How to Choose a Resin or Rubber
With so many options available for liquid tooling materials (aka resin and mold making rubber) it can be daunting to figure out what to use for your project. Should you use a quick curing resin or a slow cure? Condensation cure or additive cure silicone? Even if you can narrow down your selection to a product series there are often many seemingly similar variations to pick from. The distinctions between products are buried in technical specifications which seem better suited for a chemist than a prop maker.
You might be tempted to pick a product at random, or even worse to buy based on price alone. Don't risk wasting your money; instead follow these guidelines to select the right material.
For most uses you can focus on these specs:
- Mix ratio: Some resins are easy to mix by volume, and others require a gram scale. Obviously if you don't own a gram scale then you should pick a resin which has a 1:1 mix ratio.
- Pot life / Gel time: This is how long you have to pour the resin into your mold after you've started to mix parts A + B. Choose a resin that stays liquid long enough for you to get it into your mold(s)
- Cure time / demold time: A faster cure means you can take the finished cast out of the mold sooner. On the flip side, some strong resins need to sit overnight to properly cure.
- Shrinkage: Resin shrinks when it cures. Some resins are filled with inert materials which lessen the amount of shrinkage. Is this important? It depends on what you are making. If the piece does not need to fit perfectly with another part then you can ignore this number. For our 1:1 scale replicas this number is important to us. In fact, it is absolutely critical on projects like our working Zat Gun each piece was CNC milled to an exact dimension by the studio. Also, if you are making a cast which you will re-master and then mold again then you can prevent generational shrinkage (degradation which occures with a copy of a copy of a copy) by using a non-shrinking resin for your new master.
- Viscosity: Usually not that important, but if you are casting a highly detailed part then a low viscosity will ensure that the resin can flow into all of the detailed ares of your mold.
- Shore D hardness: This is the durometer hardness of the final cast. Unless you are trying to achieve a certain feel to your cast then you probably won't care about this number. In general, anything from 70-90 will feel hard to the touch.
Aside from these specs there are many specs which tell you how strong the material is. For display props this doesn't matter, but for props that are going to see a lot of physical use then you should carefully compare each product. Some numbers to look at include the compressive, flexural, and tensile specs.
When making molds consider the following:
- Tin/Condensation cure or Platinum/Additive cure: Tin cure rubber can be a lot cheaper than platinum cure. However, tin cured rubber degrades over time a lot faster. So if you want your molds to last a long time consider platinum cured silicone. If you only need a few casts then save some money and go with tin cured.
- Mix ratio: Most silicones will require a gram scale. However there are some 1:1 mix systems if you don't have a scale.
- Pot life / Gel time: The amount of time you have to mix and pour the rubber before it starts to set.
- Cure/Demold time: How long the rubber takes to fully set. If you have a multi-piece mold then a quicker curing rubber might be an advantage.
- Shore A hardness: Lower numbers will give you more flexibility in the mold, whereas high numbers are stiffer. Use a soft rubber if your casts are thin and could easily break when taking out of the mold. Otherwise a harder rubber is fine.
- Tear strength: If you need to stretch and pull the mold when removing casts then look for a high PLI/PPI number and a high elongation percentage. If the shape is simple then you don't need to worry about this since the rubber won't need to be stretched a lot.
- Shrinkage: Mold rubber can shrink quite a bit. This will result in your casts being a little smaller than the original. This might not matter for you, and the shrinkage is usually undetectable by the human eye since it's usually in the hundredths of an inch range. When we made the molds for our working Zat Gun it was very important for us to select a silicone with as little shrinkage as possible to ensure that all of the micro electronics would fit inside.
If you want to pick the best materials for your products then it is worth your time to learn more about all of the specs listed by the manufacturer. However, for the weekend prop warrior the above should help you narrow down your product selection.
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