A heat seal bar used in a plastic film packaging machine for cutting and sealing plastic films about wrapped products is provided with an insert having sealing surfaces and a knife cutting edge which is removably mounted to the seal bar. A first rectangular portion of the insert is adapted to be received within an elongate slot formed in a bottom surface of the seal bar for mounting the insert to the bar with screws. A second portion of semicircular cross section extends along the bottom of the first portion and thereby projects downwardly from the seal bar. A knife blade detachably mounted in the insert has a cutting edge extending below the seal bar. The cutting edge is defined by a pair of converging knife surfaces formed on the knife blade. Convex surfaces of the second portion formed respectively on opposite sides of the knife define the sealing surfaces.

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