Polyurethane Resin & Rubbers

Polyurethane resin and rubber

This page is about Polyurethane resin and rubber, check out the main project index for the rest.

Polyurethane is a man made polymer which is especially suitable for home fabrication of plastic or rubber parts. Typically Polyurethane casting products are supplied as two liquids which are mixed together and then set into a solid piece. This article details some example projects and products.

I have used a lot of Polyurethane resin in my Iron Man project, to cast the majority of the parts. I have used Smooth-on 65D resin which is a rotocast resin – this means that it will set gradually so that you get a chance to rotate the mould around with the resin inside, without it setting in one go. This will form a smooth even coating.

I also used this for plastic coating foam parts, for the same reason it will make smooth coat if you can rotate the piece around as it cures, and keep the coats as thin as possible.

Polyurethane is great in thin section without reinforcement, it’s not too brittle, although it can warp in large sections unless they are cast thick enough, so in some cases Polyurethane is not suitable if you want to make large thin very light sections, although it’s generally fine for most costume/prop pieces etc.

Polyurethane resin is considered to be toxic in liquid form, although the smell isn’t too bad, you should always wear protective clothing, use a respirator, and work in a well ventilated area. Always read the Material Safety Data Sheet (MSDS) from the manufacturer in order to understand how to handle Polyurethane resins safely.

You can also make solid casts with 65D, although the Smooth-Cast 300 range of resins are designed for this, and have variable cure times. You’ll find that typically most Polyurethane fast cast resins will transition suddenly from liquid to solid, which is of course fine for solid pieces.

You can reshape Polyurethane casts with a heat source such as a hot air gun if the piece is thin enough fairly easily. This is useful for costume armour shells which aren’t quite the right shape for one reason on another. If you attempt this then you should watch out for further Polyurethane fumes, so always work in a well ventilated area and wear a respirator.

Polyurethane thinners is Acetone, which can be used to clean up any spills – it works best when the resin is still liquid rather than trying to dissolve cured plastic.

Rubber parts can also be cast in Polyurethane, if you buy the rubber version of the product, although painting with a flexible paint can be problematic. You can buy Polyurethane combatable pigments/tints to colour the rubber or plastic. For rigid plastic parts you should wash the cast piece in soap and water, and paint with a suitable plastic primer before any final colour paint coat. I use automotive plastic primer paints from my local auto accessory shop.

The UK supplier for Smooth-on products can be found on my resources page, although there are other Polyurethane fast cast resins on the market.