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processing feature | High-temperature compounding

Entek says that materials used in extruder parts need to meet the high temperature requirements

lengths also allow more flexibility in downstream feeding and venting zones. Screws and dies must be designed to minimize dead-space, because any material caught in a dead-spot will quickly degrade. Materials of construction should also be considered.

If a process is run hotter than the tempering tempera- ture of the metal, the metal could lose hardness, which could result in premature wear, comments Craig Benjamin, design engineer at Entek Extruders. The process used to heat-treat or temper the metal must be changed in order to adjust the temperature at which the metal maintains hardness. Corrosion can also be amplified at higher temperatures, and the corrosion- resistance of extruder materials should be considered, adds Benjamin. KraussMaffei Berstorff makes twin-screw extruders with heat-treated steel designed for processing up to 420°C (790°F), or with a special design that allows processing up to 450°C (840°F). Barrel elements with a special heating and cooling system provide direct and intensive heat transfer or cooling, notes Dr Thomas Winkelmann, head of the processing department for twin-screw extruders at the company. The extruder’s barrel housings are designed to

increase the heating performance in the barrel section by using more heating cartridges and by reducing the distance to the processing chamber. This barrel design can deliver up to 50% more heating power compared to the standard version, which ensures that the required processing temperatures are reached quickly, reliably maintained and exactly controlled, says Winkelmann. In addition, the barrel sections and strand die heads

have insulating plates to limit heat losses and decrease heating energy consumption. The optimized layout of the cooling bores provides high-efficiency, counter-flow cooling with standard injection of water. Using a strand pelletizing system requires high-power heating of the

KraussMaffei Berstorff has developed

barrel hous- ings that

significantly increase heating capacity

die head and sometimes additional heating of the die plate to achieve a common temperature distribution and permit stable material flow out of the die. In addition, screw designs are chosen to add thermal energy by optimizing shear stress without degradation. Because processing windows are typically narrower with higher-temperature processing, there is a finer line between melting and degradation. Screw designs are thus targeted for different materials and different grades to balance shear and dispersion for that formulation. “Operators must be aware that different grades are set up differently,” notes Glenn Desio, technology manager for crystalline polymers at Solvay Specialty Polymers. Additives used in high-temperature formulations

must be chosen to withstand the higher processing temperatures. Matching colours can be challenging, notes Desio, because the colours must be both stable during compounding and the right colour match at room temperature and the application’s use temperature. Colorants are generally limited to inorganic pigments, adds Leibfried. There are few restrictions, however, on inorganic fillers and fibres, and a wide range, including nano-fibres, are used. Kevlar and other aramid fibres can be temperature sensitive, so care must be taken when using these, says Leibfried. Pelletizing high-temperature polymers presents

challenges as well, plus extra care must be taken because customers expect high pellet quality from these high-value materials. Strand pelletizers have been the traditional choice

for high-temperature resins. However, they typically need more space for such applications because they

14 COMPOUNDING WORLD | December 2012

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