bent front mech cage
 

So my front derailleur cage is bent, and the chain is constantly rubbing against the front cage and shifting quality has deteriorated a lot. It is a Di2 derailleur so it would be sort of expensive to replace, but I will replace it at last resort. I'm not too experienced with bike parts, but is there anywhere I can buy just the metal silver cage part for my derailleur? The cage is the only part that is bent, so if there is any way I can just replace that, could someone please let me know?

Thanks.

Stupid question, I know, but have you tried unbending it?

That's not how Shimano does things. The derailleurs are not serviceable in any conventional sense. The only replaceable parts are limit screws and the bolts for the band or braze-on mount. If it's only a little bit bent you can bend it back, but it will never be the same. I lost an otherwise perfect FD-6700 in this exact way-- caught the chain one day and got tweaked. I would bend it back, and it would be fine for a few days at most, then it would tweak again.

If the cage is rubbing, make sure it is adjusted right.

I haven't used Di2, but it should be similar to other derailleurs. Typical adjustments are:If it is truly bent, then as above, with a little patience, one can often straighten the cage.

An option might be to look to see if other cages such as Ultegra mechanical cages look the same. My guess is that the Di2 will use its own cages, but it is worth looking. If the cages are the same, then it might be simply a matter of removing and replacing rivets.

In some cases, one can also downgrade parts like a cage, so if you're running Dura Ace Di2, it may still be compatible with a lower quality (cheaper) cage.

The front pivot arm of the 11spd FD Di2 is plastic, the, rear driver arm is metal. Bending the cage would have to
take into account the relative fragility of the motor and its connection to the arms, as well as the plastic
front pivot arm. In theory the rivets holding the cage to the arms could be ground off and the cage removed
I suspect a donor cage would have to come from another Di2 FD. Access to a cluster of 10-11spd mechanical
FDs for cage comparison might point to a possible cheaper donor FD but Shimano probably didn't use
similar enough pivot arm designs for this to work. New FD ? $150 or so your best bet.

I believe I have a spare 10 spd Di2 FD but these are not usable directly on 11 spd setups. Have to find it
before I can compare the cages.

The 6770 and 6870 Di2 derailleur cages on E-Bay outwardly look similar, so it is quite possible a cage swap would work, but no guarantees. Make your own decision whether it is worth chancing the purchase of the 6770 to strip.