Blown film control

Halter, H. H., What a Blown Film Control System Should Do, PT Productivity, Series Four, pp. 11-18, 1987. 

Several more recent variations of the film-to-fiber approach result in direct conversion of film to fabric. The film may be embossed in a controlled pattern and subsequently drawn uniaxiaHy or biaxiaHy to produce a variety of nonwoven products (47). Addition of chemical blowing agents to the film causes fibrillation upon extmsion. Nonwovens can be formed directly from blown film using a unique radial die and control of the biaxial draw ratio (48)... 

The film tube is collapsed within a V-shaped frame of rollers and is nipped at the end of the frame to trap the air within the bubble. The nip roUs also draw the film away from the die. The draw rate is controlled to balance the physical properties with the transverse properties achieved by the blow draw ratio. The tube may be wound as such or may be sHt and wound as a single-film layer onto one or more roUs. The tube may also be direcdy processed into bags. The blown film method is used principally to produce polyethylene film. It has occasionally been used for polypropylene, poly(ethylene terephthalate), vinyls, nylon, and other polymers. 

Control flow Contains adjustable matching inserts, no restrictor bar Has restrictor bar or flow dividers in each polymer channel but with blown film dies control is by individual extruder speed or gearboxes... 

What process variables control the degree of orientation in a film manufactured via cast film extrusion and blown film extrusion ... 

Blown film is usually extruded vertically upward through a circular die. This forms a tube that is then blown into a bubble that thins or draws down to the required final gauge. Orientation takes place in two directions horizontally (transverse direction/TV) as the bubble is formed, and in the machine direction (MD) as controlled by adjustable-speed haul-off nip rolls. 

Coextrusion can be performed with flat, tubular, and different shaped dies. The simplest application is to nest mandrels and support them with spiders or supply the plastic through circular manifolds and/or multiple ports. Up to 8-layer spiral mandrel blown film dies have been built that require eight separate spiral flow passages with the attendant problem of structural rigidity, interlayer temperature control, gauge control, and cleaning. Many techniques are available for coextrusion, some of them patented and available under license (Chapter 5). 

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film, which is drawn down along the extrusion direction only. 

The process for making a cast film involves drawing a molten web of resin from a die onto a roll for controlled cooling. The cast film process is used to make a film with gloss and sparkle. The melt temperature in the cast film process is higher than in the blown film process. The higher the melt temperature the better are the optical properties of the film. 

Biaxial orientation leads to isotropic properties in blown film, that is, properties that are eqnal in the two primary directions the film was stretched (i.e., parallel to the flat bit ). Orientation in the machine direction of the film is controlled primarily by take-up ratio, defined above. To control orientation in the transverse direction, we measure something called the blow-up ratio, while the forming ratio provides an indication of the degree of isotropy. 

Figure 10. Growth of Listeria monocytogenes in BHI broth at 24 °C. Control (diamond), nisaplin prior to extrusion (square), nisaplin post extrusion (circle), nisaplin in compress film (up triangle), and nisaplin in blown film (down triangle).

This chapter includes, first, a general description of blown film and ribbon extrusion, including the most important parameters that need to be controlled. In... 

The stretching force is a direct consequence of the distance of polymer solidification (DPS) in both processes (i.e., FLH in film blowing processes or cooling length in ribbon extrusion). In this sense, controlling DPS offers the possibility to control the final properties of films and ribbons. Table 24.3 lists typical values of stretching force as function of FLH and DR for blown films of LDPE. 

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. 

The rapid-water-quenching system produces good optical properties, has low equipment cost, but can be difficult to use to get precise control over the water temperature. Vibrations and currents can cause little marks on the film. A critical point is the necessity for maintaining a smooth surface in the water quench tank where the melt first enters. Different devices are used to control the flow of water such as baffles with openings. It has serious limitations when high production speeds are attempted the water must be kept from carrying over into any on-line pretreatment and the finished roll. However, these problems can be controlled. This liquid bath system has been used for blown tubular film inside the blown film to improve... 

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