Provided is a process for thermoforming a gas and liquid permeable layer of thermoplastic material (12, 32) having an average thickness of less than 0.50 mm, the process comprising the steps of bringing the layer into contact with a layer of deformable non-gas permeable material (10, 30) to form a separable laminate of the two materials, arranging for the thermoplastic material to be at a thermoformable temperature and thermoforming the separable laminate, thereby thermoforming the thermoplastic material.