Sunday, January 8, 2017

Plastic Casting and Rubber Casting

Here we describe the plastics and rubbers as basic types of material to use as a casting material:

Plastic casting parts
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.
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.

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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