Thanks Puddles, glad I can contribute something back to the community
Mother Mold
Alright, so now we have to do the
mother mold. The basic concept behind this is that since the silicone mold is very soft and flexible there’s no way it can hold it’s own shape without some sort of support, so we make a second mold encasing it that is much more rigid.
There’s a lot of different materials that can be used for this, pretty much anything that can be applied into a form and hardens. The two options I see used more often are either
fiberglass shells or specialty mother-mold putties such as
Smooth-on’s Plasti-paste. The difference between them is pretty much a low price vs ease-of-use thing. Fiberglass costs a little less, but is highly toxic and incredibly messy to work with.. where as Plasti-paste is a little more due to it’s nature as a specialty product, but is a very easy to apply and non-toxic paste.
I’d really like to try working with Plasti-paste in the future, but because I already had some fiberglass sitting around and I wanted to keep things cheap; I elected to go with that method instead. There’s plenty of guides out there for both though, and the raw principles are pretty much the same.
Safety First:
As mentioned,
polyester resin (aka, fiberglass resin)
is a very nasty toxic chemical. Prolonged contact with it can cause neurological damage and runs the risk of
sensitization. This is where contact with the chemical can cause your body to develop an allergic-like response to it, one that can get so bad that even trace contact with the chemical can cause a severe reaction making your hobby very difficult.
Additionally, fiberglass matting, the material that we apply the resin onto which combine to create a rigid shell, is made up of many small fiberglass strands. These can break up into incredibly small fibers that can be very bad if you breathe in or have one land on your eye.
So with all that in mind, I’m going to be
doing this outside, wearing
nitrile gloves and a
high-quality respirator with vapor filters at all times. You should too whenever you’re working with polyester resin or polyester resin products.
Building a mold wall:
Same as for the silicone, this will be another multi-part mold and we need to create a dividing wall between the two halves. Just about any material will work for this if you release it, but aluminum foil seems to have good natural non-stick properties against polyester resin (aka, fiberglass resin).
My personal method for this is to use
aluminum tape. I like it because it comes in pretty big rolls so you can fold sections of it in half and create pretty rigid but still bendable walls that can slip right into the existing silicone seam. Additionally since it’s tape, you can just use it to tape segments together, letting it all be one single material.
You pretty much just wanna go around the entire circumference of it like I did here and tuck in into the mold seam. (Excuse the photos being from after I already have one side done as I forgot to take photos earlier on.)
A worry I had here was that since my silicone dividing line sorta went off to the side in the back to avoid the keyholes, I was concerned that the bigger half might be too hard to remove from the mother mold with all the registration keys and such creating mechanical locks. I initially tried to design a 3-part mother mold to accommodate this, but I ended up figuring it was too much hassle and I would see if it works as just a 2 part. Fortunately it did.
Getting the first half on:
After you get your mold wall on, apply a
release agent to it. I use
Vaseline, which I brush on then wipe to remove excess and get a polish-like covering. I don’t apply it to the actual silicone itself as silicone is anti-sticky enough without oiling it too, and we want the fiberglass to stick to it as much as we can, just focus on the tape.
From there I started cutting up a bunch of the fiberglass matting into roughly
4x4” chunks and then measure out a few
2oz cups of un-mixed resin. I like to mix my resin as several smaller batches so I don’t run the risk of having a bigger batch set-up and begin hardening on me before I can use all of it. The little cups only take a few seconds to mix up when you have them pre-measured.
Use either metal or wax paper cups for mixing, as polyester resin will eat through plastic.
My personal application method is to
mix up a cup of resin (10 drops per oz of resin), then use a chip brush to
pre-apply some onto the project in order to give the matting something to glue to. Then I
grab a square of matting, rough it up a little to loosen it, then dab some resin onto it to
pre-load it. I then
apply the wet matting to the project, pressing it on with a chip brush loaded with more resin. You can get about 2-3 squares done per cup of resin.
You want the matting wet enough so it's completely saturated and sticking flush against the previous layer, but not so wet that it's pooling or dripping.
I tend to get a ring around the mold wall first, then another higher up, then finally cap it off on top. Rinse and repeat. Also, if you need to take a break or finish this over a multiple days, feel free. You
don’t need to apply this all at once and letting under-coats dry won’t hurt adhesion.
For a good solid mothermold that won’t crack apart when you separate it, I find about
4 layers around the mold wall and
2-3 layers everywhere else does fine. The mold wall will take the most stress so focus on it, everything else is just there to give the silicone something to hug.
After it’s all on, let it sit overnight to get nice and hard.
Second half:
Flip over your model and remove the silicone tape. Re-apply some Vaseline or preferred release. Feel free to apply it pretty liberally as having these two halves permanently bond would be pretty catastrophic.
Since the tape is now gone, there may be a bit of a gap there now that we kinda don’t want to get filled in. I applied a ring of clay around the whole thing to seal this up.
From there on it’s the same as the first half. Try to use the same amount of layers on the mold wall so one side is not weaker than the other.
Then let it dry overnight again, or at least for a good few hours.
Cracking it open:
Now you’re gonna need to separate these two halves.
First thing I do is get a nice clean edge to the mold by cutting away the stringy edges of the fiberglass on the mold wall. I use a dremel cutting wheel to do this, but you could saw it too.
With all that off, I try to find a spot where the mold line can be seen somewhere along the edge. This can be pretty hard since the fiberglass is so hard pressed against itself, and if you can’t find it at all you might have to guess. I then do a shallow cut on it with the dremel to create a little crevice.
Stick a strong wide flathead screwdriver into it and twist. If all went well it should crack open a little bit. Get a few more screw drivers and start gradually prying it open along it’s length until it pops free.
Now you can finally open everything up and free your original master to enjoy some fresh air again. If you have difficulty separating the silicone halves (you most likely will), pretty much just try to find some visible line between them somewhere, even if you have to pull from the edges, and just slowly and forcefully seperate them from there. If you have a really tough spot, you can use something thin and hard like the back edge of an xacto knife to drag along through the seam and split it.
After all that though, give your mothermold a scrub down with some soap and a brush to remove all the resin and Vaseline residue, and also inspect your silicone mold interior for any imperfections now that it’s finally removed.
Mine had picked up some sanding dust from the deep crevices, and had a weird flashing issue on the visor I had to cut off, but other than that it was a very clean mold.
Prepping the mother mold for casting:
The last thing we need to do is get a way to connect the two mold halves. I just use nuts and bolts.
Find a size you like and matching drill bit (I used 7/16) and go to town. If you want to be fancy, you can fiberglass on the nuts to have them be fixed, that way it’s easier to remove the bolts with a single tool. I haven't done this myself yet though cuz im lazy.
After all that, you’ve got a completed mold and are ready to cast at will. Hurray.
I have already done one test casting so far that I made a few mistakes with, I'll post about it next. Past that though I'm all up to date and we're back to being a WIP.