Water Cooling and Underwater Vacuum Calibration

Water cooling begins when the profile enters a bath below the water level and enters a fixture to size and hold the profile during cooling. The profile may pass through a ring spraying water on all sides simultaneously or water nozzles prior to entering the water tank. Water cooling is very efficient, is good for short runs, and may suffer in repeatability between short runs, depending on the fixture quality, its location, and the operator skill. Options available are the tank length; the number of chambers in the tank; whether the water is applied in the form of a mist or spray, flood, or a combination of both in different parts of the chamber; temperature of the water (hot or cold); self-contained recirculation systems; and heat exchanger.

A more sophisticated cooling system is vacuum sizing or calibration under water, where a hollow profile, whether round or square, exits the die and enters a sizing or calibration unit in an underwater vacuum tank. The water acts as a lubricant in the calibrator. Lower vacuum pressure enables the air in the hollow profile to expand uniformly outward until it comes into contact with the sizing sleeve or calibrator to produce the desired external dimension. Vacuum sizing units may have several compartments where the vacuum level can be adjusted, and the water can either surround the profile or be sprayed on it at different temperatures. Normally, the highest vacuum is in the first compartment, where the polymer is being expanded to the maximum amount as the surface skin forms. Initially, the profile will be larger than the final dimensional requirements to accommodate the polymer shrinkage as the polymer cools completely to room temperature. Figure 49.9 shows a vacuum sizing tank produced by Boston Matthews [3]. All tanks are fitted with lifters and adjustments allowing the tank to be moved horizontally toward or away from the profile as it exits the die. The profile enters the tank passing down through the various compartments and exiting through the opposite end after it has been solidified in the correct cross-section. Options available on most sizing tanks include heat exchanger, full spray cooling compartments, water circulation system, multiple compartments, vacuum pumps, additional vacuum or water manifolds to attach external calibration units for initial sizing, additional vacuum and water ports that can be connected to calibrators inside the vacuum tank, and uniform water or spray temperature.