July 14, 2022

Pre-drying injection materials

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I’ve seen thought-to-be-dried materials giving such splays, randomly appearing from one shot to the other. After spending times on many other things we measured the moisture content, although granules were dried for more than 2hrs in a dehumidifier. They were not dry enough!

Because inserts are manually placed by an operator before the injection cycle you blow the tool surface with cold air, increase the residence time (furthermore not the same amount from one shot to another!) and bring potential pollution through the inserts and manual operation (even though operators usually wear gloves in this case). This makes a good number of possible root causes. I’d see the “over-shearing” as an interesting one to investigate (after moisture content of course) for 2 reasons: the PA12 you use has a high viscosity / low fluidity (it is hard to inject) + it first flows around an insert, that is made of metal. Trying to pre-heat these inserts (at least on 30 consecutive shots), this should ease the material flow near the sprue. And if it’s over-shearing you’re fighting against it should help decreasing / avoiding these splays. But you need to do it on multiple consecutive cycles, otherwise you won’t have time to see effects. Pre-heating these inserts will also help reducing the moisture that is always on insert surfaces, even when they look dry. This won’t be a bad thing at the end for these bowls.

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