I’ve done a lot of concrete molding, you do need to be very careful with curing as not to get cracks in the concrete. If you do plan to use molds, My suggestion would be to mill your item in plaster as above then make your mold in silicone, because if you were to make a negative mold in plaster it would only last once and may not even hold up to the moisture content of the concrete.
My only concern with milling plaster products would be rapid dulling of the bits. (I don’t know how abrasive it would be but I am assuming it is grater than wood?) But this is easily addressed by having a separate set of bits for plaster work.
Back when I first got into CNC I played around with making plaster molds for metal casting.
I would carve the master in blue jeweler’s wax, make a silicone mold, cast it in plaster, pour molten metal in that. (I used low a low melt casting metal I got from micromark (expensive) and silver plumber’s solder, heated with a propane torch.)
It worked, ok. Most of my attempts were flawed in some way. I had a lot to learn about mold design, optimizing the shape for metal casting, adding sprues and pour holes, etc. But because of the multi-step process and the expense of making a new silicone mode for each revision I never pursued it.
But I never considered milling the plaster directly. That would seem to be a better approach for the prototyping process. Then, if you wanted to make a lot of them, you could make a silicon mold of the perfected design and cast a bunch of them in plaster for metal casting.
Thanks for sharing this. I will have to give this a try sometime!
I wouldn’t worry about dulling your bits, plaster of paris is a very soft material unlike concrete or harder mixes that would dull your bits. Do be sure to clean your bits well as moisture in the air can set the dust left on your bits.
Thanks Arron, hope to see people experiment with more materials and really see what our machines are capable of creating. Next I want to try making and milling my own Micarta (a paper or fabric and epoxy mixture) and try milling items like these colorful knife handles in the picture below.
Just a heads up: plaster, hydrocal, ultracal is very corrosive. You have to thoroughly clean your machine, tools, etc. or you will have a mess.
Time to go big, need to find a design for this 20" x 20" x 2.5" block of plaster! It’s about 3 1/2 times the size of the last one
Ha! just saw this, still have the second block. Never got around to carving it
Why are you doing it backwards?
Most would carve something that carves well. There are several types of milling material that give good detail. Then you mold the carving. That way you can make a bunch without creating a lot of carvings on the machine.
It was just an experiment, actually carves extremely well and detailed. It’s also a lot cheaper then going through the whole molding process.
I used to make aluminum molds from plaster of paris all of the time to cast aluminum and copper. The key is that it has to get baked out in an oven above 400F for a couple of hours (depending on the size of the mold). It will give you flawless castings if you do it correctly. I used to use the lost foam casting method and then dip it multiple times. Usually used POP or Premixed Drywall mud. Both worked well.
No sir. Just have to bury it in sand to the top of the sprue and pour.
What’s was the outcome?