How to make an Medium grade Polystyrene mould for concrete casting stepping stones without sealing it.
Making a mould with unsealed medium-grade EPS foam is possible, but the foam will be a single-use, “lost” mould that needs to be broken away from the concrete after curing. The concrete will adhere to the porous, unsealed surface of the foam, even with a release agent.
Materials and Tools Needed
- Medium-grade EPS foam
- Carving tools: A sharp utility or serrated knife.
- Safety gear: Safety glasses, dust mask, gloves.
- Concrete mix
- Mixing and finishing tools.
Step-by-Step Instructions
- Carve the Mould:
- Cut your desired stepping stone shape from the foam. Keep the design simple with no undercuts, as you will need to break the foam off.
- Use a sharp utility or serrated knife for cutting.
- Sand the interior surfaces lightly. The surface quality of the final stone will be rougher than a sealed mould due to the foam’s texture.
- Mix and Pour the Concrete:
- Mix the concrete according to the bag directions to a workable consistency.
- Place the mould on a flat, level surface.
- Pour the concrete into the mould, filling it to the desired thickness.
- Tap the sides gently to release large air bubbles.
- Finish the surface with a trowel or a sponge float for a non-slip finish.
- Cure the Concrete:
- Cover the mould with plastic sheeting and allow the concrete to cure for at least 24-48 hours. The foam will act as insulation, helping with the curing process.
- Demould (Break Away the Foam):
- Once the concrete is cured, you must mechanically break the foam mould away from the stepping stone.
- Start from an edge or corner and pull pieces of the foam off. This process is messy and the foam will be destroyed.
- Finish the concrete stepping stone by sanding the surface to remove the many imperfections that will be on the cast concrete as the polystyrene texture will appear on the surface of the and it would probably look better if they were removed.
This method works for one-time use, but for repeated use or a smoother finish, sealing the mould is essential and using a high density foam such as xpsfoam.
We have a concrete craft website which you may find useful. https://www.concretecraft.co.uk