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). The sheet is advanced in increments through the thermoforming apparatus by a sheet advance mechanism (48) which alternately grasps the sheet to advance it and then releases the sheet while the forming and trimming operations are carried out. Sheet stretching guide rails (90) hold the sheet at the mold and are mounted to move outwardly to stretch the heated sheet as the mold engages the sheet. The stretch on the sheet is released as the mold disengages. The stretching of the sheet during forming compensates for the tendency of the formed sheet to shrink as it cools.
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