With the progressing development of intraoral scanners, planning software and 3D printers, interest is increasing in an in-office workflow for aligner production. For the biomechanical planning of the treatment, accurate knowledge of the possibilities and limitations of the tooth movements with aligners is essential. One parameter to consider is the desired local force application of the aligner to the single tooth. This is essentially determined by the thickness of the material. The aim of this in-vitro study was to investigate the extent and localisation of changes in material thickness in the thermoforming process. The results showed up to 50% reduced layer thickness of the aligner foils, especially in the buccogingival regions. Thus the reduced force application should be taken adequately into account in the planning process. Keywords: aligner, material, polyethylene terephthalate (PET), polyurethane (PU), vacuum forming