Flat-film production

Multilayer flat films and sheets can be formed by coextrusion, consisting of two layers (AB), three (such as ABA), five (such as ABCBA), and up to eleven layers. Each layer and particular multilayer configuration is selected for its contribution to one or more film/sheet product properties, such as gas barrier, adhesion, abrasion, and chemical resistance. 

Unlike the coat hanger flat film dies, no simple final film adjustment is possible by lip flexing. Consequently, the order of magnitude of thickness accuracy in tubular dies is 10% as compared to 5% in flat sheet and film dies. This larger margin of accuracy in blown film dies is compensated by die rotation, which permits the distribution of the thickness variation across the entire width of the product. 

Profiles are all extruded articles having a cross-sectional shape that differs from that of a circle, an annulus, or a very wide and thin rectangle (flat film or sheet). The cross-sectional shapes are usually complex, which, in terms of solving the flow problem in profile dies, means complex boundary conditions. Furthermore, profile dies are of nonuniform thickness, raising the possibility of transverse pressure drops and velocity components, and making the prediction of extrudate swelling for viscoelastic fluids very difficult. For these reasons, profile dies are built today on a trial-and-error basis, and final product shape is achieved with sizing devices that act on the extrudate after it leaves the profile die. 

Flat film identifies cast film. Other names used include chill roll film, roll cast film, slot cast film, water quench, water chill film, etc. These cast film lines require dies that yield a wide range of diverse products. Widths may range from less than 6 in. (15 cm) to more than 33 ft... 

Okopack Film S is semi-transparent with properties similar to polyethylene. Okopack C and S can be used for production of flat films, sleeve films and bags and sacks, which can be used for fruit and vegetable packaging. 

Several unique products are made by extrusion and the dies needed to make these products are classified as 1) sheet dies 2) flat-film and blown-film dies 3) pipe and tubing dies 4) profile extrusion dies and 5) co-extrusion dies. Furthermore, each product type has unique hardware downstream of the die to shape and cool the extruded melt. To aid the reader, detailed illustrations of the various die designs and the complementary downstream cooling and shaping hardware are shown. 

Fig. 3.— Production of a Flat Film Fiber Diagram. [Layer lines are numbered, and indicated by continuous lines in the drawing. Reflections along the zero layer-line are referred to as equatorial, and correspond to crystallographic planes that are parallel to the molecular axis. Meridional reflections are along a line that bisects the equator ( zero layer), and are due to crystiil planes normal to the molecular axis.]...

Another advantage of blown film (with a tight tolerance on thickness) can be in bag production. It requires, with the proper-size blown tube, one seal across the bottom of the bag, whereas with flat film either one or two longitudi-... 

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