Yep radius all your edges.
If you think about injection moulded parts they have sprues, a tapered hole in the corner of each blade allows plastic to flow and a place for air to be compressed. The sprue is trimmed off afterwards, hairline grooves are added to moulds to allow air to bleed away, but not let plastic flow.
10mm to 1.5mm, I think I just misunderstood your dimensions. If the fan backing is kept pretty close to the blade thickness you get less distortion than going from thick to thin sections within a mould.
Alternatively make the whole fan out of G10 board and epoxy it together. Then balance the fan.
Casting is way easier than injection moulding, and possibly spin casting would give you hours of entertainment.