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Get more insights on tooling from our expert authors: short.ptonline.com/toolingKH Learn more at PTonline.com KNOW HOW TOOLING chute? That often happens when the tunnel gate is machined in the moving side of the mold and the boss next to the gate is too short. The tunnel gate is basically a pre-loaded spring up until the point where it clears its bore. If the boss disengages from the mold before the tunnel gate disengages, the runner becomes a projectile. Extending the length of the boss formed by the ejector pin will usually solve the problem. For multi-cavity molds, or parts with multiple tunnel gates, it's not uncommon for one gate to come out, while another gate stays put. Usually this happens because one of the ejector pins slides past the runner during the ejection stroke. Again, extending the length of the bosses formed by the ejector pins will usually solve the problem. For tunnel gates machined into the moving side of the mold, the length of the boss formed by a shortened ejector pin should always be greater than the length of the tunnel gate—not its depth, but its hypotenuse. If you don't have the bearing surface in the mold to shorten the ejector pin by the required amount, or if there isn't enough ejector stroke avail- able (either from the mold or the machine), try putting a point on the tip of the ejector pins. Some people call this a "centering cone." The goal is to keep the boss engaged with the pin, even when the pin is beyond the parting line of the mold. When a tunnel gate is machined into the stationary side of the mold, the initial mold opening speed can influence the ability to remove the tunnel gate from its bore, and it can affect the condition of the vestige on the part. Conversely, when the tunnel gate is machined into the moving side of the mold, the initial ejection speed can influence the ability to remove the tunnel gate from its bore, and it can affect the condition of the vestige on the part. Since varying these speeds can give you different results, even if you don't see a problem, it's good practice to find out which works better—fast or slow. Then adjust your setpoints accordingly to help widen your processing window. If you have a problem with jetting, it's probably because the material isn't hitting an obstruction or impinging on some type of molding feature right away. If the material is shooting into a large void, jetting can be difficult to overcome. And if the gate is undersized, that makes the matter even worse. Increasing the gate size is the first thing you might try, because it is usually the least expensive option. Raising the temperature of the core might help, because sometimes the melt bounces off a cold core and squirrels its way farther down the cavity. If you are fighting a flaking issue, one of the best solutions is to convert the elliptical gate to a chisel gate. If an ejector pin is too far away from a tunnel gate, adding stiffening ribs to the runner will reduce its flexibility. Converting elliptical gates to chisel gates often solves a flaking problem. Determining the location of the ejector pin that forms a boss for a tunnel-gated mold that will run multiple materials is especially challenging. 52 FEBRUARY 2018 Plastics Technology PTonline.com K now How