The different steps in silicone molding carbon-fiber frames require different combinations of precision, mess, ingenuity, speed and patience. Also basic shop skills as well as learn-by-doing skills. This picture shows a lot of detail that helps explain the equipment and process. I think the glue chisel was added as an artistic touch to emphasize the manual nature of the process.

A silicone mold is made up of two basic sections, the box and the lid.  The mold box here uses blue-green silicone, while the mating lid visible in the background is made of yellow silicone.  Why the difference? I didn’t make this particular mold, so I can’t be sure, but looking closely at some details might give us a clue.

As background, different mold silicones are distinguished by different characteristics - hardness when cured, “food-safe” or not (to mold your own chocolate candies), heat resistance (you can mold pewter spoons), types of catalyst, and curing time. Softer and more flexible silicones are used for molding intricate surfaces with “undercuts”, whereas a harder silicone would hold the same part as if it were set in concrete. As a general rule of thumb, silicone molds for casting carbon-fiber epoxy parts last about 25 impressions before they become too degraded to use further, and softer silicones degrade more quickly.

It looks like this mold has been recently opened and the new treble frame removed, but it has not yet been cleaned. The evidence for this is the thin grey hardened epoxy “flash” on the top edge of the mold box where it squeezed out when the part was being made (Hand clamps are ganged up to provide about a ton of clamping pressure on the mold box during the cure). Additional flash is visible in patches here and there on the green silicone surfaces beyond. Before the mold can be used again, every last bit of this flash has to be removed to leave clean surfaces that will mate properly when the next part is made. The best tool for doing this clean-up is every wood shop’s Old Reliable - the “glue chisel”.

Unlike precision chisels that obsessive guitarmakers keep obsessively razor sharp, a glue-chisel is more like an old pet dog that generally enjoys life without complaining. Capable of being made out of any cheap old chisel with crappy steel, the glue chisel is purposefully kept a little bit dull, and in fact it won’t work properly if it is sharp. Its cutting edge is kept just sharp enough to allow it to find and follow a seam between excess glue and substrate, but also just dull enough so that it won’t cut into the substrate. This particular glue chisel meets all of these specialized criteria – cheap rusty blade, cheap plastic handle, and (bonus) a mushroomed handle revealing a clueless and perhaps angry previous owner who seriously abused it with a steel hammer. But it didn't mind, and eventually found a good home. The perfect glue chisel!

The evenly-spaced small steel dowels visible in the mold channel are for molding in the precise hole pattern for grill & insert attachment to the frame, with the dowels being exactly the right diameter for creating holes into which M3 threaded brass inserts will later be press-fit. Spending a little more time building these into the mold saves a ton of time trying to drill them individually later. The threaded inserts have a heavily serrated outer surface and a conical threaded core, so that when the first screw is threaded down into them, the insert is forced to expand and locks tightly into its hole by friction alone. The random half-rounds visible along the edges of the mold channel fit into corresponding holes on the opposite mold face, providing additional alignment precision along the mold parting line.

These copper taper pins are place every few inches around the edge of the mold box where then engage and precisely position the lid relative to the box for molding. Real machinist taper pins only come in steel, have square ends, and have very shallow taper angles. These copper taper pins are actually 260 caliber bullets with their tips filed off, and work much better. The wide rounded taper makes for easy and precise lid alignment, opening and closing.

The mold lid in the background is harder to see, but we can make out a few details. The inner surface of the aluminum plate will create the inner frame surface on the finished part. This aluminum plate supports a pattern with three oval knobs which will create the cavity for the frame joining bracket which allows the two body halves to be quickly and securely joined and un-joined. This frame joining bracket cavity creates a classic undercut in the finished part. To get around this undercut problem, in addition to the box and lid, there is a third mold “action” whereby the aluminum plate is mounted on a wooden piece that forms one of the sides of the assembled mold box. When the finished part is to be taken out of the mold, this side-action piece is removed first in a sideways direction. This clears out the frame bracket cavity and eliminates any undercut interference for when the mold lid is next removed vertically and the part taken out.

So why the yellow silicone on the lid? With the third side-action eliminating undercut issues, there is no obvious technical requirement for a softer silicone for the top half of the mold. There is another possibility - these molding silicones come in different colors, and the color choice could have just been just an aesthetic touch by the person making the mold. If, after going to all this trouble making a mold box, you had the choice of making both sides of your mold the same color, or making them different colors, all other things being equal, what would you do?