Plastics Technology

NOV 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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in the core. This is especially important when a portion of the pin butts up against plastic, and the remaining portion butts up against steel—as is the case with parting-line, "thumbnail," or "toenail" ejector pins. Thumbnail ejector pins (Fig. 1) are a great way to prevent having to use small-diameter pins. For example, instead of using a 1/64-in. diam. pin in the center of an 0.080-in. outside wall of a part, you can use a 1/8-in. diam. pin and let most of the pin extend beyond the edge of the part and seal off against the cavity. Not only will the larger pin have less chance of buckling, there is an increased amount of surface area pushing against the part. But there is a significant risk to this ejection method— one that I always try to avoid. If the pin is not perfectly flush or slightly below the parting line, the pin will indent the cavity plate or deform a portion of the face of the pin. If that pin isn't keyed and can rotate, it's going to flash parallel with the parting line. If you ever get this condition, don't try to fix it by pre-loading the pin. That will eventually make the situation worse. Any ejector pin that shuts off against steel should be through hardened—not nitrided. Nitriding is a surface or "case" hardening process, where the surface is typically between 65 and 74 Rockwell C. This high hardness diminishes gradually to a depth of 0.012 to 0.020 in. Due to the high surface hardness and softer center, nitrided pins can chip when they shut off against steel. Through-hardened pins made from M-2 steel at 60 to 63 Rockwell C are probably the best for steel-on-steel applications. To avoid chipping or other damage to any type of pin that butts up or seals off against anything other than plastic, I like to add small Bellville washers, also known as a disc springs, under the head of the pin (Fig 2). The spring absorbs the impact and compensates for any change in length, which could otherwise cause flash to develop from a small gap or cause the head of the pin to mushroom from repeated compression. If the ejector plates are not tied into the molding machine and they are not spring loaded, this Bellville washer design works extremely well for preventing damage to return pins, which in this scenario is the only means of positive ejector-plate return. A pin that is shaped or contoured with a steep angle usually wears out the through-hole in the core on the side opposite the contoured shape, due to the tangential forces applied during both injection and ejection. This is why contoured pins will develop "down-flash" before Number every ejector pin— even if they are not keyed. C-20 DeDuster® – removes dust and streamers Better systems for brilliant products and higher margins. Learn more: streamers Better systems for brilliant products and higher margins. streamers streamers Better systems for brilliant products and higher margins. No dust + No streamers = Reduced scrap rate and production costs and production costs @plastechmag 31 Plastics Technology T O O L I N G

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