In production of profiles, higher throughputs and reduction in scrap generation are becoming prominent features. However, producing plastic profiles faster often causes warping, especially with complex hollow shapes having varying wall thicknesses and wood/plastic composites or foamed profiles. To prevent warpage in profiles, it is important to cool the profile more evenly - controlling heat transfer so thin sections don�t cool down before thick sections dissipate heat. How heat dissipates through a material is a matter of the materials' thermal conductivity and thickness. As the outer skin of a profile solidifies, it insulates the inner material from the cooling medium. If a wall is thick, it becomes a more severe cooling issue.
The thermal conductivity of a polymer is its density times its ability to conduct heat. The thermal conductivity of PVC is 0.051, PP is 0.075, and HDPE is 0.16, where the higher number means slower heat transfer. Thus PE profiles require 20% to 30% longer tanks than PVC because of PE�s slower cooling. Polymers and fillers also play an important role.
Cooling methods include air racks, water tanks, and spray-cooling tanks. Air-cooling is the simplest and slowest method of heat transfer. Air has only 1-10% of the cooling capacity of water, so it�s less likely to cool unevenly and cause profiles to warp, twist, and bow. Typical throughput rates with air-cooling are below 250 lb/hr for materials like polycarbonate, rigid PVC and PS.

An air rack is typically 10-20 feet long with templates or form guides to support the profile while it cools. Clamps, bent wires, vise grips and other devices can also help to support and guide the profile. In the first 10 ft, nozzles are typically used to blow air at specific sections of the profile, while the last 10 feet use more generalized air-cooling with blowers above and below the profile.
Cooling the profile by immersing and pulling it through a water tank is faster but creates a zone of heated water around the profile, which inhibits heat transfer. The solution is to break up this insulating layer of hot water by agitation. In most cases, water-distribution bars can be installed in the corners of the tank. If this causes a warpage problem, it sometimes helps to raise the overall water temperature and slow the rate of heat transfer. This requires a water temperature-control system, usually consisting of a water pump, filter, heat exchanger, and valving to control chilled water input.
Current profile extrusion practice typically involves dry calibration tables, which can be 30 ft or more long, in front of a water immersion tank with form guides under low vacuum. In some cases, these tanks can be more than 100 ft long. Longer tanks minimize the potential for distortion. However, equipment and floor space costs are high, and even long tanks may not speed production enough. As extrusion rates increase and processors run out of floor space, they have been forced to look at more efficient heat transfer with spray-cooling tanks. The beauty of spray cooling is its heat-transfer efficiency. Nearly microscopic water droplets from spray nozzles come in contact with an extruded profile that is at more than 100 degree C or hotter. Evaporative spray cooling works best when the spray is directed onto the extrudate as soon as it enters the tank, when the plastic is hottest, within the first inch or so after the material leaves the die. If dry calibration tooling is installed, there is essentially no evaporative effect. Once the surface temperature of the extrudate drops below 100 degree C, spray droplets no longer flash off to steam. But even at lower temperature, spray cooling still has 10-20% better cooling potential than immersion cooling because of the large surface area of droplets and the turbulence of spray.
Immersion cooling removes heat faster than air, and spray cooling removes heat even faster, which makes it the most likely cooling method to cause complex profiles to warp. For some solid or irregular profiles, the answer may actually be to slow the heat transfer.
For instance, when rigid PVC profiles are cooled with high-turbulence spray tanks and properly sized water-circulation systems, chilled water in the range of 10-20 degree C has historically been used throughout. However, it may be better to temper the water in the initial tank to 40-80 degree C. This pulls heat out more slowly so the profile skin doesn�t freeze off immediately. A form-guide system can also be mounted in the spray tank to direct spray onto a thicker side to optimize cooling. Then, as the profile proceeds downstream, you can progressively drop the water temperature to complete the cooling.

Calibration tooling can be designed with independent water-temperature control. A process that typically requires three 8- to 12-in.-long dry-calibration tools can easily optimize temperature in each section, as is typical in foamed profile extrusion. This level of temperature control also allows the process to be highly repeatable the next time the run is set up.
It is also possible to create independently controlled temperature zones within a calibration tool�such as top, bottom, and even side zones�to enhance heat transfer from the most difficult profile shapes.