Plastics Technology

JAN 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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greater the chance of jetting. Instead of the material slamming into the opposing core right away, it's shooting up into the open void. But material type, injection velocity, mold temperature, and a dozen other factors contribute to a jetting problem—not just the gate type. In fact, I've seen a giant edge gate as large as 3/4 in. wide by 1/4 in. deep, jet into a wide-open cavity. As the tool designer, think about the potential for jetting and what you can change to minimize the risk. Sometimes you have to compromise between a large angle that may cause jetting, and a shallow angle that weakens the mold. Regardless of the length, angle, or type of tunnel gate you chose, the bore must be polished—in draw. The exception to this rule is if the material is soft or elastomeric. Those materials release better with a matte finish. I also recommend all tunnel gates be machined by EDM, especially in multi-cavity molds. Since tunnel gates are often used on small parts, the gates are usually small. If the orifice size of the gates are not within 0.001 or 0.002 in. of each other, the part dimensions could vary. In this business, it's always helpful to think in terms of percentages. Let's say one gate has a 0.030-in. diam., and another gate has 0.027-in. diam. That tiny 0.003 in. just choked off over 20% of the flow area. That particular cavity may have sink or voids because the gate froze off too early—all because of the thickness of a sheet of paper. D-gates and chisel gates are much better than elliptical gates for gating into shallow parts. While I don't recom- mend it, a chisel gate can also be machined to form a type of rectangular D-gate. But if you have concerns about the shallow angle required to gate into a shallow part, which can cause the gate to break off, you might be able to shift the parting line at the gate location to allow for a steeper angle. When a tunnel gate is machined into the stationary side of the mold, some mechanical means is required to pull it out of its bore. Typically, a shortened ejector pin with an undercut or reverse taper is used for two-plate molds, and a sucker pin is used for stripper-plate molds. When the tunnel gate is machined into the moving side of the mold, some mechanical means is also required to pull it out of its bore. Again, a short- ened ejector pin is typically used, but without the reverse angle. Regardless of on which side of the mold the tunnel gate is installed, the distance between the ejector pin and the tunnel gate is critical, as well as the length of the boss formed by the shortened ejector pin. If the pin is too close, the tunnel gate cannot flex. If it's too far away, it flexes exces- sively. Either condition can cause a problem. Every mold design is different, as is every type of molding material. There are no carved-in-stone rules on where to locate these ejector pins. It all depends on the rigidity of the tunnel gate and runner when the mold begins to open. The more rigid they are, the farther away the ejector pin needs to be. When tunnel gating into the stationary side of the mold, the boss formed by the shortened ejector pin does not have to be very long—typically only about the diameter of the runner. But when tunnel gating into the moving half of the mold, the boss must be at least the same length as the tunnel gate—not the depth of the gate, but its hypotenuse. One of the best lessons I was ever taught was to plan on things going wrong, and to have an option in mind to overcome the problem in case it does. When it comes to tunnel gates, rule number one is to leave a lot of bearing surface on the runner ejector pins. If you need to increase the length of the boss, but the pin is relieved, now yous have to install a stationary sleeve. I once had a mold where I wanted a tunnel gate that was machined into the moving side of the tool to detach from the part during the mold-opening stroke. Instead of incorporating an expen- Plan on things going wrong, and have an option in mind to overcome the problem in case it does. Sometimes it requires extremely long bosses to overcome tunnel-gate ejection problems, such as when the ejector pin is too far away from the tunnel gates. For shallow parts, stepping the parting line will allow you to use a larger tunnel-gate angle. @plastechmag 41 Plastics Technology T O O L I N G

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