A heating enclosure for the production of thermoformed plastic shanks having a relatively uniform wall thickness, in particular for the production of prostheses, is provided. The enclosure has the following features: a) a thermoforming insert (11) which can be fixed upright in the enclosure interior space (3) and is approximately funnel-shaped in design; b) the thermoforming insert (11) is designed as an air-guidance device, its lower opening, with the smallest diameter, defining the air-inlet cross-section and it upper opening, with the largest diameter, defining the air-outlet cross-section; and c) the thermoforming insert (11) has, directly above its upper air-outlet opening, a sheet receiver (15) for the approximately horizontal reception of a plastic sheet (17) to be thermoformed; d) an air guidance mechanism (4, 5, 6) which guides the air in a closed circuit and has at least one upper air-extraction opening (5) which extracts from the enclosure interior space (3), in particular the air emerging from the thermoforming insert (11), and to which an air-recirculation duct (4) arranged outside the enclosure interior space (3) is connected, which air-recirculation duct has an air-recirculation fan (7) and a heater (8) heating the extracted air and is connected to at least one lower air-blow-in opening (6) leading into the enclosure interior space (3).