Make elegant custom coasters using Rapidset Cement All, a cheap sandblaster, and 1/8th inch plywood pieces cut out on the laser cutter.
Step 1: Create Your Laser Cut Design on the Computer
I used Autocad and Adobe Illustrator to come up with a neat and personal design for the coasters. I took the font Goudy Old Style and converted it to a shape and exported it to Autocad. In Autocad I generated the rest of my design to be cut out of 1/8th inch plywood. If you don't have access to a laser cutter a less intricate design can also be hand cut for the next step.
Step 2: Sand Blasting
Decide what pieces from your laser cut are the impressions in the coaster and which pieces are the foreground. Take the pieces that you want to be recessed and using the other laser cut parts to set it right, secure it with spray adhesive to a block of rigid insulation board. Spray adhere the block with the wood pieces onto a piece of scrap wood. Blow on the spray adhesive or let it sit for a couple seconds before sticking it so the pieces come off easily later.
I bought my sand blaster off of amazon for around 30 dollars. Its pretty basic; it just hooks up to a compressor and and draws the abrasive (in this case play sand) out of a bucket. Make sure you dress up like a marshmallow with your goggles, gloves and coveralls if you have them. If you don't have an enclosure for sand blasting, as I don't, the sand goes everywhere. If you don't cover up you can count on your next meal being a little crunchier.
Take your sand blaster and evenly spray across the entire foam block for a minute or two. You don't want to cut too deep so the coaster functions like a coaster and not like a hot coffee hazard. If you are using text, be sure that it is backwards in the mold so the cast comes out correctly. Un-stick your wood pieces from the foam and you have the base of the cast.
Step 3: Making the Mold
I used some scrap 1/2" MDF I had lying around the garage and just cut it into a strip on the table saw. I used a 4.25" hole saw to make 4 holes in the MDF strip. Make sure you find out what size hole saw you have first before you make your laser cut. I thought the hole saw was four inches and consequently had to make a second laser cut. Then I cut the individual holes off of the strip with the miter saw. Be sure the inside of the hole is sanded smooth to avoid any undercuts and unwanted marks from the hole saw. Next, in the corners of each individual mold piece drill a hole so a screw can be placed into the foam base. Center the MDF on the foam block using the outer most laser cut piece to get it right and secure the mold together with screws. That's pretty much the extent of the mold making process.
Step 4: Casting
My favorite product for small casts like this is Rapidset Cement All. It captures as much detail as plaster of paris and has about the same working time but is much more durable and resembles concrete. The drying time is incredible as well; it sets up in 15 minutes and is structurally sound in one hour. It is a technically a grout mix that is just a blend of sand and Rapidset cement.
Best way to know the right consistency is always going to be with a little experience. For this pour it will have to be a little thinner. It should pretty easily flow into the mold but still be thick enough to hold its shape as a little mound when a little is poured into the mold.
Before pouring make sure you spray a release agent on your mold. I always use WD-40 for one piece casts. Fill your mold a little bit at a time and pound on the table in between pours to level it out, see how much more you have to pour, and to get the bubbles out. Fill up the molds almost all the way to the top but just a little short so that any spill over doesn't chip off when de-molding.
Step 5: Display Base Mold
If you want to make a base for your coaster there are of course a million ways to do it. What I ended up doing was cutting a larger circle in some MDF with a jig saw and using the same design on foam to cast the top part of the base. Then I made a shape out of insulation foam that was a 30 degree triangle with a perpendicular rectangle on the hot wire. The triangle holds the base up at an angle and the perpendicular piece holds the coasters onto the base. I made walls 3/4" tall on the triangular shape and cast two of them. I glued both of these shapes onto the back of the larger cast circle with gel super glue.
Step 6: Demolding
Wait about 8 to 12 hours before de-molding. Just removes the screws, remove the foam from the bottom and gently pop the coaster out of the mold. It will be a little dusty so just wash it off with some water and blot it dry. You should wait at least 6 hour before sealing so it is completely clean and dry.
Step 7: Sealing
I chose to use spray lacquer for this process because it takes very little time and is probably the most durable. I also wanted to easily keep the texture from the sand blaster in tact. I just sealed the top of the coasters with about 3 coast over the course of 20 minutes. You can also use water based polyurethane or any concrete sealer for this process depending on what you want from your coaster. The lacquer give it a wet look which I would usually not desire but the durability was more important to me and they turned out great. As a finishing touch I cut some cork out of a thin sheet and secure it to the bottom of the coaster with spray adhesive. That's my take on how to make some elegant coasters for your home or as a gift.