This is just a side post, or half post dedicated entirely to molding and casting. For this project I did a standard pouring mold for the eye piece, and a 2 part jacket mold for the potato using Smooth-On products. I had never used Smooth-On stuff before this project as I usually bought standard mold making silicone from a local place. Everybody has different preferences when it comes to silicone products but I do recommend you at least give them a shot. I don’t think I’ll be using anything else for a while.
So, lets start with the eye piece! After a coat or two of primer and a good sanding, I was happy with the smooth finish on the eye, and decided to mold it using the left over OOMOO 30 silicone from my original molding disaster.
To start, I took a scrap piece of MDF and fixed my eye piece to the centre of it using double sided tape. Rather than waste silicone by making a square mold box, I cut the top off a plastic pint glass. I made sure I had enough space at the top to fill in extra silicone as I didn’t want the base being too thin, and used hot glue to temporarily fix the top of the cup to the MDF. It’s a good idea to make sure you cover the entire seam in hot glue to make sure silicone doesn’t start flowing out the bottom of your mold, because if it can it will. You can also see in the background of this image the two parts of OOMOO 30 ready to go, this mixes in a 1A:1B ratio by volume which is super handy.
Pro Tip: It’s always better to mix more than you need, then run out halfway. (This applies to everything)
You can see here that when the two parts of OOMOO 30 are mixed properly, they make a lovely purple colour. When you no longer see any streaks of the original colours, your silicone is ready to pour. Make sure you pour from the lowest part of the molding box, so in my case down the side, and let the silicone build up and level itself out. The great thing about OOMOO 30 is that it doesn’t need to be put in a vacum chamber like a lot of other silicone, it has a low viscosity in comparison to others, and the air bubbles in it rise naturally. If you need to give it an extra tap that’s no problem, but my eye piece mold came out perfectly with no air bubbles what-so-ever.
I originally wanted to trim the middle bit of the mold one I’d pulled the original out, but after I de-molded the eye the I realised the part was thick enough, and the silicone rigid enough so I didn’t need to. The mold was so crisp and clean that no tidying up will be needed for any pulls. Because the potato was going to be made using Smooth Cast 305, I decided to test that out for the eye piece as well, however, because that takes 30 mins to cure, it was awkward to do a slosh cast of the eye. I did a couple of testers using simple fast curing bi resin, and smooth cast 305 and anything that didn’t work out was then used for paint tests later on.
The eye was kinda done in two parts, the first bit was the top because it needed to stay flat and preferably flat on the underside (unlike the white one in that picture), I pour in a bit to cover the top and let that cure. The second part was the sides, it took about 4 layers to get it thick enough, but I poured in a little resin at a time, and rotated the piece making sure the sides were evenly covered while it was curing. Resin sticks to resin so there wasn’t any problem with it cracking or splitting when I pulled it out. I also didn’t need to be too worried about the end of it being too tidy as it would be inserting into the potato anyway.
Next up is the Potato! Rather than using OOMOO 30 again, I decided to go for the cheaper but very effective option of using Rebound 25 for the potato.
This brush on silicone doesn’t require any vacuum chamber either, and only takes about 4 thin layers to get a good negative of your object. I started with a lump of clay, and built it up to roughly the halfway mark of such an awkward shape. The little nodules I made in the clay using a screwdriver are a key for when you’re putting the the mold back together so your mold doesn’t end up not aligning properly.
The first layer goes on nice and thinly as you can see in the picture here, making sure you get a good even coat all over, and after an hour a thicker layer can be
added and so on until you have 4 to 5 layers of silicone on that side. I added silicone thickener to one layer so that I could build up some support around the edges and where the lumps cause undercuts. Once all the layers were on I left the top half over night to cure
fully so it didn’t warp or anything if I took it off too early. The next morning when I was sure everything was completely cured I put a shell of plaster over this side of the mold, this acts as a supporting shell to help the mold keep its shape while the material you’re casting is curing. When that was completely dry, I flipped it over, removed all the clay and cleaned the surface. Once a coating of release agent and sealer was put on I basically repeated the whole thing again to get the other side. Make sure you leave a gap between the plaster layers though because they will want to fuse together.
Pro Tip: Release agent will stop the two sides of your silicone mold sticking together and help with the de-molding process too if it’s a tricky shape!
Once everything was cured and the plaster was completely dry, it was time to remove the original potato and make a new one. I cleaned out the inside of the mold and mixed equal parts A and B of Smooth Cast 305 and pored this into the bottom of the mold. Then I closed the mold and tied it shut with some straps with metal fasteners and rotated in every direction possible to make sure the entire inside of the mold was covered. I did this for about 15 mins, and then left the plastic cure for another 15. It should be noted if you’re doing this, and you’re not a whiz at mold making, or maybe if you are a whiz, chances are you might not get it perfectly sealed up, either way, always wear gloves in case some resin decides to leak out the sides onto your hands.
Oh! and make sure you remove any resin trying to fuse the two plaster sides together when it’s still soft because if you don’t you’ll have a nasty job on your hands if it cures fully. The first pull brought with it all the excess dirt and bits I couldn’t see out of the mold so I kept that one as spare and basically did the same thing again only with a little more resin to get the potato I was going to use. The next step will be cleaning this up and giving it a bit of a sand before I carve the eye hole and give it some colour!
Better get to it!