Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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the ejector plates have a guided system, the guide bushings will wear out prema- turely. It can cause ejector pins to bend and break. It can cause the through holes in the core insert to wear out with ellip- tical or egg-shaped holes. It can cause the ejector pins to push against the molded part unevenly, which can distort the part or cause it to stick in the mold tighter. And it can cause the ejector pins to vary in height relative to the parting line. Did you ever wonder why molds have those round stop pins or rest buttons between the ejector plate and the clamp plate? They are there because if any dirt or debris gets in between these two plates, some or all the ejector pins are going to protrude beyond the parting line of the mold by an amount equal to the thickness of the debris. Protruding pins can cause sticking issues when trying to eject the part. If any ejector pins are under a cam, the pin or the cam can get damaged when the mold closes. The purpose of these rest buttons is to minimize the contact area between the two plates, to allow some debris to accumulate without affecting the proper position of the ejector system. On occasion, rest buttons are moved or eliminated to make room for a core pin or some other component. This is when you can get a support problem. It is a good idea to have a rest button near or under any component that could apply pressure to the ejector plate, such as a lifter or an ejector pin that butts up against steel. Having rest buttons near the tapped holes for the machine's ejector bars is also beneficial. If the machine's retract position is not set properly, ejector bars can pull and distort a poorly supported ejector plate. Ejector housings are typically open on two opposing ends of a mold, and one of the open ends is often at the top when mounted in the machine. It is not that uncommon for small molded parts with undercuts on their cores to ricochet off the cavity upon ejection and end up inside the ejector housing. That can cause the ejector plates to try to bend the next time the mold cycles. This is easily correctable by installing an ejector housing cover or shield. Housing covers have additional benefits, as well. They are a good safety precaution for accidents caused by spring- loaded ejector plates—much more effective than painting the edges of the plates OSHA orange. A cover also helps prevent potential mold damage by personnel trying to advance the ejector plates with a pry bar when there is an ejector pin under a cam. In lieu of a shield, rest buttons, which are typically mounted between the clamp plate and the ejector plate, can be added between the retainer plate and support plate. This creates a 3/16-in. gap, so any dirt, debris or the occasional small stray part won't cause the ejector plates to try to cock or bend. I only suggest this because most processors set the ejection forward position to be flush with the back of the support plate, even when they have plenty of ejector stroke, as opposed to staying a short distance away. Always install all the Plastics Industry Association's (PLASTICS) knock- out patterns that will fit in a mold. If you don't know what they are, get a copy of AN-109 Recommended Guidelines–Interchangeable Mold Mounting Dimensions from PLASTICS. It is up to the setup man to decide which ejector pattern(s) to use. Hopefully, he chooses a pattern that will not damage the mold. Hydraulic ejector cylinders on small molding machines can have a forward ejection force of 1 to 5 tons. Larger machines can go as high as 80 tons. Now that's a force to be reckoned with! On large molds, if the knock-out bars are not spaced far enough apart, this can cause the plates to want to bend in the middle—toward the support plate. Conversely, if the knock-out bars are at the extreme outer edges, but there are a number of ejector pins pushing the runner and parts in the middle of the mold, the ejector plates will also want to bend in the middle—but this time away from the support plate. Therefore, make sure the knock-out bars are evenly spaced; or on large molds, consider using additional knock-out bars to help distribute the force evenly over the entire ejector plate. Installing additional bolts connecting the ejector retainer plate to the ejector plate will provide some added strength to prevent the two plates from trying to bend or twist. But they do a much better job of preventing just the ejector retainer plate from bending, which is important. Extra bolts placed near the return pins, lifters and any groupings of ejector pins help prevent the Thermal expansion is a major cause of misalignment. Ejector extension designs should apply the machine's ejection force to the thicker ejector plate (left) and not the weaker retainer plate (right). A single offset cavity puts an uneven load on the ejection system. 28 AUGUST 2018 Plastics Technology K now How

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