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.
01. The Comet taillights feature a large ring about ½” deep around the edge of the lens. This ring is fragile and breaks easily. This particular lens is barely holding together. We placed it on a sanded and smoothed board and traced the lens.
02. We also marked the original screw locations and marked a center hole. The three holes inside the outer circle will be drilled out and sanded.
03. We used the best lens from the Comet for the outer mold, but that one had a bad spot too. Using some Plast-Econ clay, the chip was covered up.
04. We used a razor blade and our fingers to trim away the excess, filling in the gap left by the chipped section. It is fragile at this point.
05. A ring of clay was added to the base of the lens, this will seal it to the casting enclosure.
06. The lens was pressed to the base of the plastic tray and the excess clay was trimmed away.
07. We did the same with other lens. This lens will form the inner section of the mold. In a typical two-part casting, a single enclosure would be built to capture both molds. This part is not very complex, so we skipped that part.
08. The board was placed on top of the tray. This is not necessary, but we did it anyway. Note the clay on the screw holes in the other lens, you don’t want to mold the holes, these sections would not release well and could damage the mold. We plugged them with clay and will drill new holes in the cast lenses.
09. The V-3040 silicone rubber is a 10:1 mix of the part A to part B. We mixed 550 grams of part A and 55 grams of part B. We ended up needing double that. The silicone bonds to itself, so there is no problem adding more, especially if it has not gelled. You must mix the rubber thoroughly, there is plenty of gel time with a pot life of about an hour. Vacuum degassing is recommended, but not required.
10. We normally use V330 silicone for casting, as it is a little softer and flows a little better, which helps if you are not using vacuum degassing chamber (to suck out all the air bubbles from mixing). The V-3040 has a higher durometer (hardness) and is thicker, so the air bubbles don’t rise and break on the surface. A slow pour is the key to a good mold. If you pour too fast, you will trap air bubbles and have voids in the mold that will transfer to the part.
11. After the mold had cured for 16 hours, we put it in the oven at 125 degrees for about 15 minutes, which gave us a nice post-cure solidification. If the ambient temp had been 75 degrees, then this post cure would not have been necessary. We used a pick to help separate the outer edges of the mold from the enclosure. If we had made a wooden enclosure, we would simply unscrew the sides and break down the enclosure.
12. The mold is flexible, so it peels right out.
13. Next, the mold flexed outward to release the original part. Since there is an inner ring, this part took a little more effort to release the part by gently working the mold loose from the inside of the lens.
14. The inner lens mold came out easily. This is a perfect carbon copy of the inner side. There will be some flashing on the edges; you can just peel it off.
15. Before the resin is cast, we cut some additional wood from the inner plug base and sealed it with Freeman wood and plaster sealer. Three coats, with a 220-grit sanding between each. This is the same process you would give a wood enclosure.
16. Once the last coat of sealer has dried, we applied two coats of Freeman Wax Release. This keep the resin from sticking to the wood.
17. We mixed up a cup of the Freeman 1090 Fast-cure polyurethane resin. At room temperature, this resin has a pot life of about 15 minutes, in higher temps, it can kick as fast as 5 minutes, so only mix what you use in that amount of time. This is WAY too much resin, we really only needed a couple of ounces for the one part, but we were filling some previously made molds at the same time. A few drops of red pigment provided the deep red color.
18. With the resin mixed thoroughly, it was slowly poured into the mold. You want to pour slow and careful, too much resin and the mold will spill out which creates a large amount of flashing you will have to clean up later.
19. After the part had cured for 8 hours, we put in the oven for a 2-hour post cure at 110 degrees. This helps solidify the thin parts. The resin is exothermic, meaning it creates heat during the cure process. Thin parts do not heat up enough to fully cure, so a post cure is usually necessary. If the part is thicker than about ½”, post cure is not required.
20. Once cured, the mold was flexed to break the lens free.
21. Here is the new part. This is the first part out of the mold, so there is still a little residue from the clay. The next part won’t have this. Also note the air bubbles in the middle of the lens. This happens sometimes. The part is smooth to the touch, the bubbles are in the inside of the casting. Vacuum degassing would fix most of that, but we don’t have a vacuum chamber. Less vigorous missing and a slower pour would help reduce bubbles. You have to be careful with the new part, even after a post cure, the resin can still be soft and flexible. Without post cure, it can take up to a week for the part to fully harden.
22. Here is the finished part next to the original. The new part still needs to be cleaned up (flashing and holes drilled), but it is going to look great on the back of the wagon. Now we just need to make 3 more.
A life-long gearhead, Street Tech Magazine founder and editor Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 5 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced. You can follow Jefferson on Facebook (Jefferson Bryant), Twitter (71Buickfreak), and YouTube (RedDirtRodz).