Thermoforming of plastic sheet material

In a thermoforming machine of the type having an oven through which plastic sheet material is passed to soften the plastic, a forming station including mold portions which are adapted to form the softened plastic into a desired shape, stationary guide rails mounted on either side of the path of the plastic sheet which extend through the oven to the forming station, each of the guide rails having a slot therein into which the plastic sheet can pass, the guide rails having walls defining an internal cavity opening on the slot therein and including internal engagement walls extending away from the slot, the internal engagement walls being adapted and positioned to engage with inwardly facing abutment surfaces formed on the edges of the plastic sheet to prevent the sheet from slipping out of the slot in each guide rail, whereby the plastic sheet may be held within the guide rails to prevent substantial lateral movement of the sheet but allow it to move forwardly, and drive means for driving a plastic sheet held within the guide rails forwardly through the machine, the improvement comprising:

• (a) sheet stretching guide rails mounted on either side of the path of the plastic sheet over the width of the mold portions, the sheet stretching guide rails being separated from the stationary guide rails and having a slot therein and internal engagement walls which are normally generally aligned with the slots and internal engagement walls in the respective adjacent stationary guide rails such that a plastic sheet with inwardly facing abutment surfaces formed at its edges will be passed between the stationary guide rails and the sheet stretching guide rails;
• (b) means for mounting the sheet stretching guide rails for inward and outward movement;
• (c) means for moving the sheet stretching guide rails outwardly to outwardly displace the engaged inwardly facing abutment surfaces of the softened plastic sheet to stretch the sheet slightly before engagement of the plastic sheet by the mold portions and for moving the sheet stretching guide rails inwardly after the mold portions release the plastic sheet to a position at least as far inward as the stationary guide rails to permit forward movement of the sheet.