Chill-roll quenching

It is also evident that draw resonance hardly occurs if at all during chill-roll quenching or film blowing. Chill-roll quenching prevents spontaneous lengthening of the melt mechanically, while film blowing results in a two-directional orientation, which prevents a preferential lengthening in one direction. 

Sheet can be produced by melt extmsion, but in this case a three-roll stack of quenching roUs is generally used (Fig. 2). More than three roUs may be used where necessary. The roUs may be mounted vertically or horizontally. The web is extmded through a slot die in a thickness close to the desired final thickness. The die is in very close proximity to the first chill roU or chill-roll nip. The web may be cast horizontally directly onto the upper chill roU of the stack as shown (Fig. 2), or it may be extmded into the first nip directly. The roUs quench the sheet and provide the surface polish desired. In some applications, matte or embossed roUs maybe used to impart special surface characteristics for certain functions. Where the utmost in optical (glazing) quality is desired the trend has been to mount the roU stack horizontally. The hot melt is then extmded vertically down into the first nip. This avoids problems associated with sag of a horizontal hot melt no matter how short the distance between die and quench. 

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. 

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... 

Cast film is produced by extruding the melt from a slit die and cooling it either by contact with a chill roll or by quenching in a water bath. The most popular process used to produce the flat film is with the chill rolls. Chill roll lines can be arranged in different layouts to meet different requirements. Example is shown in Figure 5.7. Water chill tank or quench film is also a popular process. 

For some operations, the chill roll method does not provide rapid enough cooling. In that case, a water-filled quench tank may be used for cooling and solidifying the plastic, as shown in Fig. 7.5. After solidification, the film is dried, trimmed, and rolled up. Drying may be accomplished by evaporation alone, or air jets, heated rolls, or radiant heat maybe used. The film characteristics are controlled by the die dimensions, extrusion rate, melt temperature, drawdown, and water temperature. This method used to be widely used for polyethylene and polypropylene, but is now much less common, since chill roll casting can provide better control over optical properties and thickness. 

The film is normally quenched in a water bath but may also be quenched on chilled rolls. The chilled rolls are expensive and their use is restricted. The quench tank for films is very similar to that used for monofilament. The molten film exits the die an inch or so above the water level and film guides route the sheet through the tank so that the tank water temperature is uniformly and constantly controlled and the water is free from turbulence. The film leaves the tank at a speed usually no greater than 45 m/min. The speed is limited because of water carry-over by the film. If the film is wet, difficulties will be encountered in drawing. Resin manufacturers control the wettability of films with additives and, consequently, the running speed. Drying methods, such as blowing with air, are applied as an added precaution just after the film leaves the water-quench tank. 

Casting Process. The flat-die, chill-roll, cast-film process is more suitable for high volume production on dedicated lines because of higher output rates obtained by wide dies and more efficient cooling on chill rolls. Cast films usually have better clarity than blown films because of rapid quenching, but imiaxial orientation can cause the film to split in the machine direction for some structures. 

Depending upon polymer, the resin is extruded or coextruded as a fiat sheet, approximately 0.75-2.5 mm thick, onto a large chill roll. For PP, the chill roll is in a water quench bath. The cast sheet is reheated and stretched in the longitudinal direction on a series of heated chrome rolls and fed into a tenter chain... 

Quench chilled rolls gap, and temperature design space ranges... 

Strapping tapes, film tapes, monofilaments, fibres and nonwovens are usually stretched immediately after extrusion to achieve considerable increase in ultimate tensile strength in the stretching direction as a result of molecular orientation. The elongation at break correspondingly decreases. The film for stretched tapes can be produced by a cast film technique using either the water quench or chill roll process. 

Water Quench. The water queneh east film process (Fig. 8.9) is similar in concept to the chill roll process and uses similar downstream equipment. A water bath takes the place of the chill roll for film cooling, and by cooling both sides of the film equally, it produces a film with slightly different properties compared to chill roll cast film. The extruder s slit-die is arranged vertically and extrudes a melt web directly into the water bath at elose range. The film passes under a pair of idler rollers in the bath and, for any given rate of extrusion, it is the rate of downstream haul-off that regulates film drawdown and finished thickness. 

Chill roll casting is the process by which a flat molten sheet of polymer is drawn uniaxially and quenched against a metal drum. Both film and sheets are made by this process. 

Films made by chill roll casting may be used in most of the applications described above for blown films. Some differences are encountered, owing to the fact that cast films are inevitably uniaxially oriented in the machine direction whereas blown films are typically biaxially oriented. Faster quenching rates are encountered in chill roll casting than in film blowing, resulting in lower crystallinity films that have greater clarity and lower stiffness. 

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