How To Make Custom Mold Mats (and why I stopped)
Calla Custom Mold Mats were flexible sheets of silicone with raised custom designs on the surface. Soap makers would place these on the bottom of tray molds, pour traced CP soap (or melted MP) on top, and let the soap harden, forming the designs into every bar. Unfortunately, I've shut down operations, so I made this page to let everyone know how to do it themselves.
Why Mold Mats?
I started looking at making single-cavity molds, but the silicone mixes were very costly, and having silicone on all sides of the soap made the molds prohibitively expensive. Mats, however, minimized the volume of silicone that was needed, along with the price.
How Did You Make Them?
The short version is that I prepped a vector file, engraved the design on sheets of plastic, set up a mold, poured mixed silicone on top, and then demolded and trimmed the piece. Detailed instructions follow.
STEP 1: File Prep
I started with an unfair advantage here. I had previously worked in graphic design, so I knew exactly how to make, clean, and prepare a 1-color vector file. If you don't know how to do this, you'll probably need help, likely at the same place you're getting the engraving done.
STEP 2: Engraving
THE MACHINE: I bought a used Xenetech rotary engraver. “Rotary” means there's a spinning metal bit that carves out the design in the plastic. Laser engraving machines are popular, but they can't handle the large pieces I wanted to make.
THE BIT: I used a “profiler” bit, which is super sharp. The "005" you see, on the packaging tube on the right, means it's 0.005 inches wide at the tip. It takes a LONG time to carve a design (2-4 hours was not uncommon for a job), but it makes for the best detail.
ENGRAVING PLASTIC: Platinum silicone (the type I used) will NOT cure if it's touching any plastic with latex in it. Even contact with scotch tape will screw it up. Fortunately, regular Rowmark engraving plastic did the job. I used clear plastic, which made it easy to see bubbles and debris in the silicone mix before it cured. (More on that later.)
ENGRAVING: Here, you can see a design getting engraved in the plastic. The silver cylinder houses the rapidly spinning metal bit. The resulting sheets became the mold masters, which the mold mats were cast from.
DEPTH: After some experimenting, I found that “40” on the machine (about 1/16th of an inch) was the sweet spot between “deep” and “detailed”. Of course, engraving that deep meant the bit had to move slowly. You may have to experiment with the settings yourself.
SIZE: I got a smaller, less-expensive engraving machine that was only capable of engraving a 12 inch by 9 inch area, but most tray molds are bigger than that, requiring me to engrave multiple mold masters.
In the job to the left, the client had an 18-bar mold, but I could only engrave 9 bars in a sheet with my machine. (A larger engraver could have done it as once piece.) This meant there was a seam between the two mold masters, which I was able to disguise with...
CUT GUIDES: I made it a default option to have “cut guides” in the mold mats. I engraved lines at the edges of all the soap bars (thankfully a built-in feature in Xenetech software; I just had to dial back the depth of the bit) so then the seam between the two mold masters blended in with the lines between the soap bars.
STEP 3: Setting Up a Mold for the Mold
Okay, you've got your mold masters: a sheet (or sheets) of engraved plastic. Time to make a silicone mold mat off them. But first, you need to make, well, a mold for the mold.
THE COMPONENTS: I used custom-cut acrylic pieces (gotten locally from the awesome Tap Plastics) for the base and sides. I used acrylic because, like the engraving plastic, the silicone didn't have any problems curing while touching it. And since almost every job was a different size (because almost every soap tray mold is a different size) I ordered acrylic pieces of different lengths that I could clamp together to make custom-sized molds for my custom-sized mold mats.
THICKNESS: ALL the acrylic pieces were the same thickness: a quarter of an inch. I would place the border walls flush against the mold masters, then pour the silicone on top of the masters until the level reached the top of the acrylic sides. The resulting silicone depth was a perfect thickness for soap mold mats.
SETTING UP THE SILICONE MOLD: I placed the mold masters on the base acrylic sheet, then arranged the side pieces flush against the edges. No gaps anywhere, or else there would be leaks! Then the side pieces were clamped down in place.
