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We are molding a 20 percent glass-filled polyetherimide at 760F, using a mold with tunnel gates (diameter .080 inch). The gates wear very quickly. Frequently, after only a few months of use, the gates become so worn (undercuts, washed out, etc.) they tear, causing plugged gates. Unfortunately, we need the gates to remain as large as they currently are. Please also recommend the optimum distance the knockout pins should be from the gates for the most appropriate runner flex.
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Last year there was a similar problem presented in your magazine that I responded to with a possible solution you published (see March 1996 1MM, p. 108). I was contacted by the company with the problem, and we have had parts in test since October 1996.
I visited this customer recently and found our original Durillium N processed components have been running continuously since they were first installed more than five months ago. Until now, the longest this mold had run was between four and five weeks. We have now done all or part of more than seven molds with no failure.
Durilhium N is a highly modified chemical vapor deposition process performed at much lower than normal temperatures, which results in no distortion and only microns of grain growth. Surface hardness will typically be 70 to 72 Rockwell C, with a transitional layer .015 inch thick returning to the core hardness. This allows for great wear resistance without any tendency to chip, crack, or flake. For a technical data sheet, contact
me.
-D. Brown, Russell Products Co., Dayton, OH, (937) 849-1601.
Gate inserts made with Crucible steel CPM 1OV will hold up very well to glass-filled and exotic
plastics.
-B. Wagner, The Mold Shop, Sylvania, OH, (419) 829-2041.
Make replaceable carbide inserts. EDM gate detail into insert. We run 40 percent glass PPS and have had good luck doing
this.
-C. Caidwell, Airborn Inc., Winnsboro, TX, (903) 629-7821.
EDM a conical hole in the gate area and insert with carbide or hardened
steel.
-B. St. Denis, Rockwell Automation, Allen-Bradley Co., Manchester, NH, (603) 625-8299.
If the problem is "washed out" gates, then harder, more abrasion-resistant gate inserts would help. A good choice is HSP23 (Uddelholm) with black diamond surface treatment. If the problem is undercuts, then you may be seeing heat checking similar to diecasting of aluminum (assuming mold temperatures are more than 400F). Then you should go to an H-13 steel with nitriding and/or black diamond surface treatment.
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