Here's how I made a mold for 32-times normal size snap-together bricks.
[Materials and tools] [Six Boards] [Pipe Segments] [Circles] [Holes and Disks] [Logo] [More Disks] [Bottom] [Top] [Alignment Pegs] [CAUTION] [Bungees] [Disks Again] [Foam] [Cost] [Mold Release] [Assemble] [Pour] [Oops Too Much] [Paint] [Results]
The basic form of the mold is a rectangular box. The six sides measure as follows:
You can cut these pieces out of four 24x48 sheets. Make the measurements and cuts as exact as you can, otherwise the finished bricks may not fit with each other in all orientations. Use the Sharpie to write "INSIDE" and "OUTSIDE" on opposite sides of each board so you don't get confused later.
Next you need to cut the pipe into sixteen 2" lengths. Start by using the tape measure and Sharpie to put marks on the pipe every 2". Wrap the sheet of newspaper snugly around the pipe so that its ends line up exactly, and secure it with masking tape. This gives you a straight edge exactly perpendicular to the pipe. Line it up with each mark and use the Sharpie to extend the mark all the way around. Remove the newspaper. Use the file to score a starter groove at each mark. Use the hacksaw to make it deeper, and the large saw to make it wider. Then complete each cut with the jigsaw.
Now get the two big 41"x21" boards. Use the tape measure to lay out the eight centers of the knobs/holes on each board. These are 10" apart and 5.5" from the edges. Measure repeatedly and make it as precise as you can. Set the compass for 3" and draw circles around each center. Get one of the pipe sections, center it using the compass circle, and use it as a guide to draw Sharpie circles around both the inside and outside of the pipe.
Now you're going to cut holes in one of these boards. The holes should be just large enough to fit the pipe sections, snugly. This means you cut along the outer circle. Furthermore you will need the cut-out disks of wood, so you can't just drill a big starter hole inside the cut. Use the small drill bit and the screwdriver to make starter slots right on the cut line. Then set up the circle-cutting gadget for that diameter and go for it. When you're done, write "INSIDE" and "OUTSIDE" on these wooden disks too. Check the fit of a piece of pipe in each hole - if it won't go in, use the file enlarge the hole a bit.
If you want some sort of logo or lettering on the knobs of your bricks, epoxy it to the insides of these disks. Remember it will come out left-for-right reversed. I made mine say "ACME BRIX", one letter on each knob. The letters are 1/2" wood, from my neighborhood hardware store.
You also need eight more wood disks that fit into the pipe sections. Cut these out of the scrap wood. It may take a few tries to get the diameter of the circle-cutting gadget set correctly. Filing the lumps off the wood will probably help. These should fit into the pipe very snugly; if you have to use a hammer or your foot to get them in and out, that's good.
Now for some assembly. Epoxy the eight smaller wood disks into eight of the pipe sections. When they have cured, get the 41"x21" board that you didn't cut holes in and epoxy the pipes onto the inside. Use the circles you drew as a guide to position the pipes exactly. This will form the bottom of the brick, although it will be the top piece of the mold - you cast the bricks upside down.
Get the board with the holes and epoxy the other eight pipe sections into it. They should be flush with the inside and stick out of the outside, so do this with the outside facing up. Do not glue the larger wood disks onto these pipes, those disks need to be removable to help get recalcitrant bricks out of the mold.
Now cut the 1/4" dowel into 1" lengths. Use the file to round off one end of each peg. Fit the mold together - see how the edges overlap? - and use masking tape to hold it temporarily. IMPORTANT: use the Sharpie to number the corners of the mold, 1 thru 8, on each board. This is because it will only fit together one way once you're done with this step. Put the 1/4" bit into the drill, and use masking tape to mark a depth of 1" on the bit. Drill holes at each corner of the mold - eight corners, three edges per corner, 24 holes total. Drill 4 more holes in the middle of the long edges. The holes should be about 2" away from the corner, on the faces of the boards, not the edges, although they will go through to the edges underneath. Hold the drill as perpendicular as you can, so you don't drill out the side of the board underneath. After drilling as many holes as you can reach, take the mold apart and glue pegs into the edge holes (not the face holes). The rounded end should stick out, not in. When those are dry, put the mold back together upside down and do the rest.
The dowels are not very strong and the boards have a lot of leverage. If you handle the mold carelessly the dowels will snap. This is actually deliberate - better the dowels break than the mold. Replacing a snapped dowel is just a matter of drilling another hole.
Make three bungee cords; one long one to go around the four sides of the mold, and two shorter ones to go around the top/bottom/long sides, in two places. Use the file to round off the edges of the boards where the bungees go, so they don't abrade.
I haven't figured out a good way to attach the disks to the knob holes. I've figured out lots of so-so ways. My current mold uses screw-eyes and rubber bands. If you come up with some obviously great idea please let me know.
Now that you have the mold, here's how to cast the bricks.
I use X-30 2-part polyurethane foam mixture, from TAP Plastics. It costs $320 for two 5-gallon buckets. The mix expands by a factor of thirty, so this makes 40 cubic feet. Each brick has a volume of 5.5 cubic feet, so that much foam makes almost exactly seven bricks.
Note that the cost-of-materials is $46 per brick, more than I like. If anyone knows a cheaper source of polyurethane foam mix, please let me know.
TAP Plastics also sells a mold-release spray. This dries to a thin sheet of plastic, and after casting it adheres to the foam but not to the mold. However, I just use Vaseline. Be sure to fill in all the joints, you don't want the foam liquid leaking out and making a mess.
Coat the inside with your mold-release compound of choice. Don't forget the top piece. Measure out the two foam components, mix well, pour in, close the mold, and hook up the bungees. After five minutes it should be hard and you can take the mold apart again.
The X-30 foam mixture seems to expand somewhat more than the factor of 30 it's supposed to. If you measured the precise quantity of foam based of this, then the foam will probably push the top off your mold. Instead, use less than the calculated quantity and if necessary do a second pour to fill in the mold. In fact, doing two pours is an excellent idea for a different reason - with a volume this big, by the time the top of the foam rises to the top of the mold, it has already started to harden so instead of conforming to the knobs there it just pushes them up. So even with precisely the right quantity of foam, you still don't get a good brick with a single pour. So, do two.
Don't forget to paint the brick, to protect it from sunlight.