Plaster of Paris moulds

Hi folks,
Has anyone tried using plaster of paris to make moulds in the compression machine? I have a 3D printed prototype which I want to replicate and I was wondering if this would work. How much pressure is on the car jack? Is it a lot or just enough to move the bed upwards?


Hi @timberstar ,

I have not tried plaster of Paris for moulds, but I would say the compression machine would most definitely break the mould.

Injection might work if you reinforce the mould, but I also doubt it will last long (most brittle materials only make one time moulds, if even that).


I’d look for another moulding material, though Plaster of Paris would make for an interesting experiment…

great stuff, helps a lot!!


Sieving the plaster of Paris, like they did the sand in the video might also up the resolution.
It should have no problem handling the heat as I have also seen it used in low/no budget alluminium melting rocket stoves.

Good point about the mould line, but the wax method of course also has its “imperfections” to deal with, like the wax-channels.

I’m looking forward to seeing more about the results of your experiments.
My money is still on the green sand, but I never mind being proven wrong 😉

excellent yes, thanks I hadn’t seen that video. What I’m planning is basically the same idea but with a lost wax investment casting instead of using greensand. I am keen to try both methods, but I think lost wax might be better for my needs because I want to retain as much dimensional accuracy as possible. Also investment casting means no mould lines, because you just smash the plaster of paris to get the casting out.
As people have mentioned elsewhere on the forum, mould making is often the most challenging part of the process – which makes it the most interesting!


I think this might be the way to go:


Simple and cheap, especially because you can make the “green sand” yourself by mixing sand with kittylitter!:

@ donald
yes lost wax casting is nothing new, but I don’t know if anyone has tried it for making aluminium moulds for plastic? I’m part way through building a furnace to melt aluminium, as I want to make prototypes of my product in aluminium too, but my end goal is to make the product in recycled plastic so maybe I can make cast aluminium moulds with lost wax casting direct from a positive pattern?
My funds are limited so I can’t develop this method as fast as I’d like, maybe someone else out there would like to give it a try?


Re degassing: I would think pushing out the air would work better than trying to suck it out, but it would be an interesting experiment…

@donald Amazon sells a small degassing vacuum chamber for under £50, for use with casting resin, I’m guessing melted plastic would behave in a similar way to resin in a vacuum, but I have no practical experience with any of this yet, I am still developing a technique that will work for my goal


Re another option: You’re basically describing the lost wax method of casting bronze, so yes it could work.

Gonna need a rocket-stove to melt the alluminum though…

Also another option I’m developing for detailed moulds:

– 3D printed original pattern object (positive) or similar – could be hand made of wood etc.
– Attach paper divider to form separation point of mould, lubricate lightly with vaseline?
– Melt candlewax and paint onto outer surfaces of pattern object to gradually build up layers
– when sufficient thickness has been reached  – at least 30-40mm, and wax is fully cooled, separate wax negative from pattern
– attach wax sprues to negatives to form pouring inlets and smaller gas outlets
– submerge the wax negatives in plaster of paris in a plastic bucket and wait until it completely sets
– remove the hardened plaster of paris lump from the bucket, bake upside down in the oven on a baking tray until all the wax has poured out
– remove wax in baking tray; turn plaster of paris upside down and bake for a few hours more to remove any traces of wax and vapour
– remove from oven; melt aluminium and pour into inlet holes while plaster of paris is still warm
– wait until fully cooled; smash plaster of paris and retreive your aluminium negatives of your original pattern object.
-Use these to make moulds for recycled plastic injection/compression moulding

Any thoughts? A bit long winded perhaps, but maybe more accessible for some than CNC-ing aluminium moulds…

Edit to note -the reason you need to build up the wax in layers is because if you cast a heavy block of wax it shrinks so much that you would lose the correct shape of the mould


Please note they used pressure instead of vacuum to get the result!

Also safer for the vacuum cleaner* indeed 🙂

*Or at least: the old ones can also can blow (I e.g. have a Kirby that can do both), otherwise an upcycled inflatable bed pump might give the same result (or a copressor, but we were talking low tech 😉 )


Some weights on the lid might be enough to keep the seal…


@donald I love the idea of using a vacuum to get rid of bubbles and get a uniform material. What about even more lo-fi, I’m thinking to form solid heavy cylindrical blanks for turning:
– Find saucepan with lid
– Cut hole in lid and fit some kind of pipe attachment
– Add some kind of rubber gasket around the edge of the lid to form a seal
– Melt plastic in oven, in the saucepan with the lid off
– Remove from oven and put lid on
– Attach vacuum cleaner to pipe connector on lid and suck.
– Hopefully don’t fill vacuum cleaner with molten plastic lava
– Cool and remove de-gassed plastic blank

What do you think? I’d like to give this a try


Those are all vallid questions and I honnestly have no idea (yet).

I think the flow could be a problem in the sillicone, as the plastic will be less fluent than the metal, but that would be a guess.

Is the heat resistant silicone stiff enough, even when contained, to not deform under injection pressure? Does it vent well?
I don’t think the low temperature metal casting challenges the silicone in these areas.

Anyone seen the white paper Form put out about this stuff? I saw the original modular mould post by  @xxxolivierxxx and thought it was the one I’d seen from Form > here

We already started testing this technique at Precious Plastic Lancaster . Our recent blog post on details that escapade which I’ll be uploading here …whenever I get the time! We’ve already been using acrylic inserts inside metal moulds which is a slight variation on what’s been done in the PP community for a while now. We then tested inserts to modify the moulds printed from recycled PET but the printed part quality varies a lot and we weren’t able to pull a final product out of it. I’m put feelers out to Lancaster University to see if we can get parts printed in High Temp resin on their Form 2 SLA which would be capable of withstanding PP moulding temperatures.

The hope is to use plastic mould inserts to quickly prove out mould ideas / products at lower cost before investing in metal moulds. If we can effectively de-risk the testing of new products we can explore product ideas more quickly and find the winning ideas faster.

While doing some further research, I came across this video:


Replace the the bismuth-tin with your favorite molten plastic lava and you’re off to the races!


Should be possible to ‘hack’ a pressure cooker into a pressure vessel, maybe even leaving out the compressor and replacing it with a hand pump.

How low-tech can you go!

I guess it’s also a matter of what you have laying around when you start.

A new project seldomly starts without leftovers from an old one 😉


I think I found the topic you refered to:

Injection Molding Tests


Seems to be part of the bigger topic:

Mould Design Community, what if…???


All very, very interesting stuff…

The most common option for this type of moulds, is to use heat resistant silicone or Epoxy, waaaaaaaay better than plaster.



Those look like some great modular moulds to have in the Bazar.

CNC is not up and running yet, but I’ll try to get a ‘flipped’ version working (two identical halves making one mould).

Plaster might not be the best filling, but it does open up a lot more options for moulding.