Week 11: Soft robotics

This week I designed and 3D printed a mould similar to the soft gripper mould in the presentation, which I used to create a soft gripper out of silicone rubber. I attempted to inflate the gripper but because of a tear in the side of the gripper, it did not inflate much. To download the file for the mould, visit: https://oscircularfashion.herokuapp.com/project/-L6SSksUvEhCAW8dIZ4v

Firstly, as a group we explored a different inflatable design - a pneumatic wrist brace. We used Adriana's pre-designed template, as below:


We cut this design in the vinyl cutter, the outer part twice in heat transfer vinyl, and the inner part once in baking paper. After cutting, we layered the vinyl pieces on top of each other with the baking paper in the middle. We then used the heat press (temperature 110 degrees, 15 seconds) to seal the vinyl pieces together, leaving the baking paper part unsealed - thereby forming the air channel. After this, we added ties through the holes and inflated it with the pump.

When trying out the inflated wrist brace, I found that it provided firm support and protection around the wrist.

I used the design shown in the presentation to create a mould in Fusion 360 that had rounded corners at the centre and pointier ends on the arms. I thought that the rounded corners might make it easier to remove from the mould and decrease the likelihood of tearing. I then 3D printed the mould using normal PLA.


To create the soft gripper, we gathered together the needed equipment - a receptacle, a stirrer, gloves, weighing scales, a flat plate, our moulds and Ecoflex part A and Ecoflex Part B. We mixed part A and part B in a ratio of 50:50 and stirred for 2 minutes.

I then poured the mixture slowly into my mould, trying to make sure the surface was even. I waited for the mixture to settle for 5 minutes to let any bubbles settle. After this, there were still bubbles in the mixture, therefore I tried to use the stirrer to pop any bubbles.

As well as the mould, it was necessary to create a flat silicone layer to be placed over the top of the gripper in order to seal it. For this, I poured the Ecoflex mixture onto a flat plate with a thickness of a few milimeters.


I placed both the mould and the plate into the oven at 70 degrees for 15 mins in order to speed up the drying process. Once out of the oven, I tried to remove my gripper from the mould. However, the gripper was very stuck to the mould and it tore and came off in pieces. I assume this was because the ratio of Ecoflex wasn't exactly 50:50, perhaps due to a human error or the weighing scales were faulty.


I repeated the process above again to produce a second gripper. When this one came out of the oven, it was much better and easier to remove. It was still a bit stuck and I had to be careful not to tear it.

To stick the flat layer onto the gripper, I used more of the Ecoflex mixture - this time to act as glue. I spread a thin layer over the flat piece and left it for 2 minutes to settle. I then put the gripper face down onto the layer - so that the air chambers were covered. I put a bit more Ecoflex around the perimeter of the entire mould to secure it.

I then put this in the oven to dry out the Ecoflex and secure the pieces together. Once out of the oven, I used scissors to cut the gripper away from the excess flat layer.


For the inflation, I inserted a needle with a tiny air passage into the centre of the gripper - where the main chamber is situated. The needle was attached to a bigger air tube that would then connect to the air pump motor.


I connected the air tube to a DC air pump motor and attached the legs of the motor to VCC and GND of the external power supply.


I tried a number of times to inflate the gripper but nothing seemed to happen - it seemed there was a hole somewhere in the gripper. I located a tear on the side of the gripper, which probably accounted for the non inflation.


I tried to close the tear by holding it down whilst trying again to inflate it. However, the gripper inflated very slightly as it seemed the air was still escaping. Here is a video of the inflation attempt.




I would say that it's very important to take care when pouring the silicone into the mould and ensure there are no holes forming. And also to be careful when handling the gripper once dry, to make sure that it doesn't tear.