Plastics Technology

JAN 2018

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"diameter," the flow area would only be 0.0028 in.², almost half of the presumed "equivalent" edge gate. The interesting thing is that when these two gate types are designed with equivalent flow areas, the edge gate is the one that has less of a chance to pack out the parts, because it is shallower. Some people think tunnel-gate orifices should be kept as small as possible so as not to increase the gate-seal time. That's nonsense. Would you make an edge gate as small as possible for the same reason? What does it matter if the gate-seal time has to be increased? The parts are still solidifying, so the time required to cool the parts doesn't change. The worst-case scenario is you have get the screw back a little faster to maintain the cycle time. Other people think the base of a tunnel gate should never be larger than the diameter or width of the runner. That's just total nonsense and should not even be considered when designing a mold. I have seen dozens and dozens of molds where the base of the tunnel gate was considerably larger than the runner—and they worked great. In fact, I often prefer a tunnel gate larger than the runner. There's a larger mass that stays hotter longer and is more flexible. The material flow is less restrictive. And such gates are less prone to sticking in their bore. Tunnel gates also get a bad reputation for leaving a high gate vestige, especially when the size of the gate is large. That's partly nonsense. While the size of the gate usually affects the size of the vestige, the predominant control- ling factor on how "clean" or how high a vestige you get is based on the angle of the wall it's gated into. Tunnel gates typically don't break away from the part. They are sheared off. The edge of the hole in the steel that is closest to the parting line does the shearing. There are exceptions to this rule, such as if the material is highly flexible, the mold design has some flaws, or if there is some type of two-stage ejection system. Using the previous example, if the tunnel gate was gated into a part with 4° of taper on the side, you can expect to get a gate- vestige height of 0.007 in. Draft angles are more commonly down around 1°, which would make the vestige height just 0.001 in. Where you get into trouble is when you gate into a part with a large taper, or almost any radius. In these cases, the gate-vestige height can be fairly large. As the gate wears and needs to be sharpened, the size of the vestige gets even higher. Always try to avoid tunnel gating into a part that has a large taper, or into a radius. The length of a tunnel gate can be very short or very long. Shorter tunnel gates can often be machined at shallow angles, whereas longer tunnel gates require larger angles for easier removal. Keep in mind, longer gates start out farther away from the cavity, which makes for a stronger mold. Gates that have a shallow angle have to be machined close to the cavity wall. Since the injection pressure at the gate is extremely high, it is often the first place to flash. Using a separate gate insert for short tunnel gates is always a good idea. It will save you from having to do a lot of welding down the road. Tunnel gates also get a bad rap for being the cause of "jetting." That's usually nonsense. No matter what type of gate you have—edge, tunnel, cashew, fan, pinpoint, even hot-runner—unless the material is crawling into the cavity, they all jet to some degree until they hit an obstruction. The most common obstruc- tion is the wall opposite the gate. A small gate does increase the chance of jetting. And because tunnel gates are frequently undersized, a 0.030-in. gate will jet more into a 0.100-in.-wide cavity than a 0.060-in. gate will. There's more room for the small stream of material flowing through the gate to snake around between the cavity walls. The distance between the gate and the first obstruction usually deter- mines whether you are going to jet or not. The reason I say tunnel-gate jetting is usually nonsense is because it depends on the angle of the tunnel gate. The angle of a tunnel gate's center line usually ranges from 30° to 50° from the parting line, but I've seen that angle as low as 20° and as high as 60°. The higher the angle, the Using a separate gate insert for short tunnel gates is always a good idea. There is no reason the base of a tunnel gate can't be larger than the runner it connects with. The height of a tunnel-gate vestige is predominantly a function of the angle of the part it is gated into. Typical tunnel-gate dimensions. Material Stiffness Dependent 0.007 in. ← 4° → → ← Cutting Edge 0.095 in. ↓ ↓ R R R 30°-60° 5° to 20° 0.08 in. Minimum 40 JANUARY 2018 Plastics Technology PTonline.com K now How

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