Calibration

Calibraition

As the melt exits the die, it swells slightly. Therefore, it requires a size adjustment, namely calibration. 

The outside diameter of a pipe or tube may be calibrated either by an internal air pressure of 1.3–2.0 

bar (19–29 psi), or by external calibration in a vacuum sizing tank (Fig. 4.46) operating at a pressure 

of 0.3–0.5 bar (4.5–7 psi). For calibration by internal pressure, the diameter of the die ring is made 

approximately equal to the internal diameter of the finished pipe. For vacuum tank calibration, the 

die ring is made up to 25% larger, and the haul-off is set for a wall thickness drawdown of up to 30%.

Figure 4.46. Vacuum sizing tank used for pipe and tube extrusion.