Screw cooling is a means through which the coefficient of friction may be reduced between the material and the root of the screw in the feed section. By introducing temperature controlled water through the core of the screw, heat that has migrated into the core of the screw via convection can be removed.
Typically for this application, the core of the screw is gundrilled through the first four to six diameters of the screw. Then once the screw is installed into the extruder, a rotary union with a siphon tube is installed into the screw core.
With internal screw cooling you must be able to monitor the amount of water flow or heat removal that is taking place. This can be accomplished by installing an immersion thermometer and flow gauge on the discharge side. It is also recommended that the water flow be controlled with a water valve on the return side of the flow. Placing the valve on the return side allows the rotary union and screw core to always be filled with water and no chance of cavitation and therefore flashing to steam.
Generally, by keeping the water flow through the screw core to an exit temperature of about 100 F to 120 F, the screw core will remain cool enough to eliminate the possibility of the resin sticking to the steel surface. Basically, this simple cooling system gives the operator another temperature cooling zone on the screw that can be used to improve the feeding mechanism of the screw. It will also make it possible for the temperature profile of the barrel heater to be increased which is higher than typical without the chance of causing a melt block in the feed section of the screw.
Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. He was inducted in the Plastics Hall of Fame in 2012. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com.
