Plastics Technology

NOV 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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non-contoured pins will. To help reduce this accelerated wear, the face of the contoured pin should have a stair-stepped relief, often referred to as "teeth." This won't keep the pin from being pushed to the side by plastic pressure, but it greatly reduces the tangential forces during ejection. Instead of stair-stepping, some mold makers use a coarse EDM finish, a cross-hatched pattern, or simply a few notches on the face of the pin to get a better grip on the part. None of these methods are as effective as a full stair-step design (Fig 3). It is often a good idea to install a shaped pin in a fixed sleeve. If the sleeve wears out, it is easier and cheaper to replace it than having to repair a worn through-hole in the core. Having an ejector pin ride in a fixed sleeve is also ideal if the core is relatively soft, or if the molding material is corrosive. If a shaped ejector pin needs to have its face polished, it should be made of through-hardened steel to prevent "dishing" in the softer center during polishing. From time to time, a threaded hole has to be added to the face of a large ejector pin or return pin. One common example is when two ejector pins are connected to a bar, and the bar ejects the part. Another example is when an ejector system is spring loaded. Trying to assemble the mold without damaging the ejector pins can be very difficult because of the necessary pre-load. Adding a tapped hole on the end of the return pins and installing a screw and washer keeps the ejector retainer plate in the perfect position for assembly. In order to tap the end of these pins, you either need a nitrided pin, which has a core hardness of 40 to 50 Rockwell C, or an equally soft through-hardened pin. Ejector-pin clearance, or relief holes, are drilled into the back of the support plate and frequently into a portion of the core insert. The clearance depth in the core insert ends at the begin- ning of the land or bearing diameter. The diameter of these clearance holes is typically 1/32 in. larger than the nominal ejector-pin diameter. That might be fine for larger pins; but for smaller pins, espe- cially any shouldered pins, it is better to reduce this clearance to just 1/64 in.—the same amount that should be used in the ejector retainer plate. The pins will be allowed to bend only about 0.008 in. before the inside diameter of the bore supports the pin and prevents it from bending any further. And yes, when a pin bends and comes in contact with the side wall of a clearance hole, it effectively reduces its unsupported length, which is advanta- geous for reducing the chances of it buckling and breaking. End of Year Sale SMAX 2 Screenless 10" x 10" Cutting Chamber DRYMAX E60-150-M (50 lb/h) GRAVIMAX G14 1 kg Blender FEEDMAX S 3-net (300 lb/h) $1,195 Used - Demo - Overstock Going fast!* *inventory not guaranteed $8,795 $8,295 $9,995 For small-diameter pins, reduce the ejector-pin clearance in the support plate to just 1/64 in. T O O L I N G K now How

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