Fabrication: 3D Printed Mold

This section contains step-by-step instructions for fabricating a fiber-reinforced bending actuator. If you are making a different fiber-reinforced actuator, e.g. one that twists or extends or combines different motions along its length, the overall steps will stay the same; the only changes you will need to make will be during the second step when the strain limiting layer and fiber reinforcements are added. This should be straightforward, using this page as a guide.

Because of the multiple molding steps, the fabrication of these actuators takes much longer than PneuNets. However, you can make multiple actuators in parallel.

There are two processes described in the following pages: the fast curing process and the slow curing process. The overall steps are the same, the only difference is the amount of time spent on curing. Leaving the cast elastomer to cure at room temperature leads to better bonds between layers, but takes about 12 hours. Alternatively, an oven can be used to speed up the curing process, but can lead to weaker bonds and can cause the finished actuator to have a pre-bent neutral position (as some of the layers contract when cured in the oven). Each of the steps described here ends with the elastomer being left to cure. At these points in the instructions, the relevant steps for each process are described.

If you choose to follow the fast curing process, please read the tips for heat curing first.

To fabricate our actuator, we will follow these steps:

  • Step 1: Mold the actuator body (a semilunar tube) out of Elastosil M4601 (image A above).
  • Step 2: Demold the actuator, but leave the inner rod in to hold the shape. Glue on the strain-limiting layer (B).  Wrap with Kevlar thread (C).
  • Step 3:  Coat an outer skin of Dragon Skin to hold the wrapping in place, then remove the inner rod (D) and dip the actuator in Elastosil to plug one end (E).
  • Step 4: Install a vented screw into the plugged end and seal with silicone glue (see image below). Plug the other end of the actuator.
  • Step 5: Seal the newly plugged end with silicone glue. Once this is cured, the actuator is complete. Attach the actuator to an air source and pressurize it to make it bend.

Bill of Materials

This section will give a list of items that are used in this project with selected links to suppliers. You can download a more detailed Bill of Materials sheet here.

Note: Many of the items listed are just examples and you can use your own discretion to substitute parts which are easier or cheaper to obtain. 

Mold Components

Steel half-round rod*, 2 sets of molds (one for actuator body, one for outer skin coating)
Note: only one set of molds is shown in the image.

* The diameter and length of the half-round rod depends on the size of the actuator you want to make. In the fabrication and CAD tutorials shown here, we use a 1/2" diameter rod (supplier link), which has been cut with a hacksaw or bandsaw to be 3-4 cm longer than the planned actuator length. After cutting the rod, be sure to deburr and file the ends so that they are smooth and will fit well into the corresponding slots in the molds.

Click here to download the .STL files  needed to 3D print the molds for an actuator that is compatible with a 1/2" diameter rod, has 2mm wall thickness, and is 170mm long.

If you would like to modify the molds (for example if you want to use a rod of different dimensions) click here to download the SolidWorks part files.

Polymer Materials

Dragon Skin 10 (or 20) 2-part silicone rubber (datasheet)

Elastosil M4601 2-part silicone rubber (datasheet)

(Note: Dragon Skin 30 is a feasible alternative to Elastosil M4601)

Silicone sealant, Wacker Elastosil E951 (datasheet) OR Smooth-On Sil-Poxy (datasheet)

Other Materials

#10-32 vented screw (supplier link) + nut Laser-cut washers, 3mm acrylic (download DXF file) Kevlar thread (supplier link)
PTFE (teflon) pipe thread sealing tape Fiberglass fabric Isopropanol (IPA)

Tools & Hardware

Thin metal rod for piercing Quick-grip spring clamps Long** hex key (compatible with vented screw, 5/32" hex)

Lab gloves X-acto knife Female threaded hex standoff (supplier link) & tubing-to-male threaded pipe adapter (supplier link)***

** The length of the hex key also depends on the planned length of your actuator. It should be at least as long as your actuator so it can reach all the way inside. Here we use a 9" long hex key (supplier link), which is 5/32" hex size to be compatible with the vented screw we listed above.

*** Hardware components for connecting to air source (alternative options discussed here)

Equipment

Vacuum chamber Benchtop vise Mass scale
Lab oven Centrifugal mixer Mixing cups
SRT_FR Mold STL Files.zip2.01 MB
SRT_FR Mold SolidWorks Files.zip17.95 MB
SRT_FR Laser Cutter DXF Files for Washers.zip6 KB
Fiber Reinforced Actuators Detailed BOM.xlsx14 KB

Step 1: Mold actuator body

Prepare elastomer

Elastosil needs to be mixed in a 1:9 ratio of Part A:Part B, by weight. The total amount you need depends on your mold. Here, we will make 50g total of Elastosil, so we need 5g of Part A.

