![[Denny] removing a plaster bust from a microwave-softened mold](https://hackaday.com/wp-content/uploads/2026/01/pla-mold-feat.jpg?w=800)
3D printing is wonderful, but sometimes you just don’t want to look at a plastic piece. Beethoven’s bust wouldn’t look quite right in front of your secret door if it was bright orange PLA, after all. [Denny] over at “Shake the Future” on YouTube is taking a break from metal casting to show off a quick-and-easy plaster casting method— but don’t worry, he still uses a microwave.
Most people, when they’re casting something non-metallic from a 3D print are going to reach for castable silicone and create a mold, first. It works for chocolate just as easily as it does plaster, and it does work well. The problem is that it’s an extra step and extra materials, and who can afford the time and money that takes these days?
[Denny]’s proposal is simple: make the mold out of PLA. He’s using a resin slicer to get the negative shape for the mold, and exporting the STL to slice in PrusaSlicer, but Blender, Meshmixer and we’re pretty sure Cura should all work as well. [Denny] takes care when arranging his print to avoid needing supports inside the mold, but that’s not strictly necessary as long as you’re willing to clean them out. After that, it’s just a matter of mixing up the plaster, pouring it into the PLA, mold, and waiting.
Waiting, but not too long. Rather than let the plaster fully set up, [Denny] only waits about an hour. The mold is still quite ‘wet’ at this point, but that’s a good thing. When [Denny] tosses it in his beloved microwave, the moisture remaining in the plaster gets everything hot, softening the PLA so it can be easily cut with scissors and peeled off.
Yeah, this technique is single-use as presented, which might be one advantage to silicone, if you need multiple copies of a cast. Reusing silicone molds is often doable with a little forethought. On the other hand, by removing the plaster half-cured, smoothing out layer lines becomes a simple matter of buffing with a wet rag, which is certainly an advantage to this technique.
Some of you may be going “well, duh,” so check out [Denny]’s cast-iron benchy if his plasterwork doesn’t impress. We’ve long been impressed with the microwave crucibles shown off on “Shake the Future”, but it’s great to have options. Maybe metal is the material, or perhaps plain plastic is perfect– but if not, perchance Plaster of Paris can play a part in your play.
I’ve done this using a heat gun rather than a microwave. If the mold is a shell rather than a solid block, it’s easy to peel away when warm, and in fact a hairdryer would be hot enough. You can do bigger pieces that way, with less risk of explosions.
I would always give plaster plenty of time to set (i.e. at least wait for it to start to cool). It will still be plenty wet for at least a day while it’s still in the mold.
Although, I’d also say wait until it’s bone dry to sand it, as you have much more control. And if you used Jesmonite or cement you wouldn’t have the choice anyway. But, you know, whatever gives the desired result.
Tinkercad is pretty good for making negatives you can pour into.
I 3D scanned the Kingston REX “redhead” tiki statue pen holder that they gave away as swag in the 90’s, but the scan wants to be a “solid”. I’m just using repetier slicer, and set the infill to zero, and my shell to 2.5 mm. The bottom shell also, but I make the top shell zero. This makes it a cup of any thickness I set. Printing in ABS so I can vapor smooth it, but the scan was actually really good, so the vapor smoothing is just for the glass like finish.
i think you meant “plastic piece”
the life of the spelling cop…..
Indeed I did!
It’s a Sisyphean task you’ve set yourself, but “We must imagine Sisyphus happy”.
I wonder if you could use something like lightning supports in prusaslicer instead of the hollowing. Seems like that’d minimize the internal structure enough to still make it relatively easy to remove later? Especially if you break out a heat gun like some of the other suggestions.