Introduction: Soft Robotic Grabber (No 3D Printer Required)

Soft robotics is a new and exciting field in robotics research. However, entering soft robotics as a hobbyist can be difficult and expensive because 3D printers are almost always required to make the robot molds. To remove this technology barrier, I developed a mold that can be easily made with cardboard and hot glue. This three-fingered grabber is just one of the many possible projects that can be made using this type of mold.

Credits: The actuators used in this project are a variation of the fiber-reinforced actuators developed by Kevin Galloway at the Wyss Institute for Biologically Inspired Engineering at Harvard University. The assembly procedure is based on the Instructable "Air-Powered Soft Robotic Gripper" by Dr. Ben Finio.

Step 1: Assemble Materials

Mold:

  • Cardboard (about two square feet)
  • Box Cutter
  • Cutting Board
  • Hot Glue Gun
  • Scotch Tape
  • Gloves (latex free)

Robot:

Step 2: Cut the Mold Components

    To make this mold you will need to cut 14 rectangles out of cardboard.

    • 6 - 2.5" x 0.5"
    • 6 - 3.5" x 0.75"
    • 2 - 9" x 9"

    Using the hot glue gun, glue two of the 2.5" x 0.5" pieces together so that they form a block. This structure will be referred to as a "block" throughout this instructable.

    Step 3: Make the Blocks

    Seal the edges by wrapping a strip of scotch tape around the block.

    Make sure the block is centered in the strip.

    Fold down the edges of the tape on both top and bottom of the block.

    Step 4: Seal the Blocks

      Spread a thin layer of hot glue over the edges of the tape for a smooth finish.

      Repeat with the remaining 2.5" x 0.5" pieces of cardboard to make two additional blocks.

      Step 5: Make the Walls

      Take a 3.5" x 0.75" piece of cardboard and wrap tape around the two short edges and the straightest long edge.

      Fold down the edges of the tape and smooth with glue just like you did when making the blocks.

      This structure will later be referred to as a "wall".

      Repeat this step with the 5 remaining 3.5" x 0.75" pieces.

      Step 6: Assemble the Mold

        Take one of the 9" x 9" pieces of cardboard and draw a dot in the center.

        Place each of the blocks so that they radiate outwards at approximately 120 degree angles with the center-most face spaced 3/4" from the center dot.

        Step 7: Glue the Blocks

        Hot glue the blocks to the cardboard in their current positions. Then trace the bottom edges of each block with hot glue. This prevents rubber from flowing into the small gaps between the block and the base of the mold.

        Step 8: Glue the Walls

        Draw lines offset 1/4" from the base of the blocks.

        Glue the walls tape-side-down on these offset lines. The wall should meet in the center corners, but do not worry if they leave a small gap. This can be addressed later.

        Step 9: Glue the End Pieces

          Cut a piece of cardboard that is 3/4" high and fits in-between the two walls.

          Glue the piece so that there is a 1/4" gap between it and the block.

          Repeat for all 3 arms of the mold.

          Step 10: Seal the Mold

          Now that the mold is assembled it is time to seal all the gaps! Trace all of the joints with hot glue. DO NOT BE STINGY DURING THIS STEP. Too much glue is infinitely better than too little. I usually trace the entire mold twice just to be sure.

          Step 11: Make the Air Channel of the Mold

          To make the air channel mold, connect all three blocks with hot glue. The resulting pattern should resemble an upper case Y. Wait for the glue to cool and repeat the process on top of the first layer, resulting in a 3 dimensional hot glue structure.

          This concludes the assembly of the top mold.

          Step 12: Prepare the Ecofelx

          Safety : Uncured Ecoflex 00-30 is labeled as a mild skin irritant. When you handle uncured Ecoflex 00-30 always wear latex free gloves. (Latex can prevent the rubber from curing properly.) Personally, I use nitrile gloves.

          Mix equal volumes of A and B together for three minutes.

          Estimate the amount of rubber you need. If you make too little, just make another batch. If you make too much, let the excess cure in the cup before disposing.

          Step 13: Pour EcoFlex Into the Mold

          Slowly pour mixed Ecoflex 00-30 into the mold so that the inner blocks are completely submerged by at least 1/8" of rubber. DO NOT FILL MOLD COMPLETELY! Watch the rubber for a few minutes to check if the mold is leaking. If this is the situation, tape a wad of paper towel over the leak.

          If needed, level the mold by placing thin objects under one side of the base cardboard. Coins are well suited for this task.

          Wait at least four hours for the rubber to cure.

          Step 14: Remove Rubber From Mold

          Remove the cured rubber starting with the outside edges. Do this by pulling the outside edges up and towards the center one at time until the rubber is removed from the mold.

