Thermoforming apparatus having a pair of plastic sheet heating stations, each including upper and lower heaters, on opposite sides of a differential pressure forming station, upper and lower plastic sheet supporting carriages for supporting upper and lower pairs of plastic sheets of differing thicknesses; transfer apparatus for shuttling the upper carriages to alternate ones of the heating stations while concurrently moving alternate ones of the lower sheets to opposite ones of the heating stations so that two of the upper and lower sheets are simultaneously heated, one in each heating station, while the other two upper and lower sheets are concurrently superposed at the forming station where they are formed into a hollow article by differential pressure forming apparatus at the forming station, and control apparatus for alternately operating the upper and lower heaters at each heating station for longer and shorter periods of time while the upper and lower heaters at the other station are being operated for shorter and longer periods of time respectively to subject the plastic sheets which are alternately moved to the heating stations to different amounts of heat.