Today I joined your caster’s club 
Why not just bying some? Well, I did, but they didn’t quite fit to my DIY Hubmotor (too tight), so I had to DIY again 
For the outer ring of the mould I used an ordinary aluminium pipe and machined a simple ring groove into another aluminium part for centering the outer ring and the inner one (being the core of my sleeve).
Basically I used visnu777’s manual and same PU.
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Application of release agent on mould (Spray, distribute with brush, spray again, wait for 30 mins):
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Assembly of mould, stiring of both PU components for 3 mins, casting slowly to prevent air bubbles)
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Curing for 2 days, easy removing from mould:
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Some tempering at 65° in order to further improve physical properties
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Magic amber sleeve is done
@visnu777: Thanks a lot! Without your post here I wouldn’t have even thought about casting wheels or sleeves at home at all! 