Walking-beam band sealer

Abstract

A machine for sealing polyethylene bags or the like of the type having conveyer bands for gripping and transporting the bags to be sealed at a constant velocity from the machine's inlet end, through a heating station, then through a cooling station to the machine's outlet end. The conveyer bands are formed from a material having good release (non-stick) characteristics with respect to the fused thermoplastic. Disposed at the heating and cooling stations, respectively, are heating and chilling bars (heat transfer bars), each being disposed in pairs on opposite sides on and in proximity to the conveyer bands. Means are provided for cyclically opening and closing both pairs of bars relative to the conveyer bands and for translating the bars longitudinally in reciprocating fashion with respect to the moving bands. As bags to be sealed are carried along by the conveyor bands, the heat transfer bars pinch closed on the traveling bands, but since the bars at this time are being translated in the same direction of travel (forward stroke) and at the same velocity as the bands, no band wear occurs. Toward the end of this forward stroke, the heat transfer bars open relative to the bands and the return stroke occurs with substantially zero force between the moving bands and the heat transfer bars. During the return stroke, the bars act as a preheat device to warm up the film prior to the forward stroke to obtain greater speed. Because the bars are of a predetermined greater length than the length of the reciprocating stroke, a continuous package seal is ensured.