Kneaders | processing feature
Peter Mapleston discusses new kneader developments with leading suppliers and with a compounder that specialises in their use
Mixing it up: kneaders move into new application areas
Kneading compounders are moving into new territory, application-wise and geographically. The latest variants of the original technology are taking this innovative equipment into areas well beyond today’s scope. Suppliers say kneaders are increas-
ingly used in areas outside their core application areas in the production of PVC compounds, cable insulation and black and functional masterbatch. These include high-end TPE compounds, barrier materials, compounds with antibacterial properties for medical applications, and electrically and thermally conductive materials for electronic components. Key requirements for compounding include precise dosing, optimal dispersion and homogeneity of the formulation ingredients, and versatility of the processing line. Alongside technolo- gies based on single, dual and multi-screw systems, compounding with kneaders is now well established in many fi elds, says Buss, the best-known kneader producer.
“Kneaders stand out for their excellent mixing
characteristics at moderate shear rates. Common among the typical applications are temperature- and shear-sensitive ingredients, highly-fi lled, high-viscosity compounds, abrasive products, and combinations of all these,” says the company. The kneader was invented by Heinz List in the 1940s.
Buss owns patents for the original technology world- wide, excluding North America where they are held by Baker Perkins, which uses the term co-kneader. Manufacturers of kneaders include Buss and X-Com- pound in Switzerland, B&P Processing Equipment in the USA, and a few suppliers in China, such as Xinda and Cenmen. Kneaders of all makes have a single screw that oscillates axially as it rotates. On the screw are various
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discontinuous kneading fl ights that create shear by passing very close to sets of pins protruding from the barrel. This combination of rotational and axial movements produces extensional fl ow with highly dispersive mixing action, breaking up agglomerates in the ingredients. The combination of radial and longitu- dinal mixing results in effi cient distributive mixing that ensures a homogeneous compound at the end of the processing section.
Shear-sensitive materials The shear gradient in the processing zone is steep enough to melt the matrix materials, such as plastics like polyvinylchloride (PVC) or polyethylene (PE), through dissipative energy (heat of friction) as well as applying the necessary energy for dispersive and distributive mixing. The shearing effect does not, however, cause thermal or mechanical damage either to the matrix materials or to the additives. For this reason, says Buss, the kneader has established itself as the best choice for compounding all kinds of temperature- and shear-sen- sitive materials. Typical examples are high grade PVC pellets for cable insulation compounds or medical
August 2015 | COMPOUNDING WORLD 35
Kneaders such as the Buss
Quantec 85 G3
provide a highly dispersive mixing action
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