Abstract: A method and apparatus are disclosed for holding and feeding a continuous sheet (12) of plastic in an operation such as thermoforming. Articulations (13), such as alternating waveforms, are formed on the edges of the plastic sheet (12) by heating and pressing the edges, producing inwardly facing abutment surface at the edges of the sheet. The sheet is then passed into slots (34) in guide rails (32) with the inside surfaces (35) of the guide rails (32) engaging the abutment surfaces on the plastic sheet (12) to prevent the sheet from slipping out of the guide rails. The plastic sheet is thereby held firmly so that it can be passed through such thermoforming operations as heating in an oven (14), forming with a mold at a forming station (40), and cutting the formed articles away from the remainder of the sheet by a trim press (53).