Taking the heat out

One of the unique aspects of working with thermoplastic resins is that, in a few feet of linear space, we consume a great deal of energy to pump heat into our raw material, then use a great deal more to take it out at the mold. Adding heat is intuitively easier than taking it out, so I’ll limit the discussion to cooling processes.

Water, or water-glycol-additive solutions, have been used for so long in mold cooling applications that no one I knew on the floor could ever remember why. Consulting my old “Heat and Mass” textbook, I rediscovered some interesting facts about removing heat from metals. One is that heat transfer occurs by conduction, convection and radiation if your hot mass is a liquid or a gas, but only by conduction and radiation for solids. That disadvantage, however, isn’t so much of a problem if the solid is a metal (i.e. a mold), because metals are intrinsically better at conducting heat than most other materials.

That led me to question why, then, don’t we equip molds with fins like a motorcycle engine, and blow air over them? Air is much better than liquids as a convective heat transfer fluid over a finned radiator, which is why your car radiator has the fins out in the air, rather than pointing in towards the anti-freeze. If it works in my Chevy, why not in a mold? The simple answer is that air moving around fins is a one-trick pony. Fluids, when flowing through the cooling circuit of a hot mold, pick up heat both by conduction across the mold’s steel-fluid interface, and by convection in the coolant itself. And compared to gases like air, fluids are typically between two and 1000 times better at convection heat transfer.

What’s the perfect mold coolant? Probably a liquid metal like gallium or mercury. Several nuclear reactor designs use liquid sodium, but like those reactors, a plastics application with liquid metal coolants would probably turn a leak or mold change into an instant toxic waste situation. Leaking mercury makes spilled hydraulic oil look like Perrier on a patio.

Turning water into vapor (steam) is a very effective way to move heat around, so why don’t we allow molds to boil the cooling fluid, then collect the vapor, condense it back to the liquid state, and recirculate it back to the mold? The only answer I can see is that cooling solvents such as R-12 and ammonia do it better–mold coolants are nothing but heat pipes for external chilling systems. If you can make evaporative cooling work, give me a call.

The astute reader may have noticed that I haven’t mentioned anything about radiation as a means of cooling molds. It turns out that heat transfer by radiation is proportional to the fourth power of the surface temperature, so doubling the temperature of the radiator increases the heat transfer rate by a mere 16 times. Only at temperatures that would melt P20 steel does heat transfer by radiation become significant. On the bright side, however, it works really well for the sun.

Jim can be reached at janderton@corporate.southam.ca