Blown-film width

The bubble diameter is normally always much greater than the die diameter. This bubble diameter divided by the die orifice diameter is called the blow-up ratio (BUR). The BUR is usually 1.5 to 4.0, depending on the plastic being processed and the thickness required. The bubble diameter must not be confused with the width of the flattened double layer of film between the nip rolls. The width of this double layer is 1.57 times the bubble diameter and is called the blown-film width (BFW). 

Knitted nets are produced from blown film or specialized extmsion lines with corotating dies. The film rolls are slit into small tapes of 2-5 mm width. Then the tapes are stretched on heated rolls (galettes) in several steps to achieve and maintain a high level of orientation. Typical stretching factors are three to five times. Thus, the materiel undergoes a strain-induced crystallization, which maximizes the crystallinity level in the tape. 

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. 

The lay-flat film is then either kept as such or the edges of the lay-flat are slit off to produce two flat film sheets and wound up onto reels. If kept as lay-flat, the tube of film is made into bags by sealing across the width of film and cutting or perforating to make each bag. This is done either in line with the blown film process or at a later stage. 

The blown-film technique has several advantages the relative ease of changing film width and caliber by controlling the volume of air in the bubble and the speed of the screw the elimination of the end effects (e.g., edge bead trim and nonuniform temperature that result from flat film extrusion) and the capability of biaxial orientation (i.e., orientation both in the hoop direction and in the machine direction), which results in nearly equal physical properties in both directions, thereby giving a film of maximum toughness. 

After extrusion, blown-film is often slit and wound up as flat film, which is often much wider than anything produced by slot-die extrusion. Thus, blown-films of diameters 7 ft. or more have been produced, giving flat film ofwidths up to 24 ft. One example is reported [16] of a 10-in. extruder with 5-ft diameter and a blowup ratio of 2.5, producing 1,100 Ib/h, or polyethylene film, which when collapsed and slit in 40 ft wide. Films in thicknesses of 0.004-0.008 in. are readily produced by the blown-film process. Polyethylene films of such large widths and small thicknesses find extensive uses in agriculture, horticulture, and building. 

All HIDM and twin screw color concentrate samples were letdown into blown film for evaluation of color, strength and dispersion quality. Standard 1.5 mil (37 pm) films were prepared by blending concentrate samples with 1.0 MI LLDPE resin at a letdown of 5.0 weight percent and extruding through a 37.5 mm laboratory film line at a blowup ratio of 2.4 1 and lay flat width of 20 cm. Standard operating procedures were observed throughout. 

TPE sheet can be produced (e.g., from TPVs) by extrusion processing. Sheet extrusion of TPEs can produce thicknesses from about 0.2 to 4 mm and widths of 2 m or more. Very thin TPE sheeting can be produced by the blown film process used for many thermoplastics. This technique allows hard TPE sheet to be made down to a thickness as low as 0.05 mm, and even soft TPEs can yield sheets 0.2 mm thick. 

Coathanger die n. A sheet-, or film-extrusion die whose melt-distribution manifold has the obtuse-isoceles outline of a coathanger. This popular die design is said to yield uniform distribution of material across the full width of the extruded web, thus producing sheet of laterally more uniform thickness. Side-fed blow-molding dies and spiral-type dies for blown film may also be spoken as a coathanger dies. 

Lay-flat width In blown-film manufacture, half the circumference of the inflated film tube. 

Extruder diameter, mm Blown film die diameter, mm Cast film die width, mm Extrusion coating die width, mm... 

An extrusion system used extensively for thin films of polyethylene and polypropylene is the blown film process. In this process the polymer is extruded as a relatively thick tubular extrusion and is then blown up by internal air pressure to form a thin-walled tube. The tube is then sUt to form a sheet of film up to 10 ft in width or is left as a flattened tube known commercially as lay-fiat tubing. To improve gauge uniformity in this process, the circular extrusion die, the air cooling ring, or both are frequently rotated. Although low-density polyethylene and polypropylene are the materials most used for this process, polyvinyl chloride, polyamides (nylon), and some polystyrenes have all been used. (See also blown film lay-flat tubing.)... 

The blown film process was briefly described in Section 1.2.6. The process is shown schematically in Figure 10.7. There are many similarities between the blown film and the fiber spinline because of the free surface and the very small transverse dimension relative to the distance between melt extrusion and solidification, and thin sheet equations analogous to the thin filament equations are typically used, although the hoop stress must now be taken into account. The equations for a Newtonian fluid were first published by Pearson and Petrie in 1970, and their approach has been used by nearly all investigators since. There are two steady-state momentum equations because variations in both thickness and width in the stretching direction are important. The mechanics of the solid region above the ill-defined freezeline are... 

Processing Into blown films The films (thickness oa. 30 W m/ width 20 cm) wore produced using an extruder (Brabender 20 mm ) under the following conditions barrel temperature 200 - 230 0/ die temperature 230 0/ extruding rate oa. 20 g/min/ air cooling at room temperature. 

Oscillating Die m(1) A blown-film die that slowly rotates about its axis in one direction about 90°, then reverses to rotate as far in the opposite direction. The effect of the rotation is to distribute evenly over the width of the rolled-up film any slight differences in film thickness at different points around the die. (2) Flat or cylindrical strand dies in which the one die lip moves to and fro so as to cross the flow channels and produce nonwoven, knotless netting. 

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