(If using a different material see the vendor instructions on mixing ratios)

Take the bottle of Elastosil Part A (red fluid) and shake it vigorously, as it tends to settle. Measure out 5g in the cup, making sure to pour slowly so you don’t overshoot. If you do overshoot, you can pour some back into the bottle, or add more part B later to compensate and maintain the 1:9 ratio. [Video]

Now add 45 g (5g x 9) Elastosil Part B (white). Being accurate within 2-3 grams is okay. If you pour too much, you can remove some material with a spatula. [Video]

Place the Thinky cup with its contents inside the cupholder/adapter. Weigh this assembly, then adjust the mixer counterbalance (spin the dial) to match this weight. Press the ‘Start’ button to run the mixing program [Video]. The program should be set to 30s @ 2000 RPM (mixing) and 30s @ 2200 RPM (defoaming). Remove the cup of mixed elastomer from the mixer [Video].

Place the mixing cup containing the elastomer into the vacuum chamber and turn on the vacuum pump to remove any trapped gas. After about 5 minutes turn the pump off and remove the mixing cup. Pop any bubbles on the surface of the elastomer.

Pour elastomer into mold

Prepare your mold parts as shown. Insert the rod into the mold cap.

Pour a layer of the mixed Elastosil into the main body of the mold, going slowly to avoid spillage and trapped air pockets. Do not pour right to the open end of the mold (where the cap will go); stop about a centimeter from the edge.

When you have finished pouring, take the rod and come in at an angle to insert it into the hole at the base of the main mold.

Once the end is in the hole, slowly lower the rest of the rod into the elastomer, then slide the end cap forward to match the mold.

Make sure to push the rod really hard into the hole because the elastomer in there will resist the rod from going in and locking properly.

Now pour another layer of Elastosil to cover the rod.

Allow the Elastosil to settle for a few seconds and pop any visible bubbles.

Position the final part of the mold and gently push it into place. Clean up any spillage with paper towels.

Attach clamps to hold the mold together. Attach the first clamp at the bottom of the mold.

Attach another clamp near to the top of the mold, and on the opposite side so that the mold can balance while standing upright.

Clean any spillage using paper towels. The cap will probably have popped up when the clamps were attached. Push it back down until it snaps into the hole.

Cure

Stand the mold upright on a paper towel or petri dish to catch any leaking elastomer.

If following the slow curing process, leave the mold to cure overnight at room temperature. Tip: leave the mixing cup containing the remainder of your mixed elastomer beside the mold so that you can see when it’s cured.

If following the fast curing process: Leave the mold standing at room temperature for at least an hour, then remove the clamps and place in the oven (lying on its side with the flat side of the rod facing upwards) for at least 30 minutes at 65°C.

Step 2: Add strain limiting materials

De-mold

In this step we will first remove the rod, with the actuator main body attached, from the mold. Then we will attach the strain limiting layer, which causes the actuator to bend. Finally, we will wrap the actuator with Kevlar thread to limit its radial expansion.

If using the slow curing approach, remove the clamps and place the mold in the oven for about 15 minutes, until it is warm and pliable. This will make opening the mold easier, and prevent the mold from snapping.

If using the fast curing approach, remove the mold from the oven.

Remove the mold cap, then peel away the thin flat mold piece. You can use a screwdriver or other tool to initially lever off the parts.

Prop the inner rod on an solid, elevated surface, flat side facing up. Near the top of the mold, press down on either side of the actuator until the mold pops off. Continue down the actuator until it is almost completely separated, then gently remove the actuator, taking care not to break the bottom of the mold.

Attach strain-limiting layer

Cut a strip of fiberglass slightly larger than your actuator (as the fiberglass will fray). Wearing lab gloves, use your finger to spread a thin layer of silicone glue along the flat side of the actuator.

Place the strip of fiberglass on top of the layer of silicone glue and press until all parts are glued down. Trim any excess fiberglass.

Variation: if you want your actuator to expand axially at some points along its length, do not attach a strain limiting layer at those points. See this page for more details.

Wrapping

Place the free end of the rod in a vise and tighten to secure. At the end of the actuator closest to the vise, wrap the Kevlar thread a couple of times and tie a knot.

Following the grooves, wrap in one direction until you reach the opposite end. 

Wrap the actuator just tight enough to stay in the groove; if it is too tight it will deform when the rod is removed.

When you reach the other end of the actuator, wrap a few times, then switch into the other set of grooves and wrap back to the start.

Variation: To make a twisting actuator, the thread should be wrapped in only one direction for the length of the section you want to twist. See this page for more details.

When you reach the start, wrap a couple of times and knot again (use a one-ended knot like a constrictor knot).

Rub silicone glue on the two knot areas to reinforce them. Allow the silicone glue to cure – if you used SilPoxy, this should only take 15 minutes.

Step 3a: Encapsulate

Mold outer skin

The next step is to mold a thin skin on the actuator, to lock the fiber reinforcements in place.

Make sure that the Sil-Poxy for gluing the fiberglass is completely cured -- it should not smell like vinegar at all. Otherwise, uncured Sil-Poxy may interfere with the curing of the skin.

Make 20g of DragonSkin 20, which should be mixed in a 1:1 ratio. After mixing, place the DragonSkin in the vacuum chamber for 5 minutes to remove any bubbles.

As before, prepare your mold parts and slide the cap onto the rod. Note that the cap for this step is different to the one used when molding the actuator body.

Pour a layer of elastomer into the main body of the mold, and then insert the rod and push the cap into place.

