We read and hear a lot about “breakthroughs” in our industry: Robots eliminate operators; auto-feeding systems never allow the machine to go dry; snazzy signal processors and transducers monitor every microsecond of the molding process. With all this gadgetry, however, are we seeing more profit and a return on investment for the money spent? Not really, because we have been dazzled by technology and ignored the fundamentals.
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Recently, I got an e-mail from a guy who had just taken over the position of lead technician. He wondered about the use of chillers and their expense. He also wondered about the quality of his products when the setup sheet used "tower water" as the main source of cooling for molds and machines.
The trouble with tower water
Let's hit the simple but often overlooked problem first—tower water. When people first build a molding plant, they decide on the number and size of the molding machines and calculate the power and cooling requirements. What they tend to ignore is what happens when additional machines are purchased, because that is covered in the “safety margins” of the original designs.
Heat exchange is necessary because:
· the machine generates its own heat; and1
· the mold will heat up because it must cool the 300°+ plastic that is injected into it.
Heated machine oil is cooled directly from the tower. The molten plastic's heat first dissipates into the mold steel, is transferred to the cooling circuits and then to the mold's heat exchanger (generically called a Thermolater, although there are other suppliers) and finally to the tower's evaporative cooling circuits.
Evaporative cooling depends on the evaporation of water. This depends on the outside temperature, relative humidity and a host of other variables. It is obvious that when the outside air changes, the temperature of the tower water also will change. As the tower water's temperature changes, your mold temperature will change, and the dimensions and quality of your parts will change.
Another reason to avoid directly putting tower water in your mold is scale buildup. With water flowing through a mold you have the perfect setup for electrolysis, where the minerals in the water will plate out onto the waterlines. Just 1/64 in. (0.4 mm) of scale buildup can reduce the heat-transfer efficiency of a waterline by 60%, even with adequate flow.
Fun facts about heat distortion temperatures
First fun fact: The ideal ejection temperature for any molded part is when it reaches 80% of the material's heat distortion temperature (HDT). Second fun fact: If you check the literature, no thermoplastic resin's HDT is so low that the 80% figure turns out to be room temperature or lower. There are some practical exceptions: Thin-walled elastomers tend to turn themselves inside out during ejection. If dimensions are not sacrificed, if you “over-cool” the part prior to ejection it can be rigid enough to conventionally eject.
These fun facts beg a simple question: If this is correct, why do we need chillers? You use a chiller in an attempt to overcome inadequate cooling in a mold.
Most molds use a Thermolator to maintain mold temperature so that the part can reach 80% of HDT as efficiently as possible. Keep in mind plastic is a poor conductor of heat. The heat from the plastic radiates relatively slowly into the mold steel. The heat-transfer characteristics of the mold steel and the water in the cooling lines are many times faster.
The weak link in this plastic-metal-water heat-transfer system is the water's flow rate. When water flows smoothly like a gentle stream, it flows in layers: This is called laminar flow. The layer that is in contact with something—the walls of the waterline or the bottom of the stream—will flow very slowly. The water at the top of the stream or the center of the waterline only has to slip past itself and flows must faster. With laminar flow, the heat transfers very slowly because it has to heat up this stationary layer before the flowing layers can pick it up and exit the mold.
The laminar flow effect stops when the flow increases. It ceases to flow in layers and begins to tumble over itself. This is called turbulent flow. With the water tumbling over itself in a waterline, it picks up heat directly from the mold steel. Turbulent flow is measured with a very complex dimensionless number called a Reynolds number that uses flow volume, the size of the flow channel, the heat of the water and the viscosity of the water. Instead of going through the calculus, a rule of thumb is 1 gallon per minute (GPM) per circuit will always give you turbulent flow in normal molding situations.