Help Me Select a Mold Material
Great, you need assistance choosing a Polytek mold material. Let’s get started. The selection process is not always straight forward, but we are going to try to simplify it as much as possible.
The mold material you are about to choose is based largely upon three criteria:
- What casting material will I use?
- How many castings do I need?
- What is my model made of?
Let’s consider casting materials first. Below is a list of the most common casting materials you might choose and the corresponding mold material we recommend:
- Concrete – use polyurethane mold rubber (Poly 74-Series, Poly 75-Series, Polygel)
- Plaster – use polyurethane mold rubber (Poly 74-Series, Polygel)
- Polyurethane/Polyester/Epoxy Resin – use silicone mold rubber (TinSil and PlatSil rubber)
- Polyurethane Foam – use silicone mold rubber (TinSil and PlatSil rubber)
- Wax – use polyurethane mold rubber (Poly 74-Series, Polygel)
Let’s consider how many castings you need for each of these types of materials.
- Concrete – you can cast hundreds of parts using properly designed polyurethane molds.
- Plaster – you can cast hundreds of gypsum parts using polyurethane molds.
- Polyurethane/Polyester/Epoxy Resin – you can cast upwards of a hundred parts using silicone rubber molds. Economical alternative – use lower-cost polyurethane rubber to cast 10-20 parts if that’s all you need. If you use polyurethane, you will need to use PolyCoat or a release agent (if you are painting or finishing your parts, you will need to remove this release).
- Polyurethane Foam – you can cast upwards of a hundred parts using silicone rubber molds. Economical alternative – use lower-cost polyurethane rubber with PolyCoat to cast fewer parts. PolyCoat can be reapplied to extend the life of a polyurethane mold.
- Wax – you can cast hundreds of parts out of a polyurethane mold.
Let’s consider your model material. There are three notable considerations here:
- Model is a human body or animal – this is a special case where Skin-Wax or PlatSil Gels are recommended.
- Model is made from sulfur clay; polyester resin; certain epoxy resins or has had tin silicone or latex rubber applied to it in the past – in this case use a polyurethane rubber or a TinSil rubber (after testing for cure). PlatSil rubbers are not recommended.
- Model is valuable, needs to remain unharmed. The mold making process can be complicated, and it is difficult to ever guarantee this to a customer or client. Putting any mold material/sealer/release in contact with a model may slightly alter its appearance. Only small scale testing can help determine to what extent. Polytek offers sealers such as PVA (a water soluble coating) that acts as a barrier between a model and a mold material. Silicones rubbers are often used in this case since additional release applied to the model is not always needed. Still, the oils from the liquid silicone rubber may affect the model’s final appearance. When in doubt, test first!
Ok, based on the above discussion, you now have a mold material picked out for your project. The next step is to determine the best mold making technique to use and what hardness rubber would be best. The following considerations are important factors in these decisions:
- Model size, shape and location. Small models lend themselves to single or multi-piece poured block molds. Medium size models can be poured or brushed. Brushing is more useful when the model has “projections” that stick-out from the average surface height over the rest of the model. Large models can be poured, brushed or sprayed depending again on surface texture irregularities. If a model is on a wall, ceiling or other location preventing it from being laid flat, brushing or spraying will be more appropriate.
- A note about rubber hardness. Each Polytek rubber product line offers a range of hardness. Generally, the more simple a model, the firmer the rubber; the more complex the model, the softer the rubber. A car tire is about a Shore A60 and the palm of one’s hand is about a Shore A30. Softer rubbers can be stretched more owing to their greater flexibility. This can allow undercuts and more difficult areas of a model or casting to demold without damage to the rubber of the object. Visit this blog entry for an overview of Shore Hardness.