Extruder flat-film orientation

Water quenching can be used for wire insulation, tubing, and pipe. Sheet and film extruded from slit dies are frequently crystallized on polished steel rolls operating at 65°C-145°C. Extruded, blown, or flat film can be uniaxially or biaxially oriented to submil thickness. Monofilament is extrusion spun into a water bath, and then oriented and heat set at an elevated temperature. 

In flat-film extrusion (particularly at high takeoff rates), there is a relatively high orientation of the film in the machine direction (i.e., the direction of the extrudate flow) and a very low one in the traverse direction. 

In the blown-film process, the resin is extruded through an annular die and internal air pressure used to expand the bubble. After the resin solidifies, the bubble is collapsed and may be wound as a tube or slit and wound as rolls of flat film. Since the film is stretched by the take-off rollers as well as by the bubble expansion, it has biaxial orientation. 

Simultaneous Biaxial Orientation. There are two predominate systems available to do this, tubular and flat film. In the tubular process (see Fig. 9), also referred to as the double bubble process, a continuous tube is extruded and quenched. Typically, an interior cooled mandrel is hung from the die inside the tube. The surface of the mandrel may greatly influence the interior surface of the tube. Care must be taken not to impart scratch lines in the melt as it is pulled down over the mandrel. Air pressure in this primary tube is very critical. The melt needs to be held out over the mandrel but not too far away. A water bath on the external side of the tube helps quench the tube rapidly. A nip pulls the tube from the die and acts to isolate the casting bubble from the air pressime in the stretching bubble (27). 

Aramid films have been in development since the late 1990s by several Japanese companies including Toray, Teijin, and Asahi. As with fibers, aramid solutions can be extruded through flat dies to form films. The conventional wet process can be employed to produce unidirectional and bi-oriented films from isotropic aramid solutions. Production of films from anisotropic solutions requires unique processes as shown by the example of PPTA film. 

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... 

---