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 (Figure 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 the roller compactor settings and the material’s densification properties). 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.
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Influences on draw-in conditions
The frictional forces between press rollers and the material are based on the product’s material properties. Lubricants have the task of reducing adhesion between tooling surfaces and the 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 active pharmaceutical ingredient (API) may cause low frictional forces with the press rollers.
Another influence on the frictional forces is the formulation’s particle size. 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 of GERTEIS’ press rollers are made of special-grade stainless steel, but the press rollers’ surface structures can be different. The press roller with the lowest ability to draw powder in is the smooth roll (Figure 2). Its surface is as implied by its name: smooth. Frictional forces are determined only by the direct contact between powder particles and the press roller’s surface. Its drawback – a minor draw-in capability – is also its benefit: the tendency for material to stick 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 in the press roller surfaces, which have different sizes 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 generally has higher levels of 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, which 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 also 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 offer 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 more difficult products suffering from draw-in problems, the axial grooved or the pocket type is often used.