Profile Cooling & Calibration Process

Cooling and sizing or calibration is used to set the part dimensions while preventing the part from warping and twisting. Part fixtures or sizing depends on the part complexity. The simplest setup has adjustable metal fingers shaping the soft polymer as it exits the die, with fans blowing air on the profile to cool it. The profile may be supported by wood forms or by a metal rack. Although this is the simplest way to cool a profile, it is also the most inaccurate, producing profiles with very low dimensional tolerances. For thermoplastics generally, more intricate sizing fixtures or calibrators are used with complex shapes in calibration tanks. The sizing unit may be a long metal fixture containing the desired profile shape where the molten polymer is pulled through and allowed to solidify, possibly in a water environment. As the part exits the die, it passes through a small air space and into the sizing unit or calibrator where it is cooled with air or water spray, flooded with water, or goes through a metal calibrator with circulating water and a heat exchanger to freeze or solidify the part to its final dimensions.

Air cooling can be used for simple shapes where tolerances are relatively unimportant. Metal fingers and jigs are used to bend or shape the profile to the desired configuration. One advantage of air cooling is that it is relatively slow; consequently, there is sufficient time to alter the profile with jigs to freeze it in the desired configuration. This is a quick approach to use with small simple profiles that do not have critical dimensions, such as thin flat stock, vertical blinds, U- or T-shaped profile. The most important criteria in these applications are a uniform melt temperature, good melt strength, and uniform cooling around the entire part. Air is directed onto the profile with adjustable position nozzles mounted at the beginning of the cooling section as the profile exits the die. Uniform cooling is required to eliminate warpage and twisting associated with differential shrinkage. Assuming the flow from the die is uniform and the profile is not being dragged across a surface in the cooling operation, profile bowing, or twisting is frequently caused by nonuniform cooling. The profile will bend or cup toward the hot surface or the side that cooled last. This fact helps identify the causes and the proper corrective actions to take in modifying the cooling operation.