A power press in which a ram is driven towards and away from a press bed so that, in use, a work piece is deformed by a tool mounted between the ram and the press bed is provided with means by which a jam in the press can be released both quickly and simply, this release means comprising a layer of heat softenable rigid plastics material which is located in the press so that the direction of the press stroke is substantially normal to the layer and so that during a pressing operation the layer is subjected to compressive stress in the direction of press stroke, and electrical heating means located so that it is capable of heating the layer of plastics material, when desired, to a temperature sufficient for the plastics material to soften whereby, under compression in the direction of the press stroke, the thickness of the plastics layer decreases. Preferably the electrical heating means comprises an element which is embedded in a metal layer which fits face to face with the plastics layer, and is preferably sandwiched between two plastics layers, to form a spacer assembly which is fitted in the press, usually between the tool and the ram, or between the tool and the press bed.