Family Molds – Automatic Runner Balancing

In SOLIDWORKS, I’m designing some parts that will eventually be injection molded. Really, I’d like to create a Family Mold; having two different parts created in the same plastic mold. This is most often done when you have two components that need to fit together, like the top and bottom half of a casing, as I’m working on, shown below.

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I’ve run a simulation of the injection of polymer in SOLIDWORKS Plastics, however I’m finding a bit of a problem. As a result of the two different shapes and relative volumes, one side of my plastic mold is filling up before the other.

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GIF 1An unbalanced fill like this can lead to different pressures at end of fill on the two parts, and therefore differences in cooling and shrinkage fro the plastic part. As a result, even though the parts are designed perfectly, they may not fit together after molding!

In order to address this, we need to balance our runners. If we adjust the sizes of the runners leading up to each part individually, we can achieve a consistent fill time, and therefore the same pressure at end of fill. But where do we start? Well, thankfully SOLIDWORKS Plastics has an automatic runner balancing system that can calculate this for us! I’ll just start the process and grab a cup of coffee as it runs a few different iterations of our runner system, honing in on the correct runner design.

I came back a couple of minutes later to a completed, balanced family mold!

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We can even look at the results confirm the changes. The left side runner was increased in diameter, while the right side was decreased slightly. This will give us an idea of what we need our final runner system to look like. If we wanted to get more in-depth, we could easily take these results and model the runner system itself.

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Runner balancing in SOLIDWORKS Plastics is fast, easy, and most importantly, automatic. I didn’t have to touch the numbers at all; it was all taken care of while I took my coffee break. It’s sure to make designs of family molds a snap. We can easily predict and eliminate differences in pressure at end of fill before we even create the plastic mold, so our parts come out just how we want them to.