We sometimes sell products wholesale to West Elm and we got an order that included 80 coasters - and we only had one master model and a few half-decent molds. It was time to get serious.
Below is a video of the whole project with all the ups and downs (for some reasons, we got an error when trying to embed it):
So we made 8 CNC'd master models, 16 silicone molds, and finished our order in time! But along the way we learned a lot about how to make these concrete products in bulk so we'll go through the steps and the things we learned.
Here's what we used:
Step 1: CNC Master Models
In the past we've mainly 3D printed master models (IE, the thing you make your silicone molds from). But 3D prints require many many rounds of priming and sanding to remove the layer lines and smooth out their surface enough to get a clean silicone mold out of them. It's so time consuming that we only made one and it took the better part of day with all the dry time involved. So we decided to try CNCing our master models on our X-Carve instead this time.
We used high density urethane foam for this because it's machinable and nonporous. For the model we cut, we used Fusion 360 to do the CAD and CAM. We have an additional tutorial here that's a beginners guide to Fusion 360 CAM if you're interested.
Step 2: Prep Master Molds for SIlicone
They came off the CNC pretty much perfect, we just smoothed out the subtle cut lines with some hand sanding. Then spray a little layer of mold release.
In our first attempt, we went straight from this to pouring some Mold Star 15 platinum cure silicone. Little did we know at the time that this type of silicone was incompatible with urethane and when we poured in the silicone, it never quiiiiiite cured (see the last photo in this step).
There's a whole saga on that in the video at the beginning of this tutorial, but here in the step-by-step we'll save you the trouble. There are 2 ways to avoid the incompatibility issue. You could use Oomoo 30 tin curing silicone from the get go, or you can coat the urethane foam with automotive primer. We did the latter and used the primer, because silicone isn't cheap and we had bought the bigger sized buckets of Mold Star 15 already.
We did three coats of automotive primer and sanded lightly in between.
Step 3: Mix and Pour Silicone
Mixing the silicone is pretty easy, and we picked up some tips when we reached out on Instagram during the whole silicone-not-curing saga. So we'll cover those here.
It's a one-to-one silicone, so you pour equal amounts of part A and part B and mix them together, stirring well. One of the tips we picked up is to stir from top to bottom, not just around in a circle, that way you fully incorporate A and B.
Before pouring, make sure there's no dust or anything in your master models. We give them a quick once over with compressed air. If you have intricate details or tight corners, you can use a brush to brush on a thin layer of silicone into those details and corners before pouring it in.
When pouring, pour as thin a stream as possible to prevent air bubbles. And after pouring, vibrate your master model to make air bubbles rise to the surface and pop. We often take a sander (without sandpaper) and hole it against the master model to vibrate it.
Let set according to instructions, making sure you leave it in an environment that is within the correct temperature range. Demold carefully (it helps to break the seal along the edge first and then gently pull the molds out)
Step 4: Mix and Pour Concrete
This is the easy part! We use Quikrete Precision Grout (full disclosure, they sponsored the video, but we've been using their products for a long time). You add water to the mix and stir until it's the right consistency. A little bit of water goes a long way, so add sparingly. You'll know it's the right consistency when it's not dry anymore but it isn't wet enough to pour out of the bucket and there shouldn't be excess water that comes to the surface when you tap it.
Spoon it into your molds, tapping the mold as you go to level it out. The tapping also removes air bubbles, but if you wanna go more hardcore on the air bubble removal you can use the vibrating sander technique we talked about earlier.
Let it set for the amount of time it says in the instructions... or you can take them out a little early if you're in a rush like we were. It'll continue to harden and cure in the open air.
Step 5: Finishing Touches
At this point, you have concrete coasters! But we like to add a few finishing touches to really make them feel, well, finished. First off, we add a gold ombre spray paint. It's nice because it doesn't have to be perfect and you can do a lot at once!
We also like to add these little self adhesive cork backings to the bottoms so that they don't scratch your table. And since we're selling ours, we add these little paper labels with our logo and name. It's the icing on the coaster! :P
Hope you enjoyed this tutorial and picked up some tips about making concrete coasters (or other concrete items) in bulk!
If you like this project, please consider checking out and subscribing to our YouTube channel. Thanks!