Introduction

This robot may sound complex - a silicone robot with intertwining interior chambers that walks using air pressure - but the process to make it is relatively simple. Even though it's got a lot of stuff going on all you really have to learn is how to mix and pour silicone and you're well on your way to having your very own quadrupedal soft robot.

This robot has two hollow interior chambers. When either of these chambers is pressurized it deforms and bends the robot's legs and sides. This bending produces the walking motion. It is similar to how a salamander walks, by balancing itself on a pair of legs diagonal from one another while moving the opposite pair forward.

The basic steps to make one of these quadrupeds are to 3D print all the molds, fill them with silicone, put the resulting parts together to form molds for the waxes, pour the waxes, put the waxes together into the alignment jig, pour one side of the silicone bot, and then pour the other. It might take some trial and error to get a perfect, bubble free robot, but if you've got any mold making or casting experience, chances are you've already got all the expertise you need to make one of these robots in a handful of afternoons spent hacking.

Gather Materials

Your first step will be to go here, download the CAD files, and 3D print them. I used a Makerbot Replicator 2 printing PLA using three shells and a 10% infill. Since these have long, flat bottoms and large, straight edges, you will want to add mouse ears, rafts, or other features to prevent curling and distortion. My method was to print each part using mouse ears on the corners, and then hold them down with the springs from the inside of laundry clips once the print head was high enough to avoid knocking into them.

While you're at it, you'll want to print out some Luer Caps and keep them to one side for later. Also, laser cut yourself a pair of these plates in wood or acrylic. The exact thickness of the plates isn't important, so long as they're strong enough to stand up to a little abuse.

Next, you'll need to gather all the tools and supplies.

Materials

  • Eco-Flex 00-50 - 1gal
  • Smooth-Sil 935 - 1gal
  • Sil-Poxy
  • Silc-Pig if you want to tint your robot (Optional)
  • Jewelry injection wax (I use the Freeman aqua wax)
  • Mixing cups
  • Tongue depressors
  • Nitrile gloves (Don't use latex... I learned that the hard way)
  • Paste wax
  • Chip brushes
  • 3" 1/4"-20 bolts x 6
  • 1/4"-20 wing nuts x 6
  • Luer fittings (These things are so useful. They form an airtight seal and are quick to assemble)
  • PVC tubing (Get the corresponding size for the Luers you buy)

Tools

These materials will run you somewhere around $300. However, once you’ve got the materials and molds in place you’ll have everything ready to make dozens of soft robots.

All the materials I’m recommending are safe to work around, though you’ve got to use common sense when using them. Work in well-ventilated areas. Wear a respirator if you’re chronically exposing yourself to solvents or vapors. Wear disposable nitrile gloves and change them in between processes. Don’t keep food, drinks, or things around that you could cross contaminate by touching with your gloves on and then touching again with your gloves off. Handling chemicals is a lot like handling raw chicken: it can be perfectly safe as long as you minimize the potential for contamination, keep good hygiene, and clean up after you finish.

Also, I tend to use Smooth-On products because I know their materials well and have worked with them for years. They're certainly not the only company out there for silicones and mold making materials, but they're the ones I know and trust. They've actually been nice enough to film some of these robots coming together and support the project with some free materials. So, I've got to admit to a bit of favoritism but feel free to experiment with other materials.

Pour your Molds

Once you've got all your 3D prints and materials on hand, you're ready to pour your molds. The terms can get a bit confusing here, since all of the parts you just printed are called molds, and the silicone parts that you get out of them are also called... molds. It's molds all the way down. However, it'll make sense once you see what they're all for.

You can think about it like this: if you want to get a complicated part made in a material like silicone, you'll need something to form that material - a mold. However, just printing a mold directly on a Makerbot won't work for a lot of shapes because they just don't want to come out of a hard plastic shell. So, I go a step ahead printing molds in plastic that, when poured in silicone, make molds that get me the final shape I want. This makes it easier to get precision parts without loads of manual labor, cleaning up seams, and flashing.

Coat the insides of your 3D printed molds in wax. This will help the final part pop out of the mold easily, in addition to sealing up any little cracks that would allow liquid silicone to pour straight through the print. Fill all the molds to the top with Smooth-Sil according to the manufacturer's directions. This video gives a good introduction to mold pouring if you haven't given it a shot before. Just pour in a thin, even stream starting the pour at the lowest point of the mold, allowing the wave of liquid that rises up to push any bubbles out.

Once the molds are all poured, wait about 8 hours to de-mold.

Open those Molds

When your silicone has cured overnight it's time to de-mold those parts. The easiest way is to get an air gun and put the nozzle in the seam between the 3D printed part and the silicone. When you blast it with air it tends to pop the parts right out. Barring that, you can work your way around the rubber, pulling the sides away from the mold, shimming the seam with popsicle sticks, and pulling gradually until the rubber gives way.

Pour your Waxes

Take a look at the silicone pieces you just pulled out of the "Glaucus_Left_Bathtub_Bottom" and the "Glaucus_Left_Bathtub_Top" prints. They fit together to form a new mold, one that will produce the waxes you need for the final quadruped. The same goes for the "Right_Bathtub" molds. There's just one thing you'll need to do before they're ready to cast: ventilate.

