GB772148A - Film extrusion dies - Google Patents

Abstract

<PICT:0772148/IV(a)/1> <PICT:0772148/IV(a)/2> <PICT:0772148/IV(a)/3> In a film extrusion die of the type comprising a pair of hollowed out blocks separated by spacer plates which define a reservoir and a slit extrusion orifice, one block has a pair of vertical bead-forming grooves connecting with the ends of the slit orifice and a pair of vertically adjustable bead-controlling members extending into the lower end of said grooves, and at least one block carries two vertically-adjustable dam members having curved-bottom doctoring surfaces. The die produces film with beaded edges and the adjustable dam members control the contour of the flashing on the outer sides of the beads. The die blocks may be provided with internal channels for the circulation of a heat exchange medium to maintain the temperature of the extrusion mixture. The film is extruded on to a rotating casting drum which may be grooved to receive the beads and whose temperature also may be controlled by internal circulation of a heat-exchange fluid. As shown in Fig. 1, the extrusion die comprises a pair of hollowed out blocks 10, 13 separated by spacer plate 11, and beaded film is extruded on to the cooled casting drum 34, grooved at 35, and is removed by correspondingly grooved roller 36. The die is shown in cross-section in Fig. 2 and in expanded view in Fig. 5. The two blocks 10, 13 and spacer plates 11, 12 define an entrance 141 and reservoir 14 for the molten material which passes over a pair of converging surfaces 15 and emerges through a slit orifice in film form. Beads are formed near each edge of the film by additional material which passes down vertical grooves 16 formed in one or both blocks at each end of the slit orifice. The size of the beads is controlled by varying the effective aperture of the grooves by means of a pair of adjustable members 22 carried by block 13. Each of these members has an extension which enters the respective groove 16 so that vertical adjustment of such member on block 13 varies the flow of material through the groove. Adjacent each adjustable member 22 is a vertically adjustable member 21, and these, together with horizontally adjustable members 28, Fig. 2, carried by the block 10, control the contour of the flashings formed on the outside of each bead. The thickness of film formed may be varied by using spacer plates 11, 12 of different thicknesses, and also by altering the rate of draw-down as determined by the relative rate of extrusion and peripheral speed of the casting drum. The die blocks are preferably made of corrosionresistant steel which can be annealed, hardened and highly polished. Suitable metals are stainless steel, alloy steels, vanadium alloys, bronze, brass or titanium or titanium alloy. The spacer plates 11, 12 may be of steel, bronze, brass, copper, aluminium or alloy. The preferred film-forming material is polyethylene terephthalate, but other materials which may be used are resins and superpolymers, e.g. nylon, polyethylene, polystyrene, polyvinyl halides, polyvinyl acetate, rubber hydrochloride, cellulose derivatives, polyesters from glycols and aliphatic dibasic acids, and polyesteramides. Specification 746,386 is referred to.