Extruder roll cooling

The principal use of LDPE and LLDPE in building products is as a film water barrier under below-grade floors as a wall vapor barrier, though PVC is typically preferred and as temporary enclosure film during construction. The film is made either by extruding a thin-walled tube, which may be slit or wound up direcdy, or by extmsion through a slot die and cast direcdy on to a cold roll, cooled, then wound up. The former method is more widely used. A much smaller use for low density polyethylene is in piping. 

Another way of obtaining the profiled films is by melt-embossing. The hot melt is extruded from a linear die into the nip formed by a plain roll and a V-grooved embossing roll. Cooling of the melt occurs on the cool roller and the embossed film is stretched and heat-set in the conventional way. The cross-section of the yams is uniform but the shape depends not only on the size and shape of the grooves but also on operating conditions and may approach the circular cross-section of spun filaments. 

Film is usually extruded down onto a chill-roll assembly (Fig. 5 44103) gjjjj ig cooled and drawn by two or more water-cooled rolls while the surface of the chill roll imparts a finish to the film. Alternately, the film can be extruded down into a trough of water (Fig. 5.45 ). In the water bath, the film passes under a guide shoe before it is raised out of the water. Since chill rolls cool more efficiently than the water bath, they increase transparency and gloss, decrease haze, and provide a better surface finish to the extruded film. Consequently, chill rolls are preferred even though they are more expensive than a water bath. Water cooling is limited to films, such as tarps, where good surface finishes are not required. Chill rolls (Fig. 5.46 ) are also used in sheet extrusion. However, the sheet is typically extruded horizontally into nip between two chill rolls and then wrapped around another roll. 

Another commercial application for flat-die coextrusion is biaxially oriented multilayer films (11) made with the tentering process to improve mechanical properties. Tentered film is biaxially oriented by stretching in the longitudinal and transverse direction, either sequentially or simultaneously, at uniform optimum temperature. In sequential stretching, the multilayer extrudate is cooled to a suitable orientation temperature on a first set of rolls and then stretched in the machine direction between a second set of rolls which is driven faster than the first set. The uniaxially stretched film then enters a tentering frame, which has traveling clips that clamp the edge of the film. The clips are mounted on two... 

Fig. 11 Foamed sheet extrusion line. 1 Primary extruder, 2 blowing agent metering and flow control, 3 screen pack/breaker plate, 4 secondary extruder, 5 annular die, 6 cooling mandrel, 7 guide rolls, 8 winders. [38]...

Since the binder systems are solid at room temperature, they can be produced by the existing methods used for powder coafingsd Solid resins, pigments, photoinitiators, and other additives are premixed, then melted and dispersed in an extruder at 100 to 130°C (212 to 266°F). The molten blend is then squeezed into a thin ribbon between chilled rolls. This ribbon is further cooled to near room temperature on a water-cooled cooling belt. The cooled ribbon is broken first into flake and then ground into a fine powder ready for use. The process is illustrated in Figure 7.15. 

To maintain a steady and uniform temperature within the cylinder the surface of the disks or rouleaux must not be allowed to cool during cutting or other operations preceding loading. To prevent such cooling, in many factories the powder sheets are cut or rolled on tables heated with hot water and the material is then immediately loaded into the cylinders of the press. If the disks or rouleaux should happen to cool down, the tubes extruded by the die are uneven, often shredded, and should be rejected. 

Figure 12.15. Two types of extrusion pelleting equipment, (a) Screw-type extruder for molten plastics The die is turned 90° in the illustration from its normal position for viewing purposes. The extruded material is cooled and chopped subsequently as needed (U.S. Industrial Chemical Co.), (b) Ring extruders material is charged with screw conveyors to the spaces between the inner rolls and the outer perforated ring, the ring rotates, material is forced through the dies and cut off with knives.

Polypropylene (PP) films were first produced by extrusion casting. Polymer is extruded through a slit or tubular die and quenched by cooling on chill rolls or in a water bath. Cast films can be sealed over a wide range of temperatures and do not shrink in a steam autoclave, Polymers with melt flow rates below 5 dg/min are usually used to maintain the stability of the extra date. Higher clarity films are produced using random copolymers. 

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