Mold Maintenance & Repair

OCT 2014

Mold Maintenance & Repair

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October 2014 7 CONTRIBUTOR: Craig Kovacic has been with DME Co. since 1995 and is currently global manager of hot runner systems. In this position, his primary responsibilities are to troubleshoot hot runner systems and to manage technical service, electronic engineering, application engineering and assembly teams. For More Information: DME Co. / 800-626-6653 / / • Some materials require very high melt and mold temperatures. In a valve-gated hot runner system, seals in the valve pin actuator are adversely affected by higher tempera- tures. With all valve-gated systems, leave mold cooling on after the hot runner has been shut down. This allows removal of heat from the hot runner system and does not allow the mold plates to absorb the residual heat from the heated components. Removal of the heat from the system protects seals and o-rings from damage. Exposing the seals and o-rings to high temperatures repeatedly will accelerate their degradation. Exposure to heat beyond the specifcations for the seals will degrade them to a point that they will fail, causing leakage of the air or hydraulic fuid used to actuate the valve pins. • Hot runner systems are much like the barrels on a molding machine in that they require a soak time once the temperature reaches the set point. This soak time allows all material within the system to reach a temperature at which it is thoroughly melted. • Many hot runner systems are damaged when injection pressure is allowed to go to the maxi- mum the machine is capable of—trying to push unmelted plastic through the hot runner. If you have followed all these steps, you will produce better parts, prevent future problems and keep your molds running at optimal levels. this process until plastic starts coming through the gates. Once you see plastic, clear the gates and mold surfaces of material and start molding. 11. Purge material from the previous day's cycles. If material sticks to the mold at the start of the day, it's because material was left in the system and it degraded overnight. This problem is especially prominent when work- ing with heat-sensitive material such as nylon. When this happens, heat up the nozzles to compensate for loss of heat soak. Remember to turn the temperature down to normal process temperature after about fve minutes to avoid overheating the material. Eliminate this prob- lem completely by purging the system with a commodity material such as polypropylene or polystyrene at shutdown. 12. Don't use a hot runner to melt the material. A hot runner system is an extension of a machine barrel; it's meant to keep melt fow the same as it enters the mold. If you use hot runners to get all the material up to temperature, many problems will occur. Gasses won't free-off and you'll have spitting or bubbling plastics and/or gas coming out of the nozzle tips. In addition, material residence time may be increased because the material is too hot going into the mold. 13. Pay attention to proper maintenance. Once you have your hot runner system set up properly, you'll want to keep it that way. Here are some things to consider: • Wear due to the abrasiveness of the material adds to the maintenance frequency a tool requires. Filler and additives are commonly used today to enhance part performance. In these cases, preventative maintenance frequency should be increased beyond the normal recommendation. Cycle time, injection pressure and velocity of the plastic through the hot runner system can also have an infu- ence on the maintenance frequency required for each mold. Much like a car, the harder and faster you push it, the greater the frequency of maintenance. If you have already implemented a control system and checked the grounding for the thermocouple, but still believe there may be a discrepancy between the temperatures displayed and the actual temperature, do a manual check.

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