a knife stop plate having a heat seal block opening for receiving a heat seal block of a heater assembly of the heat sealing machine therethrough, wherein the knife stop plate is a single unitary component having a top surface, a bottom surface and no portion extending below the bottom surface;

a clamping plate having a knife opening and a clamping shoulder extending downward from a bottom surface of the clamping plate and surrounding the knife opening, the clamping plate operatively connected to the knife stop plate to allow movement of the clamping plate toward and away from the knife stop plate;

a plurality of knife position springs biasing the clamping plate away from the knife stop plate; and

a knife having a shape complimentary to a backing sheet for a blister package that is heat sealed by the heat sealing machine, the knife being mounted to and extending downward from the bottom surface of the knife stop plate, encircling the heat seal block opening, and aligned within the knife opening such that the plurality of knife position springs are compressed and the knife extends through the knife opening to cut the backing sheet from a backing material web after the clamping shoulder engages a clamping gasket support shoulder of a nesting tray assembly of the heat sealing machine, wherein no portion of the knife stop plate extends downward within an opening defined by the knife, and wherein the heat seal block of the heater assembly passes through the opening defined by the knife to seal the backing sheet to the blister package.