Bultmann's Roll Compaction-site
on Dry Granulation

Some frequently asked


Q: What is the advantage of movable gap systems (compared to fixed gap systems)?
A: The aimed product is a ribbon produced at a) always the same pressure and b) with the same thickness in order to obtain ribbons of constant porosity.
While fixed roll systems can easily provide ribbons of uniform thickness, they can hardly provide same pressure during the process while powder is applied that exhibits local changes in powder density (and usually powder used for roll compaction is non free flowing and shows such changes). Thus one time denser powder is applied to the gap causing high pressure, some moments later powder with low density is to be processed which causes a drastic decline in pressure.
Since pressure contributes much more to ribbon porosity, it is favourable to keep the pressure constant all the time and allow the gap to change a little bit (movable roll systems). This itself gives more homogeneous ribbons concerning porosity.
If in addition to this, an intelligent gap control algorithm is used, actually both of the above mentioned demands are fulfilled.

Q: How to find the right settings for all those parameters?
A: It’s easy: concentrate on the key parameters: force, gap (adjusted by the auger speed) and roll speed. Start the process in manual mode and switch to automatic mode if steady state conditions are reached and/or you are familiar with the machine.

Q: What about validation of a movable gap system when auger speed changes all the time?
A: Validation means to guarantee that always product of the same quality (here especially porosity) is produced. It doesn’t mean to guarantee that the process is driven by constant process settings while nobody cares for the resulting product’s quality.
 The first (constant product quality) can easily be achieved by using the gap control algorithm. If it is necessary to change auger speed setting to compensate for inhomogeneous powder density, this must not only be permitted, it even should be claimed in order to obtain a constant porosity. Because: How can you guarantee constant product quality without doing so?

Q: What about roll adhesion?
A: On the one side, roll adhesion means that adhering powder is compacted many times (while the actual number of recompression cycles is irreproducible. Thus you end up with a non validable process), which is unfavourable because of poor recompactibility. On the other hand, adhering plaques disturb gap control algorithm, which shoule be avoided. The solution is to use a little bit (less than used for tabletting) of lubrication agent (Magnesiumstearate or equivalent).

Q: What happens to powder when it is compacted for a second or third time in the compactor?
A: While the amount of fines slightly decreases, the compressability (needed for subsequent tabletting) suffers a lot. If possible, any recompaction step should be avoided. Moreover two instead of one compaction cycle decreases the process efficiency by about 50%!

Q: What about thermal considerations?
A1: When rolls of 2.5cm width are used, a temperature increase even at high pressure is hardly found. This is because a) the surface of those flat rolls is quite large, which leads to a quick heat exchange with the suroundings and b) a lot of “cold” powder is brought to the compaction area and can take a lot of heat without considerable temperature rise.
A2: When wider rolls are used, heat exchange with the surroundings can not easily be accomplished by the roll surface anymore. Therefore rolls can be equipped with an active cooling system.

A lot of questions regarding (supposed) hardware malfunctions are answered by the MAX Expert System, which is available (only for Gerteis machines).