Mold Maintenance & Repair

OCT 2014

Mold Maintenance & Repair

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6 Mold Maintenance & Repair F E AT U R E used a particular nylon, for example, may have required different settings than another job using a different nylon material. 7. Check nozzle orifce size. Off-the-mark orifce size can cause high or inconsistent pressure, which can burn material and create silver streaks on parts. To avoid this problem, start by ensuring the molding machine nozzle radius is the same size as the nozzle seat radius of the manifold. Proper sizing prevents material "drooling" and creates a leak-proof seal. Ad- justing the machine nozzle orifce to match will drop the pressure and allow more material fow, preventing burning and streaking problems. 8. Inspect the nozzle tip and hot runner seat. Ensuring that the molding machine nozzle tip and the seat of the hot runner system are properly centered can prevent leakage. An easy way to check nozzle centering is to put a sheet of paper between the nozzle tip and the seat. Move the barrel forward so the nozzle tip touches the seat, and look for a clean circle cut in the paper. Make adjustments to the molding machine carriage as needed. Compression nozzles leave room for the expansion that occurs with excessive process temperatures and absorb it without placing stress on the components. Here, an operator checks a machine's nozzle tips to ensure they're properly centered in the manifold nozzle seat to prevent material leakage. 9. Start the hot runner properly. Damaged manifolds caused by plastic leaks are usually the result of improper startup, a common error. Molders in a rush sometimes turn on all zones of the manifold at the same time. The nozzles heat up fast and then snug up to the manifold—which is still heating up to operating temperature and needs room to expand. To avoid this problem, heat the manifold frst to allow expansion. When it reaches the proper temperature, a green light will appear on the con- trols. Then turn on the nozzle zones. Improper startup won't cause problems right away, but after about six months, you will get a burr, and a damaged manifold is inevitable. This is especially true if you're using a seal ring on the nozzle heads. 10. Check that the machine is pre-flled be- fore startup. A hot runner system must start full of material, but pre-flling can be diffcult because some molding machines are not built to extrude plastic with the mold open. If that's the case with your machine, use this process to pre-fll the hot runner system: Close the mold, jog the screw for- ward, open the mold and look for plastic. Repeat

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