Mold for 3inch compliant wheels

Last crawled date: 4 years, 2 months ago

"Gietmal ribbelwiel 3 inch" is our newer design for a 3"wheel with a profiled rim. We designed this to be used with softer (shore 15) silicone, because anything stronger would probably be impossible to get out of the mold.
Many people asked us to include an example of a hub, we have now included a STL for an example hub ("hubje 3 inch ribbelwiel anderhalf hoog zonder gear"). This hub has a 4-hole pattern ready to accept M4 bolts with 16mm spacing (commonly used by GoBilda/Tetrix/REV
The original mold:
This is the mold FTC team 3954 made to pour our own compliant wheels from 2-component silicone. Our aim was to make the wheels lighter and add custom made hubs to suit our needs. It was a fun side project for a few of our "intake-engineers".
Printing the mold:
material, infill, no of layers or layer resolution does not matter much. As long as it is a smooth surface on the inside. The largest forces on the mold are when the wheel is pushed out afterwards.
Designing your own hub:
Hub should have a 1 or 2 mm high, 6 mm diameter cylinder to plug the hole in the bottom. This cylinder is used to keep the hub centered in the mold. We designed it with a 4mm diameter hollow shaft, so we could use a long M4 bolt as an axle.
Hub should have a diameter of no more than 22mm to make sure silicone can flow freely around the hub.
Hub can be over 10mm high, but of course only 10mm will be covered in silicone. We designed ours with a 16T sprocket for #25 chain on top.
Hub MUST have holes or "inlets" to "capure" the silicone. we found out silicone does not stick to the PLA of the hub, so you must design a way to physically attach the silicone to the hub. We used a row of thin pillars around the outside of the hub.
Prepare for pouring silicone:
Make sure the molds are slick and free of printing artifacts (blobs/strings). Use a sharp hobby knife or a small file.
Spray the molds with anti adhesive for silicone. we used:https://polyestershoppen.nl/siliconenrubber/lossingsmiddel-voor-siliconen-305.html
Spread the anti-adhesive with a small paintbrush.
Mix the silicone and hardener and mix according to the instructions of supplier. MIX WELL! We added pink coloring (of course).
we used:https://polyestershoppen.nl/siliconenrubber/siliconen-gietrubber-1-1-shore-40-594.htmlhttps://polyestershoppen.nl/siliconenrubber/pigment-voor-siliconen-316.html
Each wheel takes roughly 20 mL of silicone, depending on the volume of the hub you designed.
Pouring the silicone:
"Pouring" silicone is hard, so we used a large syringe (60mL) to suck up the silicone (slowly) and inject it into the molds. Although the silicone may seem thick, is DOES flow quite well into the crevices of the hub AND it levels. So make sure your mold is level, otherwise the wheel will be thicker on one side.
We had a problem with lighter (less infill) hubs because they would "float" out of the center hole, so we prevented the hubs from floating with a small dead weight on top of each hub.
We did not use a vacuum chamber to de-air the silicone. We did find some small air pockets in our wheels but nothing serious. We did mix the silicone more gently at the end to prevent trapping more air in and we tapped the molds on the table a couple of times after they were filled to help free trapped air pockets.
Removing from the mold:
After the suggested curing time (supplier) you are ready for the last stage.
All surfaces in the mold are 6 degrees slanted to assure the wheel comes out easily.
By pushing a 6mm pin (bolt) into the hole you should be able to push the hub out of the mold far enough to grab the silicone. Resist the temptation to push the whole wheel out this way, the silicone CAN tear or come loose from the hub, in both cases the wheel is worthless. just keep pushing and pulling until you can get hold of one of the spokes and pull the wheel free.
THERE you have it! Your own Compliant wheel!