A sealing machine for polyethylene bags, having bag clamping and conveying chains, a pair of bag heating bands, a pair of bag cooling bands downstream of the heating bands, the adjacent heat transfer runs of both the heating and cooling bands having ends offset from each other in the direction of band movement, the band mounting wheels at each end of both bands being similarly offset, the adjacent heat transfer runs of the heating and cooling bands also lying parallel and flush against each other from end to end, stationary heat bars adjacent the heat transfer runs of the heating and cooling bands at which the bars respectively generate and absorb heat, and each heat bar having a plurality of very lightly spring pressed heat transfer elements or slides continuously engaging and transferring heat between the adjacent band and heat bar.