With molds like this there's lots of internal geometry for air bubbles to get trapped in. The best way to make sure air bubbles don't get trapped in the mold and end up messing up your casting is to ventilate the mold. Take the mold perforating tool I described in the materials section and poke it through all the places in the top half of the mold (the one with the hole on top) where you think bubbles will collect in the cast wax. When the tool erupts from the surface of the mold, pick out the string of silicone that comes along with it out so that it won't get stuck back in the hole or clog the tool.

When everything's well vented, you're ready to fit your funnel and pour the wax. I used a dollar store kitchen funnel with a bit of electrical tape wrapped around the small end to form a nice seal when pressed into the mold sprue.

My wax pouring cup is a steel beer can I bent a spout into. A bolt through a piece of dowel on the back made for a nice handle. I place the can on the stove for melting the wax, which is admittedly a little dangerous. Wax vapor is flammable, and spilling liquid wax over an open flame would put you in quite a pickle. I'd recommend a hot plate or similar ignition-free heating element for melting your wax. At the least use a double boiler if you're going to put it on the stove.

When your wax is heated, place something on top of the mold to add compression. I use a scrap of 1/4" plywood with a bunch of holes in it from little laser cutting projects. It's got a big enough hole in it that I can fit the funnel in the mold and still compress the mold pretty easily. You'll want to add compression to the mold go get the seams to press together and reduce the chances of wax running out and creating flashing you'll have to clean up later.

Situate the funnel in the mold sprue and pour in one continuous stream until the funnel starts to fill up. Leave the funnel in for about thirty seconds, making sure to top it up if the wax level drops below the mold's surface. After thirty seconds, pour the remaining liquid wax in the funnel back into the wax pot, and reset everything for the second mold. Repeat the process. Remember not to leave the funnel in the cooling wax or else it'll be a hassle to clean once the wax has set.

Clean your Waxes

If everything went well with your pour, you should have some nice, clean waxes. If not, chances are there's some flashing around where the seams between the two mold halves were. It should be easy to scrape off with a fingernail or snip away with a pair of wire cutters. If you have little bubbles, don't worry about it. If you have big bubbles that could compromise the structure of the wax (you don't want anything breaking off when casting the quadruped) you could repair it by melting more wax into the gaps. I use a quick and dirty wax pen, which is just a soldering iron with a metal sculpting tool held into it with steel wire. I'll often touch up the surface of the wax with a heat gun to smooth things out a bit and melt away any tiny flashing that could flake off or distort the core. If things break completely off, you can rearrange them back into the mold, melt the seams with a wax pen, and freeze the hot wax in place by turning a can of Dust-Off upside down and gently dribbling some of its accelerant on the seam.

Take your pair of waxes and interleave them together. They should fit without too much finagling and there should be an even space between them. When they're ready, you can put them into the silicone alignment jig, and then you're ready to start casting the final quadruped. If the little tubes on the back of the waxes stick up off the surface of the alignment jig, hit them with a heat gun until they stay flat.

As long as the silicone has had about 24 hours to cure you shouldn't have any problems with the molds locking together or sticking. If you cast earlier than that there's a chance enough catalyst will still be active in the molds to adhere them to the new pour of silicone.

Pour Side One

Take your clamp plates, screws, wing nuts, the top mold, and the alignment jig with the waxes securely in place and secure them all together. I find it easiest to put the alignment jig into a clamp plate, insert the screws, and then lay the other mold components on top like I'm playing a game of Towers of Hanoi. Once everything's stacked and aligned, lightly screw on the wing nuts. Inspect the edges all around to make sure you've got good clamping force on everything and there aren't any gaps between the silicone molds.

If the tubes on the ends of your waxes still aren't flush against the alignment jig's surface, you can put a small segment of tongue depressor between them and the top mold. Remember to remove the tongue depressor before doing the final pour or else you'll have to get it later and fill the area in with adhesive.

I use a vacuum chamber to suck all the air bubbles out of the silicone after it's mixed and again after it's poured into the mold. This isn't essential. I've gotten dozens of successful castings out of these molds without it, but I wanted to have absolutely, positively no bubbles whatsoever in my castings.

Now all you've got to do is pour the silicone. I'm not going to go into much detail on pouring silicone here, as there are dozens of good tutorials out there. Just make sure go go slow, tap the mold to make sure all the bubbles get out, and keep checking in ten minute intervals so you don't miss the opportunity to pour the next layer while the silicone's still tacky.

Pour Side Two

When you can drag a tongue depressor through the cup of silicone you just mixed up without any tearing or liquid rubber coming away on the stick, you are ready to replace the alignment jig with the last part of the quadruped mold. For me, this is usually between one and three hours after casting.

Disassemble the mold and take away the clamp plates. Gently peel the alignment jig away from the top mold being cautious not to disturb the silicone that you just cast. If it's super squishy like jello, you should let it sit for a bit longer. When you can pull the alignment jig off and the silicone remains stuck to the top mold you know you're at the right stage.

