I've been experimenting with directly printing water-soluble molds for casting metal pieces, with some success. The mold is printed from PVA, the molten metal is poured into the printed mold, and then the mold is dissolved away in hot water. This allows very complex shapes to be cast. I have uploaded some models for printing molds to my 3d printed molds for metal casting collection at printables.com.

It may seem surprising that you can make a metal casting using a plastic mold, but the metal used is a low-melting point alloy of tin and bismuth (essentially, a relative of pewter), and the metal cools quickly on contact with the mold before it has a chance to deform.

There are two parts to making a cast metal part from a 3d-printed mold: designing the mold, and then making the part using the mold.

Designing the mold

Starting from an STL or other 3d file of a piece that you wish to mold, the steps are outlined as follows.

• Choose a good orientation for the mold. There are some limitations, for example: the piece can't be oriented with any upward-facing "pockets" because the mold will then have a downward-facing "finger" that can't be supported because you can't put supports inside the mold. In most cases though it is possible to find a good orientation that avoids this limitation.

• Form the mold, which consists of a hollow body whose outer surface is a constant distance away from the piece to be cast and whose inner surface is the piece to be cast. The molten alloy will be poured into the void in the hollow mold.

• Identify the high points on the piece that will need to be vented so that air can escape as metal is poured into the mold, and add the vents to the mold.

• Identify a good location to feed the molten metal into the piece. Typically I like to use a point somewhere near where the piece is angled at around 45 degrees upward, and ideally not on an area of critical detail because the remnants of the feed have to be filed and sanded off. Then add a channel and a funnel to the mold to pour the molten alloy into.

• Design supports for the mold. As mentioned above you can't put supports inside the mold, but the good news is that you can add any needed support to the outside of the mold without affecting the quality of the result. I haven't found supports made by the slicer to be completely satisfactory for this application so I design my own.

After spending a lot of time designing these elements by hand in a CAD program, I decided to write an interactive program to semi-automate this process using various open-source mesh processing libraries. It's not ready for general use, but if there's interest I'll try to clean up the worst of the rough edges and post it here.

Making the part

The steps are outlined as follows. These steps are desribed detail here.

• Slicing. This requires selecting the correct infill and perimeter settings, as well as appropriately selecting and tweaking the filament and printer profiles.

• Printing the mold, including bed prep for getting good adhesion with PVA.

• Melting and pouring, including appropriate temperature of molten alloy and use of water bath to help cool the part and ensure that the mold doesn't sag before the metal solidifies.

• Dissolving the mold. While PVA is water soluble, it does so slowly, and patience together with some thermal and mechanical encouragement are needed.

• Finishing the part, including removing the vents and feed, filing, sanding, polishing, and applying surface finishes.