Plastics Technology - Dedicated to improving Plastics Processing.
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During the frst 14 years of my career, when I was designing and building injection molds, gating and runners were not really a focus. I was concerned with building a robust tool and had no idea what was going on over at the molding side. In 2002 when I made a move from the mold shop to the molding foor, gating and runners jumped out at me right away as lacking standards. I started studying gates and runners when I was developing in-mold degating with tab gates. To create a clean break, gate geometry and gate land were critical. I saw there was nothing consistent in gate geometry between material or part size from mold to mold. I saw large parts with gates half the size of the gates on a smaller part running the same material. I observed that the main focus in mold designs was on gate location on the part, and gate orifce geometry was basically shoot- from-the-hip. For in-mold degating, I needed to have the gate as thin as possible and a minimal land to assist with a clean break. I started learning a lot about plastic pressures while I was welding and modifying gates on numerous tools. I remember the concerns people expressed about my making the gates thinner, but they gave no specifc data to back up their concerns. It was very confusing when there was no standard for gates or runners, and sizes and geometries were all over the map. The Importance of Gate Geometry GATE SIZE VS. SHAPE Through the process of modifying gates on various tools I learned some very interesting things. Just because you decrease the gate size or thickness, it does not always mean that plastic pressure will increase. And it increasing a gate thickness or size does always not mean your plastic pressure will drop. I mention this because it goes against common assumptions. Typically, when a gate is made smaller you can expect to see a change in plastic pressure because you have reduced the volume of the gate orifce. But a change in gate size or volume does not always produce a notice- able change in plastic pressure. I found out that when I decreased the gate thickness and had an increase of plastic pressure, I could widen the gate to compensate and reduce the plastic pressure. I also learned that the gate land can afect plastic pressure. It is important not to focus on the gate as the only restriction to plastic fow. Many people overlook the role of hot-drop tip orifces. I have often seen people open up gates or runners when the hot-drop tip was the restriction—as would occur, for example, when a hot runner feeds into a cold runner. BREAKING UP JETTING Before I get into some gating details, I want you to understand my basic concept. Most of you readers understand fountain fow in mold flling. Let's assume you have a round runner and experience textbook fountain fow. That's what happens when you take this volume of fow and ram it through a small round orifce into the large space of the cavity. Sure, you could gate into a side wall or have a restriction in front of it to break up the concentrated fow of plastic. But what if you could change the gate geometry to improve the transition from the runner to the part? My focus here will be primarily on cashew and sub-gates, but I learned the concepts when working with tab gates. I like to use a pressure washer as an example to help paint a mental picture. If you were to put a tip with small round orifce on your pressure washer, what type of stream Gate size and shape—unlike gate location—don't often get the attention they deserve. Many common assumptions about gate geometry deserve to be challenged. By Randy Kerkstra A cashew gate with no taper is an industry standard, but not necessarily the best design in terms of fow restriction and pressure to fll. The author prefers a thin and wide cashew gate with a mass taper down to the orifce as that minimizes jetting and fow restriction for the lowest pressure to fll. "Just because you decrease the gate size or thickness, it does not always mean the plastic pressure will increase—or vice versa." 34 DECEMBER 2014 Plastics Technology PTonline.com K now How TOOLING