Ejector pins with diameters of 0.250 inch or larger make the best and longest-lasting ejector pin gates. The disadvantage of the wedge shape is that it cannot be used on small-diameter ejector pins as well as it can be with larger-diameter ejector pins. For a typical part with a wall thickness of approximately 0.100 inch, I make the thick portion of the wedge 0.150 inch thick and the thin part 0.090 inch.Ĭosmetic problems such as sink can be caused by improperly shaped ejector pins.Īnother advantage of the wedge shape is that the subgate can be opened up to help fill and pack the bigger part, and since the ejector pin gate is usually clipped off and ground up or just left on the inside of the part, we don’t care if the subgate tears or peels. The thick part of the wedge should be where the subgate is located and the thin part of the wedge should be where the plastic enters the cavity. The wedge shape allows the material to go from thick to thin as it flows through the ejector pin on the way to the cavity. I prefer to use a wedge-shaped ejector pin gate in every situation I have ever run across. The problem with cutting off part of the pin is that part cosmetics in the area where the plastic enters the cavity are always going to be visually unacceptable due to gas marks, blush, or sink on the part directly above the gate sliver. This was done to leave a flow path for the material to be injected into the pin and flow from the subgate up through this thin sliver section and into the part cavity. This looked to me like a gate size problem on the largest part only.Īs I looked at the subgates, I saw another problem: They were feeding into an ejector pin that had a thin vertical section cut off on its end to allow for material flow. The second-largest part was fairly free of sink and both smaller parts had no sink at all. I say this because the biggest part was the one with sink on the cosmetic side. However, I do not think the problem with these parts lay in the differences between sub- and edge gates. Proper sizing is a little more difficult to do with subgates, and much easier with edge gates. This was a four-cavity family mold with each part a different size and weight. Besides, I didn’t have a processing sheet to check the pressures and mold temperatures. So I needed to look at the gate sizes, the nozzle orifice, and the rib thickness-pretty much in that order. Sure, it could have been a mold water problem, a gate that was too small, or a nozzle orifice on the small side. Now, sink is mostly caused by a couple of things: Either the molder is not using enough pack and hold pressure or the ribs are thicker than they should be. The sink was exactly opposite the ribs on the inside of the part. The appearance was not very good: I saw sink all over the front side of the big part and what appeared to be glossy spots in the EDM finish. The black parts were fairly flexible, probably made of polypropylene. has more than 45 years of experience finding solutions to processing challenges. Consultant Bob Hatch of Bob Hatch & Assoc. This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. (I used to get notes, but the last couple of years I have been getting phone calls or e-mails instead.)īusiness was slow on this particular day so I decided to go ahead and work up a review on the contents of the box. I also found four parts in the box with the runner but no note. I opened the box and removed a runner with its sprue attached. In just a couple of minutes I was taking delivery of a fairly small shipping box that was sent to me from someone I used to talk with on a regular basis, but lately not so much. It was a quiet Tuesday afternoon until I heard a delivery truck pull up downstairs. Some of the most obvious cosmetic problems, such as sink marks, may not be caused by the most logical suspects.
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