Returning to moldmaking

Sorry I got the moldmaking off-track for a few days. I thought it would post the last part as soon as I got back home from the show, but instead I have been playing catch-up all week. (Not very successfully, I might add!) But when I left off, I had my rubber positive of the smaller Celtic Pony bead. This will become my master to make prodution molds. Right now the positive image is flush with the Lego frame, so once again I’ll need to build mold walls to hold the material I will be pouring.

I only need to build the walls as tall as I want my finished plaster mold to be, which in this case was three layers of bricks. I don’t need any releasing agent because neither the rubber nor the Legos will stick to the plaster. I can just pour the plaster directly into the Lego box. Once the plaster has set up I will once again break my box in half. A thin screwdriver is good from breaking the seal between the two sets of Legos so they break apart neatly. This will reveal the first half of the plaster mold.

If I was making a press mold, all I would need to do is remove the Lego frame. But I want to make a slipcasting mold, so I need to make a lid. I will also need to cut natches to key the bottom half to the lid. In the picture above, I made the circular natches by twirling a coin in each corner. (This works a lot better with newly-poured, damp plaster. It takes a lot more force to cut them after the plaster dries.)

Because I am pouring plaster against plaster, I need mold soap to form a barrier between the two pieces. The soap makes the plaster waterproof, so it only goes on the non-design area. The last thing I want is a mold cavity than cannot draw the water out of the slip I pour! Usually I fill the design area before I paint on the mold soap, but I couldn’t find my tiny rubber insert. (I usually pour a rubber version of the design so I can temporarily fill the design when I pour lids.) Here I’m painting without the design filled because I am still hoping the insert is lying around somewhere. In this picture, I have given up and just filled the design area with moist pottery clay. 🙂

And once again, I build up the walls – this time with upside down Legos. I will pour the plaster lid in this box and let it set up. Then I can take off all the Legos, break the two mold pieces apart and remove the clay insert. After drilling a pour hole in the lid, I have a working slipcasting mold.

Unfortunately I didn’t think to get a picture of the finished slipcast mold. But this is a finished (and slightly used) press mold. You can see that I sanded the outside (no Lego brick pattern) and rounded the corners. I started rounding the corners and finishing out my production molds so I could tell at a glance that they were production molds. I leave my ‘waste molds’ – the molds where there is only one copy – raw. It’s a bit of craftsmanship overkill, but I handle the molds a lot, and I like having tools that feel pleasant in my hands.

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