Method for continuously simultaneously thermoforming a plurality of individual plastic sheets in moulds to form an identical number of individual spatial objects, the method comprising: - simultaneously picking-up said plurality of sheets, each from its individual sheet stack located in a supply station near an inlet side of a transport track and depositing said plurality of sheets on a common holder located on said transport track; - simultaneously passing said plurality of sheets on the holder from the supply station through at least one heating station, a moulding station and a removal station; - heating the sheets on the holder in the at least one heating station; - thermally deforming the sheets on said holder in the moulding station so as to form a plurality of spatial objects; - simultaneously removing the plurality of formed spatial objects from said holder in the removal station; - transferring the upper frame of the holder from the removal station to the supply station; - whereby a new moulding cycle is started for successive series of a plurality of individual sheets in their holder after said holder has been transferred from the removal station to the supply station; characterised in that: - each of the common holders comprises a middle frame and an upper frame, and which can be temporarily coupled or decoupled from the middle frame, the middle frame being adapted for supporting the sheets to be situated thereon with their outer edges, the upper frame being adapted for clamping said plurality of sheets between said middle frame and said upper frame by means of clamping means (18, 23, 24, 25); - each holder further comprises a plurality of transverse clamping pins and a clamping frame which is analogous to said middle frame and has elongated openings and protruding lips near edges of said openings, and wherein the middle frame comprises suitable holes for receiving said clamping pins; and in that the method further comprises the steps of: - positioning the upper frame above the middle frame and extending said clamping pins of the upper frame through the holes of the middle frame and through the elongated openings of the clamping frame for fitting the clamping frame against an underside of said middle frame; - laterally displacing the clamping frame of the holder in the supply station such that during this displacement, a suitable tensioning of the lips occurs in order to securely clamp the edges of the sheets; - laterally displacing the clamping frame of the holder in the removal station, such that during this displacement, a loosening of the lips occurs in order to release the edges of the moulded object. - simultaneously picking-up said plurality of sheets, each from its individual sheet stack located in a supply station near an inlet side of a transport track and depositing said plurality of sheets on a common holder located on said transport track; - simultaneously passing said plurality of sheets on the holder from the supply station through at least one heating station, a moulding station and a removal station; - heating the sheets on the holder in the at least one heating station; - thermally deforming the sheets on said holder in the moulding station so as to form a plurality of spatial objects; - simultaneously removing the plurality of formed spatial objects from said holder in the removal station; - transferring the upper frame of the holder from the removal station to the supply station; - whereby a new moulding cycle is started for successive series of a plurality of individual sheets in their holder after said holder has been transferred from the removal station to the supply station; - each of the common holders comprises a middle frame and an upper frame, and which can be temporarily coupled or decoupled from the middle frame, the middle frame being adapted for supporting the sheets to be situated thereon with their outer edges, the upper frame being adapted for clamping said plurality of sheets between said middle frame and said upper frame by means of clamping means (18, 23, 24, 25); - each holder further comprises a plurality of transverse clamping pins and a clamping frame which is analogous to said middle frame and has elongated openings and protruding lips near edges of said openings, and wherein the middle frame comprises suitable holes for receiving said clamping pins; - positioning the upper frame above the middle frame and extending said clamping pins of the upper frame through the holes of the middle frame and through the elongated openings of the clamping frame for fitting the clamping frame against an underside of said middle frame; - laterally displacing the clamping frame of the holder in the supply station such that during this displacement, a suitable tensioning of the lips occurs in order to securely clamp the edges of the sheets; - laterally displacing the clamping frame of the holder in the removal station, such that during this displacement, a loosening of the lips occurs in order to release the edges of the moulded object.