Calibration Table & Advanced Cooling

The calibration table can be moved up or down, or from side to side, to align the calibrator with the extrusion die and the downstream equipment. A good calibration table is required with high-tolerance parts to guarantee that the profile meets the final dimension specifications. The process should not be speed limited by the calibration unit length or its ability to hold dimensions. Although the calibration unit is expensive in comparison to the extruder, die, and raw materials, it is a relatively inexpensive part of the production line. Profile rate limitation by either cooling capacity or the ability to hold dimensions established by the calibrator is a problem that is normally easily corrected. At high rates, the calibrator in complex profiles may be 5 ft long, or multiple calibrators with different cooling along the way may be required to produce a suitable profile at high throughput rates in good yield. Calibrators may operate at slightly lower throughput rates compared to either sizing units or air cooling because of the higher friction between the calibrator surface and the moving profile. This is more than compensated for with improved dimensional precision.

Some company has introduced supercold nitrogen gas to cool parts more rapidly in less space than water does. Since cooling is quicker, line speeds can be increased and calibrators and cooling tanks shortened. Liquid nitrogen is converted to a chilled gas, and it is circulated around the profile at rates up to 2000 cfm. The liquid nitrogen itself never comes into contact with the hot extruded profile, preventing thermal shock from altering the material performance. Liquid nitrogen is converted to gas when it enters the cooling chamber, expanding rapidly to replace any room air. The chilled nitrogen gas is circulated with a high-velocity blower around the extrudate, producing turbulent air flow, aiding the heat transfer from the extruded profile. Temperatures in the cooling zones can be controlled between −300 and 75 °F (−184 to 24 °C). Multiple cooling zones can be used during calibration and cooling to optimize the process efficiency. Nitrogen cooling has been successfully used to cool window, fence, and tubing profiles in less space at higher throughput rates.

Since all thermoplastics shrink on cooling, the die has to be properly cut and the calibrator properly sized to allow for shrinkage as the product cools to room temperature. Additional shrinkage occurs with cooling and postcrystallization if the profile is not completely cooled down to room temperature before it is cut. Shrinkage is balanced with the drawdown to ensure the correct dimensions in the final part.