Chill roll operations

The desired shortening flake product dictates the chill roll operating conditions and additional treatment necessary before and after packaging. However, some generalizations relative to chill roll operations and product quality can be made. 

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. 

Cast film is generally produced by downward extrusion of the melt onto chilled chrome rollers, which are highly polished to impart good surface characteristics to the film, as shown in Fig. 7.4. The extrudate contacts the first chill roll tangentially, and then typically travels in an S-pattern around two or more chill rolls. The first chill roll typically operates at a temperature of at least 40°C (104°F), with subsequent rolls operating at successively lower temperatures to cool the film enough that it can be trimmed and wound. An air knife is typically used to pin the plastic against the first chill roll. The film dimensions are controlled primarily by the die dimensions, extrusion rate, and take-off speed. Film produced in this manner is... 

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. 

Sheet is usually defined as being thicker than film, or thicker than 1 to 4 mm ( 0.003-0.010 in). Sheet thickness can be at least 2 mm (0.5 in.), and widths can be up to 30 m (10 ft). Basically, hot melt from a slit die is directed to a combination of an air knife with two cooling rolls, or, a more popular choice, to a three-cooling-roll stand (Fig. 3-28), which cools, calibrates, and produces a smooth sheet. To aid the chill rolls, end sections of the die are operated at a higher heat than the center (Fig. 3-12c). Cooling rolls require this type of heat control from their ends to the center. 

In extrusion coating, a molten web from a film die is applied to paper, film, or foil as shown in Fig. 4.12. The distance between the die and the pressure and chill rolls is usually quite short between 75 and 125 mm. The polymer melt temperature in this region has to be very hot for the melt to adhere to the substrate. A small IR sensor can easily measure the polymer melt temperature in this small space. The operator can monitor and adjust the die heater and the chill roll temperatures either manually or automatically. 

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). 

Polymer morphology and its response to processing parameters will affect haze values. For example, polypropylene homopolymers have higher haze than random copolymers, with ethylene. The haze values of PP copolymers, in cast film operations, will be affected by such parameters as die temperature, chill roll temperature, line output, and sample thickness. 

Polymer surface morphology will affect the value of gloss. Processing parameters as well as mold flnish (for various molding operations) or chill roll finish (in cast film operations) will also affect gloss values. Typically, rubber-modified polyolefins have a lower gloss than highly crystalline polyolefins. 

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