Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 41 of 75

ABOUT THE AUTHOR: Jim Fattori is a third-generation injection molder with more than 40 years of molding experience. He is the founder of Injection Mold Consulting LLC, and is also a project engineer for a large, multi-plant molder in New Jersey. Contact; not—to support the weight of the ejector plates. If the ejection stroke is very long, it's often wise to have the guide pins extend into the opposing plate, so they are supported on both ends, instead of being cantilevered from just one end. It makes sense that two components rubbing against each other will have less friction if their surfaces are smooth rather than rough. Depending on the manufacturer and type of ejector pin, the surface finish can be as low as 2 micro- in., or as high as 0 micro-in. Even the latter is more than adequate for most molding applica- tions, although if you're running over a million cycles a year, a finer finish may help extend your preventive-maintenance interval. The most common method of installing a through hole in a core for an ejector pin is to drill and then ream the hole. The surface finish of a reamed hole will usually suffice for most molds. Back in the day, many tool shops honed or barrel lapped the holes after reaming for a more precise size and a finer surface finish. A honing stone rotates in the bore as well as moves in and out. This leaves a cross-hatched pattern to help retain a fine film of lubricant. Today, when the size or surface finish must be finer than a reamed hole, jig grinding or wire EDM are typically used. Both methods have excellent position, diameter and circular accuracies down to a couple of tenths of an inch. That's important when having to install a pin close to the edge of a side wall, or precisely in the center of a thin rib. I recommend using either of these two methods when there is a large quantity of small ejector pins in a mold, simply because the laws of probability predict at least one of them will cause a problem. There is a slight advantage to the wire-EDM method: It leaves very fine peaks and valleys, parallel to the central axis of the ejector pin. These peaks and valleys help retain lubri- cants, as well as reduce the surface contact area on the ejector pin, which in turn reduces the frictional heat. Wire EDM costs more than reaming a hole. To help reduce that cost, pre- drill the hole about 0.002 in. to 0.004 in. smaller than the desired final size. Then the wire EDM only has to make a precision skim cut or two. The table on p. 39 shows the range of average roughness values for various types of machining operations. Depending on a number of factors, each range could be coarser or finer than those listed in the table. Galling can be greatly reduced with proper lubrication. 40 OCTOBER 2018 Plastics Technology T O O L I N G K now How

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