Step 3: Building The Mold
The best material to use to build your mold is 3/4" Melamine since it is perfectly smooth and straight and, if properly built, will hold water without swelling up. You'll need to use a tablesaw with a fine tooth blade so that it will rip down the melamine without chipping or splitting it.
The minimum recommended depth of a concrete countertop is 2" if you're going to add some sort of metal support for it. Since I don't have any significant overhangs, I'm just using standard concrete wire mesh. If your countertop is less than 2" thick, you risk having problems with "ghosting" from your supports - lighter areas in the concrete that are patterns of whatever metal you used.
We ripped several long strips of melamine down to 2 inches for depth and then used a miter saw to make our cuts to length. Then lay the template on the mold.
I CAN'T STRESS ENOUGH TO MAKE SURE YOU TURN THE TEMPLATE UPSIDE DOWN - YES, THAT'S RIGHT, THE TEMPLATE SHOULD BE FACE DOWN.
This is because you're pouring the countertop upside down. The top of your countertops will be the bottom of the mold. If you forget this step, as I did, your countertops will most likely not fit and might become a pretty little mini-patio in your backyard. Thankfully, I caught my mistake before the pour and was able to rework the mold.
We assembled the sides of the mold by predrilling holes every 3 inches or so then used a countersink bit so our screws would be flush with the top. The top of the mold needs to be perfectly flush so you'll be able to screed the concrete during the pour. Drywall screws are the easiest to use, but be very careful drilling them in. If you drill them too hard, they will split the melamine and ruin that side piece. This is especially so when you drill too close to the end of the piece. We reinforced the corners with blocks of scrap melamine since they would be under alot of pressure.
Due to the size of the island countertop I opted to do it in 2 pieces instead of one big one. There are a couple of reasons for this - one being that it makes it much easier to handle without an army of people. The other reason is that large spans of concrete are sensitive to shifting of weight over time and need intentional "stress fractures" (Go outside and take a look at your sidewalks). By separating it, you're ensuring that if there's any stress on the countertop, it will crack in the seam, which can be easily repaired and ground back down.
Instead of having a seam go straight through the middle of the countertop, I wanted to do it an an angle and highlight it with an aluminum strip. That way I could refer to it as a "decorative feature" instead of a "stress fracture". I bought a 1/8" x 2" x 12' strip of aluminum at Lowe's and propped up on the mold to decide what would look good, then marked it on the sides and template. We then used an angle guide to transfer the proper angles to the miter saw, where the sides and sink cutout were cut and then screwed in the mold with a 1/8" gap to allow for the aluminum strip to slide into the mold.
Another challenge was a big curve that I wanted to put in the countertop for a little barstool nook. To accomplish this, we cut the desired curve with a bandsaw out of two peices of melamine that were screwed together. We then took the two side pieces of melamine that connected to the curve and used a table saw to cut a 1\8" by 2" notch in each of them. We cut a piece of 1/8" piece of plexiglass down to 2" then prepared to glue the plexi into the notches we cut in the 2 side pieces. Before you try gluing plexi to anything, it needs to be sanded so the glue will stick. We used 5 minute epoxy to glue the plexi in the notches and clamped them. After that set, we had 2 side pieces with a flexible piece in the middle that would conform to the curve we needed to make. We screwed in the curved melamine into place and spread epoxy over the back side of the plexi. We then bent the plexi around the melamine, clamped it into place, and then screwed the 2 side pieces into place.
I wanted to have curved corners on the island so you wouldn't impale your gut if you walked a little too close. To do this, we took 2" PVC pipe and cut it into 1/4th with the band saw. By using a belt sander, we were able to sand down the edge till they were paper thin on either side. We then used epoxy and taped them down in the corners. This worked like a charm and was part of Mr. P's evil genius showing through. Mr. P Note-Evil is as Evil does-etoN P .rM
The last detail is a cutout hole for the faucet. One thing to keep in mind is that most kitchen faucets are made for 1.5" countertop depth. Not only is my countertop 2", but there's another 3/4" of plywood below that. For the cutout, I used PVC that the OUTSIDE diameter was smaller than the base of the faucet, but was big enough to get the hoses through. I cut it to 1.5" in length and then used a piece of 1/2" plywood circle so that when they were attached, they would be 2" tall. This allowed for a proper depth for the faucet, while also giving access for wrenches to screw it down.