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temperature rise from where the heat-transfer fuid enters the mold to the point where it exits the mold. If the temperature delta is less than that specifed by the mold maker (or less than experience would suggest it should be), this may signal that something has happened to limit fuid fow. Some TCUs are equipped with a fow meter showing gallons or liters per minute, or one can be plumbed into the system temporarily or permanently. If you don't have a fow meter, you can still check fow rate by running the process fuid into a pail and timing how long it takes to fll. Divide 60 by the number of seconds it took to fll the pail and multiply by the volume of the pail and you'll have a good approximation of your fow rate in gal/minute. •Do you have turbulent áow? Remember that the fuid fow rate, in combination with the length and diameter of the cooling channels and a few other variables will determine if the coolant fow is laminar—fowing mostly in neat straight paths with a smooth, consistent motion—or turbulent. Turbulent fow is characterized by random eddies, vortices, and other fow instabilities that ensure the maximum amount of water comes in contact with the walls of the cooling channels, enhancing heat transfer between the fuid and the surface. Anything that reduces fow rate potentially limits turbulence and will dramatically reduce cooling effciency. If you have reason to believe that the fow rate is inadequate to achieve turbulence, you have a couple of options. If you want to use a portable chiller, it is best to use one that has dual pumps to ensure consistently higher fow to the mold. One pump is dedicated to recirculating fuid through the chiller evaporator, while the other provides the increased process fow to the mold. Alternatively, or if you are using a central chiller, a TCU can be used not only to fne-tune mold temperature, but also to act as a booster pump, ensuring adequate pressure and volume to create turbulent fow regardless of the load on the overall central chilling system. •Is the TCU properly sized? Temperature-control units, as their name implies, have both heating and cooling capabilities to maintain a preset mold temperature over time. Early in a run, before the mold has absorbed heat from the polymer, the TCU circulates hot fuid to bring it up to temperature. Later, it can heat or cool the circulating fuid as necessary to maintain the correct temperature. Thus, TCUs are generally specifed for their pump size (in hp or kW), their heater capacity (kW) and their cooling-valve size. Pump size dictates overall fow rates and pressures, while heater capacity will determine tool heat-up time and the maximum temperature that is attainable at a given fow rate. The cooling valve allows chiller or cooling-tower water into the system to keep mold temperature down. If your TCU is not properly sized—if, for instance, it is normally used with another mold with different heat-transfer requirements, its heating or cooling capabilities may be inadequate. SUDDEN APPEARANCE OF COOLING PROBLEMS Now assume that you started up your molding machine, established heat-transfer conditions that resulted in quality parts, and that you have been operating for some time without incident. All of a sudden, however, you begin to experience a heating or Flow, temperature, and pressure indicators on process fuid lines help confrm that conditions are right for making quality parts. cooling issue. You've checked the setpoint and actual readings on the molding machine and the material temperatures, injection pressures, and cooling times are still within acceptable limits. So, you begin to suspect a heat-transfer issue. Most of the issues that might have caused you problems during start-up also apply here, except that you can assume all your equipment is properly sized and that the basic confguration of the system is correct. A sudden change in heating/cooling operation signals a malfunction: A heater has burned out, a pump has failed, or there is a leakage or blockage in the fuid-circulating system. To troubleshoot, you do the same things you might have done during start-up to confrm that your system was operating properly. Go back to the mold and check hoses and connections to make sure there are no holes or kinks in the hoses. Check the operating condition of the TCU and/or chiller to make sure each of their components is functioning. Check your cooling tower to make sure its fans are operating and that water is fowing to and from the heattransfer equipment. If you are drawing city water into your system Plastics technology february 2013 25