User Tools

Site Tools


Soft Quadruped Robot

Author: Yeeun Kim, Elyse Chase Email: [email protected], [email protected]
Date: Last modified on 08/08/16
Keywords: Soft Robotics, Quadruped, Molding, Silicone

The photo above depicts a quadruped. The big picture problem is how to make it easy to replicate. Solving this partially or completely is important because this silicone robot walks with intertwining interior chambers using air pressure. This tutorial shows you how to make it and takes approximately one week to complete.

Motivation and Audience

This tutorial's motivation is to increase familiarity with soft robotics. Readers of this tutorial should have the following background and interests:

* Know how to use a 3D printer
* Perhaps also know how to mix and pour silicone
* This tutorial may also attract readers who want to know about soft robotics

The rest of this tutorial is presented as follows:

  • Parts List and Sources
  • Construction
  • Final Words

Parts List and Sources

US-based vendors to obtain material to complete this tutorial include Amazon and Smooth-On.
To complete this tutorial, you'll need the following items

Eco-Flex 00-50 Smooth-On 30.10 1
Smooth-Sil 935 (trial size) Smooth-On 34.73 4
Sil-Poxy Smooth-On 28.71 1
Jewelry Injection Wax Amazon 8.25 1
Luer Fittings Amazon 1
Nitrile GlovesAmazon
Silicone Mold ReleaseAmazon
Mixing CupsAmazon
Hot Plate (or Stove if you have access to one)Amazon
Toaster Oven (or a Conventional Oven if you have access to one)Amazon
Heat Proof GlovesAmazon
Metal Measuring Cups (or any cup that can be used to melt the wax)Amazon
Bulb Pump with TubingAmazon
Paint StirrersLowe's, Home Depot, Ace Hardware, or similar storeFree10
3“ 1/4-20 Bolts Hardware StoreVaries6
1/4-20 Wing NutsHardware StoreVaries6

Additional Items
-Vacuum chamber or another tool to remove bubbles in the silicone (optional)
-Acrylic with holes drilled to match the pattern on the 3D-printed molds


This section gives step-by-step instructions along with photos describing how to make the quadruped.

Step 1
1. Pour your Molds

*2) Once you've got all your 3D printed parts 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. However, it'll make sense once you see what they're all for.

*3) Mix the silicone according to the directions on the packaging. Once thoroughly mixed, use a degasser (if available) or other vibration method (we used the vibrations created by a sander) to remove bubbles from the material. While this step is optional, bubbles in the molds can cause issues with the parts created from within the mold.

Step 2
2. Open those Molds

  • 1) Allow the silicone to cure overnight, then de-mold the parts.

Step 3
3. Pour your Waxes

  • 1) 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.
  • 2) Heat your wax in the metal cup until a vapor begins to appear from the surface of the wax. If you are using a hot plate, use the highest heat setting available.
  • 3) You now need to pour the wax. You can use a popsicle stick or a funnel to direct the wax into the mold sprue. Pour the melted wax in one continuous stream until the mold appear to be full. Watch the mold for a few minutes, making sure to top it off if the wax level drops below the mold's surface.

Step 4
4. Clean your Waxes

  • 1) After 30 minutes or so, your waxes should be ready. If everything went well with your pour, you should have some nice, clean waxes.
  • 2) If not, it should be easy to scrape off any excess wax with a fingernail or snip away parts with a pair of wire cutters. If you have little bubbles, don't worry about it - those won't affect the final product. If you have big bubbles that could compromise the structure of the wax, you can repair it by melting more wax and adding it into the gaps.

  • 3) Take your pair of waxes and interweave 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.

Step 5
5. Pour Side One

  • 1) Gather your clamp plates (we used two acrylic sheets with 6 holes that aligned with the holes in the silicone molds), screws, wing nuts, the top mold, and the alignment jig. Thoroughly spray the silicone molds with mold release (this is VERY important - if you don't spray it thoroughly, the new and old silicone will stick together).
  • 2) Secure the waxes in place. If it has been a while since you poured the waxes you may need to heat them up slightly in order to bend them into the jig without breaking them. We used a heat gun on the “cool” setting to accomplish this.
  • 3) Now, use the screws and wing nuts to attach all of the components together. Make sure these parts are tightly attached. If it is too loose, the silicone will pour out of the mold through the sides, instead of creating the desired shape.

  • 4) Now all you've got to do is pour the 2nd kind of silicone. Again, if you have a degasser you should use it prior to pouring into the mold. Additional bubbles in this mold can cause major issues, as the internal air chambers created by the wax are what creates the quadruped's motion. Just make sure go go slowly as you pour, and tap the mold to make sure all the bubbles get out.

