to cooperate with the step 24 so that the member 21 has its annular periphery clamped between the rings 13 and 22 if so required the ring 22 can be provided with a sealing ring 26 when the mouthguard shell 16 is to be deformed to form a mouthguard for the patient from which the impression has been taken the shell 16 is heated and then placed on the portion 17 the apparatus 10 with the shell 16 is then placed in a machine which applies a pressure differential across the member 21 more particularly the machine includes a piston that engages with the upper surface 27 of the ring 22 and moves the rings 13 and 22 down into contact with the surface 28 of the base 11 the piston sealingly engages the surface 27 and applies a gas preferably air under pressure to the upper surface of the member 21 the member 21 then deforms about the shell 16 and forces the shell 16 onto the portion 17 to plastically deform the shell 16 the pressure above the member 21 is released and the rings 13 and 22 raised so that an operator may then remove the formed mouthguard in an alternative embodiment the rings 13 and 22 may be moved into contact with the surface 28 and then air withdrawn from below the member 21 so that air pressure above the member 21 exerts a force on the shell 16 to deform the shell 16 in an alternative embodiment the member 21 may be preformed so as to be convex as illustrated in fig4 this would aid in minimising or eliminating forces generated by stretching the member 21 over the shell 16 in such an arrangement only the pressure differential across the member 21 would cause deformation.