  In this paper, we present direct mass measurements of neutron-rich $^{86}$Kr
projectile fragments conducted at the HIRFL-CSR facility in Lanzhou by
employing the Isochronous Mass Spectrometry (IMS) method. The new mass excesses
of $^{52-54}$Sc nuclides are determined to be -40492(82), -38928(114),
-34654(540) keV, which show a significant increase of binding energy compared
to the reported ones in the Atomic Mass Evaluation 2012 (AME12). In particular,
$^{53}$Sc and $^{54}$Sc are more bound by 0.8 MeV and 1.0 MeV, respectively.
The behavior of the two neutron separation energy with neutron numbers
indicates a strong sub-shell closure at neutron number $N$ = 32 in Sc isotopes.
