GB1085949A - Improvements in and relating to films of thermoplastic material - Google Patents

Abstract

1,085,949. Perforated films. T. J. SMITH & NEPHEW Ltd. Jan. 22, 1965 [Dec. 18, 1963], No. 49896/63. Heading B5B. In a method for making a film of thermoplastic material having a desired pattern of perforations therein, molten thermoplastic material is extruded from a slit die on to an indented or perforated supporting surface moving with, and at the same speed as, the extruded material, and, when the material has changed to a semi-molten condition and whilst continuing to move, exposing the layer of material to the action of a flame or a jet of heated air or other gas, which has a temperature at least equal to the m.p. of the thermoplastic material, such that the portions of the semi-molten layer over perforations or indentations on the supporting surface are re-melted, while the portions in contact with the supporting surface remain unmelted, and cooling the thus-perforated layer to form a perforated film. Before cooling, the film may be biaxially stretched. The supporting surface may be heated above the temperature of the surrounding atmosphere, and may comprise a hollow cylinder or an endless band. In one embodiment, (Fig. 1) (not shown) the thermoplastic material is extruded directly on to a moving perforated cylinder (5) on which it is exposed to flame (10), the tips of which contact the material. In another embodiment (Fig. 2), the molten flat layer is delivered on to an endless band 16 operating between two heated driven rollers 17, 18, and passes over heated bridging member 19 before contacting perforated roller 5. An upper heated driven roller 20 may be provided to reduce the thickness of the layer of almost molten material.