Avoiding Cooling Problems During Plastics Injection Molding
If quality concerns arise when injection-molding plastic parts, consider the heat transfer equipment, including chillers and temperature control units, and avoid over- or under-cooling.
Mold temperature control is just one of many process elements that can affect the quality of a plastic injection-molded part and, even within that narrow slice of
molding technology, there can be many variables.
Heat-transfer-related quality problems tend to arise from improper polymer flow due to over-cooling or under-cooling. The latter condition is evident when parts are not completely solidified and stick in the mold or deform after being ejected. Over-cooling can be suspected when some or all of the mold cavities do not fill properly. Surface detail may not be perfectly replicated or — worse — parts can be incomplete (short shots). Shrinkage or warping can result from over- or under-cooling, or uneven cooling.
For the purposes of this article, it is assumed that the injection-molding machine is correctly set up, that cooling times and overall cycle times are reasonable, and that the resin is properly heated prior to injection. I’ll consider only cold sprue molds and quality problems that are related to mold temperature.
If quality issues are observed at the beginning of a production run and heat transfer problems seem to be the culprit, here is a checklist to review.
Check the Cooling System. Starting at the mold, check all the components in the system. All hoses should be connected properly and there should be no leaks. If there is a mold temperature control unit (TCU) between the mold and the chiller, make sure the heater and pump are working and that the cold water source (chiller or cooling tower) is also operating correctly.
Check the Tooling. When changing the tooling, heat transfer settings from the previous job must be adjusted so that the system can work efficiently. Settings that work well with one tool or process may not be capable of handling another mold effectively. Also, mold cooling lines must be emptied of any air before startup because air in the tool will inhibit proper heat transfer.
Check the Specified Temperatures and Flow Rate. Use a contact or strap-on thermometer — or an infrared temperature gauge — to double-check the temperature at the mold and at several other points in the system. If the temperature differential
(T) is less than what is specified by the mold maker, this may signal that something has happened to limit fluid flow. Some heat transfer units are equipped with a flowmeter (measuring in gallons per minute or liters per minute), or one can be plumbed into the system. If there is no flowmeter, you can run the process fluid into a pail and time how long it takes to fill. For an approximation of the flow rate in gallons per minute, divide 60 by the number of seconds it takes to fill the pail, and multiply the result by the volume of the pail.
Check for Turbulent Flow. Fluid flow rate — in combination with the length and diameter of the cooling channels and a few other variables — will determine if the coolant flow is laminar or turbulent. Turbulent flow is characterized by random eddies, vortices and other flow instabilities that ensure the maximum amount of water comes in contact with the walls of the cooling channels, enhancing heat transfer. Anything that reduces flow rate potentially limits turbulence and will dramatically reduce cooling efficiency. If the flow rate is inadequate to achieve turbulence, a portable chiller with dual pumps can be used to ensure consistently higher flow to the mold. If a central chiller is being used, a temperature control unit can be used as a booster pump, ensuring adequate pressure and volume to create turbulent flow.
Check Temperature Control Units Size. Temperature control units have both heating and cooling capabilities to maintain ideal mold temperatures over time. Early in a run, before the mold has absorbed heat from the cooling polymer, the temperature control unit circulates hot fluid to bring the mold up to temperature. Later, it can heat or cool the circulating fluid as necessary to maintain the correct temperature. Generally, temperature control units are specified for:
• Their pump size (in horsepower or kilowatts), which will dictate overall flow rate.
• Their heater capacity (in kilowatts), which determines tool heatup time and the maximum temperature for a given flow rate.
• Their cooling valve size, which determines how much chiller or cooling tower water can be fed into the system to keep mold temperature down.
If the temperature control unit is not sized properly, its heating or cooling capabilities may be inadequate.
A sudden change in the heating or cooling operation after the system is up and running usually signals a malfunction. Perhaps a heater has burned out, a pump has failed, or there is a leak or blockage in the fluid-circulating system.
A sudden change in the heating or cooling operation after the system is up and running usually signals a malfunction.
To investigate, begin by going back to the mold and checking hoses and connections to make sure there are no holes or kinks in the hoses. Check the operating condition of the temperature control units or chiller to ensure their components are functioning. Check the cooling tower to make sure its fans are operating and that water is flowing to and from the heat transfer equipment. If you are drawing city water into your system, investigate whether the water system pressure is at normal levels.
The gradual development of quality problems can be more difficult to diagnose. In fact, you may have been compensating for a growing heat transfer issue — by changing other process settings, for instance — long before the problem became too serious to ignore. When the process is operating properly, it is helpful to keep a detailed log of the heat transfer system parameters and conditions that result in a stable process and quality. Useful data would include:
• Composition of the heat transfer fluids in the system (e.g., water, 30 percent propylene glycol or oil).
• Process fluid flow rates, temperatures and pressures in and out of all heat exchangers, tools and processes.
• Equipment in the system, including pump motor horsepower, actual equipment electrical amp drawn, temperature control unit heater sizes and cooling valve size.
• Ambient temperature when all of the previous data was recorded.
• Resin type and throughput rate.
Regular review of the log could signal the following problems before they become troublesome:
• Fouling of internal water lines in the mold. Suspended or dissolved solids can build up on channel surfaces, reducing fluid flow and limiting heat transfer rates between the steel and cooling water.
• Fouling of lines in the cooling equipment. Solids also can build up in heat exchangers and piping in chillers and temperature control units. Using filters can remove many suspended solids, but dissolved solids like lime need to be addressed by a water quality specialist.
• Cooling tower water contamination. Cooling towers can become contaminated with airborne debris and algae. A full-flow strainer on the outlet from the tower can eliminate many problems, and a properly sized pump tank will allow solids to settle out of the water before it is pumped into the heat transfer system.
• Environmental issues. In hot, humid weather, heat transfer to ambient air is reduced, requiring more heat to be removed by the system. Higher temperatures and humidity also can affect outdoor cooling equipment efficiencies. Condensation that forms on cooling lines reduces cooling capacity. Proper ventilation around a mold can cool the mold, reducing the burden on the system.
• Refrigerant leaks. A gradual loss of refrigerant pressure due to a slow leak will degrade chiller performance, resulting in repairs and potential environmental hazards.
Efficient operation of heat transfer systems depends upon the equipment being in good working order. Establish a regular maintenance program and keep a maintenance log. Most reputable suppliers of heat transfer systems can help set up such a program or provide service personnel to manage it.
Finally, make sure operators are well trained in the operation of heat transfer equipment. The more they understand about its importance to process efficiency and part quality, the better they will be able to use it properly and maintain the operation and maintenance logs.
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