A method of manufacturing a mouth piece by 3D printing a 3D printer readable file encoding the mouth piece , the method comprising steps of: obtaining a scan file comprising physical data representing a user's teeth and gum line using three dimensional scanner means; removing outlier data from physical data in the scan file ; generating representative geometrical data for the curvature of the user's teeth and gum line from the physical data; generating a virtual base model representing the mouth piece from the physical data and the representative geometrical data, the virtual base model having customisable dimensions; removing selected regions from the mouth piece represented by the virtual base model ; from the virtual base model , generating the 3D printer readable file encoding the mouth piece for printing on 3D printing means, the method characterised by steps of: operating a graphical user interface comprising an adjustable slider operable to reduce or increase the included height of the user's gum line represented in the scan file , operating a graphical user interface operable as adjustable value sliders or value shifters to position a cut line for selectively removing regions of the teeth and gum line in the scan file , in which the value sliders or value shifters when manipulated by a user show a graphical representation of the regions that will be excluded when generating the mouth piece , and in which the graphical user interface is further operable for selectively removing portions of one or more of: a frenum region, palate region, upper and/or lower gum line region and sharp edges from the mouth piece in the virtual base model , in which the graphical user interface provides a graphical representation of the regions of the teeth and gum line before and after removal from the virtual base model , and the adjustable value sliders or value shifters are further operated to adjust the smoothness, thickness of walls, scale, positioning and desired dimensions of the mouth piece in the virtual base model . obtaining a scan file comprising physical data representing a user's teeth and gum line using three dimensional scanner means; removing outlier data from physical data in the scan file ; generating representative geometrical data for the curvature of the user's teeth and gum line from the physical data; generating a virtual base model representing the mouth piece from the physical data and the representative geometrical data, the virtual base model having customisable dimensions; removing selected regions from the mouth piece represented by the virtual base model ; from the virtual base model , generating the 3D printer readable file encoding the mouth piece for printing on 3D printing means, operating a graphical user interface comprising an adjustable slider operable to reduce or increase the included height of the user's gum line represented in the scan file , operating a graphical user interface operable as adjustable value sliders or value shifters to position a cut line for selectively removing regions of the teeth and gum line in the scan file , in which the value sliders or value shifters when manipulated by a user show a graphical representation of the regions that will be excluded when generating the mouth piece , and in which the graphical user interface is further operable for selectively removing portions of one or more of: a frenum region, palate region, upper and/or lower gum line region and sharp edges from the mouth piece in the virtual base model , in which the graphical user interface provides a graphical representation of the regions of the teeth and gum line before and after removal from the virtual base model , and the adjustable value sliders or value shifters are further operated to adjust the smoothness, thickness of walls, scale, positioning and desired dimensions of the mouth piece in the virtual base model . <EOS>