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troubleshooting mold cooling troubleshooting Mold temperature control By Tim Miller, Conair Group Mold-temperature control is just one of many process elements that can affect the quality of an injection molded part. even within that relatively narrow slice of molding technology, the number of different variables is almost innumerable. We will limit scope of this article, therefore, to some of the basic causes of three different classes of quality problems: problems that appear during start-up, problems that appear suddenly, and problems that develop over time. We will assume that we are dealing only with cold-sprue molds, as hot-runner systems only add more variables to an already complex subject. We will also assume that the injectionmolding machine is correctly set up, that cooling times and overall cycle times are reasonable, and that the resin is properly heated prior to injection. In other words, we will assume that any quality problems observed are related to mold temperature. In a perfect world, anyone running a particular mold would know what the mold designer set as the ideal temperature and fow rate for the cooling water. In reality, operators and setup personnel don't always know these settings and/or something changes so that it is necessary to re-establish them by trial and error. Quality problems related to heat transfer in the mold tend to arise from improper polymer fow in the tool due to over-cooling or under-cooling. The latter condition would apply when parts are not completely solidifed and therefore stick in the mold or deform after being ejected from the mold. Over-cooling may be suspected when some or all of the mold cavities do not fll properly. Surface detail may not be perfectly replicated or, worse, parts may be incomplete (short shots). Shrinkage or warpage could result from over- or under-cooling or could be due to uneven cooling. POOR PART QUALITY AT START-UP If you begin a production run and immediately notice problems that you suspect are due to a heat-transfer problem, here are a few questions to ask: •Was the cooling system checked? It sounds ridiculously simple, but is your heat-transfer system actually working? Check that all hoses are properly connected and that there are no leaks. Then, 24 february 2013 Plastics technology Keeping a detailed log of the heat-transfer conditions associated with each mold can help you recognize gradual changes that signal trouble. starting at the mold, make sure all the components in the system are working correctly. If you have a mold-temperature control unit (TCU) between the mold and the chiller, make sure the heater and pump are working. Until the mold is up to temperature, the TCU will need to heat the heat-transfer fuid and it can't do that if the heater isn't functioning. Then check the cold-water source (chiller or cooling tower) and make sure they are operating correctly. •Was the tool changed? If so, then make sure that all temperature and fow rates are optimized for the new mold. Different molds often require different heat-transfer settings and if the tooling was changed but the heat-transfer settings from the previous job remain, the system won't be able to produce good parts. It is also possible that a TCU that works well with one tool or process may not be capable of handling a larger tool effectively. And don't forget to make sure that the mold cooling lines have been purged completely of any air before start-up. Air in the tool will inhibit proper heat transfer. •Are you actually getting the temperatures and áow rates speciàed? Simpler chiller and TCU controls don't always display actual temperatures and fow rates, so it is a good idea to double check. Use a contact or strap-on thermometer (or an infrared temperature gauge, if you have one) to check the temperature at the mold and at several other points in the system. Check the