Chill rolling method

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. 

Extrusion with wide exit slits is a special case. Planar films of 20-100 m in thickness, for example, are produced with wide exit slits. The film is then chilled by cold rolling or in water baths (melt-molding or chill-rolling method). Wide exit slits are also used in what is called extrusion coating of paper or cardboard with poly(ethylene). Papers thus treated can then be heat-sealed. 

Zinc rolling slabs have been cast successfully by semicontinuous direct-chill casting methods. This is the preferred method for superplastic zinc alloys which, because of their large freezing range, display unacceptable surface shrinkage when cast in open molds. 

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. 

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. 

The film is normally quenched in a water bath but may also be quenched on chilled rolls. The chilled rolls are expensive and their use is restricted. The quench tank for films is very similar to that used for monofilament. The molten film exits the die an inch or so above the water level and film guides route the sheet through the tank so that the tank water temperature is uniformly and constantly controlled and the water is free from turbulence. The film leaves the tank at a speed usually no greater than 45 m/min. The speed is limited because of water carry-over by the film. If the film is wet, difficulties will be encountered in drawing. Resin manufacturers control the wettability of films with additives and, consequently, the running speed. Drying methods, such as blowing with air, are applied as an added precaution just after the film leaves the water-quench tank. 

The advantage of calendering and extrusion methods is that they generally do not use any solvents—a factor making more economical production possible. The most common method for producing film (and sheet) is by extrusion from a melt. Virtually any thermoplastic can be extruded into film and sheet with an extruder, air knife, chill roll, and turret take-up winding. When used with an appropriate film die, materials up to 6 ft wide can be extruded at thickness as low as 0.001 in. The material can then be stretched to produce thickness as low as 0.00025 in. 

Casting is the second most popular method of producing EVA film. Dual chill roll systems are used to roll the EVA copolymer into sheets of thin film. Additives are sometimes needed to alleviate problems with blocking and poor chill roll release. Sheet extrusion is very similar to cast film extrusion except the EVA copolymer product is much thicker. Sheet extrusion usually uses three or four chill rolls stacked one on top of the other instead of the two generally used for casting. 

In the rolling experiments discussed above, the scale was always in contact with the roll surface, which has a chilling effect on the scale. The chilling effect was more profound when the contact time was longer, as also reflected in Equation (8.10). To eliminate this effect, Li et al [140] and Eilatov et al. [141] used a sandwich rolling method to examine the deformation and... 

A fourth method of assuring good contact to the chill wheel is to mechanically nip the melt against the chill will. This nip roll may also have to be cooled. The surface of the nip usually needs to have some flexibility in it. The cast web may not be perfectly flat and a nip roll will need to press on the entire surface of the melt. High temperature rubber sleeves on the nip roll are most often used, but these may result in producing a rough surface. 

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