Chill-roll film extrusion

FIGURE 2.26 Sketch of chill-roll film extrusion. (After Lukov, L. J. 1963. SPE 13, 10, 1057.)... 

FIGURE 14.5 Schematic drawing of chill-roll film extrusion equipment. (Data from... 

Web processes Processes, such as chill roll film or sheet extrusion, extmsion coating of paper, foils, or fabric, and lamination are aU web processes. Thickness is typically the critical... 

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. 

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. 

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. 

Cast film extrusion. In a cast film extrusion process, a thin film is extruded through a slit onto a chilled, highly polished, turning roll where it is quenched from one side. The speed of the roller controls the draw ratio and final film thickness. The film is then sent to a second roller for cooling of the other side. Finally, the film passes through a system of rollers and is wound onto a roll. A typical film casting process is depicted in Figs. 3.53... 

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. 

The die is maintained in close proximity (typically 40 mm to 80 mm) to the chill roll so that the low-strength melt web remains unsupported for a minimal distance and time. If the die is too close, there is insufficient space for thickness draw-down and widthwise neck-in (Figure 5.8 where m = width at die, f = width on chill roll, m - f = total neck-in) to take place in a stable manner. With neck-in a beading occurs on both edges of the film. Down the extrusion line these beads are later trimmed away.200... 

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. 

Different types of downstream processing equipment are necessary for the hot-melt extrusion process. For extruded film preparations, chill rolls are used to cool down and control the film temperature before it is taken up by the roller. The thickness of the film can also be controlled by adjusting the rotating speed of the chill rolls. Control of the chill roll temperature... 

A homopol5mier of vinylidene fluoride was ex-trudedt J using a coat hanger die with a width of 590 mm to produce a sheet at a thickness of 150 pm at a die temperature of 270°C. A chill roll at a temperature of 70°C chilled the hot film. This film was biaxially oriented by four times the original length in the extrusion (machine) direction and six times in the transverse direction (perpendicular to the extrusion direction). The film was at a temperature of 100°C while it was oriented at a stretch rate of 10 mm/min in both directions. 

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. 

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. 

The equipment used for extrusion coating is similar to that used for the extrusion of flat film. Figure 2.30 shows a typical extrusion coating setup. The thin molten film from the extruder is pulled down into the nip between a chill roll and a pressure roll situated directly below the die. The pressure between these two rolls forces the film on to the substrate while the substrate, moving at a speed faster than the extruded film, draws the film to the required thickness. The molten film is cooled by the water-cooled, chromium-plated chill roll. The pressure roll is also metallic but is covered with a rubber sleeve, usually neoprene or silicone rubber. After trimming, the coated material is wound up on conventional windup equipment. 

Flat film extrusion through a linear slit die on a rotating chill roll. 

Fig. 3-26. Schematic of chill roll system for flat-film extrusion line.

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. 

Air gap n. (1) In extrusion of film, sheet or a coating, the distance from the die opening to the nip formed by the pressure roll and the chill roll. (2) In the radio-frequency heating of plastics and corona treatment... 

Cast film extrusion See chill roll extrusion. 

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