Bultmann's Roll Compaction-site
on Dry Granulation

THE COMPACTOR

Compactor types

Although the general layouts of roll compactors look alike, there are some features that differ from compactor to compactor. These lead to a type classification:

· Roll assembly: fixed / movable rolls
· Sealing: Side plate assembly / rim roll assembly
· Auger design: constant diameter and slope / decreasing diameter or varying slope
· Auger assembly: one / two augers
· Roll layout: vertical, horizontal, skewed

Roll layout

The relative position of the rolls towards each other is mainly a matter of design and therefore it only plays minor role (the 30° skewed position of Gerteis compactors for example results from the wish to easily harvest long ribbons during the running process while the integrated milling equipment is running).

                    

Figure: Different roll assemblies: horizontal (left), skewed (middle) and vertical (right)

Only the vertical assmbly might give some reasons for concern:
· If the draw in angle is fairly small, powder that is fed to the nip area might remain at the gliding zone since gravity force is not overcome by friction to the roll surcace (see next fig., left).  Thus particles that stay at this position might show an increase in their temperature and are susceptible for increased degradation.
· The situation concerning the direction of force by friction and force of gravity is completely different for the two rolls. Therefore the nip angles at upper and lower roll differ.

                                       

Figure: Powder might stay in place if nip angle is quite small giving reason for concern regarding thermal degradation (left) and major differences in nip angles should be taken into account when using vertically assembled rolls.
 

Fixed vs. movable rolls

The rolls of a compactor might be assembled next to each other in a fixed position. This results in a fixed gap size, which is the result of the distance between the roll centers and the roll diameter. Therefore, this gap size can not be changed during the process. The force applied during the compaction process results from the amount of fed powder per time and the powder’s compression properties but cannot be controlled properly.
Compactors with movable rolls are equipped with one roll, that is assembled in a fixed position in respect to the housing (master roll) and a movable roll (slave roll). The slave roll either slides on tracks or pivots on a lever. The compaction force is applied by hydraulical units and might be adjusted, changed or controlled during the process independent from other process settings.
 

                    

Figure: Roll assembly: fixed rolls (left), and movable rolls (sliding rolls, middle; pivoting rolls, right)
 

Side sealing

In order to prevent the powder from finding its way around instead of through the compaction area, the rolls have to be sealed at their sides. This might be achieved either by a side plate assembly or by rimmed rolls.
A typical side plate assembly consists of two strong side plates (one for each side of the gap) that are slightly pressed against the rolls. In order to prevent metal-metal friction and wear, a PTFE-platelet is positioned in between.
Since the degree of densification might be quite high in the compaction area, the side plates might be bent to the sides by the high transversal forces. This leads to insufficient sealing which in return allows powder to escape from compaction. Therefore the ribbons edges might be weak and/or crumbling.
The other sealing system is the so called rim roll assembly where one roll is equipped with two rims (one at each side of the roll) while the other roll runs within this cavity. Since the rims are mounted to the roll, they can stand high transversal pressure without losing their sealing capability. Therefore ribbons produced on rim roll systems exhibit strong, non fragile edges.
 

Auger design

The auger screws might be of cylindrical or conical shape and also the slope might be constant or changing and any combination thereof.
                                       

Figure: Different auger designs

By varying the slope and/or a conical outline, a predensification of the powder might be intended (see figure). One has to keep in mind, that by densifying powder that shows local changes in powder density, those changes are not eliminated since the powder is only densified including those inhomogenities. Other precautions must be taken in order to provide  powder of a homogeneous density to the compaction area.

Figure: Predensification by the tamping auger. If the slope doesn’t change, the  feeding speed reains the same which results in a predensification from V1 to V2 when the powder is moved forward for half a revolution
 

Auger assembly

The feeding principle of the powder might rely on gravity transport in combination with a single feeding auger. Other manufacturers use combinations of two augers in order to convey the non free flowing powder to the nip area. The benefit of a two auger layout can be effective air vent and a more uniform powder transport (expressed as mass per time unit) if proper control algorithms are used.

                             

Figure: Different feeding sytems using one or two augers. The auger design can also differ.

Next to the above mentioned topics, different brands show major differences in the PLC process controll algorithm and performance.



last chapter BRC-homepage next chapter JMB Publishing, Software, Consulting and more...