Surface finish chill roll

The rolls should be of close grained chilled cast iron. The surface of the rolls must be ground to a perfectly true diameter free from pinholes and should be highly polished to assist the production of first quality defect- free sheeting with a high degree of surface finish. 

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. 

Film is usually extruded down onto a chill-roll assembly (Fig. 5 44103) gjjjj ig cooled and drawn by two or more water-cooled rolls while the surface of the chill roll imparts a finish to the film. Alternately, the film can be extruded down into a trough of water (Fig. 5.45 ). In the water bath, the film passes under a guide shoe before it is raised out of the water. Since chill rolls cool more efficiently than the water bath, they increase transparency and gloss, decrease haze, and provide a better surface finish to the extruded film. Consequently, chill rolls are preferred even though they are more expensive than a water bath. Water cooling is limited to films, such as tarps, where good surface finishes are not required. Chill rolls (Fig. 5.46 ) are also used in sheet extrusion. However, the sheet is typically extruded horizontally into nip between two chill rolls and then wrapped around another roll. 

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. 

Chill Roll n A metal roll - a shell within a shell with a relatively small clearance between the two — with water or other heat-transfer medium circulating through the so-formed annular space, used to cool an extruded or cast film or sheet prior to winding or cutting. The surface of the roll may he polished or textured to impart a finish to the plastic. 

Cast film is produced by extruding the polymer from a slot die onto polished metal chill rolls. The rolls maintain a more uniform thickness (gage) and the more rapid cooling and smoother surface finish provide superior film clarity. On the other hand, cast film is oriented (stretched) only in the machine... 

Three materials are used for the surfaces of rolls—chilled cast iron, cast steel, and chrome plating. For the manufacture of rigid PVC and for polished unplasticized films (particularly transparent or translucent), chromium-plated rolls are standard. Cast steel has some advantages in terms of strength but, probably because of the difficulties of modifying surface profiles, has fallen from favour by far the most popular roll surface for producing plasticized PVC with a matt finish is chilled cast iron. 

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