With 3D printing, you have the ability to take something you've thought of, make it in a virtual setting, and then make it a reality. It's not a new idea, but it's so much more accessible. You can print after-market parts for your car, fancy or "pop-culturey" knobs for your kitchen, customized badges for your car, and so much more. The possibilities are endless for creativity, but 3D printing can take a long time. If you have friends, family, or potential customers all interested in one model that's really time intense, it'll take a while to print it all. The solution: use 3D printing to make a mold.

One of the most fascinating things to me is mold making and casting; the ability to take anything that exists and being able to duplicate it is really satisfying. There are a number of ways you can make molds, from materials to techniques, so we are going to delve into this one technique at a time, with a focus on open face silicone molds.

What is an Open Face Mold?

Open face molds are the most basic form of molds. These aren't specific to any medium so you can use plaster, silicone, sand, clay, aluminum, etc.

Some tips on deciding whether or not you should be using an open face mold:

  • Short, flat backed models made of extrusions are great for this method.
  • The key with open face molds is no overhangs. Technically you can have brief small overhangs, but anything major like a 20 sided die is pretty much going to be locked in there.
  • Tall extruded objects, like a tall cylinder, can actually get stuck in the mold, even if it doesn't have overhangs. The vacuum created by trying to remove the part, is just like trying to remove a full trash bag out of the garbage and trying to fight to pull it out. Solve this by adding draft angles if you can; (drafts are slight angles to the vertical faces. Think of a pyramid as exaggerated drafts on a cube).

Making the Mold Box

Once you've decided on the object you want to mold, you need to build a box to contain all the silicone. A good measure for the box is to have half an inch of space on all sides of your 3D print, which includes the top. I'm going to be molding the MatterHackers logo, which measures in at 4.5 inches x 5 inches x 0.375 inches, so the box needs to be 5.5 x 6 x 1.375 (with some room on top so it doesn't overflow). To actually make the box is pretty easy; foamcore cut to size and hot glue are cheap and easy to use. Just make sure the entire box is sealed and glued together by running a bead of glue along the inside and outside seams. 

Measuring Out Your Silicone

Traditionally, to measure the volume of an object before you mold it would mean you would need either a rough estimate or using displacement. To use displacement, you would:

  1. Measure out 1L of dry rice.
  2. Remove the rice from the container
  3. Put the object in the container
  4. Pour the rice back into the container
  5. Mark the change in volume
  6. Subtract 1L from the new volume to find the volume of the model.

This works fairly well for small objects, but when you get to a model that utilizes the full build volume of a 3D printer, it becomes impractical to find a container that large or a bag of rice big enough. But since a 3D print needs a 3D model, you can use that 3D model to get an exact number. There are a variety of programs that can tell you the volume of a model, but I use netFabb. By loading a model into netFabb, part of the information that is immediately displayed in the sidebar is its volume cubic centimeters. With a box volume of roughly 473 cubic centimeters and a model with a volume of 37 cubic centimeters, I need 436 cubic centimeters of silicone, which equates to 436 mL. 

Preparing Your 3D Print for Molding

Moldmaking of any kind is excellent at capturing the surface detail of the master model - especially silicone. Any blemish, skipped perimeter, scratch from removing support, etc. will be captured in any and all castings. Before you get to molding, check out our article on how to finish 3D prints, so you can get the best casting possible. Once you're satisfied with the quality of your master, use a little hot glue or super glue to stick it to the bottom of the mold box. You should be able to peel off hot glue after all the molding is said and done, but the super glue's stronger hold would mean your model will have paper foamcore stuck to it. Use either Smooth-On Mann 200 Mold Release or Rustoleum Gloss Clear Coat. Both apply in a fine mist and help break the seal between silicone and your model for easy demolding.

