First things first: Making your own Lego ice cube trays is emphatically not cheaper than buying them straight out from from Lego. It is probably more fun, and since I wanted to learn mold-making, it gave me an excuse to buy a few materials and take a stab at it. Details after the jump.
First, I ordered a pint of ArtMolds MoldRite 25. This is a two-part tin cure silicone rubber molding compound that’s food-safe. I also picked up a can of Ease Release 200 to see if it made a difference in demolding. Once the MoldRite came in, I went on an attic excursion. My Legos have been in storage until my Geeklet is a little older, so finding them was actually the most time-consuming part of the project. I got out my large blue base plate and an assortment of 1, 2, 3, 4 and 8-stud blocks.
With the blocks washed and dried, I assembled the mold by building a 2-brick-high square wall, then laying single eight-stud (4×2) bricks inside, spaced one stud apart. I sprayed the mold with Ease Release, then mixed the silicone and catalyst, and filled the mold. Then, realizing that I’d mixed about twice as much compound as I needed, I quickly built two additional molds using two- and one-stud blocks, and filled them. As an experiment, I did not coat these with release compound. I also did not de-air the molding compound, since I wasn’t casting anything particularly detailed. Then I put them all aside to cure for 24 hours. 
As it turns out, they were set enough to release in 12 hours, so I pulled them out the next morning. As you can see, they’re not perfect. There’s a fair amount of sprue flash that needs trimming off with the old Xacto. Still, the molds were accurate, right down to the Lego logo on the top of each stud! So I went ahead and froze up a batch to see what would happen. I’d classify the results as fair, maybe even good after I trim up the edges. The bottoms of the "cubes" are far from flat, which I attribute to (a) sprue flash and (b) the fact that I overfilled them a bit. My daughter didn’t care, though…she just thought it was cool to have "blocks" in her water! So in that respect, the project was a total success. Lessons for next time, or for anyone else who’d like to try this:
- MoldRite 25 is fine, but it’s pretty viscous and doesn’t pour easily. I’d like to try SkinRite or an alginate mold compound.
- Release compound isn’t required. The silicone released from the bricks very easily. That said, I plan to try using a thin coat of petroleum jelly next time, to seal up the gaps between bricks. Oh yeah…be sure to press your bricks together tightly. Less
sprueflash means less cleanup. - If you use a kitchen measuring utensil to measure or mix your molding compount or catalyst, clean it up right away. I forgot to rinse the cup I measured the catalyst with, and the Geekspouse was not impressed by what it took to clean it out. In that vein, also remember to put down a piece of paper so you don’t spill liquid rubber on the countertops.
- Build your mold to accomodate slightly more compound than you plan to use. Filling the mold only partway up a brick results in a smoother edge.
- When freezing up your ice, place the molds on a cookie sheet or other flat surface to help keep them even.
- Next time, I might try boiling up some distilled water to see if I can get clear blocks, or dye the ice with food coloring (mmmm…black ice to match the black Legos).
- Finally, I’d like to try pressing the brick forms into the molding compound from above, rather than casting over the studs-up bricks. I think this would get you better control over the size of the cubes, which are actually pretty small. I think I’ll try making 2×8 cubes next time.
On the whole, I’d say things went well for a first attempt at casting a mold. If you’re looking for a project to learn casting — and entertain/involve your Geeklet(s) — this is a great way to start.
UPDATED: Change “sprue” to “flash.” Thanks Joel…GeekDads rock!
