I appreciate all the great help here.
Our team is thinking of making wheels out of the polypropylene sheet and would like to drill holes in them to lighten them. Since they only have the one sheet, they would like some advice on loss of stiffness if this is done. Is it similar to plywood which they are familiar with, which does not lose much stiffness? Will it turn floppy with a few holes? Normally we would drill holes and check it as we go, but they are going to use a water jet cutter and that strategy won't work.
Thank you
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Since polypropylene was included in the BEST Ckit materials, I've never seen it used for wheels at Dallas BEST, or Texas BEST, so I doubt there is any experience here with it.
Since polypropylene is slippery, it seems an odd choice for traction wheels, but maybe a good choice for hubcaps, to allow sliding along edges of the field or other obstructions.
Polypropylene is already less dense that plywood, so it may not need further density reduction.
The wiki page for polypropylene mentions Young's Modulus in physical properties.
The wiki page for Young's Modulus defines it as stiffness, with a sortable table of various materials including 'wood' (which I'm sure varies widely)
The values for polypropylene between the two pages don't match that well, and the units are different.
If the learning experience is o use a water-jet cutter on polypropylene, maybe a robot component more intricate and precise than a circle would be even more educational
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During last year's competition, our team used the polypropylene sheet to make a scoop/dump for collecting various commodities. The motor that operated this scoop pulled directly on a strip of Polypropylene less than 2" wide and 4" long. We experienced little to no warping/bending on this piece. I would estimate its pliability to be somewhere between 1/4" and 3/8" plywood. (Just my opinion and experience, unfortunately I have no math or science to back it up) 
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