Lip heater for plastic extrusion die

Claim 1

Apparatus which is suitable for use in producing plastic film by extrusion, wherein: the apparatus includes an inner die (60) and an outer die (80); the dies have complementary surfaces which comprise the respective inner (6) and outer (8) lips of an annular nozzle (4); the outer die comprises a massive bulk of material of generally cylindrical-annular form, having an upper face; the upper face is annular in form, having an inner diameter and an outer diameter; the surface which comprises the outer lip extends down into the outer die from the inner diameter of the annular upper face; the outer die includes a means for heating the bulk of the die to a first temperature; the outer die is provided with a deep annular groove, having a base, an inner wall and an outer wall; the depth of the groove extends down into the bulk of the outer die from the annular upper face; the inner wall of the groove is in close proximity to the surface which comprises the outer lip; the apparatus includes an outer lip heater, for heating the die to a second temperature, which is higher than the first temperature; the outer lip heater includes an annular ring (82), and a means (84) for heating the ring; the ring (82) is made of a good heat-conducting material, whereby all locations around the annulus of the ring are constrained to be at substantially the same temperature; the form of the ring in cross-section taken at one radius of the ring is in substance identical to the form of the ring in cross-section taken at every other radius of the ring; the apparatus includes a heat-conduction means for ensuring that heat from the ring (82) is transferred easily and evenly to the outer lip (8) of the nozzle (4); the apparatus includes a heat-insulating means for insulating the bulk of the outer die (80) against heat transfer from the said ring (82); the ring is received inside the deep annular groove; and the depth of the groove is such that substantially all of the ring (82) is effectively isolated inside the groove against the impingement against the ring of drafts in the surrounding air.