          Step 15: Make Second Mold

            After the rubber is removed place it on the second 9" x 9" piece of cardboard.

            Trace the rubber with hot glue, leaving about 1/4" between the hot glue and the rubber. After the first layer cools, place another layer of hot glue on top to make a two layer wall surrounding the rubber. This will serve as the mold for the bottom section of the robot.

            Step 16: Make the Paper Cutout

            Trace the rubber on a sheet of printer paper with no offset and cut it out.

            Step 17: Fill the Second Mold

              Make a new batch of Ecoflex.

              Pour just enough Ecoflex into the second mold to cover the bottom.

              Center the paper cutout in the mold and press down to remove any air trapped underneath.

              Completely fill the mold with Ecoflex. Wait at least four hours for the Ecoflex to cure before removing.

              Continue immediately to the next step.

              Step 18: Repair First Rubber Piece

              Check the inside of the first rubber mold. If the cavities are smooth skip this step.

              If there are imperfections such as lines or cavities in the rubber it will make weak points that can cause the robot to pop when inflated.

              To prevent this, pour a small amount of rubber into the cavity of each affected arm and spread it evenly so that it fills any imperfections.

              Wait at least 4 hours for the rubber to cure.

              Step 19: Remove Rubber From the Second Mold

              After the rubber from Step 17 has cured completely, carefully remove it from the mold using the same technique as before.

              Take extra care not to rip the paper layer inside.

              Step 20: Glue the Two Halves Together

              Put the bottom piece back into the 2nd mold.

              Mix a new batch of Ecoflex.

              Spread the Ecoflex in an even layer about 1/8" thick on top of the mold.

              Place the top piece on top as shown, ensuring the air cavity is between the two layers, and press down lightly.

              Step 21: Glue the Two Halves Together

              Push any excess Ecoflex (rubber that is on the outside of the two molds) against the outside walls of robot.

              Wait at least four hours for the Ecoflex to cure.

              This completes the robot's construction. To power the robot you will need to make a simple DIY air bulb as described in the next step.

              Step 22: DIY Air Bulb

              1. To make the bulb you will need:
              • A plastic bottle with lid (a least one liter)
              • Nail
              • Hot glue gun
              • 1/8" OD Pneumatic tubing (about three feet)
              1. Puncture the lid of the bottle with a nail.
              2. Thread about 0.75" of tubing though the hole in the cap.
              3. Surround the tubing inside the cap (as pictured) with hot glue.
              4. Before the glue cools, screw the cap onto the bottle, block the free end of the tube with your thumb and squeeze the bottle. The pressure inside the bottle will push the hot glue though the any gaps and seal the bottle.

              Step 23: Connect the Robot and Test the Air Channels

                Use a nail to puncture the inside corner of the robot. Make sure the nail reaches the center cavity.

                Cut the free end of the tube with scissors so that it forms a point.

                Remove the nail and insert the tubing so that it reaches the central cavity. Lightly squeeze the bottle to test the air channels. If an arm does not inflate, an air channel may be blocked. (trouble shooting for this can be found at the end of the instructable)

                Remove the tubing for the next step.

                Step 24: Wrap the Arms

                  To wrap the arms of the grabber, you will need a non-stretching material. I have found curling ribbon to work well because it is less likely to shift when the robot is inflated.

                  Cut about 15' of ribbon.

                  Tie the middle of the ribbon to the end of one of the arms. Wrap one side of the ribbon up the arm counter clockwise and the other side of the ribbon up the arm clockwise . Wrap each ribbon so that they overlap on the top and cross on the bottom. Tie off and trim the excess ribbon when the arm is completely wrapped.

                  Do this for all three arms.

                  Wrapping the arms is mostly trial and error. Usually higher wrapping concentrations are better than lower wrapping ones. The ultimate goal is for all three arms to close at the same time.

                  Step 25: Glue in the Air Tube

                  Make a small batch of Ecoflex.

                  Use Ecoflex on the tubing, being careful not to get it near the tip. Insert the tube back into the robot.

                  Immediately continue to the next step.

                    Step 26: Add a High Friction Layer to the Grabber

                    The ribbon wrapping is slippery and not good for gripping things so you need to cover the bottom of the robot with a thin layer of Ecoflex.

                      Fill the second mold about half full with Ecoflex.

                      Put the robot in the mold, crisscross side down. and wait at least four hours for the Ecoflex to cure.

                      After the Ecoflex is cured remove the robot and trim the excess Ecoflex using a sharp scissor.

                      Step 27: Good Luck!

                      Congratulations you have just completed your own soft robotic three fingered grabber. As stated earlier this is just one of the many projects you can do with this type of mold. I look forward to seeing what you can come up with!

