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The photo at right shows an ugly gate scar from an elliptical tunnel gate machined into the stationary side of a two-plate mold. Notice how the gate scar is rough and somewhat indented. There are also some drag marks on the side of the part below the gate. One of the obvious reasons this happened is because the cutting edge of the gate is worn. A second, not-so- obvious reason is that the tunnel-gate bore is rough or pitted. A third, not-so-obvious reason is because the gate is rather large, and the runner may be flexing during mold opening. Large gates have a significant amount of holding force on a part. It's often enough force to pull a flexible runner out of the moving side of the mold during the mold-opening stroke. Once the holding force of the bent runner overcomes the holding force of the gate, it pulls or rips the gate off the part. Since the inner wall thickness of the part is usually warmer than the frozen gate, it breaks off at the warmer or weaker portion—leaving an indentation in the part. The solution to this combination of problems is to sharpen the gate, draw polish the bore, and increase the rigidity or holding force of the runner. The most common cause of a runner being too flexible is because the shortened ejector pin that forms the boss is too far You can also profile your injection velocity—going very slowly from right before the gate until it makes a small molten puddle just beyond the gate. The puddle acts as an obstruc- tion to the material behind it. Once the puddle is formed, you should be able to go back to the desired injection velocity. The downside to this "solution" is that you are now processing around a tooling problem. If you want to fix the root cause, ask if a small core pin directly opposite a gate can be added, specifically to create an obstruction to stop the jetting. The core pin does not need to extend all the way to the cavity, which would form a through hole. It can be left short—with a height approximately equal to the depth of the gate. The mold designer will select the type of tunnel gate, where to put it, what size it should be, and what method to use to remove it. This is all derived from what he has been taught and learned over the years. But no two molds are alike. What worked 100 times before may not work the 101 st time. When something goes wrong, the designer and the tool shop would benefit from your input on how to resolve the problem. But first, you must define or describe the problem to them. Nothing does that better than a video taken with your cell phone, played back in slow motion. 525 East Stop 18 Road Greenwood, Indiana 46142 Phone: 317-887-6352 Email: sales@TempTek.com For information online www.Temptek.com Factory Phone 317-887-6352 Outdoor Central Chillers • Complete Outdoor Chilling System with Refrigerant Zones and Pumping System • For Use In Most Outdoor Climates • Central Unit for Plant Wide Cooling for Many Processes • Integral Air-Cooled Condenser Indoor Portable & Central Chillers • Use Inside Plant Facility for Single Process or Plant Wide Cooling • Microprocessor Controller and Display • Small Footprint to save floor space • Environment Friendly Refrigerants • Internal Coolant Reservoir And Cirulation Pump Water Chillers with New Product Warranties, industry recognized components for Maximum Reliability, Economically Affordable capital equipment providing perfect bottom-line returns, and a network of factory trained technicians for Unbeatable Service. • Environment Friendly Refrigerants • Microprocessor Instrument Controls • Non-Ferrous Construction • Small Footprint • Other Products: Temperature Control Units, Pump Tanks, Cooling Towers, Material Dryers, Loaders, Granulators No two molds are alike. What worked 100 times before may not work the 101 st time. 54 FEBRUARY 2018 Plastics Technology PTonline.com K now How