Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/918111
There are many different types of cold-runner gates used to fill the cavities in a two-plate mold. Here I will be focusing on just one type—the tunnel gate—because of all the gate types, tunnel gates are the most misunderstood. An angled, conically shaped tunnel gate, also known as a subgate or submarine gate, is one of the best—and worst—types of gating methods. One of the reasons it's the best is because a tunnel gate automatically separates or degates from the part when either the mold opens or when the parts and runner are ejected. This reduces labor—often from the equiva- lent of one operator down to just a quarter of an operator. And those savings add up fast. Another advantageous feature is their ability to be machined at almost any angle. This is very convenient when you want to gate into a location that would be inaccessible with other gate types, such as in the thickest section of the part. There are various types of tunnel-gate designs. The four most common are: Tunnel Gates for Mold Designers • Full conical gate, which leaves an elliptical gate mark; • Truncated cone or D-gate, which leaves a "D"-shaped gate mark; • Spherical or ball-nosed gate, which leaves a perfectly round gate mark; • Chisel gate, also called a flare gate, which leaves a rectangular gate mark. All four have a knife-edge section of steel on the side nearest the parting line. That's the edge that does the shearing. But the D-gate and ball-gate also have a knife edge on the opposing side. Those edges can wear out quickly during injection, especially if the material is filled or abrasive. Tunnel gates have a bad reputation for freezing off too early, not being able to fill or pack out a part, or generating excessive shear. Those reported problems are almost always due to the tunnel gate being under- sized. It's common practice to make the depth of a standard edge gate 60% to 70% of the part's wall thickness, and the width of the edge gate is typically twice the depth. The width affects the length of material flow, while the depth affects the ability to pack out the parts. Let's say you have a part with 0.080-in. wall thickness. You might design an edge gate that is 0.050 in. deep × 0.100 in. wide. The flow area for this gate is therefore 0.0050 in.². If you wanted to use a tunnel gate instead, you would probably use the same 0.050 in. as the "diameter," because you assume that the same rule of thumb as an edge gate applies: 60% to 70% of the wall thickness. And that's where the error occurs. For these two gate styles to have an equivalent flow area, you have to do the math. The flow area of an elliptical tunnel gate is equal to: (Pi × Height)/2 × Width/2, or (Pi × Height × Width)/4 If the tunnel gate in this example were on a 45° angle, the height (major diameter) of the ellipse would be 0.096 in. and the width (minor diameter minus the size the tool maker measures with a gage pin), would be 0.067 in.—not 0.050 in. If you used the 0.050-in. Avoid tunnel gating into a part that has a large taper. Of all the gate types, tunnel gates are the most misunderstood. Here's what you need to know to choose the best design for your application. Get more insights on tooling from our expert authors: short.ptonline.com/toolingKH Learn more at PTonline.com KNOW HOW TOOLING By Jim Fattori PART 1 The four most common types of tunnel gate designs are (r-l) eliptical gate, D-gate, ball-gate and chisel gate. 38 JANUARY 2018 Plastics Technology PTonline.com K now How TOOLING