How Much Heat Do You Need?

Plastics processing is incredibly diverse — injection molding, extrusion, rotational and blow molding, vacuforming … the list goes on and on. The common thread to all, however, is heat. We add heat to “melt” resin, and take it out to yield usable parts or compounded pellets.

Last month I scratched the surface of the primary technology that processors use to measure temperature: the thermocouple. What thermocouples do is display temperature. What they don’t do is display heat. What’s the difference?

To a scientist, temperature is a measure of an average of molecular energies in a closed system, like a pound of resin. The same scientist might think of heat as the total amount of energy stored in that same mass. You can use a huge amount of energy to raise the temperature of some materials by just a few degrees, as anyone who owns a swimming pool will attest. On the other hand, you can raise the temperature of the head of a pin to hundreds of degrees with the heat of a match in a couple of seconds. The simple math is:

Heat in = mass of resin x specific heat of the resin x the desired change in resin temperature

“Specific heat” is just a number that’s unique to each material. Engineers look it up in standard handbooks and plug it into the equation. Why do we need to know? The important part is that the amount of heat you add depends on only two variables: the resin mass, and the temperature difference between ambient and the desired melt temperature.

And it gets better. The waste energy of the screw’s motion in propelling and mixing the melt is eliminated mostly as heat, which is what we want in the barrel anyway, so energy efficiency is high. Similarly, band heaters, like all electrical resistance heating, convert almost all the power input into usable heat.

So why does it take so much energy to run your equipment? Two reasons: The first is that nothing’s perfect when you’re converting from one energy form to the other. In a conventional extruder or I/M press, electricity pumps hydraulic fluid which spins the screw, losing a fraction of the input power with each conversion. All-electric machines cut out the hydraulic middleman for some savings in energy.

The second reason is that as fast as energy is put into the system, i.e. the melt stream, it’s leaking out through conduction through the barrel walls and runners and radiation into the atmosphere.

On paper, radiation is the scary one because it’s proportional to the fourth power of temperature difference. Double the temperature difference and the amount of heat lost to radiation goes up by eight times! Fortunately, plastics processes operate at temperatures low enough that radiation isn’t a major problem, but conduction certainly is, especially since the surfaces that the resin contacts in its journey from throat-to-part are heat-conducting metals. Insulation helps, but it also increases the surface area exposed to the surroundings, encouraging both radiation and convection heat loss.

We commonly add heat all the way to the mold gates, where we frantically draw it out using chillers, as well as any ambient air conditioning needed on the plant floor for part consistency and worker comfort. It’s a mess, isn’t it?

In theory, we’d want a pure radiative heating source, like a big microwave oven tuned to a frequency that resonates the resin in use, and a very short L/D. A ceramic insulating sphere with a paddle mixer would be close to ideal, with a chiller that recycles the heat extracted from the mold and uses it to preheat the resin at the throat.

There are many reasons why we can’t do it this way, and as a result, energy is a major cost in resin processing. But don’t expect your customers to be sympathetic!