Sunday, November 23, 2008

The last pieces

Now that all the rubber has cured, it's time to take the Lego boxes apart. I tend to pour the rubber so that the last few hours of curing happen in the middle of the night, or while I will be away from the house, because I get impatient to see the mold and want to unbox it too early.

Here it is so far - the two large 'framework' sides and their plaster supports. The light brown oval on the top is the clayed-up poll piece.

Here I've used rubber bands to hold the framework pieces in place. These bands don't exert a lot or pressure, because I don't want to risk distorting the mold. Mostly they are there to ensure that the pieces don't slip apart later when I am pouring the inner pieces.

Now I can remove the clay which as been acting as a placeholder for my inner pieces.

Because the foal has a textured coat, and because his small size made this opening rather tight, I had to disassemble the mold to clean clay the remained on the original. Now that the mold and original are clean, I'm ready to add my pour hole.

Like the inner mold pieces, the pour hole is formed with clay. Because ceramic molds need a relatively wide "mouth", the pour spout usually extends to each side of the gusset opening which splits the gusset into two pieces - front and back. (You can see this typical pour hole opening on the Finn mold in the previous post.) Imp presents a bit of a problem in this regard because his left side legs are too close together to run the pour hole through them. Instead, as the picture to the right shows, I've run it off to one side.

I've also chosen to make this pour hole more uniform in width. That's because - for this mold at least - I'm only using this inner piece configuration to pour my rubber insert foal. Because I'm lopping off some legs, I need a few extra steps before I arrive at my final "mold of a mold". If I were working on what would be the final plaster mold, I would need a wider mouth to prevent clogging, but I'm going to take advantage of the fact that I can use something smaller for now. (This will all make more sense as we get to those steps - or at least I hope it will!)

Here I have poured the gusset. Once it is set I will flip the mold over and pour the poll piece. I don't have to wait until the first piece truly cures, just that it be set firmly. (This process is pretty messy, as this picture shows!) Since I am going to experiment a bit with my mold, I plan to pour more than one gusset piece. That way if something I try doesn't work, I will have a replacement piece without having to set up to pour again.

With luck I will be able to pour the rubber insert on Monday. I have an idea I want to try to see if I can get a more faithful (mostly bubble-free) casting, and I'm anxious to see if it will work!


Becky Turner said...

Hey this is really getting interesting! Im so glad your doing this step by step! Im curious..are you using mold release to coat the rubber before you pour the rubber for the gusset piece? and what kind of rubber are you using for this? it looks really stiff and firm rubber..not like silicone at all or even the brush on urethane I use.. also.. when you do the next mold is it made of regular plaster of paris or a different kind of plaster? or ceramic? Im sure I'll have more questions for you as you do this! thanks again!
Rebecca Turner

Lesli Kathman said...

Yes, I use a mold release at each step. For master molds I use a polyurethane rubber with a pretty high Shore rating. Silicone is wonderful for really small castings because it degasses, but it's too flexible for this kind of thing. Master molds need to be pretty rigid so they don't distort.

When I get to the plaster part, I'll be using white pottery plaster.