DESIGN:
I decided it might be interesting to try to cast lego bricks, especially to see if I can get multiple bricks to stack together properly.
First I modeled a 2×4 lego brick:
Then I split that brick along the bottom of the “roof” of the lego. So, essentially I have one mold for the “roof” of the lego with the positive bumps and one mold for the “walls” of the lego with the interior structure.
I’m a little unsure if the pouring hole will work properly as the poured material might not distribute properly through the ringed/ribbed structure on the interior of the brick. Also, I did include air holes, but again I’m unsure how well they will work since there is no way to connect them to the interior structure.
MAKING THE MOLDS:
TOOLS/MATERIALS:
shopbot
1.5″ foam
gesso
soap
oomoo 25 (1:1 mix ratio, 15 minute pot life, 75 minute cure time)
MILLING THE POSITIVE:
The first mold I had to make was a positive of the lego brick within an enclosed space, considering future pour holes, ventilation holes, and alignment notches. I made this mold out of foam using the shopbot. I used a quarter inch ball nose bit and had the spindle running at 9000 rpm with a pass speed of 10 in/s.
Roughing:
Finishing:
MAKING THE FLEXIBLE NEGATIVE:
The next step would be to make a flexible negative mold of the lego brick into which I could then pour a final positive brick. First, I sanded down the surface a bit to remove residual roughness from the milling process and then I gesso’d the surface of the foam as the foam is porous on the interior and thus must be sealed. I applied two liberal coats of gesso and let it dry over night. I the prepared a mixture of OOMOO 25 (the silicone smooth-on product) and poured it into each half of my foam mold (I applied a bit of soap to the surface before pouring to ensure easy removal). After about two hours, the OOMOO was set, and I peeled it out of the foam mold. I was now ready to make the final lego bricks.
Gesso’d foam:
OOMOO mold:
CASTING:
TOOLS/MATERIALS:
gloves
protective eye-wear
fume hood
soap
smooth-cast 305 (1:1 mix ratio, 7 minute pot life, 30 minute cure time)
Using the smooth-cast 305, I poured three bricks. First, I applied some soap to the interior surfaces to ensure easy removal and then I attached the two halves of the mold using rubber bands. Once I prepared the plastic mixture, I poured it in through the pour hole and let it set for half an hour. The results were mostly successful except for a few air pockets that tended to be created on the “top” wall of the brick. My guess is that the ventilation holes weren’t sufficient or weren’t properly placed. The bricks themselves were also a bit rough in appearance, seemingly mostly due to the initial milling process. If I were to repeat it, I would not have used a ball nose bit (because the beveled edges prevented true lego snapping) and I would have perhaps milled out of wax or higher density foam to achieve a smoother final surface.
Bricks:
COMPLETE FABRICATION 
