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PLA model shrinks when vaccuum forming dental retainers

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I´m producing my dental models for making dental aligners on my UM2+ wich by the way works perfect. I use PLA anN infill of 25-30% but when trying to make the aliogners on the vacuum machine (2.5-3 BAR) they shrink. Should I increase the infill? or any other change in the process to avoid this ?

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Do you make them on a vacuum thermoforming machine where the model and the plate to thermoform are both placed under a heater, and the hot molten plate is then pulled over the dental model, and sucked vacuum? (Such as the Erkodent thermoforming machines?)

In that case, probably the heat is the main problem. PLA starts getting soft from around 50°C, and your thermoforming plate is heaten to maybe 120 or 150°C or so.

So I would suggest that you try printing the teethmodel with 100% infill, just for testing, and that you put that model in the freezer prior to thermoforming.

Do not yet insert the model in the machine while this is heating up, but only insert it at the very last moment when the heating is finished, and when the molten plate is ready to be pulled over the teeth model. So that the model is still ice-cold when the hot plate is pulled over it. (But only if your machine allows this manual handling, of course; since this may not be possible on fully automatic machines.)

Then try to cool the whole thing as fast as possible, with a cold fan blowing over it, or so. Try if that works, or do you still get deformation?

Otherwise, try printing in a material that can withstand much higher temperatures.

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