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processing feature | High-temperature compounding Preventive maintenance is crucial for high-tempera-


Above: Gala says that mechanical seals require less mainte- nance and


endure faster speeds and higher


temperatures


Right: Gala’s Heat Flux Die Plate is


designed to reduce or eliminate die-hole freezing


He adds that another technology that is beneficial for high-temperature processing is a mechanical seal to prevent water from entering the pelletizer motor. Compared to radial shaft or lip seals, mechanical seals require less periodic maintenance and better endure faster speeds and higher temperatures, says Tate. While everyone wants to minimize


downtime, it is particularly important to reduce the polymer wasted during downtime when running expensive engineering resins, comments Tate. He claims that Gala’s EAC (Electronically Ad- vanced Cutter) technology increases uptime and reduces blade and die-face wear. The system incorpo- rates automated blade advance, periodic blade sharpening, and blade monitoring. Underwater pelletizers have an advantage for


high-temperature resins with low viscosities that can cause problems when trying to draw a strand. “The temperature difference from the molten material to the cooling water, which is at 80-90°C in these applications, creates a skin around the molten material, and low-viscosity materials are then easy to cut,” notes BKG’s Simon. Underwater pelletizers are closed systems, which


can be an advantage compared to open, strand systems for some materials in which evaporation of volatile components may be an environmental risk. Underwater systems also offer the general benefits of throughput, automation, and a smaller footprint. Underwater systems are broadly used with polyam- ides, and are increasingly used with PEEK, PPS, and PSU, for example. Some grades of LCP have been run successfully in small-scale and larger commercial systems, but other grades of LCP remain a challenge, note suppliers.


18 COMPOUNDING WORLD | December 2012


ture processing because there is less tolerance in all aspects of the process. Because the cycle from room temperature to processing temperature is more extreme, expansion and contraction of metal parts is also more significant, and regular maintenance should be done at shorter intervals. This maintenance should include tightening barrel bolts, checking alignment, and checking that elements on the screw shaft are tight, says Entek’s Benjamin. If gaps develop between the screw elements, polymer could leak through onto the shaft, which would make pulling the screw more difficult. “These are all mainte- nance items that processors should normally do, but they may need to be done more frequently,” he recommends. Cleaning is also important. While


standard, commercial purge compounds may not work well in high-temperature processes, a few high-temperature purge com- pounds are available. For example, Sun Plastech’s Asaclean purging compound comes in high-tempera- ture grades. Asaclean SX is designed


to clean in the temperature range of 300-390°C (570-735°F). Asaclean PX is


designed to clean at temperatures up to 420°C (788°F) with low levels of smoke and processing odour. Some use neat resin as a purging compound between different grades of that resin. When making extreme colour changes, the most


effective technique is still to pull the screw and clean it, says Solvay’s Desio. With PEEK in particular, adds Berstorff’s Winkelmann, mechanical cleaning of screws and barrel borings is necessary. It takes special training and care to clean and


operate a high-temperature processing line. Not only is it hot, but there can be a fire risk because processing can be close to the auto-ignition temperature of materials. Operators must be comfortable working with high-temperature equipment and must take appropri- ate precautions. Training and documentation of procedures are critical.


Click on the links for more information: ❙ www.asaclean.comwww.bkg.dewww.entekextruders.comwww.gala-industries.comwww.kraussmaffei.comwww.solvayspecialtypolymers.comwww.victrex.com


www.compoundingworld.com


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