Again, make sure to push the rod in very forcefully so that it goes all the way into the hole. There will be a lot of resistance from the very viscous elastomer you have to displace. If you don't push in the rod far enough, you will not be able to close the end cap of the mold because the actuator is sticking out the top.

Pour another layer of elastomer to cover the rod and actuator main body.

Gently push the final part of the mold into place.

As before, attach clamps to the mold, working from the bottom up and balancing the clamps so that the mold can stand on end.

Cure

Stand the mold upright on a paper towel or petri dish to catch any leaking elastomer.

If following the slow curing process, leave the mold to cure overnight at room temperature. Tip: leave the mixing cup containing the remainder of your mixed elastomer beside the mold so that you can see when it’s cured.

If following the fast curing process: Leave the mold standing at room temperature for at least an hour, then remove the clamps and place in the oven (lying on its side with the flat side of the rod facing upwards) for at least 30 minutes at 65°C.

Using the slow curing approach is encouraged for this step as it results in a better bond between the actuator body and the outer skin.

Step 3b: Plug one end

De-mold

As before, remove the clamps and clean the outside of the mold with a paper towel. Warm the mold in the oven, then peel it open using the same method as before.

Remove inner rod from actuator

Place the end of the rod in a vise, so that the actuator is not caught in the vise.

Twist the actuator 90 degrees – this reduces the contact area between the actuator and the rod, and therefore the friction.

Inject IPA for lubrication.

Pull, with one hand behind the actuator and one hand gripping the front half.

[video]

Cap one end of the actuator

Wrap Teflon tape around one end of the actuator – starting at the edge and continuing up about an inch. Do not cover the end face.

[video]

Mix 20g of Elastosil (1:9 ratio). Leave the Elastosil in the mixing cup, and dip the Teflon tape-wrapped end of the actuator in it. You can use a laser-cut guide plate to hold the actuator upright in the cup, or strategically lean it against something.

[video]

Leave overnight to cure. It is possible to put it in the oven to cure faster, but this seems to lead to a weaker bond, and the actuator will fail at this seam.

Trim cap of actuator

Cut away the mixing cup, leaving the actuator standing in a blob of Elastosil.

Trim away most of the Elastosil using an X-acto blade, leaving a thin island around the actuator.

Peel this layer down so that it’s sticking beyond the end of the actuator, like flipping an umbrella inside-out, and trim this excess away.

Peel off the Teflon tape, then cut a few mm off the end of the actuator to make it nice and smooth and to make sure your vented screw can go all the way through the cap (i.e. cap isn’t too thick compared to screw length).

[video]

Step 4: Install vented screw

Prepare actuator and screw

Pierce the actuator end using a thin rod and push it through until it’s sticking out the far end of the actuator.

Place the hex screwdriver in a vise, pointing upwards. Place the vented screw and small laser cut washer on the tip of the hex screwdriver.

Insert screw

Push the piercing rod into the hole at the end of the vented screw. Using the rod to guide the screw into place, slide the actuator down over the entire assembly until the screw comes through the hole at the far end.

If there is a lot of resistance, the screw has probably lost contact with the guide rod and is trying to puncture the wrong location – retract the assembly and try again.

Place the other laser cut washer over the screw, adding some more glue between the actuator end and the washer. Finally, add the nut and hand-tighten it.

[Video]

Remove the actuator from the vise assembly and tighten the nut further using pliers. Do not over-tighten as the end cap will start to compress and bulge.

[Video]

Paint the washer and actuator end with a thin layer of silicone glue. Leave it to cure.

Step 5: Cap other end and finish

Cap other end

Follow the exact same capping procedure as before to cap the remaining open end of the actuator:

  • Wrap Teflon tape around 1” of the actuator’s end.
  • Mix 20g of Elastosil (9:1 ratio) and leave it in the cup.
  • Dip the Teflon-wrapped end of the actuator into the cup, brace it so that it stays upright, and leave it to cure overnight at room temperature.

Paint the newly capped end with silicone glue to reinforce it. Let it cure.

Attach air source and actuate

The attachment method depends on the air source you use. If the air source has a threaded output, simply use threaded adapters to connect it to the #10-32 threaded vented screw. Depending on the air source's thread size and type (female/male), you can use female x male pipe adapters or coupling nuts. If the air source's thread is male and also #10-32, you can use female threaded standoffs, which are generally cheaper than coupling nuts and have more options available - just make sure the standoffs you select are fully threaded.

If the air source output is tubing, use threaded barbed connectors. Insert a tubing-to-male threaded pipe adapter into the air source, and connect it to the vented screw using a coupling nut/standoff.

A simpler but more expensive option is a tube-to-female threaded pipe adapter, attached to the vented screw and the barbed end inserted into the air source's tubing. 

Be careful not to over-inflate the actuator, as it may spring a leak or burst. These actuators can typically be pressurized up to 40-50 psi.

If the connection to the air source is leaking, you can wrap the threaded sections of the connector components 1-2 times with PTFE (Teflon) thread sealing tape before screwing them together. If the leak is in the actuator body itself, patch the leaking area with silicone adhesive.