Simple molds for plaster and concrete
Simple mold boxes are constructed with readily available materials and provide a flexible working method during design.
Many examples and considerations in this post work on a smaller scale suited for model building where considerations like hydrostatic pressure are secondary.
First of all: Creating good castings takes time!
It involves making a positive, a negative, a casting, and, most of the time, some clean-up. The process can be more enjoyable and successful if you use the right combination of mold and casting material and learn how to hide seems in your model to allow for more complex assemblies.
Mold types and basic considerations:
The construction of the mold depends highly on its complexity. Undercuts and enclosures require careful planning of the casting process.
(Gallery with diagrams)
- Molds with simple contours
- Molds with complex contours
- Molds with enclosures
- Molds with undercuts
- Post-processing of castings (drilling holes, etc…)
- No mold
- Undercuts / Enclosures
-Try to avoid undercuts and enclosed parts, and consider splitting your shape into different parts
-Undercuts and enclosures can lead to trapped air and incomplete castings - Disassembly logic
-Build your molds with disassembly in mind. - Wall strength and minimal sections
-Try to avoid long and narrow sections as much as possible - Material combinations
-Use material combinations that make sense. Cardboard and concrete are difficult to combine - The structural integrity of your mold
-Build your mold strong enough and make sure that it does not leak. Fill your mold with water before casting to identify possible weak spots. - Release agents
-Separating agents can lead to cleaner castings with less surface defects if applied correctly
Examples
The examples below should serve as a guide explaining some of your possibilities when designing a mold. Consider from the start what type of details are part of the mold and what can be done with some simple post-processing.
[Image → Block of plaster with a stick inside; the stick can not be removed] incl short description
[Image → Block of plaster; a hole is drilled by hand] incl short despricption
Materials
Good mold-making materials are easy to assemble, are non-porous, and provide the necessary flexibility to make changes on the go. Selecting a suitable mold-making material depends highly on the material that you want to cast and the result that you want to achieve.
- Cardboard (raw, coated) → works for plaster
- Foam board and hot glue → works for plaster and concrete
- 3d print – spiral vase → works for plaster
- Polystyrene → works for plaster
- Acrylic glass with engravings → works for plaster
- Vacuformed part → works for plaster and concrete
- Styrofoam – glued with contact cement → works for plaster and concrete
- MDF → works for plaster
- Phenolic plywood → works for concrete and plaster
Separating agents
Materials like MDF and cardboard are porous and unsuited for creating molds without applying separating agents to them.
Separating agents range from simply coating material in tape to applying shellac or wax to seal the surface.
Some of the more commonly used separating agents are: (should be accompanied by an image)
- Tape – Suitable for separating plaster from cardboard
- Talcum – Ideal for separating plaster from clay
- Shellac – Perfect for separating plaster from
- plaster – Ideal for sealing MDF before casting
- plaster – Suitable for sealing cardboard before casting plaster
- Beeswax (melted) – Ideal for coating cardboard, MDF, and other porous materials that are not heat-sensitive
- Vaseline – Ideal for coating styrofoam
- Release Oil – Suitable for coating wood before casting concrete / Perfect for coating phenolic plywood before casting concrete
Checklist Release Agent when pouring plaster into a mold:
