TABLETING
Finally, the press roller surface itself has an influence on the frictional forces. Te press rollers are made of a specific type of stainless steel because they are in contact with the product and must endure high forces during the roller compaction process. Tere are press rollers with different surface finishes available designed to address various draw-in problems.
PRESS ROLLER SURFACE AND FRICTIONAL FORCES All Gerteis press rollers are made of a special grade stainless-steel, but the surface structure of the press rollers can be different. Te press roller with the lowest ability to draw-in powder is the smooth roll (Fig. 2). Its surface is as implied by its name: smooth. Frictional forces are determined only by the direct contact between powder particles and surface of the press rollers. Its drawback – a minor draw-in capability – is also its benefit: the sticking tendency of material to the press rollers is small. Other types of press rollers are available for the Gerteis roller compactors; for example, power grip, knurled, axial grooved and pocket type. Te difference is the press roller surfaces, which
have different size and shape indentations. Tose indentations are filled with power particles and as a result the frictional forces change from powder-metal friction to powder-powder friction. Powder-powder friction in general has higher friction than powder-metal friction. Terefore, with materials that exhibit draw-in problems with the smooth press rollers, these draw-in conditions can be improved by using other press roller types. Te power grip press rollers have a special
surface finish that has a higher roughness than the smooth press rollers. Te power grip indentations are still rather small. Te knurled press rollers have the same basic surface but additionally have a diamond- shaped waffle structure that is very precisely manufactured. Axial grooved and pocket type press rollers have even deeper and larger indentations, which allow for more powder to penetrate and therefore offers the best draw-in efficiency. In practice, the most commonly used
Fig. 2. Ranking of the press rollers concerning their draw-in efficiency [1 – low to 10 – extremely high]
press rollers are the smooth and knurled press rollers as well as a combination of both. For really tricky products suffering from draw-in problems, the axial grooved or the pocket type is often used.
Fig. 1. Roller compaction process
The Macro-Pactor from Gerteis
Hartmut vom Bey is with Gerteis Maschinen + Processengineering & Barbara Fretter is with Solids Development Consult.
www.gerteis.com
www.scientistlive.com 51
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