Plastics Technology

JAN 2019

Plastics Technology - Dedicated to improving Plastics Processing.

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Most people think runner shutoffs (RSOs) are needed only for family molds. That's not true. Many people use them in dedicated high-cavitation molds. Let's say you have a four- or eight-cavity mold, and one of the cavities has a problem. Sometimes it's advantageous to be able to quickly and temporarily block off that cavity—in the press—and continue molding. Your profit margin just dropped by 12% to 25%, but you are still able to fulfill a promised delivery date to your customer. I cannot stress how important it is in this situation to have the mold repaired at the end of the production run, so as not to have a cavity blocked off at the start of the next run. In addition to the loss of profit margin and extended time to make the production quantity, you also expose yourself to potential mold damage. Most molders have a process setup sheet for every mold. That sheet is based on using all the cavities in the mold. So, what happens when you dial in the shot size for eight cavities, but only six or seven are open? Flash, overpacked parts stuck in the cavity, broken ejector pins, or a cracked cavity insert are just a few of the problems you might encounter when you accidentally mold a "pancake." RSOs can do a lot more than just shut off a cavity or divert material flow. Quite often, when a cavity in a family mold is blocked off, the material flow and fill pattern change, as do the packing pressures to the other cavities. The same change in flow and fill can occur if the mold is used to run more than one type of thermoplastic material—especially if they have very different Tricks of the Trade on RTOs, RSOs viscosities. An RSO can be deployed to change the gate location to the open cavities to overcome a flow issue. Changing the gate location can potentially improve or eliminate things like an undesirable weld-line location, excessive sink, or trapped gas. I had one mold that required some parts to be run in acrylic and others in rigid PVC. The acrylic parts were no problem, but the process became pressure- limited when running the more viscous PVC. An RSO was added to the mold in order to open a second flow channel, which led to a second gate to feed the part. Say you have a two-cavity family mold, and one of the cavities is an interchangeable insert that makes a different part. The mold has an RTO to run one cavity or the other, but usually both cavities are run at the same time. When you debug the mold using one of the interchangeable cavity inserts, everything is fine; but when you install the other cavity insert, the balance is way off because the part volume, wall thickness or flow length of the new part is considerably different. Adjusting the size of the gate to either one of the parts will result in a host of other problems—primarily the gate-seal times and different pack pressures in the cavities. So instead of using a small round runner turn-off (RTO), a long, rectangular RSO insert could be extremely helpful in this scenario. It will allow you to adjust the flow and associated fill pressure to the gate by changing the runner size. The longer the runner you have to work with, the easier this rebalancing will be. Therefore, if you made two of these RSO inserts, each with a different size runner cut into it, you can now run either of the interchangeable cavities without the original flow-imbalance problem and without having to change the gate diameter. One runner acts as a flow enhancer, and the other acts as a flow restrictor. If you have the real estate in the mold to mount the RSO insert off-center, you can cut both runner sizes in the face of just one insert, as shown in Fig. 1. Use the one runner size for one of the interchangeable cavities. Rotate it 180° to run the other interchangeable cavity. Or flip the RSO over to completely block the flow of material to the cavity. RTOs don't take up a lot of room, and they are not very expen- sive to make or buy. They have a lot more to offer than shutting off a cavity. Let's take a look at what else they can provide. By Jim Fattori PART 2 FIG 1 An RSO with two different runner sizes machined into one face to balance interchangeable molds. FIG 2B RTOs often rotate during production. FIG 2A 34 JANUARY 2019 Plastics Technology PTonline.com K now How TOOLING

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