The electrostatic force between the metal plates of an isolated parallel plate capacitor C having a charge Q and area A, is

If the electric field produced by one of the plates is E, then force experienced by the other plate is QE where Q is charge on the other plate.
Electric field produced by any plate is $E=\frac{\sigma }{2{ϵ}_0}$where $\mathrm{\sigma }$ is the surface charge density.
So, it is independent of distance between the plates.