Process and Mechanisme to get Green Sheet on the Hatschek Machine

The success of fibre cement by the Hatschek process :( From paper mill Cylinder Mould Type )

• Formation in thin paper like films that are placed one on the other until the desired sheet
thickness is reached.
• Formation of the sheet by this means distributes the reinforcing fibres in two dimensions
taking best advantage of the reinforcing fibres to increase the in-plane strength of the
sheet.
• Thus the strength of sheets made in this fashion is approximately 50% greater than
sheets formed to full thickness in one action in the filter press process.

Sheet formation on the Hatschek Machine occurs in 4 stages1. Initial formation of a filter layer on the surface of the sieve
2. Building of a very watery layer of fibre cement over the filter layer as the sieve rotates
in contact with the slurry in the vat
3. Low intensity dewatering of the wet film as it transfers to the felt and
4. High intensity dewatering of the film as it passes through the nip of the accumulator
roller.

Introduction The Hatschek machine :( Modification and using cylinder mould machine (frangky )
1. First developed for the production of asbestos cement in the 1890’s when it was patented by
the inventor, Ludwig Hatschek.
2. The machine is still used in the same basic form today and although modern Hatschek
machines are much more productive than the early models

Mechanism to build green sheet on the Hatschek machine as follows
1. Clean sieve is pulled under the slurry in the vat, water from the slurry runs through the
sieve depositing a soft porous film of fibre and cement on the surface of the sieve.
2. The sieve carrying the film exiting the vat is brought into contact with the felt stretched
tightly across the sieve. This removes much of the water from the film by forcing it back
through the film. The solid film floats on this layer of water and is transferred to the felt
partly in response to the effect of removal of water and partly because the felt has a greater
affinity for the film than the sieve.
3. The film is carried on the felt to an accumulator roll to which it is transferred by further
removal of water at high pressure.
4. A sufficient number of films are wrapped on the accumulator roll to form a sheet of the
desired thickness, the stack of films is then removed from the roller and laid out flat to form
the sheet. The action of dewatering successive films in contact with each other under
pressure is sufficient to bind the films together to form a contiguous solid sheet.


Detailed Mechanism of Film FormationFormation of the film takes place as follows
1. A filter layer of fibres forms on the surface of the sieve within a short distance the
immersion of the sieve into the water.
2. The film continues to build up on the sieve but now contains a lower proportion of fibres
and a greater proportion of the non-fibrous materials.
3. The film is dewatered and stripped from the sieve on to the felt driving the sieve.


The position of formation of the filter layer Two possibilities exist,1. The fibre orientation screw runs counter to the sieve direction and throws the slurry onto
the sieve above the immersion point. In this case most of the formation of the filter layer
occurs before the sieve enters the slurry.
2. The fibres orientation screw runs in the same direction as the sieve that may be fitted with
a rubber flap extending 50 mm or so beneath the surface of the slurry. In this case the
formation of the filter layer takes place just below the rubber flap.

Raw material in to processThe feed to a Hatschek machine is
1. Fibres
2. Portland Cement
3. Ground minerals
4. Water.

Formation of the Filter LayerFormation :
Typical sieve apertures are around 0.4mm (400 um) and clearly the non-fibrous material is significantly smaller than the sieve apertures and so would wash through the sieve.
Fibres on the other hand are able to bridge the wires of the sieve although any fibre presenting perpendicular to the sieve surface could also pass lengthwise through it.
Entrapment of the non-fibrous materials therefore depends on the formation of a filter layer of fibres on the surface of the sieve.