Plaster should be close to ideal for this application. Before you give up on it, here are a few considerations and ideas.
Even at a thickness of 1/4" or 3/8", plaster should be strong enough not to easily break up. Check these possibilities that may be the cause:
- Not using a good ratio of water to plaster (especially too much water or way too little water).
- Not waiting long enough for all the plaster to get saturated with water, or not mixing adequately.
- Demolding too soon and the plaster isn't yet strong enough.
- The plaster is full of air bubbles (use more vibration and tapping to get them out when you pour).
- The plaster is old. Plaster absorbs moisture from the air every time you open the container (or through the container if it isn't airtight). The moisture allows some of the plaster to hydrate or partially hydrate before you use it, so the casting isn't as strong.
You can also make the casting stronger by reinforcing it (good if the item is large but thin). Line the mold with a layer of plaster and get the bubbles out. Then lay in some drywall repair mesh and cover with another layer of plaster, or saturate some coarse cloth with plaster and layer it in the mold.
Assuming you can get the plaster out of the mold without it breaking, you can add a lot of strength by coating it after the plaster dries. Dilute PVA glue with a little water and paint the surface. When that's dry, give it a second coat. This will help with handling the casting, but it may be of limited value for carving it.
Alternate casting materials
If something about the mold is just problematic for casting it with plaster, the next thing to try would be a stronger plaster-like material that is soft enough to easily carve with a hobby knife.
- Mixing the plaster with a small amount of PVA glue adds a lot of strength.
- Hydrocal is an enhanced plaster that is stronger.
- Jesmonite or its competitors, like Hydroflow, is a stronger casting material that uses plaster with latex acrylic resin rather than water. It's a bit pricier than plaster, though.
A few other alternatives
Plaster-like casting materials can be demolded within hours and are fully cured within a day or two. If you have the time, there are air-dry materials that will also work.
- Air dry clay or paper mache clay can be applied and dried in relatively thin layers. It is very strong, so you can often create a shell, with the thickness governed by how much carving you need to do, rather than casting a solid block. Some formulations are very good at retaining fine surface detail from a mold. Some air dry clay recipes allow you to demold almost immediately. Others do better if you wait until it is fully dry.
- You can make a clay or putty by mixing sawdust and PVA glue. Press it into the mold to create a relatively thin shell and let it dry for several days. If you need more thickness, add another layer after it is fully dry. The result can be carved or machined similar to wood. As long as the mixture is putty-like, rather than crumbly, the glue will fill the surface detail when you press it into the mold, and the result will have a surface similar to a resin casting.
Use hot melt glue. Apply a thin layer to the mold. Use a heat gun at a low setting to melt all the glue to a very liquid consistency so it flows well into all the nooks and crannies. When it has cooled, add another layer (don't use a heat gun after the first layer; you want to minimize heating of previous layers). Build up layers to the needed thickness using thin layers and allowing it to fully cool before applying the next layer. Hot glue shrinks when it cools, so if you fill a mold of any significant size with hot glue all in one step, it may distort as it cools. Doing it in thin layers, allowing each to completely cool, avoids that. Hot melt glue can be painted, and at room temperature is easily carved or drilled (for any drilling or machining, just be careful to not let the bit get hot).