Plastics Technology

JAN 2019

Plastics Technology - Dedicated to improving Plastics Processing.

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Let's assume you have a multicavity mold that requires a vali- dated molding process. Murphy's Law says that at some point in time one of the cavities is going to have an issue. That would require you to immediately shut down the press and make the necessary repairs. It's not much of a problem if you have a spare cavity-and-core set sitting on the shelf that you can simply swap out and get back into production. But what if you don't? Can you afford to be down for days, or even weeks? Running anything other than full cavitation is not only a non-validated process, there would most likely be a flow imbalance that could throw some of the other cavities out of specification. But what if the mold had an RSO like the one in Fig. 1, which can improve the flow balance to the other cavities? During the initial mold sampling to establish the best process to submit for validation, you could intentionally block off a cavity and establish a second process, with less than full cavitation. Both processes could then be submitted for validation up front. Obviously, there would be increased cost to validate two processes instead of just one, but doing so will keep you up and running if there's a problem. Granted, it won't be 100% efficient, but it buys you time to machine the necessary components in order to get back to full cavitation. I have always preferred RSOs to RTOs. Most RTOs have a spring- loaded ball plunger, an O-ring, a Bellville washer, or some other type of component under their heel to prevent it from rotating as the molten material flows through the runner channel at high velocity and high pressure. Over time, either the spring in the ball plunger fails or the O-ring gets stiff, which allows the RTO to rotate freely (Figs. 2A, 2B). That's when someone on the production floor will grind a point on the end of an old ejector pin and peen a dot between the RTO and the mold plate, to prevent it from rotating. This will elicit bad language from the next person who needs to rotate the RTO to a different position. Conversely, due to an accumulation of gas and rust around their body, RTOs can become nearly impossible to rotate at all. Despite these shortcomings, RTOs don't take up a lot of room in a mold and they are not very expensive to make or buy. Injection mold supply companies offer various types of standardized RTOs. I couldn't find anyone who offered a standardized RSO. I guess that's because there really is no such thing as a true standard RSO design. Most of them are tailored specifically to the individual mold. The three most common runner configurations machined into an RTO are a "T," "L," or straight-through, but others are possible (Figs. 3A, 3B). Obviously, at some point in time, an RTO needs to be rotated to a different position. The people on the production floor will use anything at their disposal to do this. I like to reduce their options to just one tool: a hex-key wrench. EDM'ing a hex into the RTO has several benefits (Figs. 4A, 4B). It provides a lot of bearing surface; it's easier to orient the RTO into the proper position; and it minimizes the chance of damage caused to the RTO and the adjoining areas when using some other type of method, such as a steel screwdriver or a pair of gate cutters. If the RTO is opposite the sprue bushing, the hex also functions as the all-important cold well to catch and retain any semi- solidified material residing in the machine nozzle tip. For full-round runners, the RTO on the cavity side is typically the sprue bushing. I try to avoid this configuration and use two RTOs or RSOs just beyond the sprue bushing. The amount of plastic at the end of a sprue bushing is already massive. The addition of the hex, as well as multiple runner channels, make this location considerably more massive. This extra mass can require the cycle time to be extended by a few seconds to prevent the sprue from breaking away from the runner and sticking in the sprue bushing when the mold opens. Another reason I try to avoid the configuration cited above is that production personnel often forget to back the machine carriage off before trying to rotate the sprue bushing. The hex can get stripped because the machine's nozzle-touch force prevents it from being rotated. Lastly, the production personnel don't know if there is a dowel pin or a key under the head of the sprue bushing that prevents it from rotating on its own accord. If the bushing is in a mechanically fixed position, any attempt to rotate it in the press with a hex key will be futile. RSOs are often an afterthought when designing an injection mold. When you understand all they have to offer, the folks in the trenches and in the office will appreciate having them when a problem occurs. FIG 3A FIG 3B RTO for feeding two or four cavities. ABOUT THE AUTHOR: Jim Fattori is a third-generation injection molder with more than 40 years of molding experience. He is the founder of Injection Mold Consulting LLC, and is also a project engineer for a large, multi-plant molder in New Jersey. Contact; FIG 4A FIG 4B RTO with a hex EDM'd for a full-round runner (left) and a trapezoidal runner (right). 36 JANUARY 2019 Plastics Technology T O O L I N G K now How

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