Films

摘要

In the manufacture of films by extruding film-forming material and setting the extruded material, the extrusion aperture is fed from a stream of the material by diverting material from the stream through inlet means extending along the path of the stream for a distance substantially equal to the length of the extrusion aperture, the undiverted part of the stream being recycled to a point up-stream of the diversion and the material that enters the inlet-means at any given point along its length issuing from a corresponding point along the length of the aperture, the paths between all these pairs of corresponding points being substantially of the same length. The weight ratio of diverted to undiverted material is preferably from 1 : 1 to 3 : 1. Open or closed hoppers and fixed or adjustable extrusion apertures may be employed. As shown (Fig. 1), the film-forming material is circulated from tank 15 through one of two branches 18, 19 provided with filters 21, 22, then through heat-exchangers 28 or, when no temperature control is necessary, through by-pass 33 to hopper 32, excess material being returned through conduit 37 to tank 15. Pressure drops across hopper 32 and the rate of material-circulation are regulated by a valve 38. The hopper 32 (Fig. 2) comprises a body 39, a extrusion aperture 45, a longitudinal throat 42 and a longitudinal bore 41 containing a centrally disposed conduit 47. A portion of the material passing through conduit 47 flows through a slot 54 extending substantially the length of the extrusion aperture 45 into annular space 55 and then through throat 42 to aperture 45. A heat-exchange jacket 56 may be provided. In a modification (Fig. 4) the hopper is of triangular cross-section and a portion of the material passing through conduit 69 flows through registering slots 71, 72 to the extrusion aperture 67. Slot 72 is yieldably closed by a spring-pressed member 73 (Fig. 6), the member being forced away from the slot when the pressure in conduit 69 exceeds a predetermined minimum, thus ensuring adequate material-circulation. Instead of a longitudinal slot, the conduit may be provided with a plurality of holes. The film-forming material may be employed as a melt or in solution in a volatile solvent and numerous suitable cellulose ethers and esters and polyvinyl compounds are specified, to which plasticizers and dyes may be added.