Plastics Technology

NOV 2018

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even though the number of diameters increased, the land length and the amount of surface area decreased, as I suspected would be the case. The accompanying table lists the results of these calcula- tions. I'm not recommending you use these values. They have yet to be validated. But you definitely should consider an ejector pin's land length and the resulting amount of surface area, instead of arbitrarily using some rule of thumb that has little or no empirical validity or is based on incorrect assumptions from previous failures. Since there is an exception to every rule, always leave a consid- erable amount of land length for any pin that ejects the sprue puller, runner, or various types of gates. As mentioned in a previous column, long bosses are often required in order for them to perform their intended function. If there isn't enough land length to extend the length of a boss, that's going to turn an easy fix into an expensive one. If it ever comes down to having to make a choice between increasing the bearing length or accepting an exces- sive unsupported length—increase the bearing length. Theoretically, if all of the holes are perfectly aligned and there are proper clearances, you should not get any friction between the pin and the core. But that is in a perfect world—one without temperature variations, material outgas- sing, injection forces and machining tolerances. As a precaution, molds with very small ejector pins, or a lot of ejector pins, should initially have a short preven- tive-maintenance interval. Once some history has been established, the interval can be adjusted accordingly. I spent a lot of time researching how much land or bearing length should be used in a core for various sizes of ejector pins. All I was able to find were several ambiguous rules of thumb. Some say the land length should be two times the pin diameter for pins under ¼ in. and one and a half times the pin diameter for pins over ¼ in. Others say the land length should be between ¾ in. and 1.5 in. Several sources say small-diam- eter pins need a "lot" of land length. All of these "rules" seem far too general to me. And the rule about needing a long land for a small-diameter pin seems to me to be an incorrect conclusion as a result of an unsupported length issue. So I started to think about the pros and cons of various land lengths, or as my father used to say, "Think in terms of extremes." If a land length is excessively long, it would cause greater resistance for air and gas to vent out. And because of the increased surface contact area, it would generate a proportionally larger amount of frictional heat and abrasive wear, which could cause a pin to gall. Conversely, if a land length was excessively short, it could prematurely distort the hole in the core because of insufficient bearing surface to resist the forces of pin deflection, misalignment, and the associated side loads. There must be a logical, mathematical solution to this age-old conundrum. LAND-LENGTH EXPERIMENT I decided to make a chart starting with a 1-in. diam. pin and assigning it a land length of 1.5 times its diameter. I then incrementally increased the number of diameters by 1% until I reached the smallest standard diameter pin—1/32 in. Interestingly, Why Ejector Pins Break and How to Prevent It In this installment the focus is on bearing length, clearances, keying, and machining. By Jim Fattori PART 4 FIG 1 Example of a thumbnail ejector pin. Pin Size, in. Number of Pin Diameters Land Length, in. Surface Area, in 2 1/32 2.80 0.09 0.009 1/16 2.74 0.17 0.034 1/8 2.63 0.33 0.129 3/16 2.53 0.47 0.279 1/4 2.43 0.61 0.477 5/16 2.33 0.73 0.716 3/8 2.24 0.84 0.991 7/16 2.15 0.94 1.295 1/2 2.07 1.03 1.625 9/16 1.99 1.12 1.976 5/8 1.91 1.19 2.343 3/4 1.76 1.32 3.113 7/8 1.63 1.42 3.910 1 1.50 1.50 4.712 Empirical Calculation of Land Length vs. Pin Diameters 28 NOVEMBER 2018 Plastics Technology PTonline.com K now How TOOLING

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