We Break for Brakes: Recreating Broken Taillights with Polyurethane Casting
Unless you are building a 32 Ford or a ’69 Camaro, chances are you have run into a broken part that nobody reproduces. While you can repair many things, certain objects like taillights and emblems are much harder to repair convincingly. Such was the case for our 1962 Mercury Comet wagon project, RDR5P62. The 4 round taillights are 62-model only and we can’t find a decent set anywhere, so we were stuck—either go custom or make our own reproductions. Having worked with casting in the past, that was an easy decision. A little silicone rubber and some liquid polyurethane is all you need to recreate or build your own custom parts.
Plastic casting may seem out of reach, but in reality, it is affordable and if you follow the instructions, it is not difficult at all. First you make a mold of the original (or prototype) part, then you cast it, it really is that simple. For the mold, silicone rubber is the material of choice. We use Freeman Manufacturing and Supply for all of our casting supplies, they handle all orders, small and large. Once you have the mold, you can create as many 100 parts before having any degradation issues with the mold. The nice thing about silicone molds is that they have a natural release agent—silicone—built right in, no need for additional release agents or waxes. The liquid silicone will reproduce every detail, down to 220-grit sanding scratches. Even hairline cracks in the original part will show. Depending on your project, you may want to seal the part with a smooth finish, or leave the part as is, which will lend a certain authenticity to it, hairline cracks will only be there in looks, the part will be stable.
There are many different types of mold castings, but we are going to talk about Simple Mass Casting and Two-Part Mass Casting. Simple Mass Casting is the process of taking a mold of an exterior or interior of a part, such as emblem. Here, one side of the part will be open, much like a cake or muffin pan, detailed on one side, and open on the other. Two-Part Mass casting is used for creating more complex parts, such as a taillight, where both sides of the part have details. This requires take two molds, an internal and external, that when assembled together, form a cavity to create the new part. Two-part molds are more difficult to create, so you should practice with a simple mass casting before attempting a Two-Part Casting.
The polyurethane materials vary widely, and you need to determine what you need before you start casting. For taillights, you need clear resins, which can be tinted using special pigments that are transparent. For other items, such as emblems, door handles, arm rests, etc, an opaque resin is better suited. These can be tinted as well, using pigments designed for opaque formulations. We used Freeman’s V3040 clear polyurethane resin with red pigment for the Comet taillights. The results were dark, rich red that will shine bright and they look original.
You will need a few items to ensure that your casting project goes well the first time:
Electronic scale with tare feature– All of the chemicals (each product is a 2-part mix) must be mixed by weight. These are available for about $20 at any grocery store, just make sure it stays a dedicated shop tool, once you have these chemicals on it, it can never be used for food.
Casting enclosures– You have to contain the liquid in an enclosure to form the mold itself. You can use anything from Tupperware to custom built wood (use MDF wood or smooth-finish laminates only)boxes. If you use wood, it has to be sealed and waxed before pouring the silicone.
Mixing cups– Any plastic or glass cup will do, plastic Dixie cups work great and are cheap.
Mixing paddles– Wood paint sticks are the best.
Lots of time– Both the silicone and the polyurethane resin require lots of time to cure. The silicone we used takes 16 hours to hard cure and another 48 hours for final cure. The resin sets up much faster, within 30 minutes (even faster in hot weather), but needs another 6 hours before it can be de-molded.
Vacuum chamber– This is an optional tool. If you have a part that must have no air bubbles in it, then the vacuum chamber is suggested. By pulling a vacuum on the resin (and the silicone rubber), all the air is removed from the mixture. You can get by without a vacuum chamber by not whipping the mixture, keeping bubble creation to a minimum. It just takes a little practice.
Oven (optional)– When it is cold out or you are in a hurry, you can speed cure the silicone. Some silicones are designed for it, others are not, pay attention to the spec sheets. Speed curing may also increase shrinkage. For these taillights, the outdoor temp was well below the 75-degree suggested temp, so we let it cure for 16 hours, and then did a final cure in the oven at 100 degrees for 30 minutes. Some thin castings, such as emblems, may require a short heat cure to fully set the resin.
Once you get the hang of casting, the possibilities are endless. This is the same process that R&D shops use to create many rapid prototype parts and it used in custom hot rod shop everywhere. With silicone casting, you can make your own custom emblems, arm rests, shift knobs, just about anything. So read up, order some materials for yourself, and get casting.
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