Step 2: Mold Stiff Ends

The next step is to make the stiff end pieces of the sensor. To begin, inside a fume hood, spray the center-displacement piece with mold release. Make sure to spray the side with the raised ridge. Sorta-clear is very stiff, so without mold release the 2 mold pieces will be glued together Mix a small amount of Sorta-clear elastomer in a cup. Not much is needed, only roughly 10 grams. Refer to the Bill of Materials section for vendor information.

Pour stiff rubber

	Insert velcro pieces into mold through the slots on the ends with the fuzzy size face down.
	With a tongue depressor, apply elastomer beneath the exposed velcro. Make sure the fuzz is thoroughly coated.
	Align the velcro using the pegs, then apply elastomer to the top of the velcro as well. Excess elastomer should be left in the end circular areas of the mold.
	The image on the left shows a mold with fully coated velcro end pieces. Note how there is some velcro sticking out of the ends of the mold.

Degas elastomer

	Place the mold with velcro and elastomer into a vacuum chamber and degas (set the vacuum chamber to -90 kPa). Release and restore the vacuum several times to help pop many of the bubbles. Then leave the mold in the machine for a few more minutes until bubbles start dying down.
	Remove the mold from vacuum chamber and inspect for bubbles, particularly ones near the interface between the velcro and elastomer, as these will turn into failure points. To remove any remaining bubbles, dip tweezers into the elastomer and slowly “push” the bubbles towards the edges and “lift” them out of the elastomer.

Seal and press mold

	Put on the top piece of the bottom mold, raised side down. Press it down gently so the pegs align, then apply gentle finger pressure from the center moving outwards. This process will keep the elastomer out of the middle portion of the mold, which is where the softer elastomer will go later.
	Place the molds in a 60 oC oven and put weights on top. Remove from the oven after 15 minutes.

Check for defects

	After taking the mold out of the oven, peel off any excess elastomer from around the edges. Pry off the top using a tool to gain leverage, such as a screwdriver. The pieces should be non-tacky but brittle (it will take a day to reach full strength).
	There should be no gaps or bubbles and the interface/edge of the stiff elastomer should be clearly defined. Peel excess elastomer off the pegs to make next step easier.