Chill-roll process

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

Flat films processed through slit dies are cooled principally by using chilled rolls. Many different resins are used, with thicknesses ranging from 15 to 200 fim (Figs. 3-26 and 3-27). Alternatively, certain plastics go directly into a water tank, but that creates many technical difficulties in production. Thus, the chill roll process is preferred and film up to 3 m in width will have output rates of at least 120 m/min. 

Fig. 3-27. (a) Chill roll process used in oriented film tape line, (b) Example of performance of oriented PP based on orienting heat and stretch ratio. 

Pipes, film, profiles, cable insulation, and knot-free nets are produced by extrusion. Extrusion with broad slits, for example to produce 20-100-/Ltni-thick film, is a special case. The film is then chilled by cold rolling or by water baths (melt-molding or chill rolling process). Broad slits are also used in what is called the extrusion coating of paper or cardboard with, for example, poly(ethylene). Papers thus treated can then be heat sealed. 

Figure 7-46. Top An example of the downstream equipment used with the extruder. Bottom The processing steps in the chill-roll process used in an oriented film-tape line.

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. 

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. 

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. 

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. 

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. 

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). 

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. 

In this process the plastic is directed against the surface of the substrate immediately before its entry into the nip between a pressure roll and a chill roll. The extrusion rate and the line speed of the substrate determine the coating thickness. The chill roll controls the surface finish of the coating. Extrusion coatings are typically applied in thin layers, ranging down to 0.005 mm. 

The complete equipment usually consists of a mixer such as a Banbury mixer followed by the heated rolls, chilled rolls, and finally a windup roll.3 The windup roll controls the tension on the film or sheeting as it moves through the calender rolls. Calenders are generally designed to meet the specific needs of the customer. Once installed and operating continuously, the cost per pound of film or sheet is lower than by any other process such as extrusion. 

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