Ocarina of Time Coasters: Part Three

Click here for Part Two: Sanding and Finishing

Welcome back!  In this post, I'll cover the process I'm using to create the molds for my coasters.  So far, creating the molds has been the easiest part of this entire process, as the issues I've had mostly stem from me doing a bad job of finishing the originals (see Part Two for details).

Part Three: Making the Molds

Since these coasters are flat and really don't have anything complicated going on, I'm making simple one-part molds.  I start by building a little cardboard box to contain the liquid silicone, trying to leave about a half-inch around the edge of my coaster to give the silicone some strength.  Some people use acrylic sheets to build their mold boxes, but I'm getting so many things shipped to my home lately that I'm just about swimming in cardboard, so it's the cheapest option I had.  If you follow in my footsteps, just keep in mind that cardboard is going to absorb moisture, which will make it a little tough to remove the box once the mold sets up.  To seal the box, I used a gratuitous amount of hot glue around the inner and outer edges.  I also used a ring of hot glue to attach the coaster to the bottom of the box, but I'm sloppy with hot glue, so I still had some silicone to trim off the bottom when I was done.

Derpy little cardboard box

The next part was super easy, despite my fear that I was going to completely screw it up.  I took my two-part silicone (Smooth-On Mold Star 15 Slow, as you can see from the picture), mixed equal parts A and B, and slowly poured it into a corner until it covered the model with about a half-inch thickness.  I kinda guesstimated the amount of silicone to use by measuring the box itself, getting its volume, and subtracting an approximate volume of the coaster to get 200 mL of silicone needed.  After pouring, I tapped on the sides of the box for a couple minutes in the hopes it'd help release any bubbles trapped on the surface of the coaster, and then let it cure until the instructions that came with the silicone said it was safe to touch.

Apparently I can't decide on a camera orientation when I take these pictures...

After it cures, you can just rip the cardboard box apart to free your mold.  I had to gently trim the edges of the mold away from the cardboard to loosen it, as it soaked into the cardboard along the top edge.  Once the mold was free, I gently pried the coaster out and admired my sloppy handiwork.

Yay!

So far I've had two main issues with my molds-- using a rough original, and not using a flat surface when I pour the mold.  The former is obvious, but the latter I didn't even think about.  Since my crafting table dips inwards in the center and my porch seems to be at a slight slant, both test molds have been very crooked.  The severity of this issue didn't become obvious until I had a cast spill over one edge but not fill the other side of the mold.  I'm not quite sure what to do about this at the moment, other than perhaps take note of the alignment of the mold relative to both surfaces and be sure to keep it in the exact same orientation when I cast, which sounds pretty error-prone.  Trying to get my table perfectly level might actually be easier.  I'll have to do some more tests to figure this issue out.  Until then, may the super glue be with you, not on you!


UPDATE 8 November:  Today I learned that I've been butchering my molds.  Trimming the edges around the original is a terribly dangerous thing to do, and can leave you with rips, dents, and other oddities that sure as hell will show up in your casts.  Since hot gluing the original to the bottom of my mold box has been the culprit, I'll be using some spare (sulphur-free) clay as an adhesive for my next mold.  Hopefully that will make it easier to get a nice clean edge.  Replacing my X-Acto knife blade ought to help, too, now that I think about it...