I am using a very tiny motor (GM15, to be precise) for an application where optimizing weight is the biggest challenge. The motor weights around 1.1 g. I opened up the motor and weighed all the different parts and found that the outer metal casing of the motor (along with the bronze bushing) alone is around 650mg. For those who are curious about weights of different parts: copper coils - only 120mg, permanent magnets ~ 60mg, gearbox - 220 mg.
So my question is on the exact utility of this casing. I know iron is used inside motor rotors because of their high magnetic permeability but for the purpose of outer casing, does the use of metal offer any advantages other than structural robustness? If I were to take out all the motor parts and assemble it back in a plastic casing with half the weight, would the performance of the motor be the same in terms of torque production and current cunsumption?
I couldn't find much info on the subject with a quick google search. I would appreciate if someone can point to any standard literature on this.