Deciding on a Silicone

All objects can be put on a scale measuring its shore hardness, or durometer. For example, ABS filament has shore hardness of 70D and TPU filament has a shore hardness of 95A. When an object is rigid, it's shore hardness is measured on a Shore D scale, and when it's fairly flexible it's measured on the Shore A scale. The silicone's Smooth-On carries can be as hard as 60a or as soft as 2A (a rubber band is 25A). Whether to use 60A, 30A, or even 10A is completely dependent on the complexity of the model being molded. There's also the choice of tin cure vs platinum cure silicone rubber. Tin cure actually uses tin to catalyse the reaction and cause the silicone rubber, and the same happens with platinum cure silicone. As you might expect, platinum-cure silicone is a lot more expensive than tin-cure. Platinum-cure silicone lasts for more castings than tin-cure silicone, all else being equal, and platinum cure silicone resists shrinking as it cures much better than tin-cure. But between tin-cure and platinum-cure silicones of the same shore hardness, master model, room temperature, etc. they will otherwise behave the same. For our logo which doesn't have any overhangs, is very flat, and has very simple geometry, BJB TC-5060 is a good choice; it's a platinum-cure silicone, decently rigid, and comes out as a nice blue.

Mixing Silicone

To mix silicone depends on brand, tin-cure, and other factors. Some mix based on weight and others mix based on volume. BJB TC-5060 mixes 1:1 by volume, so two cups filled to the same marker is perfect, in this case 220mL in each cup. A third cup is necessary to the mix the two in; dump one cup, making sure to scrape the sides clean, then dump in the second cup and do the same. Once you've done that, the clock starts ticking and you have a limited amount of time until the silicone starts to set solid. This is called "pot life" and for BJB TC50-60 that's 45 minutes. With both parts thoroughly mixed together and the sides and bottom of the cups scraped clean and mixed completely, there are two options for pour this on this model:

  1. Pour the mixed silicone slowly and at from a height high enough to have the silicone pour in a very thin stream. This does a pretty good job at popping any bubbles that may have been whipped into the silicone. Bubbles are the enemy of a good mold, anywhere there is a bubble in the mold, there will be a small pimple on all your castings. The "bombs away" method will get rid of most bubbles, but not all.
  2. Take the cup with the mixed silicone, make sure the volume of the container is 4 times as large as the volume of the silicone, put it in a vacuum degasser, and raise the pressure to 29 inches of mercury. This will burst the bubbles in the silicone by super inflating them from the lack of pressure to the point that they pop. If your container isn't large enough, you may need to "burp" the silicone so it doesn't overflow but the bubbles have enough time to burst. You'll know it's working when the silicone rises almost to the top, ripples, and collapses.

From here, both techniques are the same; you want to pour the silicone onto the lowest point of the mold and let it rise up to fill other cavities. With the mold full of silicone and a pot life of 45 minutes, I had plenty of time before it started solidifying, so some gently taps with the side of my mixing stick rose any bubbles to the top. After that, it's time to cure.

Curing the Silicone and Post-Cured Treatment

DON'T. TOUCH. IT. As tempting as it may be to poke the silicone, if you poke it while it's curing, you may pull silicone away from the surface of the model deep in the silicone and morph the final shape of the mold. Wait the full cure time of the silicone, in my case 6 hours, and add more time. I left the mold at the end of the day and came back to it the next morning.

Once it had cured fully, I could start tearing apart the mold box. You can either do it gently to try and save the foamcore, or you can tear at it like a kid on Christmas. Once all the foamcore is off, you can pry out the master model and be fairly aggressive with it. You don't need to go really rough with it, but the silicone can go pretty far before tearing. With the model out, there may be a little silicone that seeped under the model, this is called flashing. A nice sharp art knife will make quick work of it and give you a clean edge. Don't pull the silicone taut as that will actually skew how much you are cutting off. 

Use a little soap and water to wash out the release agent from the mold, wipe it dry, let it air dry, and you're all set for casting. That's it!

From here you can cast using a variety of materials with various dyes to achieve the base look you want. From dense foams, squishy foams, rigid foams, fluorescent filled, iron filled, red, blue, green, etc. there's so many different materials you can cast with, and with proper care, you can use this mold for a long long time. Quick tip: the quicker a casting resin, epoxy, etc. sets, the shorter life your mold will have. Quick curing resins means they give off more heat than a slower curing resin and will dry out the silicone quicker, to the point that it can completely crumble off a fateful last casting. Don't try and mold chocolate or ice in a BJB TC-5060 mold, as you need specific food grade silicone for that. 

We hope that this article was informative and has inspired you to create your own molds with the help of 3D printing. Stay tuned for more ways to use 3D printing to make molds! 

Is there something you'd like to see Alec create using 3D printing? Let us know in the comments below, or send an email to support@matterhackers.com