A ratio of 1(HDPE): 3 (paraffin wax) ended up giving us the best results for milling. We heated our paraffin up first, to about 300°F then added the HDPE chips in slowly, while raising the temperature to 370. Additionally we found this ratio worked even better when remelted.
We have found that we have much fewer bubbles when allowing for the material to cool in the same container that it’s heated in. Although we can’t de-gas or tap out hot material to rid it of bubbles, all of the bubbles rise to the surface and we are able to machine them off by trimming down about 4 mm. Also we were pleasantly surprised that it cuts cleanly on a bandsaw (can be a little gummy for the blade, we use an old blade).
what it looks like to mix in liquid form
An unsurfaced block made from paraffin and HDPE, this block doesn’t have any recycled wax so it has more inconsistencies than a a block containing HDPE and paraffin that were heated and mixed more than once.
This is the same block surfaced, the more opaque areas are more dense areas of HDPE plastic that we were able to get rid of in the following batch by heating the material for longer and by using material that had already been melted down once.
Fiber- reinforced actuator made from a soft robotics toolkit tutorial, with a kevlar restraining layer, and directionally wrapped in kevlar thread we have found to make the sturdiest actuators. This one has no weak spots from receiving too much air and appears like it will continue to last after many inflations. The thicker walls and consistent air chamber (cast around a 1/2″ 1/2 round rod), as well as the fixed hardware connection to the tubing are the biggest factors in making this a soft robot with longevity.
cleaning wood chips out of the HDPE chips.
Harrison put a fresh bag in the dust collector when milling HDPE and gave us the chips. The line for the dust collector is long so there is some wood dust and other residue but it’s been surprisingly easy to clean out.
We are trying a 1:3 ratio this time.
Melting point of Paraffin wax : 115-154 F
Flash point of Paraffin wax without additives: 199°C (390°F)
Melting point of HDPE: 265-280 Degrees F (U.S. Department of Labor)
Flash point of HDPE: 649 Degrees F (U.S. Department of Labor)
more plastic makes it cool very quickly!
Here is a link to the Instructables recipe for machinable wax where we got the idea to make our own.
We started with heating it in a toaster oven (thank you Ryan!) but then switched to a hot plate because it got much hotter and we were able to stir more easily.
stirring HDPE/paraffin mix
Melting back down because it is too thin, saving some as sample of a 1(HDPE): 6 (paraffin wax) ratio. The more opaque areas are where the HDPE didn’t melt down completely, seems like we need to keep it hot for longer.
we milled our first two gripper tests, for a three-armed and four-armed gripper out of paraffin wax on the Roland MDX-40a.