Frame device for retaining and positioning sheet materials (S1, S2) to be thermoformed to obtain objects made of plastics material, said device (1; 101) comprising: - a first tubular element (2; 102) defining internally a first vacuum chamber for grasping by aspiration a first sheet material, - a second tubular element (3; 103) defining internally a second vacuum chamber, separate from and independent of said first vacuum chamber and configured for grasping by aspiration a second sheet material, - wherein said first tubular element (2; 102) and said second tubular element (3; 103) are fixed together permanently so as to define a grasping frame-structure (FS; FS') configured as a single piece; - a first gripping slot and second gripping slot that extend peripherally on respective mutually opposite faces and are suitable for receiving internally, by an aspirating action, perimeter portions respectively of said first sheet material and of said second sheet material, said first gripping slot and second gripping slot being in fluid communication respectively with said first vacuum chamber and said second vacuum chamber, - in said grasping frame-structure (FS; FS') a third vacuum chamber being defined, separate and independent of said first and second vacuum chambers, and configured for removing air from the zone that is interposed between said first and second sheet material, - an abutting element, which extends along the inner perimeter of said grasping frame-structure (FS; FS'), configured for restingly receiving portions of said first and second sheet materials and configured for controlling the stretching stress and ensuring correct positioning of said first and second sheet materials, - in said abutting element at least one aspiration opening being obtained that is in communication with said third vacuum chamber. - a first tubular element (2; 102) defining internally a first vacuum chamber for grasping by aspiration a first sheet material, - a second tubular element (3; 103) defining internally a second vacuum chamber, separate from and independent of said first vacuum chamber and configured for grasping by aspiration a second sheet material, - wherein said first tubular element (2; 102) and said second tubular element (3; 103) are fixed together permanently so as to define a grasping frame-structure (FS; FS') configured as a single piece; - a first gripping slot and second gripping slot that extend peripherally on respective mutually opposite faces and are suitable for receiving internally, by an aspirating action, perimeter portions respectively of said first sheet material and of said second sheet material, said first gripping slot and second gripping slot being in fluid communication respectively with said first vacuum chamber and said second vacuum chamber, - in said grasping frame-structure (FS; FS') a third vacuum chamber being defined, separate and independent of said first and second vacuum chambers, and configured for removing air from the zone that is interposed between said first and second sheet material, - an abutting element, which extends along the inner perimeter of said grasping frame-structure (FS; FS'), configured for restingly receiving portions of said first and second sheet materials and configured for controlling the stretching stress and ensuring correct positioning of said first and second sheet materials, - in said abutting element at least one aspiration opening being obtained that is in communication with said third vacuum chamber.