Step 6
6. Pour Side Two

  • 1) After the silicone has dried according to the directions, disassemble the mold and take away the clamp plates. Gently peel the alignment jig away from the top mold being cautious not to break 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.

  • 2) It's ok to remove any silicone that has gotten under the wax in the alignment jig. The second pour of silicone will fill in all those holes so that the final quadruped is one solid piece.

*3) Now you need to secure the second half of the mold with the half-molded quadruped. Again remember to use plenty of mold release and to secure the mold tightly.

*4) Mix the silicone and again use a degasser if available. Pour slowly and tap the mold to remove any additional bubbles. Allow to dry fully.

Step 7
7. Demold & De-Wax

  • 1) Go around the seam between your two silicone halves and gently peel them apart. If your wax has pushed up to the surface of the quadruped or if you've got a big hole formed by a bubble in the mold, go ahead and brush some Sil-Poxy into those areas.

  • 2) Snip off the sprue end of the quadruped just where the waxes end.

Step 8
9. Wax Removal

*1) If you are using the toaster oven, preheat the oven to 300 Fahrenheit. If you want to use the toaster tray again, be sure to spend this time by covering the tray with aluminum foil thoroughly.

*2) Place the quadruped on the tray and into the oven. You may want to keep the silicone off of the tray so that it doesn't get covered in melted wax. We accomplished this by creating aluminum pieces that held the quadruped off the tray and slightly tilted to enable the wax to flow out more easily.

*3) Bake the quadruped for 20 minutes. Remove from the oven. Use oven mitts to hold and squeeze the wax out. Return to the oven and repeat until all wax has been removed. Keep in mind that one of the wax pieces has to go up and around to exit through the two sprues. If you are confused by the movement of the wax, go back and examine the wax pieces/molds.

Step 9
9. Hook Up & Done
*1) Stick the tubes from your sphygmomanometer pumps into the holes in the back of the quadruped, make a ring with zip tie around the back of the quadruped, and try inflating it.

*2) As you practice with different speeds, pressures, and amounts of air, you will begin to make your quadruped move.

Final Words

*#1. Initially we did not have an oven/toaster oven in the lab, so a heat gun was used. But this method, doesn't allow for even heating of the entire body, which hinders wax removal. So it took more time than expected to melt the inside wax and all of the wax could not be removed.

*#2. When you make the 3D printed mold, it is recommended that you to set your 3D printer high quality. This process requires a precise mold. Otherwise, when you pour the silicone your quadruped would not have the same body thickness. And it results in only partial inflation.

*#1. If the bottom tub part is big compared to the bed of your 3D printer, you can modify the file by adding an edge piece to the bottom. Once that portion has been printed out, you can clip it to the bed using a clamp or a binder clip. This will prevent the mold from shrinking upward.

*#2. Although you can bake your quadruped in the toaster oven, it is still hard to remove all the wax inside of the quadruped. If you find this too be an issue, you can boil the quadruped in oil on low heat. Then, the rest of the wax will melt out easily. However this method can be quite dangerous, and is not recommended except as a last resort.

*#3. When you insert the wax frame into the silicone mold, do it when the wax it relatively soft so that it doesn't break while fitting into the frame.

*#4. When you pour the body part silicone, make sure to remove the air bubbles thoroughly. When you remove the air bubble, you can use palm sander instead of air chamber.

It is likely that your first, second, or even third quadruped will not work as expected. Molding is an art and it can be quite difficult for beginners to create a perfect version of a multi-step mold such as this. That being said, every attempt allows the user to understand more about the process and what can cause end-failure of the mold.

The majority of issues that we have are due to two main factors: air bubbles and wax placement.

Air Bubbles: As discussed previously, we did not have a degasser, which resulted in final quadrupeds that had many holes. Although we used a lot of sil-poxy to seal many of these holes, this is not a great solution for larger holes. This can result in popping later on when you inflate the quadruped. Additionally, some air bubbles may exist that link the two separate wax pieces together. The separation of the two air segments is quite important for the later inflation and movement of the quadruped.

Wax Placement: You need to be quite careful when placing the wax into the alignment jig. First, if the wax breaks because it is cold and not malleable - this can create small spaces between parts of the wax that were previously attached. Silicone can and will leak into those places. This will stop the wax from draining out of the quadruped and will stop the inflation of that blocked-off portion. Second, the placement of the wax within the silicone directly relates to the way in which the quadruped will inflate. So, if the wax is not placed in the middle of the silicone, the quadruped will inflate unequally resulting in the quadruped not moving.

For questions, clarifications, etc, Email:


soft_quadruped_robot.txt · Last modified: 2016/08/10 13:20 by yeeunkim