The invention relates to a device for heating preform bodies or flat or preformed semi-finished products from thermoplastic material (3), comprising at least one base body (1.0, 2.0), on the surface of which at least a first layer or coating is formed, which comprises at least one electric heating resistor in the form of a flat, geometrically arranged conductor loop. The base body also has contact elements, by which the at least one heating resistor can be connected to a power source, wherein, as a result of the geometrically arranged conductor loop(s), a defined temperature profile can be generated, which can be transmitted contactlessly to the preform body or the semi-finished product by an outer contact layer formed on the first layer or coating, by contact the surface of a preform body or semi-finished product and/or by using convection or thermal radiation. During a transmission of the defined temperature profile, the preform body or the semi-finished product is arranged at a distance to the contact layer(s).

An inner cannula (20) for a tracheostomy tube is made by extruding tubing (41) and cutting into separate lengths or preforms (42). Each preform (42) is placed in a mould (50, 51, 52) and blow moulded to expand the preform into contact with an inner surface (53, 54) of the mould which is shaped to produce a pattern on the outside of the preform of two intersecting diagonal corrugations (211, 212) that act to strengthen the blow moulded preform against lateral forces. The inner surface of the mould may also be textured so that the blow moulded preform is given a texture (213) to reduce friction with the outer tube (1).