How to fix 4x6 thermal label drift on a busy parcel packing line

I ran into this in a parcel carrier handoff where packers printed hundreds of 4x6 thermal labels per hour. The issue was labels slowly drifted downward until the barcode clipped at the bottom edge. At first I blamed a bad roll of labels, but it showed up whenever operators swapped between two label suppliers during the peak shift. This caused reprints delayed the conveyor and some cartons…