Chilled roll

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. 

Film manufacture also requires special considerations in the case of polypropylene in order to ensure small crystal structures and hence high clarity. Chill-roll casting processes may be used and these give films of high clarity and minimal thickness variations at high rates of production. Blown film processes can, however, give superior mechanical properties and in addition equipment costs are lower, and in consequence the process is more economic for lower tonnage production. 

There are different coating techniques. The woven and nonwoven fabrics normally involves three steps as it passes from the unwind roll to the rewind roll. Powder is metered onto the fabric, heated in an oven (gas or electric) that are usually divided into several heating zones, and cooled by a chill roll. 

We use the casting process to make polymer films (less than 0.3 mm thick) and sheets (more than 0.3 mm thick) for such diverse end uses as cling wrap, merchandise bags, roofing membranes, landfill liners, and the interior walls of refrigerators. During chill roll casting, molten polymer is extruded as a curtain from a slot die onto a chilled metal roller where it solidifies. The product is transported over a series of rollers to a winder where it is wound up. 

Cast film extrusion of polyolefins has been developed to obtain flexible films with a high level of transparency by freezing the amorphous polymer structure of the melt on a chill roll. Cast films are mono-oriented in extrusion direction. 

A mat chill roll with a high surface roughness should be used to minimize sticking problems. The chill roll temperature should be kept at about 30-40°C for Ecovio whereas Ecoflex can be run at lower chiU roll temperatures. 

Example 2 Film extrusion on a chill roll line. In order to avoid sticking to the chill roll of Ecoflex or Ecovio we have to use an appropriate amount of antiblock, e.g., at least 1% of Ecoflex Batch AB 1. Slip agent, e.g., 500-1,000 ppm of erucamide should be used if the final Aim properties require a low coefficient of friction. 

The melt film leaves the die in a perpendicular direction, entering the gap between chill roll and first downstream roll. The height of the die above the gap should be minimized to maximize coating width, which is reduced by the neck-in effect [27]. Between die exit and the contact line on the chill roll, the film is... 

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. 

One of the requirements of this process is that the melt maintain good contact with the chill roll, ie, air must not pass between the film and the roll. Otherwise, air insulates the plastic and causes it to cool at a rate different from the rest of the plastic and this spoils the appearance of an otherwise satisfactory product. The melt should not emit volatiles, which condense on the chill roll, reduce heat transfer, and mar the film s appearance. The cast film process allows the use of a higher melt temperature than is characteristic of the blown film process. The higher temperature imparts better optical properties. 

Fig. 9. Extrusion coating A, unwinding of substrate B, pressure roll C, chill roll D, die E, stripper roll F...

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. 

Film. The blown film process is most commonly used in the production of PB film from resins with melt indexes from 0.3 io 10 g/10 min at a melt temperature of200-215DC using conventional equipment. Mechanical properties of blown PB film depend on the degree of orientation and other processing parameters. PB film can be sealed ai 100 220 (1. Another technique for the PB film production consists of film casting from the melt on polished chilled rolls and co-extrusion or lamination with other films. 

A schematic diagram of a commercial-scale thermal gelation polymer precipitation process is shown in Figure 3.16. The hot polymer solution is cast onto a water-cooled chill roll, which cools the solution, causing the polymer to precipitate. The precipitated film is passed through an extraction tank containing methanol, ethanol or isopropanol to remove the solvent. Finally, the membrane is dried, sent to a laser inspection station, trimmed and rolled up. 

Polymer sheets are cooled without stretching by convected cold air (or an inert gas), by immersion into a fluid bath, or by passage over chilled rolls. Flat films are usually stretched and oriented uniaxially and cooled by either of the methods previously mentioned. Films are also cast and cooled on rolls for optimal clarity purposes. 

In many processes based on extrusion the material is subjected to further manipulation after leaving the die—as examples, by stretching or casting on chill rolls in the manufacture of film. In all such cases it is essential that an extrudate withstand the forces applied to it and not tear—in other words, while there should be some strength and elasticity the main requirement is that the molecules of which it is comprised can flow relative to each other (in this sense its viscous behaviour is the most important feature). 

Two examples may be used to illustrate the complexity of problems of this kind. When film is made by extrusion followed by casting on chill rolls there can be a tendency for the extruded web to shrink inwards towards the centre of the rolls—the phenomenon known as neck-in . The edge of film concerned becomes thicker than the rest. It has been found that more elastic melts, capable of keeping a tension in the direction of extrusion, are less liable to exhibit this fault. 

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. 

Figure 5.7 Schematic of flat film chilled roll-processing line...

---