THE BASE: Here's the view underneath the silicone mold. The base acrylic is resting on top of a square riser, to allow room for the clamps, which stick out below the base piece. Then that whole assembly sits on a lazy susan, so that I can turn the piece around in a circle for inspection and cleaning.
You can see the final setup below. Yes, the "clamps" keeping everything in place are actually binder clips. They were cheaper than shop clamps.
DIMENSIONS: You may have noticed numbers written on the pieces. I wrote the length of every side, on the back, in sharpie. This made it easy to pull the right pieces for the job.
GLOVES: Any time when I might have skin contact with the silicone mix (including handling the acrylic mold pieces) I put on disposable gloves. The silicone manufacturer didn't say this was necessary, but better safe than sorry, as far as I'm concerned. Make sure to use VINYL gloves! (Remember how platinum silicone doesn't like latex?)
STEP 4: Mix and Prep the Silicone
I used Platsil 73-15 from Polytek. Benefits include a long working time, a short cure time, a very clear color and a super high-quality silicone when it's done. The mix is also very low viscosity, which made it easy to get fine details in, and bubbles and debris out (more in that later). It's also an easy 1-1 mix by weight.
One problem is that since this is such a soft and stretchy silicone, large pieces can stretch under their own weight. (I didn't offer mats larger than 12 inches by 16 inches.) Another problem is that the mix is very temperature-dependent. If it's under 60 degrees in the room, the silicone takes forever to cure, but over 75, it cures too fast, which is bad when you're trying to work bubbles out.
When it comes to mixing, I followed the manufacturer's instructions. Before I poured, I placed the mix in a vacuum chamber to remove most of the bubbles made in mixing. (This is technically optional, but it helps.)
STEP 5: Pouring
I poured the mix onto the mold masters, slowly and around the surface, until the silicone reached the top of the side bars.
STEP 6: Cleaning the Poured Silicone
This was the part of the procedure that ended up being a big headache: cleaning out debris and bubbles from the silicone before it cured. It required careful inspection and a lot of time. At least the defects were easy to spot when using clear silicone on top of clear engraving plastic.
BUBBLES: There were always bubbles at the bottom, especially in detailed designs. While some bubbles floated to the surface on their own, not all did. I had to dig these out of the details, one at a time, with something pointy. Even when the bubbles got to the surface, I sometimes had to pop them individually so they didn't end up in the final mat.
DEBRIS: Silicone mix is super-staticky, which meant nearly anything floating in the air nearby (like cat hair) tended to zip right into the mix. I also got dirt and bits of cured silicone in the mix, which had been stuck to the inside of one of the mixing cups. They would cause lumps and dimples in the final mat, unless I picked them out.
THE FINAL RESULT: Here's the filled and cleaned silicone mold, ready for curing. The cure time depends on the ambient temperature, but I allowed 24 hours at least.
STEP 7: Demolding and Trimming
DEMOLDING: The binder clips came off, and the sides were removed.
PEELING: The silicone mold mats were peeled off the mold masters.
THE RESULT: You can see the raised silicone, which will make a recessed design in the soap.
TRIMMING: I had to clean up the mold mats: anywhere the silicon leaked, overflowed, or otherwise went out of bounds, I had to cut it off with a pair of scissors.
THAT'S IT! If you have any more questions about how I made them, feel free to email me at email@callacustommolds.com. (Please allow some time to reply.)
As a bonus, here's the forms and instructions I used for my business, which might provide some additional help. The "spatula scrape" demonstrated in the instructions is an important step for bubble-free soap:
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So, Why Did You Stop Making Them?
Bottom line, the business was profitable, but not profitable enough. My potential customer base was also very limited: most soap makers either used loaf molds (which don't work with the mats), or custom soap stamps (which don't look as good but are much cheaper), or single-cavity molds (and singe-cavity inserts ended up being even more work). I wasn't making enough to cover my time, and I didn't think I could get away with charging more, since most soapmakers were turned off by the price as it was.
Now, COULD someone make it work where I couldn't? Maybe. Hopefully. If you want to try making your own, I wish you the best of luck.
