How to use: Molds for plaster forms
The printed mould is a mould for a mould: you cast a plaster slipcasting mould inside the print. The print is reusable — one set will give you many plaster moulds. Below is the whole process, from unpacking the parcel to your first cast.
- 01
Check the set and clean the print
Lay out all the parts: the wall halves, the insert (the positive of the vessel) and the base. Remove any filament strings and check that the alignment keys slide in freely. Wash the print in lukewarm soapy water and dry it.
- 02
Assemble the mould
Join the halves on the alignment keys and seat them in the base. Clamp the wall joints with rubber bands, a hose clamp or masking tape. Centre the insert precisely — this is what gives an even plaster wall thickness.
- 03
Seal the joints
Tape every gap on the outside and run a thin roll of plasticine or petroleum jelly along the inside. Plaster is very fluid and will find any leak.
- 04
Apply a release agent
Coat the inside of the mould and the insert with a thin layer of release agent: potassium soap, technical petroleum jelly or a soft soap solution. Wipe off the excess with a sponge — a thick layer shows on the plaster surface.
- 05
Mix the plaster
Use ceramic (pottery) plaster. Follow the ratio on the bag, usually about 100 g of plaster to 70 ml of water. Always sift plaster into water, let it slake for 2-3 minutes, and only then stir — slowly and near the bottom, so you do not whip air into the mix.
- 06
Pour the mould
Pour in a thin stream into one spot, down the wall, until the mix fills the whole cavity. Tap the mould against the bench for a dozen seconds so the air bubbles rise to the top.
- 07
Let it set and demould
Plaster heats up and sets after 20-40 minutes. Wait until it clearly cools down (usually 45-60 minutes), remove the tape, open the halves and gently lift out the insert. Do not lever with metal — the plaster corners are still soft.
- 08
Dry and use
Dry the plaster mould for 2-4 days at room temperature, or a day in a dryer up to 45 degrees. Only a dry mould absorbs water from the slip. Sand the edges with fine paper and you are ready to cast.
Frequently asked questions
- Which plaster should I use?
- Ceramic (pottery) plaster, not builders plaster. Builders plaster absorbs too little water and crumbles quickly after a few casts.
- How many casts will the print survive?
- The print is used for casting plaster and, with a good release agent, lasts dozens of cycles. It wears far more slowly than the plaster mould itself.
- Plaster leaked through the joints — what now?
- Usually too little tape or missing plasticine in a corner. Scoop the spill before it sets, wash the mould and pour again with more careful sealing.
- I can see print layers on the surface
- That is normal with FDM printing. For a smoother mould, sand the inside with fine paper (600-800) or apply a thin coat of acrylic varnish before the release agent.
- Can I wash the mould in a dishwasher?
- No. High temperature deforms the print. Wash in lukewarm soapy water and dry at room temperature.
- How long does the finished plaster mould dry?
- Depending on thickness, 2-4 days indoors. The mould is dry when it returns to a light, matte colour and no longer feels cool to the touch.