Hartmut vom Bey & Barbara Fretter detail the draw-in capability of press rollers within the roller compaction process
During the dry granulation process, material is densified between two counter rotating press rolls – the core of a Gerteis roller compactor. Material is conveyed by the dosing system to the rolls, but the dosing system is not responsible for material movement during densification. The press rolls draw the material towards the gap (the smallest distance between the press rolls), and the draw-in is based on frictional forces between material and press rolls. As these frictional forces vary due to product properties, Gerteis offers different press roller surfaces that have different capabilities with respect to draw-in.
Draw-In And Friction
From the moment the powder is drawn in, the densification process starts. This area is called the nip area and is defined by the nip angle α (Fig. 1). The nip angle is not constant: it varies due to roller compactor settings and material properties. From this nip angle α onwards, the material is driven by the two press rollers towards the gap. The frictional force between the surface of the press rollers and the material is responsible for drawing in the powder. Above the nip area, there is a gliding area. If the conditions for draw-in are not met, the press rollers will glide through the material causing slippage and ultimately an increase in the temperature of the product. If a large enough frictional force is achieved however, the material will be drawn in.
Draw-in problems arise when the nip angle is larger than physically possible (due to settings of the roller compactor, and the densification properties of the material). In other words, the densification process would have to start within the gliding area, but since the material cannot be pulled in by the press rollers in this area, the press rollers cannot densify the material.
Influences On Draw-In Conditions
The frictional forces between press rollers and the material are based on the material properties of the product. Lubricants have the task of reducing adhesion between tooling surfaces and material. They also reduce the frictional forces between the press rollers and the material. Therefore, lubricants should be used sparingly in roller compaction blends and less than is commonly found in tableting. Depending on the material properties, it is possible to roller compact certain materials without any lubricants. For other materials, some lubricant will be required to prevent too much product sticking to the rolls. Even if there is no lubricant in the product blend, certain excipients (or even the API) may cause low frictional forces with the press rollers.
Another influence on the frictional forces is the particle size of the formulation. Coarser particles have less direct contact with the press roller surfaces and therefore create less overall frictional force.
Finally, the press roller surface itself has an influence on the frictional forces. The 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. There 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. The 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. The difference is the press roller surfaces, which have different size and shape indentations. Those 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. Therefore, 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.
The power grip press rollers have a special surface finish that has a higher roughness than the smooth press rollers. The power grip indentations are still rather small. The 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 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.
Hartmut vom Bey is with Gerteis Maschinen + Processengineering & Barbara Fretter is with Solids Decelopment Consult