HIPS filament expands by how much?

Hey I am a middle school tech teacher who is having my students customize phone cases to sell for a fundraiser at out school. I have been downloading blank phone templates to test them out before we begin customizing them but I am having a few problems. First off, we are only printing in HIPS because we get it super cheap and it hardly ever fails. The problem is that it seems to expand slightly when it cools. This is a no good because none of the phone templates we are downloading seem to fit the phones… If I just knew how much to scale up a print before making it, I feel like we could probably make the cases fit. What do you guys think? I am still a bit of a printing novice and would love some advice. We are printing with Lulzbot Minis and a Taz 5 if that info helps. Thanks!

Phone cases might be a tricky item. Maybe a keychain would be a better fundraiser item, at least for a first try.

The plastic shrinks as it cools. The actual dimensional change in the part depends on its shape, material, tool path and printing temperature. The function of snap-fit items, like phone cases, are tricky because their function is very sensitive to error in certain dimensions. If the part was originally tested on a different brand of printer, then it is likely to need some trial-and-error to find the scale factor to get a correct print on a Lulzbot machine. Since HIPS is relatively stiff, it increases the difficulty of getting a snap-fit to work well.

you should be getting nearly identical shrinkage to what you would see with ABS, about 1% which the firmware already compensates for. if the part is too large by an appreciable amount there may be something else going on. SOme pictures would help diagnose.