It's ok to snip the silicone wherever it's gotten under the wax in the alignment jig. The second pour of silicone will fill in all those holes so that it appears that the final quadruped is just one solid piece. Chemically and structurally it is, as the silicone will form polymer chains that bond all the way between the two sides. To provide for better ventilation, you can trim off the silicone that has flowed into the air vent underneath the quadruped.

When you've got the top half mold looking like the one below, reassemble the mold switching out the alignment jig with the bottom mold and pour up the second side just as before. This side has a tendency to catch bubbles more than the first, so you might want to go extra slow and spend a lot of time tapping and jiggling the mold to prevent them from forming.
Allow everything to cure for at least four hours. Keep the cup you mixed your silicone in around so you can poke it to see how far along the cure is. Heating up the mold will speed up the cure if you're in a rush. I tend to set it in front of my radiator so it gets gentle, even heat.

When everything's cured, you're ready to de-mold. Just go around the seam between your two silicone halves and gently peel them apart. You're looking to get the soft Eco-Flex away from the harder Smooth-Sil at its own pace so it doesn't tear. Even though it looks like the quadruped is solid and robust, there's only 1/8" wall thickness between the surface and the wax cores and if you pull too quickly it might rip.

When you've got the entire part out, give it a once-over. Now's the easiest time to plug holes by adding some adhesive. If your wax has pushed up to the surface or you've got a big hole formed by a bubble in the mold, go ahead and brush some Sil-Poxy into the offending area. Make sure to extend a film of adhesive out from the site of the repair so its got lots of surface to grip. Even though Sil-Poxy is pretty strong once it's dried, it's not super sticky. If you put it on as if you were gluing a broken coffee cup back together, with adhesive only on the seam between the broken surfaces, it'll pop together pretty quickly. Get a little paintbrush and get glue deep into the hole, all up on the underside, and spread glue out on the surface.
This process can be time-consuming and messy. Don't let it get you down. It's actually pretty fun if you don't mind getting a little wet.

Snip off the sprue end of the quadruped just where the waxes end. This should open up those two holes at the bottom. Set your oven for 300F and pop the quadruped in on a disposable pan or a tray with a lot of aluminum foil for 20 minutes. After it's heated up, start squeezing it over a sink with your heatproof gloves on. By massaging the body of the quadruped, you should be able to get a whole lot of the liquid wax out. After that, pour some soap in the holes, add some water, and return it to the oven at 200F. Be careful not to get it too hot as wax and water can suddenly pop- like dropping water into boiling oil. You don't want to get burned. It would also probably be a good idea to put on your science safety goggles.

After getting the soapy water out, keep filling the quadruped with water and rinsing it out. No need to return it to the oven after a second heating as the wax that's left will be broken up into little pieces. I made myself a little sink nozzle to inflate the quadruped with water. When the water sloshes back out it carries a lot of the wax bits embedded deep in the inner recesses away. When the water comes out clean, you know your robot is ready to go. Leave it to dry in a warm oven or on a dish rack.


Hook Up & Done

When your quadruped has had a chance to dry out, it's time to give it a test drive. Stick the tubes from your sphygmomanometer pumps into the holes in the back of the quadruped, make a ring with your fingers around the back of the quadruped, and try inflating it. Look over the robot while it's inflating to check for air bubbles or problem spots that could use more adhesive. You could put a zip tie around the back of the quadruped to do some more thorough tests without having to glue any additional pieces on.

Once you're satisfied with the inflation, it's time to glue on the final part: the luer seat. Start by inspecting the back of the quadruped and doing a dry fit. Chances are you'll have to take a pair of wire cutters and open up the holes in back a bit. The quadruped should sit all the way into the luer seat without too much manhandling. When you're satisfied with the fit, clean up the whole area that will be glued with some rubbing alcohol on a q-tip.

Take a little paintbrush and coat the whole interior of the luer seat with Sil-Poxy. Try to get a even coat so that it won't glob up in places disturbing the quadruped's fit. When it's ready, ease the back of the quadruped in and set the quadruped up in the corner between a table and a wall to hold the seat on straight. When the glue dries in about an hour, you should have a working quadruped on your hands.

Hook up your luer fittings to some lengths of hose, twist those luers into the back of the bot, and you're ready to play around. You could power it electronically using the same air system as my trefoil tentacle, keep using the sphygmomanometer pumps, or use a system you devise.

Colophon

The documentation for this tutorial was taken during the production of several different prototypes of the same overall design. You may have noticed the colors of the silicone molds, waxes, and quadrupeds changing from photo to photo. The same goes for the structure of the waxes. I chose, from the available documentation, the best photos to represent the process during each step.

If you're curious about details, I encourage you to read the blog entries about this process and check the Soft Robotics Forum. I also have detailed write-ups on my other soft robotic projects, photos of the quadruped process, and more information on my robotics design company Super-Releaser.

Thanks so much for reading.
This guide was first published on Mar 24, 2014. It was last updated on Mar 24, 2014.