Gap Former

Fig. 3. Paper formers (a) top-wire former (b) Roll and Blade gap former. (Valmet Corporation)...

As long as paper is produced with fibres and water, foam will always potentially exist. The greater the turbulence experienced by the stock, the greater the chances of air being entrained. Unfortunately, modem trends accentuate this problem as machine speeds increase and greater use is made of gap former technology. 

Machine configuration Twin wire, gap former or multi-ply vat machines all have different requirements in terms of free drainage and retention. 

An example is shown below for a gap former maehine that changed from oxidised (anionic) size press starch to enzymatic conversion of native tapioca starch. The graph shows the accumulated lost time due to size press breaks over a 120-day period - 60 days before the change and 60 days after. 

Increased drainage shear on high speed gap formers requires the use of higher DS starches. 

Hydrauhc headboxes enabhng higher machine speeds, higher throughputs and easy apphcation of twin wire gap formers... 

Fig. 6.34 State-of-the-art headbox with dilution control for a gap former (source Voith).

Twin wire gap former, which is the state-of-the-art wire section in high speed web forming, mostly dewatering the suspension to both sides. 

In a twin wire gap former (Fig. 6.38F), the suspension from the beginning is led between two wires operating at the same speed, and is drained through one side, or mostly both sides. One of the driving forces in gap former dewatering is the... 

Fig. 6.44 State-of-the-art gap former for high-speed paper production (source Voith).

Figure 6.81 shows a schematic of a modem paper machine for LWC paper production started up in 2004. It includes a gap former wire section, a press section with a double-felted first press nip and a second press nip with felt and transfer... 

High machine speeds require a twin wire gap former to ensure high uniformity of the product and sufficient dewatering capacity. Twin wire gap formers are available in different configurations. One is shown in Fig. 6.82 which depicts a state-of-... 

As the fourdrinier vtire sections have sufficient speed potential gap formers are seldom used in cartonboard production. Nevertheless gap former installations can be found for certain applications of specialty board like gypsum board. 

The base sheet forming concept for specialty papers is the fourdrinier. It can be used for the whole basis weight range and up to a machine speed of about 1200 m mimf In order to achieve best results concerning formation and dimensional stability Dandy rolls or hybrid formers and effective vidre shaking units are frequently used. New machines however can exceed the speed limit of fourdri-niers, so gap former concepts are used in this case. 

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