As any seasoned installer will tell you, custom fabrication takes a long time. No matter how simple the custom part, it still takes a lot longer to design, engineer and build something as opposed to pulling some pre-made product off the shelf. For a car audio dealer, the problem with doing custom fabrication is making it profitable. With each new vehicle, it takes time to figure out what fits, what works and what doesn't. Even if you work on the same vehicles, each custom part must be made from scratch. Many times, the job ends up taking much longer than originally anticipated, costing the shop time and money. Although smarter shops and installers make jigs and templates and keep notes on popular vehicles to make things more efficient, a lot of fabrication is still required to duplicate one system from another. After many hours are spent shaping and sanding a custom panel or sub enclosure, you still only have one panel/enclosure, and a second panel/enclosure would take almost as long to build. If only there were a way to make duplicate parts in half the time.
Moldy Oldie
Plaster mold making has been around for over 7000 years dating back to ancient Egypt (there's your history lesson for the day) and is used in many professions and industries, but it's very uncommon to see plaster in a car audio install bay. Some may have played with it in grade school or art class, but most haven't applied it to an installation project. The team of installers that build the incredible demo vehicles at JL Audio have been using plaster mold for years and are experts in plaster mold making. They have a very informative tutorial on their website that should be referenced by anyone interested in this technique. Those of you out there looking to cut down on fabrication time and increase profits should thank the ancient Egyptians (and JL Audio) and go buy some plaster.
The basic concept behind making negative molds is to build your original piece once, take a mold of it and make copies of it in a fraction of the time later. Negative molds can be made with a variety of materials including plaster, fiberglass, latex and silicone. The benefits of making molds with plaster over the other materials include cost, cure time and rigidity of the mold.
The most important step when making a negative mold is to make sure that the original positive piece or "plug" is the desired shape and finish that you want the end result to have. Plaster can transfer amazing levels of detail, so it is critical that the plug is smooth and finished. A project for a beginner should have a relatively simple shape that will allow the mold to be easily removed. Complex and curved shapes can "lock" themselves into place unless the mold is made in sections that separate.
The Process
The first step is to prepare the plug by applying a mold release wax or spray. Certain scenarios may require parts of the plug to be covered with masking tape before applying the mold release. Once the plug is protected and coated, the plaster powder is mixed with water (to the manufacturer's specifications) and a "skin coat" is applied over the plug with a 3" bristle brush. Plaster, like fiberglass, cures after going through an exothermic process of heating up and cooling down. Once the plaster has hardened, but is not completely dry, more plaster is applied. This time, hemp fibers are used to reinforce the plaster, much like the way fiberglass mat or cloth reinforces the fiberglass resin. After the plaster has cured overnight, the mold can be separated from the plug using plastic wedges. If the plug was properly prepared, the mold should release with minimal damage. The surface of the plaster mold is then sealed with shellac. Small imperfections in the mold can be repaired with body filler or clay. Three coats (or more) of wax are applied in preparation of making the fiberglass part. The surface is then sprayed with PVA(poly vinyl alcohol) which creates a cellophane-like film between the mold and the fiberglass part. It's a good idea to apply one more coat of wax after the PVA has dried so that the fiberglass piece releases easily, but this must be done carefully avoid tearing the PVA.
Once the plaster mold is fully prepared, it's time for fiberglass. The first step is to apply a 1/32" to 1/16" thick layer of gel coat to the surface of the mold. It should be sprayed on for even application, but can also be applied with a brush. The gel coat should begin to set after 45 minutes. A fiberglass "gel" consisting of resin, chopped mat and resin thickener is applied in the corners and tight spots then fiberglass resin and mat is laid up over the entire area. After the resin has fully cured overnight, the duplicate part can be removed from the mold using plastic wedges.
With this plaster molding process, exact duplicates of the original shape can be made in a fraction of the time without any effort on sanding or shaping the piece to fit. Although this process takes practice and patience to master, the results and time saved are well worth the investment. With a master mold, parts can be made before the car's arrival, saving time and making the job even more profitable.