The present invention relates to a thermoforming machine having a plurality of stations, i.e., a charging and a discharging station, a heating station and a forming station, arranged in a circle and a like plurality of clamping frames for thermoplastic material, which frames can be pivoted from station to station. More particularly, the present invention relates to such a thermoforming machine wherein the forming station is provided with a height adjustable mold table to accommodate a deep drawing mold, and a likewise height adjustable blow box which serves to produce an area enclosed on all sides and disposed underneath a plate of thermoplastic material clamped in the forming station so that the known process steps of preliminary blowing or preliminary suction become possible.
In such forming stations, the lifting movement of the mold table is known to be produced by means of pneumatic or hydraulic cylinders. If these cylinders are disposed below the mold table, this requires the installation of pits in the fabricating halls which is expensive and not always feasible. Alternatively the support or foundation structures for the forming station would have to be given the shape of podiums which makes operation more difficult.
Various solutions for this problem are known. For example, it has already been proposed to mount the cylinders of forming machines for plate material on a fixed clamping frame and to pull up the mold table. In this case, the cylinders are disposed either to the side (German Utility Model Patent No. 6,605,530), at the four corners (German Offenlegungsschrift (laid open) application) No. 1,779,752) or in the rear (German Utility Model Patent No. 7,501,702). All of these solutions fail, however, in machines of the type initially described above which require rotary clamping frames whose rotary movement would be prevented by the cylinders in each of the above-mentioned cases. In a drive for the mold table as proposed in German Offenlegungsschrift No. 1,951,913, the size of the lift depends on the length of the table which usually is incompatable because of the size relationships involved.
With a chain drive for the mold table as proposed in German Offenlegungsschrift No. 2,727,820, problems exist regarding fastening of the chains if a height adjustable blow box is to be installed. A likewise known scissors hoist for the mold table does not have a uniform sequence of movements or a constant lifting force over the entire stroke, and additionally is expensive.