Here we'll share a new method of 3D printing of sand molds for metal casting. The advantage of this new method is that it's much cheaper than alternatives and also provides better surface finish. In fact, for most applications, right after the printing, the surface is good enough for casting without any post processing. This saves a lot of manual labor. It also allows 3D printing of very complicated molds in one piece, so that you don't need to separately design cope, drag and cores. You can just give the model of your casting with the feeding system to the printer and it will create a mold for it. This also avoids the leaks between the parts of the molds or assembly errors - it's just one piece. Another advantage of this method is that the mold material is porous, so the air is not trapped in the cavities, and thus doesn't cause any pouring defects. In addition to that, the mold material is collapsible, so it prevents cracking of the casting when it cools down. This new method is called "Selective Powder Deposition", or "SPD" for short. Originally, SPD has been used to 3D print metals. But recently we have discovered that it also works great for making sand molds for metal casting. And we can use any of the existing SPD 3D printers, but with slightly larger nozzles.
First, an SPD printer selectively deposits shell sand and normal sand into a crucible, such that the shell sand is placed where you need the mold, and normal sand is placed everywhere else as a support material. When it finish filling the whole crucible, you warm it up to about 200°C. This temperature doesn't effect the normal sand, but it solidifies the shell sand and turns it into a solid object. When it cools down you take it out of the sand and pour out the sand that was inside the cavities, if any. This solid object can be used as a cope, drag, core, or better yet all-in-one mold, as described above. The 3D printing of sand molds video might help visualizing it.
| Lost wax | Binder Jet (ExOne, VoxelJet) | SPD (iro3d) | |
|---|---|---|---|
| Cost | medium | high | low |
| Speed | slow | fast | medium |
| Surface finish | good | bad | medium |
| Collapsibility | bad | good | good |
| Permeability | low | medium | high |
| Can print all-in-one molds | only without cores | no | yes |