Mold Maintenance & Repair

OCT 2014

Mold Maintenance & Repair

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4 Mold Maintenance & Repair F E AT U R E By Craig Kovacic Follow these tips to set up your system and keep it running smoothly. For years, injection molders have reaped the benefts of hot runners because of their ability to reduce material use and produce faster cycle times. And now that more and more mold- makers are adding presses for sampling and production purposes, they, too, are seeing the benefts of hot runners from an entirely differ- ent perspective. All that said, one drawback of hot runners is their complex startup process. But there are nu- merous things molders and moldmakers can do proactively to ensure the process goes smooth- ly, and there are best practices to consider for keeping your hot runner system up and running once you've got it set up to your liking, Here are a baker's dozen checkpoints for minimizing problems and maximizing effciency and uptime in the operation of your hot runner: 1. Determine whether you need grounded or ungrounded thermocouples for your tempera- ture control. A lack of communication between a temperature control supplier and a hot runner end user can be a source of many problems, including fuctuating temperatures after reaching setpoint and material burning. Some temperature controls work well with grounded thermocou- ples, others with ungrounded. One of the safest ways to go is with a universal controller, which works well with both. This is not just a consideration for new molds, but may be appropriate for inherited molds as well. Your thermocouples may have loose contacts with the molds, and they can become ungrounded as temperatures rise. You can use an ohmmeter to check whether your shop is using grounded or ungrounded thermocouples. You can also take a red or white lead and touch a bolt on your hot runner. If you get a direct short, your thermocouple is grounded. Hot Runner Running Effciently? The key to trouble-free operation is to provide proper design, setup and operating parameters when you specify the system. Perhaps an easier way to answer this question without testing, however, is to ask your supplier. 2. Isolate grounded thermocouples. Start your project by using an integrated control system that has the ability to isolate grounded thermocouples. This ensures the thermocouples don't have the opportunity to interact, leading to false readings (i.e., improper temperature con- trol). If allowed to interact, thermocouples can also provide a path to ground when the heaters leak current into the thermocouple inputs, creat- ing permanent damage to the control system. A broken lead on a thermocouple is hard to detect and will lead to off-specifcation tem- peratures in your hot runner system. Isolating the thermocouples will help keep the system operating at optimal effciency. 3. Seek safety features in control systems. Some integrated controls are available with built-in mold connection systems that break the thermocouple connections if they detect leak- age currents and essentially prevent "frying" of your control system. This is a key feature to seek out before purchasing and implementing a control system.

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