Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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There are dozens of reasons why ejector pins break. Very few of them have anything to do with the pins themselves. This month I will discuss how the molding machine and the ejection system can be the culprit. If the molding machine has a bent ejector cross (also known as the machine's ejector platen, ejector plate, or if you're old school like me, the dog bone), or if the cross's thin guide bushings are worn, it will travel at a slight angle and apply an uneven load to the mold's ejector plate. This condition is usually obvious because the machine's knock-out bars may not align with, or will rub against, some of the through holes in the moving platen. Steel crosses that are bent can be straightened out in a hydraulic press, but cast-iron crosses will probably crack. Why Ejector Pins Break … and How to Prevent It A more common condition that will apply an uneven load to the ejector plate is if the molding machine's knock-out bars are slightly different lengths. Bars that are too long or of different lengths are the primary reason for bent ejector crosses. If the mold has knock-out extensions attached to the ejector plate, and they are not the same length, or one of their retaining bolts comes loose, this too can cause an uneven load on the ejector plate. Several mold-compo- nents suppliers sell standardized knock-out extensions. If you have your own design, make sure the extensions do not butt up against the back of the ejector retainer plate. Those designs are notorious for bending or breaking the retainer plate. If the mold is equipped with a coil-spring ejector-return system, but one or more of the springs are broken, the ejector plates are going to try to twist or bend. This will also happen if the springs are of different lengths, or if the amount of preload varies due to varying counter-bore depths. I try to avoid using ejector- return springs. I once had a spring-loaded ejection system bind in the forward position. The foreman gave the ejector plate a hard tap with a brass rod. It broke free and slammed back against the support buttons—thankfully not against his fingers. Someone once told me he loves springs. "It speeds up the setup time because we don't have to connect the machine's ejector bars." I'm guessing he doesn't connect his seat belt either. Another thing that can cause an uneven load on the ejector plate is the location of the ejector pins and of the forces required to eject the part. This is always the case when there is a single offset cavity. When there is an uneven load applied to the ejector plate, it will try to bend, cock, or twist. This causes a chain reaction. If If you have an ejector-plate deflection issue, the root cause of the problem is probably not the thickness of the plate. In part one of this four-part series, we focus on the molding machine and the ejection system as culprits. Get more insights on tooling from our expert authors: Learn more at KNOW HOW TOOLING By Jim Fattori PART 1 If the molding machine's ejector cross is bent, it will apply an uneven load on the mold's ejector system. 26 AUGUST 2018 Plastics Technology K now How TOOLING

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