3D Printing: Extending Arc Using Air Pressure (TfCD Project)

Flexible filament for 3D printing has made it possible to print bendable parts. This could be used for soft robotics and making artificial muscles. The principle for making artificial muscles could also be used for making other sorts of moving parts. For instance an opening and closing arc-shaped ‘wall’.

The muscle, or in this case wall, is hollow and actually an air chamber that makes the object opening and closing when being exposed to different air pressure. Through a cable the pressure of the air inside the wall is increased; forcing it to straighten out.

To make this yourself you will need:

• Access to a 3D printer with the possibility to print flexible materials
• Ninjaflex or other flexible filament
• Cable (4 mm diameter if you choose to use the .stl file provided in this guide)
• Waterproofing coating spray
• Bicycle pump, or something else to increase the pressure within the wall

First you need to create a .stl file to print. You can either download the file used here or you can make your own shape. Here are some things to keep in mind if you choose to make your own:

• Make the object thin and hollow and the walls around 1mm thick
• Check the size of your cable and make the nozzle accordingly. Also make sure that the nozzle is not too short. Otherwise the cable will not be stable attached.
• Make the nozzle at the bottom of the .stl file. This will prevent an overhang in the model.
• Make the shape somewhat ‘rolled in’ so it can easily stretch out
• Don’t make it too wide. It is hard to print thin and high things with the flexible filament; there is a risk of falling

When you have the .stl file, open it in Cura and prepare the print.

To print this flexible object the flexible plastic Ninjaflex was used. Flexible material is a little bit more tricky to print with than regular filament. Check your print early to make sure that it is really printing a hollow shape. In this case it took a lot of tries before it turned out right. In the pictures 3-5 you can see some examples of failed prints. We tried change a lot of settings, such as:

• temperature
• fill density
• make sure the filament thread runs freely
• use the ‘spiralize function’
• make top and bottom layer thicker

The most influencing setting turned out to be the spiralize function that was set in Cura. For the printer Ultimaker 2 and Ninjaflex filament the following settings was used.

Settings in Cura

• Layer height: 1 mm
• Shell thickness: 0.8 mm
• Bottom/top thickness: 1 mm (but could be thicker, since our model wasn’t totally watertight at the bottom and top)
• Fill density: 100 % Print speed: 20 mm/s
• Check the box for function ‘spiralize the outer contour’ (expert > open expert settings)

Settings on printer:

• Printing temperature: 220
• Bed temperature: 65

The final print did not turn out perfect either. The top and bottom layer was still to thin (picture 2) and the nozzle did not print well (picture 3). The print was however good enough to just fix up a bit.

The nozzle that in this case did not print well was cleaned up with the help of a drill fastened in a vise. When the nozzle is clean, attach the cable.

To make the high pressure air stay within the hollow the model should be ‘air proof’. This can be done by using a waterproofing spray. In this case Pasti Dip rubber coating was used. If you have any part that was printed too thin, make sure to put some extra spray on that area. Also make sure to spray thoroughly around the cable at the nozzle. You might have to spray more than once. In that case make sure to wait at least 20 minutes (or what the label of your spray says) between each spray. Use gloves and mask and spray outside or in a spraying cabinet.

Depending on your size and model it might be enough to just blow into the cable to test it. To control the opening and closing connect the wall to something that can adjust the air pressure within it, for instance a bicycle pump. When increasing the pressure (pushing the pump together) the wall will open and when releasing it will roll together again. See the wall stretching out in the video.