In the last battle of DoFP, future Magneto and Storm create a massive explosion (by electrically charging and blowing up the future Blackbird) and destroy a vast amount of Sentinel ships.
This results in a massive amount of shrapnel and debris headed straight towards the mutants which Magneto is able to stop using his powers.
However, Trask mentioned that the sentinels were made out of a special polymer, "no metal". Why would these Sentinels then build their ships out of metal? Why not the same polymer?