Pouring concrete using 3D print molds
Step 1: Digital
Soon to come.
Step 2: Using 3D Prints As Molds
3D prints are great ways to create interesting geometry in molds. Regardless of the complexity, any part is no more costly to make than the next, generally speaking. It is however not the always the best method for producing multiple copies of parts and material options are somewhat limited to what can be printed. Instead of printing an object directly, we can use 3D printing to produce the mold itself. Using a standard desktop printer, an ABS print was produced with a flat base. A perimeter of wood, in this case MDF board, encloses the print with a 1" skirt was left around all sides. Using wood screws, the piece is assembled with all 90 degree connections. Ideally a funnel can be printed at the exact thickness of the gap so to have it built into the mold for a easier final step. You want to ensure that the walls are tall enough to account for all the silicone that you will pour on top. In preparation for this, mark off on the side of your mold the height that you will need to pour to.
Step 3: Silicon Options
Two mixers were used here the rubber molds, I'm using Smooth-On's Mold Star 15 SLOW two-part silicone product and TAP Silicone RTV with thinner. Both these give you just under an hours worth of work time. Using both these before, I wanted to make a side by side comparison.
Pour a thin stream from several inches above your part, concentrating on its lowest point. This helps ensure that the silicone seeks its own level as it rises up and over the model, while displacing air to reduce the presence of bubbles as much as possible. After you pour both halves, checking for leaks in the walls and carefully check that your 3D print isn't moving around inside.
Step 4: Take Off the
Give a conservative amount more time than listed on the product, nothing is worse then not even getting past the mold stage. Slowly remove one wall at a time inspecting for non cured areas, worse case you can just screw that wall back on. The silicon can get pretty stuck to the walls some force is ok to apply. Remove the printed parts from the silicone molds, ideally without breaking it. If the geometry had undercuts this may be difficult but remember that the mold is more important as is will yield many more parts than just the print.
Step 5: Check and Compair
The Smooth-On was much more forgiving with delicate pieces as it is more flexible while the TAP Silicon may be more durable with its more robust rigidity. Checking the edges to see if there are any hang pieces is always a good practice. Comparing the two halves is always important as well, though if you were carefully with measuring, they should not differ.
Step 6: Set Up for the Pour
With all my materials and layout space ready, it is time to pour. The set up is crucial so to give yourself ample time for the mixing and later careful pouring of the concrete into the very narrow hole that has been left of the liquid. In this case the usual paste like consistency normally desired for concrete needs to be a little more watered down. Using a 3 to 1 (concrete to water) ratio, begin mixing water into the concrete slowly while stirring making sure to not leave any edge of the vessel overlooked.
I used the same size wood bases to clamp to for even pressure. Be careful not to over tighten as the silicon will lose its shape and will be warped/leak. Once you have topped off the mold, give it a bit of a shake and twist is as much as possible without it causing any leaks.
Note: Concrete is a messy process and can be very harmful what ever it is poured on and especially for yourself. Ideally you have a waterproof surface or just being outside is good. Remember to use a respirator mask so that the very airborne dust does not get into your lungs. Also, do not rinse anything with concrete dust in your sink as it will immediately solidify and ruin your plumbing
Step 7: Demolding
Most pieces this size need at least 36-48 hours to fully set. Let the piece stay in the mold for a couple hours even after prying it open allowing the rest of it to dry as silicon really does keep in all the moisture.
Unfortunately due to the complex shape with having undercuts, this first try did not work as well. It was a lesson to make sure to shake my assemble up more to give it even more coverage inside. Not a problem, now having the mold, another piece can be poured again.
Step 8: Finally
Rinse and repeat until you start to get perfect pieces out.