Formation of Films on Hatschek Machines

摘要

The success of fibre cement manufactured by the Hatschek process is largely due its 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 stages. 1. 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. This paper examines the second of these processes in detail. For fixed set-up of the vat, the thickness of the film formed during the passage of the sieve through the slurry is found to be proportional to the square root of the solids concentration of the slurry in the vat divided by the speed of the machine. Data from operating Hatschek machines is presented to demonstrate the validity of this relationship. The significance of the relationship for control of sheet flatness and of the machine overall particularly with respect to sheet thickness is discussed.