the die's exit. The increased flow pattern reduces the stored melt elasticity which decreases the probability of melt fracture.