Canadian Plastics

Cooling Considerations

P lastics is a simple business...add heat, squeeze into your desired shape and take heat away. One of the consequences of the Laws of Thermodynamics (which, if you're lucky, you will never have to kno...

July 1, 2008   By Jim Anderton, Technical Editor



Plastics is a simple business…add heat, squeeze into your desired shape and take heat away. One of the consequences of the Laws of Thermodynamics (which, if you’re lucky, you will never have to know) is that putting heat into things is an easy task. Electrical resistance adds heat, as does friction between resin and barrel/screw as well as the heat rise from the compression of the melt in the barrel.

Ever rub your hands together on a hot day to cool them off? That’s the dark side of thermodynamics: as easy as it is to put heat in, it’s a little harder to take heat out…in fact in engineering terms, it’s impossible.

You can, however, move it from place to place, which is the job of chillers. For one of the more controllable variables in the molding process, it’s a little underappreciated, although there are ways to create needless cost and frustration for the careless or unaware. Inconsistent cooling or inadequate mold cooling is, surprisingly to many, rarely caused by a breakdown in the chiller itself.

Chillers can fail, but they’re simple machines at heart and as most suspect, they’re related to process cooling as well as food refrigeration equipment. So how come your home fridge never needs maintenance, lasts 25 years and only costs $600? Because it’s similar, but different. The main difference is that the refrigerant loop cools a working fluid, which in turn cools the mold. The obvious question is, where do you look when the mold won’t cool properly? I start simply…with my eyeballs. How much fluid leakage is acceptable? None, yet I’ve seen bonus-hungry operators tie a rag around a bleeding manifold and keep going rather than call me for a repair.

The fluid loop must have adequate volume to transfer heat properly. The second common mistake I’ve seen involves portable chillers. Remember where I said that you don’t remove heat, you move it? Where it’s removed is a radiator-like component called a condenser. Push the chiller up against a wall, stack cartons against it or block the air flow through the unit and you will have poor cooling performance. Similarly, the tiny fins of the condenser are easy to clog with dirt and grease, also blocking cooling air. The solution is simple: clean the condenser. In a perfect world, I’d flush with a garden hose from behind, but there is enough electronics in the cabinet these days to make this a really bad idea. Compressed air (from the inside out!) and spray solvents designed for electrical work are good, as is a small stiff bristle brush. Clean is good, but so is straight. The tiny fins aren’t there for show; each one is a tiny radiator, rejecting heat to the ambient air. Smash the fins flat and they can’t get the flow of air needed to surround every square millimetre of fin surface area.

This is more common than most molders realize and it gets worse if the millwright or mechanic fails to replace the guard after service or cleaning. If your fins are beaten up, there’s a simple tech tip below to get them working again. Chiller manufacturers have made the guts of their units so robust that many jobs shops treat their portable chillers like the forklift…out of sight, out of mind, but see what happens when you don’t have one available! CPL


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