Abstract
In this overview forming covers all processes from the dilution of thick stock into a mix, using recirculated white water in the short circulation, to the dewatering in the wire section.
Grammage non uniformity in the paper web is to a predominant degree generated by the forming process, and especially the small scale variations summarized in the term formation. The term mass formation is recommended when only grammage is considered and not the optical impressions there of. The forming process also generates the main part of the large scale variations, that is the MD-,CD-and residual grammage variance.
Mass formation has traditionally been evaluated using beta radiography, combined with micro densitometry or image analysis. A new technique involving the direct recording of electron beam transmission is underdevelopment, with promises off aster processing, perhaps even on-line, and high geometrical resolution. Characterization techniques based on the co-occurrence matrix, applicable to image analysis, can be a useful complement to the traditional power spectra techniques.
It has recently been conclusively demonstrated that in flowing fibre suspensions, flocs are kept together by the bending forces of interlocked fibres. To study the dynamic behaviour off lowing fibre suspensions, modern video techniques, high speed movie pictures and image analysis are applied.
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 fibre flows are drawn should be allowed in the short circulation. Further the consistency of each material component in the recirculated white water should be controlled, if the content of the different components in the sheet produced is to be held constant.
First pass retention is shown to be a badly defined retention value, and should only be used in comparisons for one paper machine when no changes of the material content in the long circulation occurs.
In the headbox, the tapered manifold is the commonly used means for achieving the coarse distribution across the machine width. From the cross distributor, a tube package can lead either to a stilling chamber, or directly into the out let nozzle. The nozzle has to be fed using maximum open area to avoid flow instability, and the nozzle contraction must be large enough to reduce velocity streaks and relative turbulence to an acceptable level. Mathematical models are now being applied to the calculation of water flow patterns in headboxes.
Local slice lip adjustments, especially on headboxes without let nozzles of low convergence angle, can cause considerable sideways flow on a Fourdrinier wire, and this will have a large effect on the CD grammage variations as well as on the final sheet anisotropy profiles.
There are two basic headbox designs for stratified forming. In one of these, thin,pointed vanes separate the different furnishes. In the other, thicker separation walls generate air wedges, which may separate the furnishes up to the actual starting point for dewatering. In the first case, layer mixing can start already at the headbox, while in the latter four new surfaces between air and mix jets are created, all four being potential sources of disturbance generation.
High consistency headboxes have been developed, with which paper is formed according to an extrusion process. To reduce mix flocculation, various channel shapes are used inside the headboxes.
Sheet build-up generally takes place according to a filtration process, which has an inherent self healing effect. Therefore, the mass formation of a laboratory sheet is better than that of a random sheet. For a machine-made sheet, the comparatively high mix consistency causes floc generation, which may result 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 from the same furnish may be expressed as the Forming Efficiency.
The Kozeny-Carman equation describing flow through porous beds can not be used to predict filtration dewatering rates during web forming. This is because of the gradual compression of the web by the dewatering forces, the closing of some pores, turbulent flow situations etc. Dewatering capacities must so far be predicted using empirical equations, and parameters evaluated on the basis of dewatering experiments.
The development of forming wires has led to multi-layer designs where both the paper side and the wear side can be optimized simultaneously.
Pressure pulses in hydraulic headboxes are detrimental to Fourdrinier dewatering, since attenuation due to standing wave generation on the wire can create large MD grammage variations.
In Fourdrinier dewatering, several new dewatering elements have been developed, allowing a better control of the activity in the mix on the wire, and thus also of the mass formation of the web formed.
In conventional twin-wire forming,the dewatering pressure is generated by wire tension according to one of two basic principles: roll dewatering with constant or blade dewatering with pulsating dewatering pressure. A combination of these two principles may result in an improved combination of mass formation and retention. Recently a new method for the generation of dewatering pressure has been demonstrated, in which the pressure along the forming zone can be controlled freely, since it is generated by application of local forces and thus not by wire tension.
Multiply products manufactured through simultaneous forming are now used for low grammage products. The problem is to achieve acceptable layer purity as well as layer mass formation. Controlled pressure pulse dewatering could provide the means to reach optimum dewatering conditions.
The influence of forming conditions on product properties is a vast area within which two subjects are discussed: the interrelationship between mass formation and paper strength and finally fibre orientation anisotropy.
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