Here we describe the plastics and rubbers as basic types of
material to use as a casting material:
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| Plastic casting parts |
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| Plastic parts cast in molds |
Rubber or elastomeric: Rubbers range from super soft to
hard, almost like plastic. The hardness are ranked from Shore OO-5, very soft like
a rubber fishing lure to Shore A to Shore D of 80 or 90, which is hard like a
skateboard wheel. Rubbers can be
silicones, urethanes (also called polyurethanes), latex, PVC based, and others.
The two recommended for most castings is urethane rubber or silicone rubber,
and there are a lot of different options based upon the other properties you
are looking for such as amount of elongation, strength (typically listed as
tensile strength), tear strength (how easy it is to rip after a rip has
started), color and feel, and how easy it is to make. Thermoset resins require mixing two parts
together and depending on the resin, this ratio will be different. If the
instructions for a particular resin do not say specifically, they are typically
mixed by weight, although many will give the mixture ratio as both by weight
and volume. Each one of those rubbers will be discussed in future blog posts.
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| Rubber casting of an X, made with silicone rubber |
Plastic: we hear a lot of people ask for plastic that acts
like the plastic of an Army soldier. Army soldiers that you buy are made from
thermoplastic resins, but you will likely be casting with thermoset resins. The
difference is thermoplastics are melted into a thick goo and squeezed into the
molds. Whereas the thermosets are two part resins that are mixed together as a
liquid (or paste) and poured into your molds. It’s a lot easier to make your
own castings with thermosets because it can be poured. Thermosets, after they
are cured, cannot be re-shaped. Thermosets
can have very similar properties as compared to thermoplastics.
Different types of plastics include urethanes, epoxies,
polyesters, vinyl esters, and others. For home or small industrial, the main
plastic resins are epoxies and urethanes. Polyesters are much cheaper, but the
smell and toxicity are extremely detrimental. It is also more complicated to
use.
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| Rubber Casting Material |
“Exotherm” of a Casting Material is the heat of reaction
produced as a resin or rubber cures. If the casting is thin then heat is easily
dissipated in the mold, but thicker pieces will create more heat. To
compensate, large pieces can be poured in intervals or add fillers to the
casting material.
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