For this project, I will be making a production mold for the smaller Celtic Ponies. This rubber mold will be used to make a simple two-part plaster medallion mold, one piece for the design and one piece acting as a “lid” across the flat back of the design.
To start, I will take my original and glue it to a base. In this case, I’ve used a 4×4 glazed tile because it separates easily from the rubber. Then I build a box around the design using Legos. To keep the Legos from sliding on the slick surface of the tile, I stick them down with double-sided tape (the kind used in scrapbooking). My Lego mold box is then filled with silicone rubber.
Normally I would key the back of the rubber, as I did in this post, and then pour a plaster base. The base keeps the rubber from distorting when it is clamped inside the mold boards. But here I’m not going to pour a base because I won’t be removing the Legos. They will act as my stabilizer instead. But I do need to pull the tile off and expose the negative of my design.
Now I have a negative image of my design, encased in Legos. My next step is to pour a positive image, also in rubber. I won’t pour just the design, though. I want to pour the design and the base – all in one piece – so that I won’t have to glue the design to a tile like I did in the first step. But to do this I need to box the area around the positive image, since right now that area is flush with the Legos. This is where the Legos come in handy because I can extend my box (in either direction) by adding more blocks.
Okay, so I’ve built my mold box up(side down) and I can now pour the positive piece.
This was the trick that it took me an embarassingly long time to discover. I didn’t have to take the rubber piece out and rebox it. I could just build the box up. Of course, I’m actually building the Legos upside down, but that works just as well. In fact, because Legos can be added to the top or bottom of my box, I never really have to remove them from the rubber. They can function as the stabilizer (eliminating the plaster back), and serve as a base to add mold box walls. And since the rubber is never removed from the box in which it is poured, the box never has to be re-sealed. (Resealing mold boxes is my least favorite job.)
I did key this one, and pour a plaster base, only because my kids were beginning to notice an odd shortage of rectangular Legos. Here I poured the rubber flush with the top (the dark grey Legos), cut the keys, added another layer of Legos, and then poured plaster.
(Bad plaster pour there – look at all my air bubbles!)
I have both a negative (inside the bottom half of the Legos above) and a positive (in the top half). I can now split the stack apart, and reveal my positive.
Now I have an original that gives, yet will hold up to repeated pours of plaster (the amber piece to the right). My negative (the light blue piece on the left) can remain in its Lego frame in case I ever need to pour another positive. In the next post I’ll use the new rubber original to make a working plaster mold.