Abstract
In this overview, forming is defined’more generally to include all processes from thick stock dilution with recirculated white water (mix production), to the final dewatering of the wet web on the wire.
Grammage variations in the finished paper are generated mainly during the forming process. These variations can be expressed as mass formation in the small scale range and MD-, CD- and residual variance for large scale variations.
Mass formation can be evaluated off-line using beta radiography, combined with micro densitometry or image analysis . A new technique involving direct re cording of electron beam transmission is under development, with promises of faster processing, perhaps even on-line, and higher geometrical resolution. Characterization techniques based on the co-occurence matrix, especially suitable in image analysis, can be a useful complement to the traditional power spectra.
It has recently been conclusively demonstrated that in flowing fibre suspensions, flocs are kept together by the bending forces of the fibres involved. To study the dynamic behaviour of flowing fibre suspensions, modern video techniques and image analysis are being applied.
The build-up of the fibre mat is considered to be a filtration process under normal conditions. This process posesses an inherent “self healing” effect on local grammage variations . The mass formation of a laboratory sheet is for this reason superior to that of a random sheet. The higher mix consistencies used for machine made sheets cause floc generation, and usually results in a worse large scale mass formation than that of the random sheet.
When evaluating the mechanical and optical characteristics of a machine made paper sample, its properties relative to those of a laboratory sheet formed from the same furnish may be expressed as the Forming Efficiency.
To improve grammage uniformity, the mix should be fed to the headbox directly after the dilution of thick stock with white water. No processes like screening or cleaning from which uncontrolled reject flows are drawn from the measured fibre flow should be allowed . Further, the material content in the recirculated white water should be controlled to a constant level if the composition of the paper produced is to be held constant.
In todays headboxes, the tapered manifold is the dominating method of distributing the mix flow into a tube bundle across the entire wire width. The distributing tubes can exhaust either into a stilling chamber, or directly into the outlet nozzle. In the latter case, the tube outlet area must be maximized to avoid excessive wake effects. Further, the nozzle contraction ratio must be large enough to reduce the degree of relative turbulence to an acceptable level.
There are two basic headbox designs for stratified forming . In one of them, thin, pointed vanes separate the different furnishes. In the other, thicker separation walls generate “air wedges”, which may separate the furnishes all the way to the initial dewatering point . In the first case, layer mixing can begin be-fore dewatering; in the latter case, four new interfaces between air and mix are created, and all are potential sources for disturbance generation.
Mathematical methods are now being applied to the calculation of water flow patterns in headboxes, which may eventually lead to designs with improved flow stability.
High consistency headboxes have been developed to form paper according to an extrusion process. To obtain acceptable mass formation, various channel shapes, causing mix deflocculation, are used.
Because of the complex interactions between dewatering forces, water flow, material movement and resulting web structure and elasticity, no models have so far been developed for the prediction of filtration dewatering rates. The Kozeny-Carman equation, describing flow through porous beds, is a too simplified model to be of any great value in this situation, and therefore empirical equations are applied instead.
The development of forming wires have lead to multi-layer designs where the topography of the paper and the wear sides can be optimized simultaneously.
For Fourdrinier dewatering, several new dewatering elements have been introduced, allowing a better control of the activity in the mix on the wire, and thus also of the mass formation.
Fourdinier dewatering can be especially sensitive to pressure pulses from the hydraulic headboxes since amplification due to standing wave generation on the wire can create considerable MD grammage variations.
Local slice lip adjustments, especially on hydraulic headboxes which normally have a low conver-gence nozzle, can cause considerable cross flows on the Fourdrinier wire, and this will have a large effect on the grammage profile. Further, local changes in fibre alignment will be generated, a problem which has not yet been given due consideration.
In twin wire forming, the dewatering pressure is generated by wire tension according to either of two basic principles : roll dewatering with constant; or blade dewatering with pulsating dewatering pressure. A combination of these two principles has resulted in the best mass formation and retention. Recently a new method has been demonstrated, in which the dewatering pressure is not generated by wire tension but instead can be controlled to the desired pressure event along the forming zone.
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