                      Step 28: Trouble Shooting

                      I have glued the tube in the robot but it still leaks. The easiest way to prevent the air from leaking out near the tube is to pinch the area around the tube with some sort of clip.

                      One (or more) of my arms will not inflate: This means that the air channel connecting the arm was blocked with Ecoflex. This can be fixed by using a piece of tubing as a stent to open the airway. First, use a nail to puncture the corner opposite from the arm you want to fix. Next, push the nail through the robot until it punctures the blocked channel on the other side. Cut a piece of tubing and fit it though the hole in the arm made by the nail. Use the flat end of the nail to push the tube through the blocked air channel so that it connects the arm cavity to the central cavity. Lastly, using Ecoflex, glue the initial incision point closed and wait at least 4 hours for the Ecoflex to cure. (This needed to be done on the robot featured in the instructable. The stent can be seen in the second picture on Step 23.)

                      The paper layer on the bottom of the robot ripped: In general a few rips will not have a noticeable effect on the robot's performance. Continue as usual.

                      Comments

                      author
                      Graphene6968 (author)2017-04-24

                      Hi Harrison, Thanks for the great post. Wonderful instructions. The prior questions posted by Graphene6968 were posted by my youngest son. He is doing this as a school project and I am trying to help where I can.He built two prototypes but they had leaks for various reasons and then he ran out of ecoflex. He has since built two more and the last one was quite good. No leaks and all three legs inflated. However, the project is due tomorrow and still won't fully close to grab anything. One leg curls more than the others (about 1/2 way) and the other two only curl something less than 1/4 of the way. We've tried re wrapping the ribbon a number of times but to no avail. Now, the paper has torn on all legs and big bubbles appear between the tears whenever he inflates it. He's going to make one more but my feeling is that the paper is too thin. Do you think it will help if we reinforce it by using two sheets and mapping putting some masking or painters tape over the paper before putting it into the mold? Also, any more guidance on how much to fill the top mold (we are doing it a little more than half way but not all the way to the top. Seems good and expands well without bursting) and any more detailed guidance on how to wrap? Can you explain what the wrapping is doing so that we can better understand how to use it to encourage curling of the arms? also, are you using an air compressor or a 2 litre bottle in the videos? I am guessing that it's a compressor. We can't seem to get that kind of inflation with a bottle.

                      Thanks so much and, again, great Instructable.

                      author
                      Harrison_89 (author)Graphene69682017-04-24

                      Hi,

                      For a stronger bottom layer you should be able to strengthen the paper layer with tape. Additionally you can replace paper with plastic labels from 2 liter bottles.

                      You should fill the top mold so that the center boxes are underneath about 1/8th inch of rubber. It also help to have the bottom layer be thicker than the top layer. This helps prevent the bubbles you are describing.

                      I did use the two liter bottles as a squeeze bulb. However, I needed to put my full weight on it in order to fully inflate the grabber.

                      Here is another video I made that goes over the wrapping processes in more detail. https://www.youtube.com/watch?v=wOlQnLXZ4No&t=1s

                      Hope this helped

                      author
                      Graphene6968 (author)2017-04-19

                      What happens if ran out of eco flex when making the high friction layer? Can I melt scraps down again? If that doesn't work, what can I use?

                      author
                      Harrison_89 (author)Graphene69682017-04-20

                      Unfortunately, there is no way to melt the the rubber after it has cured. You could try using tape to cover the fibers. High traction tape would be best but duck tape would probably work as well.

                      author
                      the r (author)2016-07-31

                      Have you considered using pvc as a mold to create three hollow tubes and bonding them together and to the claw to add a tactile arm to your assembly? You might even have two sections of these tri-tube assemblies in series to give it the ability to bend in multiple places, which would make it harder to control by hand, but more versatile. Just an idea to try, but by no means my own.

                      author
                      PedroR125 (author)2016-06-28

                      I´ve seen actuators done combining ecoflex and carbon powder to end up with a carbon grease. have you tried it? do you think they do a good mixture?

                      author
                      Harrison_89 (author)PedroR1252016-07-27

                      I have not tried mixing the two so I unfortunately can not comment good it is. However, I would be interested in learning more. Do you remember where you saw it?

                      author
                      Wmzemka (author)2016-01-05

                      is about two pounds (about one of the yellow and one of the blue) enough to make one grabber? Thanks!

                      author
                      Harrison_89 (author)Wmzemka2016-01-07

                      Yea two pounds is more than enough to make the grabber.

                      author
                      mechanic_koli (author)2015-12-23

                      nice and very well explained too.

                      author

                      Thank you!

                      author
                      seamster (author)2015-12-23

                      Very cool idea and technique, and presented very well too. Nice work!

                      author
                      Harrison_89 (author)seamster2015-12-23

                      Thank you!

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