Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

Issue link:

Contents of this Issue


Page 49 of 91

you beginning to see how important the unsupported length is? If you think you might have an issue with an excessive amount of unsupported length in a mold, how do you reduce it? The first thing to do is estimate how much ejector stroke is needed to safely eject the part off the core. For a new mold, this can vary depending on the mold design, part geometry, material and other factors. Typically, you add about a half an inch or so to the overall height of the part, but this all depends on ejection speed, undercuts, mold design, robotics and various other factors. When you order (or design) a mold base, you specify what height you want the ejector-housing rails to be. That dimension is equal to the desired ejector stroke, plus the thickness of the ejector plates and the rest buttons. You shouldn't want to go any longer than your estima- tion, and you obviously never want to go any shorter. If it turns out that the mold design does not have the minimum stack height required for the intended press, which is common for large flat parts, it is very easy and rela- tively inexpensive to add spacers to the back side of the ejection clamp plate to make up the difference. Adding a spacer plate to the molding machine's platen, instead of spacers on the mold, is rarely a better alternative. Let's assume that when the mold goes into production, an ejector-pin buckling problem starts to develop. If the ejector stroke can be shortened, do not add stroke-limiting spacer blocks. They won't help reduce the unsupported length of the ejector pins. You need to reduce the height of the rails and the length of all of the ejector and return pins by an equal amount. Ejector-stroke limiters are beneficial only if you don't have a buckling problem, but you want to increase the longevity of the mold by preventing production personnel from over- stroking the ejector plates. If you ever add ejector-stroke limiters, mount them in line with the machine's ejector bars. I cannot stress this enough. It is probably the most common cause of bent ejector plates. I like to make the stroke limiters out of a nylon or a hard urethane to help absorb the impact caused by momentum or an improper machine setting. As shown in Tables 1 and 2, the smaller the ejector-pin diameter, the shorter the unsupported length needs to be to prevent it from buckling. Any pin 7/64 in. or smaller should be stepped or shouldered to reduce the pin's unsup- ported length. The shoulder on stepped pins is 1/8-in. diam., and is available in ½-, 2-, 3- and 4-in. lengths. You want the shoulder of the pin to enter the support plate when the ejector plate is back against the rest buttons. If a 4-in. shoulder is still not long enough to enter into the support plate, you can either have custom pins made or you might be able to use an ejector-pin extension. Now here's the weird thing: Several mold-component suppliers offer extensions for ejector sleeves, but no one that I know of offers exten- sions for ejector pins. Maybe someday they will realize this is something molders occasionally need. Until then, we will have to make our own extensions, and there are multiple ways of doing so, as shown in Fig. 1. In Fig. 2, the ejector pin on the left is mounted in an ejector-pin extension, because the shoulder is too short to enter the back of the support plate. Other pins are depicted with bearing inserts on the bottom of the support plate, on the top of the support plate, and on both top and bottom of the support plate. These bearing inserts are a simple way to reduce an ejector pin's unsupported length. They can be made of various types of metal—from bronze to heat-treated tool steel, whatever is your preference. The ejector pin on the right uses a thin- Ejector Pin Diam., in. 1/32 1/16 5/64 3/32 7/64 1/8 1 Max. Unsupported Length, in. (Slenderness Ratio = 100) 0.8 1.6 2.0 2.3 2.7 3.1 25.0 Minimum Force to Buckle, lb, at Max. Unsupported Length 91 115 233 394 601 1453 93,019 Force on Ejector Pin, lb, at 40,000 psi Plastic Injection Pressure 31 69 123 192 276 491 31,416 Cross-sections of various ways to make an ejector-pin extension. TABLE 3 Injection Force on Ejector Pins Vs. Buckling Force FIG 1 Get more insights on tooling from our expert authors: Learn more at KNOW HOW TOOLING If you have to use ejection-stroke limiters, nylon or hard urethane are good material choices 48 SEPTEMBER 2018 Plastics Technology K now How

Articles in this issue

Links on this page

Archives of this issue

view archives of Plastics Technology - SEP 2018