Quench tank extrusion

Quench-tank extrusion An extrusion process wherein the extrudate is conducted through a water bath for rapid cooling. 

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. 

Monofilament is usually extruded using a conventional screw extruder of rather small size, through a die consisting of a series of holes. The monofilaments are extruded downward into a tank of water (quench tank) from which they go to pull-rolls to he drawn and oriented. The physical properties of the strands depend on the conditions of extrusion, on the degree of stretching, and on the temperature at which they are stretched. For HDPE, a melt temperature of between 260 and 290 °C is usually best. For LDPE, about 30 °C lower is better. Strength increases with draw ratio and generally levels out at ratios between 9 1 and 12 1. 

When the extrudate exits the die, it has to be quenched and possibly sized (drawn through a fixture) to maintain its final shape. Depending on the extrusion process, different methods are available to quench the final product. Cast film and sheet are quenched on rolls and in water baths, blown film is quenched by air in a blown film tower. Solid profiles, pipe, and tubing are quenched in calibration tanks filled with water and in some cases connected to a vacuum system. Polymer strands and monofilaments are quenched in air or water baths while wire coating is done horizontally in air or water. In small and large part blow molding, the melt is quenched in molds as the extruded tubular parison is inflated. 

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