TABLETING
Figure 1: Schematical drawing of forces upon draw-in for one roll depending on the considered area
DRAW-IN, LUBRICANTS AND ROLL SURFACES The maximum possible angle α at which powder will be drawn in depends on the extent of the frictional force between powder and roll surface. Adding lubricants like magnesium stearate is often accompanied by draw-in problems because they reduce the friction and in consequence, the maximum possible nip angle. This has also been found experimentally [3].
This
means, to prevent draw-in problems, lubricants should be avoided or at least minimised in roller compaction formulations. Another possibility for improving draw-in is to use rollers with rough surfaces or indentations to change the powder-to-metal friction to the larger powder-to-powder friction. More details can be found here [4]
.
EXPLAINING DRAW-IN PROBLEMS It is possible to select roller compactor settings that would require larger nip angles than is actually feasible. But if the theoretically required nip angle cannot be achieved, the applied specific roll force will cause a smaller gap than the set one, because the forces within the roller compactor must be balanced. When running with gap control, the feed system will try to provide more material into the area between the rolls to open the gap. But the rolls are already conveying the maximum possible amount of powder at the maximum possible nip angle. Then, the powder will be accumulated in the gliding area. This will cause an
increase of the tamp auger torque and finally, an overload of the tamp auger drive. To counteract draw-in problems, smaller gaps and lower specific roll forces can be used. But be aware that changing gap or specific roll force result in different ribbon densities.
SUMMARY By visualising the forces acting on the rolls, draw-in and its associated problems can be explained. Larger gaps and higher specific roll forces increase the risk of draw-in problems, because the necessary nip angle might exceed the maximum possible nip angle. The limitation of the nip angle depends on the frictional forces between the powder and the roll surface. Increasing these frictional forces, by, for example, choosing different roll surfaces, helps to avoid draw-in problems. Reducing them, for example, by lubricating the powder to avoid sticking, may enforce draw-in problems considerably.
Authors: - Barbara Fretter, Managing Partner, Solids Development Consult GmbH
- Dr. Robert Frank Lammens, Managing Partner, Solids Development Consult GmbH
- Michael Schupp, Head of Process Engineering, Gerteis Maschinen + Processengineering AG
For more information visit:
www.gerteis.com/en/
Press roller draw-in efficiency
Literature: [1] Barbara Fretter, Michael Schupp, Understanding the Nip Angle, Eurolab (Dec 2023),
https://content.yudu.com/ web/15ex3/0A2nilh/EurolabDec2023/html/
index.html?page=52&origin=reader
[2] Barbara Fretter, Michael Schupp, Understanding the Nip Angle, Eurolab (June 2024),
https://content.yudu.com/ web/15ex3/0A2nilh/EurolabJune2024/html/
index.html?page=54&origin=reader
[3] A.M. Miguélez-Morán, C.-Y. Wu, J. P. K. Seville, The effect of lubrication on density distributions of roller compacted ribbons, Int. J. Pharm (2008) 362(1-2):52-9
[4] Barbara Fretter, Hartmut vom Bey, Press Roller Properties, Eurolab (June 2022), https://
content.yudu.com/web/15ex3/0A2nilh/ EurolabJune2022/html/index. html?page=50&origin=reader
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