Help Me Select a Mould Material

Great, you need assistance choosing a mould 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.

Your choice of mould material is based primarily upon which casting material you plan to use and how many castings you wish to produce.

Polyurethane mold rubbers are excellent for casting concrete, plaster, wax and limited resins and are very economical.

Silicone mold rubbers are the best choice for reproducing numerous resin castings (polyurethane, polyester, epoxy) and polyurethane foam parts. Silicones are more expensive, but have greater chemical resistance and longer mold life when resin casting.  They usually don't require release agents.

Polyurethane and silicone rubbers are most popular as molds can be completed within a day!

The mold material you are about to choose is based largely upon three criteria:

  1. What casting material will I use?
  2. How many castings do I need?
  3. 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) or Silicone moulding rubbers
Polyurethane/Polyester/Epoxy Resin – use silicone mold rubber (PlatSil rubber)
Polyurethane Foam – use silicone mold rubber (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 Resins – 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:

  1. Model is a human body or animal – this is a special case where Hydrogel, Skin-Wax and PlatSil Gels are recommended.
  2. 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.
  3. 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 tyre 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.

 

  • Each of these models can be placed in a “box” or other easily constructed liquid-tight container so a one-piece block mold can be poured.
  • Notice no severe undercuts or long projections. There is relatively uniform surface texture/height.
  • Each of these models can be reproduced using a two-piece poured block mold.
  • This type of mold requires using clay to define a parting line that separates the two halves of the mold.
  • Rubber is poured over the clay parting line which creates a false floor for the first half of the mold.
  • Once cured, the mold if flipped over, clay removed; then the second pour is made against the first cured half

  • Each of these models can be molded with a one-piece blanket mold (poured or brushed).
  • The blanket mold can be supported with a one-piece mold shell.
  • Each of these models can be molded with a two-piece brushed blanket mold.
  • A two-piece mold shell also makes for easier demolding.
  • Each of these models can be molded with a two or more piece poured blanket mold and a two or more piece mold shell.
  • This type of mold requires the greatest level of skill, but once this technique is perfected, anything is possible!

 

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