Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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somewhat greater than what is required to eject the parts under normal operating conditions. Limiting the amount of ejector force can save you a lot of time and money in the event there is an issue with an ejector pin, such as binding or breaking. It can inter- rupt the molding cycle and prevent further damage. If the ejection- force value can be tied into an alarm on your machine, the same as you might do with the overall cycle time or screw-recovery time, all the better. If the force value is not monitorable, perhaps the ejection time is—which will be almost as effective. Maybe someday a flow-analysis software program will be able to calculate the force required to eject a molded part, so we can precisely determine the size and quantity of ejector pins to use. Given the endless list of variables, such as part geometry, material characteristics, draft angles, surface finish, pack pressures, mold temperature, gate location, undercuts, etc., I doubt we will see such a program anytime soon. However, Torsten Kruse, president of Kruse Analysis Inc. and an industry expert on plastic flow analysis, notes that "some CAE software programs can give mold designers insight as to where to place ejector pins based on calculating a part's differential shrinkage and associated internal forces. Ejector pins can break due to differential part shrinkage requiring different or unequal amounts of force on different ejector pins." Based on Kruse's statement, every ejector pin will probably require different amounts of force to eject a part. For example, the pins near the gate, near an inside corner, or at the bottom of a deep rib will most likely require more force than the pins on the runner or at the end of fill. This should be taken into consideration when deciding the ejector-pin locations, sizes and quantity. In addition to the potential causes for ejector-pin breakage discussed in the first two parts of this series (in August and September), there are two other common reasons: (1) the force required to eject the part from the core exceeding the strength of the pin; and (2) galling. The force required to start the ejector pins moving forward is controlled by static friction plus the adhesion of the part to the core. Static frictional forces are usually higher than kinetic frictional forces, which are the forces required to keep the pins moving once they've started. To minimize the initial shock to both the pins and the parts, the ejection velocity should start off relatively slow. After about 1/8 in. or so of travel, it can then ramp up to the desired ejection speed. The best way to minimize ejector-pin failure because of exces- sive force is to use the largest-diameter ejector pins possible and a sufficient quantity of them. Unfortunately, ejector pins can limit your ability to locate cooling channels ideally, so a compromise often must be made. Regardless, nothing increases the amount of force required to remove a part from a core more than an over- packed condition—especially if there is a lot of vertical surface area machined into the core, such as thin, deep ribs. The force required to eject the part can become more than the strength of the plastic itself, and the ejector pins will push right through the part. Note: Large-diameter pins are beneficial when trying to eject both very thin and very thick-walled parts. A small pin can indent, pierce or leave a stress mark on a weak, thin-walled part. It can do the same thing to thick-walled parts, which are often very hot and soft in the center upon ejection. In both cases, the cycle times are increased simply because the ejector pins are too small. If your molding machine allows you to limit the amount of ejector force acting on the ejector plate, it is a good idea to set a value Why Ejector Pins Break and How to Prevent It Various factors can cause an ejector pin to collapse. Here we focus on forces, friction, surface finish, and lubrication. Get more insights on tooling from our expert authors: Learn more at KNOW HOW TOOLING By Jim Fattori PART 3 Ejector pins can buckle and break for many reasons. 34 OCTOBER 2018 Plastics Technology K now How TOOLING

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