Web-forming process

The water moves in a cycle to reduce the water consumption of a wet weh plant. Only the water that is carried out of the plant with the web has to be replaced. The water industry has to recognize and satisfy these high demands. Because the web-forming process itself filters the water, filtration is not necessary. 

To produce a nonwoven out of a fiber web produced in the web-forming process, the fiber web has to be compacted. This can be done mechanically (needles, water jet), chemically, or thermally (Fig. 6.22). A typical example for this type of nonwoven is a woolen felt. 

Cellulose fibers suspended in water have the natural tendency to aggregate and create structures called floes. Flocculation effect affects several technological operations during paper production (especially paper web forming process). There are many factors that determine the effect of flocculation including fibers concentration, level of turbulence and fibers charge. 

It can be easily found that the chance of forced collision is very high for very low pulp concentration (e.g., below 0.1%]. This makes it difficult to maintain pulp as homogeneous and stable fibrous suspension. As a result, flocculation is responsible for the local variations of the pulp concentration and further variations in paper properties. On the other hand, this effect—if properly controlled—supports paper web forming process. Flocculation must be carefully controlled particularly in the headbox of the paper machine. Different chemical agents are usually used for this purpose. 

The final chapter (Chapter 10) focuses on web and sheet forming processes. It demonstrates how the statistical techniques can be applied to evaluate process and control performance for quality assurance and to acquire fundamental insight towards the operation of such processes. 

In another type of dryer, steam is used to transfer heat into large steel rolls over which paper is passed at high speed to remove moisture added in the forming process (3). Typically, the paper web temperature is held to about 170°F. Steam is introduced to the rolls at about 50 psig, with about 2.2 pounds of steam needed to remove each pound of moisture. Process boilers are commonly fired by either residual oil or coal, augmented by waste material such as wood chips and spent pulp liquors. 

Fig. 6.11-15 is a diagram of these routes indicating the applicable raw materials and their preparations, the web forming and bonding, the post-treatments (processing of non-wovens) and the type of products [B.107]. 

In the majority of cases they are produced by needle punching a batt of fibres (a number of layers of carded fibre web formed by means of a cross-laying mechanism) on to both sides of a woven base-cloth or scrim. This may be carried out in a continuous process or by attachment of a preformed and pre-needled batt produced in a separate operation. After tacking the fibre to the scrim, the assembly is consolidated in a secondary, more intensive needle-punching operation, usually with the aid of finer needles. This operation frequently addresses both sides of the felt simultaneously in a single double-punch process. 

Air-laid nonwovens n. Fabrics made by an air-forming process. The fibers are distributed by air currents to give a random orientation within the web and a fabric with isotropic properties. 

Molds may be either male (raised) or female (recessed). Female molds are preferred, with plug assist process. A female mold will provide a thicker flange area, sharp definition on the outside area, and easy ejection. A male mold is usually cheaper to construct and provides for a deeper draw. Disadvantages include a tendency to web, form a weak flange area, and provide an extremely difficult release. 

A recent development of the dry-laid techniques can be ascribed to the preformed GMT produced by a fluidized bed process [3]. In this case, fiber rovings are cut and mixed with thermoplastic powder, conveyed and deposited by air stream onto a mold. The loose web formed on this mold is heated by hot gas stream prior to being transferred to a matched die compression mold. So, in this case, the web (i.e. the GMT preform) is consolidated and formed simultaneously in the compression molding... 

Gong, R.FI., Fang, C.Y., Porat, I., 2000. Single process production of 3D nonwoven shell structures part 1 CID modelling of the web forming system. Int. Nonwovens J. 9 (4), 20-24. 

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