Roll casting

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. 

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. 

Fig. 7. Extmsion of roll-cast film A, die inlet B, cast-film die C, air gap with molten web D, casting roll E, stripping roll F, idler roll G, edge-trim slitter ...

Fig. 1. Schematic portrayal of continuous aluminum roll casting process.

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... 

POSTER TITLE Mechanical properties of AA7068 aluminum alloy fabricated by twin roll casting and rolling processes (PAPER ID 8851)... 

FIGURE 9.33. Integrated CIC rolls (cast-in carbide) for intermediate stands of wire rod mills or finishing stands in bar mills up to 30 wt% binder WC grain sizes 3-5 pm. By courtesy of Sandvik Hard Materials, Sweden. 

Film Processing. Film can be extruded in two ways blown (lay flat) film and chill-roll cast film. The tubular film process Is... 

Some measurements were made on an unoriented, commercially extruded, isotactic PPH film (25/xm, Enjay chill roll cast) and on an oriented commercial PPH monofilament [150/xm, Amtech Inc., Odenton, U.S.A., 90% residual elongation at break, stabilized with 0.24 wt % octa-decyl 3-(3, 5 -di-ter -butyl-4 -hydroxyphenyl)propionate]. The unprocessed PPH powder also was pressed into film ( 70 /on). All samples were Soxhlet-extracted with acetone (48 hr) and vacuum dried before use. The stabilizer in the commercial fiber was not removed by this treatment, although additives were extracted rapidly from low orientation samples (5). 

Films can be produced using chill roll casting. Injection blow molding and stretch blow molding are used to produce bottles. PET is also used in extrusion coating, and PET sheet is often thermoformed. 

Cold Cast or Chill Roll Cast 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. 

M. A Villar, D. R Ania, and E. L. Thomas, "Study of Oriented Block Copolymer Films Obtained by Roll-Casting," Polymer. 43, 5139-5145 (2002]. 

B. J. Dair, A Avgeropoulos, N. Hadjichristidis, M. Capel, and E. L. Thomas, "Oriented Double Gyroid Films via Roll Casting," Polymer. 41, 6231-6236(2000]. 

Cast film extruders n. Equipment used for preparing thin polymer films by use of one or two free-rolling casting and polishing rollers. 

Fig.3.1-5tt Typical mechanical properties of (1) continuously cast and rolled strip and (2) strip produced conventionally by hot-rolling and then cold-rolling cast billets of AlMnl (3103) [1.34]...

Albalak, R. J., and Thomas, E. L., Roll-casting of block copolymers and of block copolymer-homopolymer blends, J. Polym. Sci. B Polym. Phys., 32, 341-350 (1994). 

Coextruded films are produced by a tubular-blown film process and a flat-die, chill-roll casting process. Capital and operating costs for blown-film vs cast-film coextrusion lines are strongly dependent on product mix and utilization. Equipment suppliers provide comparative economic evaluations for specific products. Practical cast-film equipment has been discussed previously (3). Coextrusion dies are unique. Extruders used before the die and take-away equipment used afterwards are standard equipment for single-layer film manufacture of blown or cast film (see Extrusion). 

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. 

Flat Dies (Slit Dies). Flat dies, also called slit dies because the orifice is a wide rectangular opening, are used in chill-roll, cast-film coextrusion. These dies are used almost exclusively for multilayer coextrusion with sheet thickness >254 /um, as well as in coextrusion coating processes (2), where a multilayer web is extrusion-coated onto a substrate such as paperboard, aluminum foil, plastic foam, or textiles. 

Film Lines Using Chill Roll Casting. Thin films are often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the extruder, the film die, the chill roll imit, the thickness gauging system, the surface treatment imit, and the winder. 

---