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sive two-stage ejection system, a parabolic runner was machined into the stationary side of the mold. The end of the runner had undercut or "puller" ribs with a slight angle machined into the side of the runner. As the mold opened, the runner flexed at the same time the ribs pulled the tunnel gate out of its bore. As the mold continued to open, the undercut ribs disengaged from the stationary side. A center sprue puller retained the runner, so that it could be ejected on the moving side when the mold was fully open. Just like any other type of gate, tunnel gates should be fed from a runner that has an overflow, or cold, well to prevent the gate from becoming blocked or unwanted material from entering the cavity. And just like any other gate, tunnel gates should have generous radii where the gate intersects the runner and where the runner intersects the ejector pin. If the material is brittle, there is a good chance it will break off in one of those two locations. Instead of using one shortened ejector pin per tunnel gate, I like to use two shortened ejector pins mounted equidis- tant from the gate. It costs a little more, but I have had great success with this design. Now let's discuss gate flaking, or as I call them, mold killers. To repair an injec- tion mold that has been damaged by flaking, you must grind the parting line to remove the hobbed surfaces and refit any shutoffs. But now the overall height of the part is shorter and possibly out of specifica- tion. Additionally, the wall thickness at the top of the part is now thinner, which can lead to a "racetrack" or a "back-fill" condition. To eliminate that condition, you must grind off the top of the ABOUT THE AUTHOR: Jim Fattori is a third-generation injection molder with more than 40 years of molding experience. He is the founder of Injection Mold Consulting LLC, and is also a project engineer for a large, multi-plant molder in New Jersey. Contact jim@injectionmoldconsulting.com; injectionmoldconsulting.com. Puller ribs were added to a parabolic runner to detach a tunnel gate from the moving side during the mold-opening stroke. Add a generous radius at all sharp intersections to prevent tunnel gates and runners from breaking. Using two ejector pins equidistant from a tunnel gate helps reduce the possibility of the gate breaking off. cores, but you are still left with a shorter part. To get back into spec- ification, you also must machine the cavities deeper and re-polish. Therefore, to correctly repair the damage caused by flaking tunnel gates, you almost have to rebuild the entire mold. That's why I call them mold killers. So how do you prevent flaking? To understand how to prevent flakes, you have to first under- stand what causes them. The photo below left shows a small flake at the tip of an elliptical tunnel gate. As the tunnel gate is withdrawn from its bore, it flexes. This flexing "spring loads" the tunnel gate, which will try to spring back to its original shape when it's fully out of its bore. The flake is created at the split second before the tunnel gate is withdrawn. The spring force is greater than the strength of a small segment of rigid plastic at the tip of the tunnel gate, which causes it to fracture. Sometimes it's held onto the tunnel gate by a whisker. Sometimes it falls down into the chute. But thanks to that devil, static electricity, it usually adheres to the face of the parting line. Elliptical tunnel gates are the biggest culprits for flaking, especially if the gate is long and the angle from the parting line is over 45°. That's when the major diameter of the ellipse is the largest and the structural integrity of the tip is the weakest. D-gates are pretty good at not flaking. But chisel gates are the best because they are wide, and their structural integrity is consider- ably stronger. Chisel gates also leave the least amount of gate vestige because they are the shallowest of any tunnel-gate type— assuming they have equivalent flow areas. But before you make your selection of the type of tunnel gate to use, check the material supplier's design manual. Some types of materials demand a particular gate, and it may not be one of the four described in this column. Tunnel-gate flakes are created by the tip of the gate breaking off at the split second before it exits its bore. To correctly repair the damage caused by flaking tunnel gates, you almost have to rebuild the entire mold. 42 JANUARY 2018 Plastics Technology PTonline.com K now How