When working with wet-layup carbon fiber, cleaning, prepping and loading up the mold is an exceedingly messy process, as is cleaning up the part after it comes out of the mold – filling voids, filing flash, a dab of Bondo here and there, sand-sand-sand-sand, then finally spray paint some cool color from the hobby shop, until all sins are forgiven.  But right in the middle of all this mess, there is one beautiful, magical moment, and that is when the mold box is opened. There, the new part lies before you, like a grandchild asleep, and you are about to wake her, to join your world. Your mind is filled with images of all of the work and heartache that went into reaching this moment of creation, and you feel fulfilled. But you haven’t seen all the little goobers yet.

When a silicone mold is young, the parts come out beautifully and the flash is very thin, almost transparent though dyed black. As the mold gets reused again and again, the silicone mold surfaces seem to shrink a little bit, and grow brittle.  The flash on the part gets progressively thicker with each use, and the work needed to clean it up properly increases until, at around twenty five impressions, it is less work to make a new mold than to continue fighting with the parts coming out of the old one.

In these two pictures the mold lids have been removed, and you can see the array of taper pins around the mold edge that precisely align the box and lid.  These taper pins are actually 0.260” target load bullets that you can buy in bulk for cheap, and they can be glued perfectly down into the 6mm holes you have drilled for them around the edge of your mold box. To make a perfectly fitting lid, you first drill matching 6mm holes in the lid at each bullet pin position, clamp the lid down, then dribble a little bit of 5-minute epoxy into each hole from above. The epoxy flows down around each bullet, and after a few minutes it has hardened, and then the lid is removed. The flared profile of the bullet provides a steeper draft angle than store-bought taper pins, making the lid very easy to put on and take off while providing extremely precise fixturing. The right to bear arms can have unexpected benefits.

Opening a mold like this is not as straightforward as you might think. After you have removed all of the clamps, what you are faced with is a highly compressed heavy block - a sandwich of wood/silicone/carbon-fiber epoxy/silicone/wood all jammed together overnight with about a ton of clamping pressure. As you probably know, epoxy just loves to glue things together really, really well and permanently, and it will cause the upper and lower mold surfaces to fall in love if it is permitted to. To prevent such an improper union, before the mold is filled, its surfaces are sprayed liberally with “mold-release agent”, which is a fancy name for aerosolized Vaseline (yes - think of the possibilities!). The idea is that the spray should prevent the epoxy from bonding the two silicone halves of the mold together.  It actually works pretty well, as long as you remember to do it. 

Sometimes that sucker just doesn’t want to open up, and you try to remember – Did I spray-Vaseline it? You can just about see your part in there, waiting to be exposed, but it just…won’t…open, and the sweat breaks out on your brow, and you go get out your glue chisel and mallet and go to work trying to pry that lid off. This of course dings up the box-lid joining surfaces, which will have to be repaired for the box to ever close again properly. To avoid damaging things, you should proceed slowly and deliberately, but time has now become your adversary, as it is important to get the mold open before the epoxy inside has fully cured. If you do things right, when you take the part out and clean it up, the flash still has a slightly leathery texture, and cuts easily with an X-Acto blade. But woe unto you if you err and don’t open the mold until after the epoxy inside has fully hardened. Then, when you finally get it open, the enclosed part and its attached flash come out together like some kind of mutant porcupine wearing tough razor blades instead of quills, and you have no choice but to remove those blades by any means necessary, and you will bleed.

Here is what one of those mutant porcupines looks like:

To prevent such unpleasantness, there is a little magic trick you can do that involves drilling a little hole into the underside of the mold box about at its middle, up through the wood, then up through the silicone, right up to the inner mold surface, then fitting it with a little flexible plastic hose that you push down through, finally pulling it flush on the inside. Before the mold is loaded, the inner end of the hose is filled with a little bit of artist’s clay, which is smoothed to match the local mold face surface. Then you forget all about it as you go through the whole process of prepping, loading and clamping the mold overnight.

After a good night’s sleep, the next morning you come into the shop, look down at the closed mold box and smile, because you have outsmarted it. You take off all the clamps, lift up the mold box and fish out that little secret tube you put in the day before. Then you look over at your shop’s air compressor, and your smile widens. You push the rubber point of the air compressor hose’s end fixture hard into the end of that little plastic hose, and pull the trigger. There is a little chug and some crackling sounds from the mold, and you see the mold lid move ever so slightly, and your smile changes from one of mastery to one of relief.  

What has gone on inside the mold is that the compressed air has first pushed out that little clay plug you put in there, and then spread out between the joined mold faces, multiplying its force many times over as the parting surface is infiltrated. Quite quickly, resistance becomes futile, and the surfaces separate. You carefully lift off the lid, and there your part lies, beautiful, dark, and glistening in all her aerosolized Vaseline glory.