Plastics Technology

FEB 2018

Plastics Technology - Dedicated to improving Plastics Processing.

Issue link: http://pty.epubxp.com/i/929876

Contents of this Issue

Navigation

Page 51 of 91

Tunnel gates can be temperamental. Sometimes they work flaw- lessly. Other times they're a real challenge. In the last column, I addressed issues from the point of view of the mold designer. Here, I'll cover the more common problems with tunnel gates from the processing perspective, and offer some proven solutions to those problems. Four very common problems associ- ated with tunnel gates that you may run into are: • They don't want to come out of their bore; • They break off; • They shoot across the mold upon ejection; • One gate comes out, but another stays put. If the tunnel gate is machined into the stationary side of the mold and it doesn't want to come out, open the mold about ¼ in. and check to see if the runner is being pulled out of the moving side of the mold. If so, you need to retain the runner better. Increasing the length of the boss near the tunnel gate, or increasing the amount of undercut formed on that boss, will usually solve the problem. But don't rule out a worn gate or an unpolished bore as contributing factors. The more common scenario of why a gate doesn't want to come out is because it broke off where the gate meets the runner, or where the runner meets a boss formed by a shortened ejector pin. This is very common with rigid and filled materials. The culprit is almost always a sharp inside edge at their intersec- tions. The solution is to radius the heck out of these areas. It may not look pretty, but it works. In addition to reducing the notch sensitivity in these locations, you'll also increase the mass Tunnel Gates for Processors of plastic. This extra mass stays hotter longer, and allows the material to be more ductile. If there is room available, you can also increase the included angle of the tunnel gate to further increase the mass of plastic at the parting line. Increasing the mold temperature on the side of the mold that the tunnel gate is machined into often works, but at the expense of having to extend the cycle time. Some people increase the size of the runner in and around the gate location to increase its flex- ibility. I don't recommend doing that on multi-cavity molds because it can affect the cavity balance. Before you try any of these suggestions, make several shots at a faster cycle time. The faster the cycle, the hotter the mold, and the hotter and more flexible the gate and runner become. If you're still making good parts at a faster cycle and the gate sticking and breaking issues went away—you're a hero. Have you ever gotten grease on your parts because the runner bounces off the leader pins or tiebars before it goes down the What you need to know to overcome problems out on the production floor. PART 2 By Jim Fattori If you have a problem with jetting, it's probably because the material isn't hitting an obstruc- tion or impinging on some type of molding feature right away. If an ejector pin is too close to a tunnel gate, increasing the length of the boss and increasing the mass of plastic at the parting line often fixes the problem. 50 FEBRUARY 2018 Plastics Technology PTonline.com K now How TOOLING

Articles in this issue

Links on this page

Archives of this issue

view archives of Plastics Technology - FEB 2018