Plastics Technology

NOV 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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While the table lists the standard ejector pin sizes, ejector pins are readily available in 0.005-in. over-size diameters. If you shop around, you will find pins are also available in diameters as small as 0.010 in. and in increments of 0.0001 in. If you can't find the exact size pin you need for a application, you can also consider using a metric pin. Whenever an odd-sized ejector pin is used, it should be engraved or otherwise marked to avoid mistakes during assembly. One trick is to coat the head of the pin and a portion of the retainer plate with a thin layer of metal stain. It's good practice to number every ejector pin—even if they are not keyed, or don't vary in size or length. I once had preven- tive maintenance performed on a mold that ran a low-viscosity material—nylon 66. It just needed a good cleaning and some fresh grease. There was nothing wrong with the mold or the parts it produced. After the PM was performed, we developed flash running down several of the ejector pins and couldn't figure out why. It turned out that the diameters of some of the pins were on the low side of their tolerance, and they were installed in some through holes that were on the high side of their tolerance. The tolerance stack-up was enough to cause a problem for a material that flashes at a clearance of just over 0.0005 in. For molds running a low-viscosity material, each ejector pin should be measured individually to four decimal places, as well as the holes in the cores they go into. Keep in mind that an ejector pin never stays perfectly in the center of its hole. It will always shift to one side or the other. For this reason, I recommend that the amount of clearance between the hole in the core and the ejector pin should be slightly less than the vent depth recom- mended by the material manufacturer, but not too much less. A tight fit is one of the biggest reasons why an ejector pin will gall. In extremely precise molds, the backs of the heads of the ejector pins should be ground so that they are all the exact same thickness, and perfectly perpendicular to the centerline of the pin and the direction of travel. This eliminates the chance of a lateral force being applied to the pin and the through-hole in the core, which can cause several different problems. SLEEVE EJECTORS Trying not to stray too far off topic, land-length consideration also applies when you are using a sleeve ejector— both on the outside diameter and the inside diameter. The bearing length on the OD is the same as you would use for an ejector pin of that diameter. The bearing length on the ID is a completely different story. The tip of the core pin inside the sleeve should never disengage from the sleeve's internal land, or bearing length, regardless of how generous the lead-in angle might be—typically 1° per side. Therefore, a sleeve's internal land length should be equal to the ejector stroke plus at least an additional 1/8 in. Otherwise, the tip of the core pin can wear as it rubs against the tran- sition section. The transition section is the area between the relief diameter and the land diameter. While this may seem like an excessive amount of bearing length, you must consider that the internal bearing length decreases as the sleeve advances to eject the part. Typical internal bearing lengths for standard ejector sleeves are 1.75 in. and 2.25 or 2.5 in. Nine times out of 10, the sleeve needs to be shortened to a required length. This reduces the bearing length. For long-running molds, custom sleeves with an increased bearing length may be warranted. If the ejection system isn't guided and you have an ejector sleeve, add at least two dowel pins in opposing corners connecting the ejector plate and the ejector retainer plate to ensure they are properly aligned. This is also a good idea whenever a component such as a lifter is mounted in the ejector plate. Some ejector pins must have their position oriented due to a shape or contour machined on their tip. There are several ways to do this. What's important is that when you key an ejector pin, you usually lose lateral float in at least one direction. Therefore, these pins require greater positional accuracy. There is an advan- tage to keying ejector pins. They remain angularly oriented to any uneven wear on the pin and in the corresponding through-holes Get more insights on tooling from our expert authors: Learn more at KNOW HOW TOOLING (L. to R.) Cross-hatched, partial and full stair-stepped ejector-pin faces. Notice the down-flash primarily on one side of the partial stepped pin. FIG 3 Bellville springs absorb impact and prevent damage in metal-to-metal applications. FIG 2 30 NOVEMBER 2018 Plastics Technology T O O L I N G K now How

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