Operating factors

The formation of paper made on fourdrinier machines is influenced by almost every thing that happens to the fibers from the harvesting of the wood, through the digester (or grinder), and on to the reel. However, it is generally conceded that the processing area of most importance, as concerns formation, is from stock preparation to when the sheet leaves the couch.
The literature reveals many studies in this area; some topics have been thoroughly investigated while others have scarcely been touched upon. One drawback, which held up investigations of factors affecting formation, was a good quantitative measurement of formation (beyond the subjective look-through). Optical scanning coupled with electronic analysis of variations has opened new channels for such work.

Stock variables
Different types of pulps affect formation differently. Variations due to wood type (Hardwood vs. Softwood), species (Eucalyptus vs. Aspen), geographical source (North vs. South), etc.

all exert some influence on the formation of the web. The short hardwood fibers are more beneficial to formation than the longer fibers of softwoods. Northern softwoods with a high percentage of spruce tend to form more easily than pulps made predominantly from pine and fir.

Pulps with more fibers per unit weight usually produce a sheet with a tighter, more uniform formation. In any case, it is important to use the same furnish run-to-run on a particular grade to reduce formation variations caused by differences in the pulps.
The main stock variables that affect formation are pulp uniformity along with the kind and degree of treatment given the long fibered pulps. Formation improvements are largely attributed to better and more uniform methods of refining or otherwise preparing the stock.

Pulp uniformity plays an important role in formation and the closer the peak of the fiber length distribution is to the mean fiber length, the more regular is the look-through of the paper.
Fines also play a part in the formation of paper. When some fines fractions are removed from the sheet, the look-through is poorer. Using superior pulps that blend easily is also important to good formation. This can be seen when hardwood kraft pulps replace hardwood soda pulps and measurable formation gains are achieved.
It has been demonstrated many times that beating and refining influence paper formation on the machine as much or more than any other factor. The role of short fibered pulp in formation studies shows that when used with light beating, formation is improved. If only long fibered stock is used, the stock requires more severe refining and cutting to obtain good formation. An additional advantage of using short fibers is the resiliency (compressibility) which they contribute to the structure. This resiliency is very important in printing papers.
For softwood kraft pulps, as the freeness decreases the formation usually improves. This is in line with the previous statement that harsher beating of long fibered pulp is needed to insure good formation. However, if the beating is predominantly of the fibrillation type to develop strength properties, then beating and refining may actually cause the formation to deteriorate.

Headbox consistencies are often adjusted to promote formation and compensate for limited refining that may be due to the particular balance of strength properties required in the finished sheet. More or less water may be needed, depending on the forming character of the different pulps, degree of
refining and other factors.

High water usage at the headbox helps keep the fibers well dispersed and tends to keep flocs from forming. Also, using more water tends to move the stock further down the forming table to take advantage of any turbulence from table rolls and foils.
Another factor often overlooked is that the air content in the stock should be as low as possible. If not eliminated, entrained air passing through the slice comes out of the stock slurry when it is exposed to atmospheric pressure. These bursting air bubbles can be a prime cause for pinhole, formation and basis weight defects.
The angle of impingement the jet makes with the forming fabric is also critical to formation and its requirements vary with the type of paper being made and the particular machine.

On tissue machines, a heavy breast roll discharge may be justified to give rapid water removal at the expense of sheet formation. On other machines or paper grades, headbox instabilities dictate low consistencies to prevent flocculation and some breast roll discharge must be taken to handle the extra water needed. If at all possible, a breast roll discharge should be avoided when making papers whose formation characteristics are critical to the end use.

If a breast roll discharge is necessary, it should never be greater than that which can easily escape between the breast roll and forming board. “Boiling” below the forming fabric causes great instability in the flow. Another undesirable feature of breast roll discharge is double forming which is when the jet impinges at two points on the forming fabric. This phenomenon is extremely sensitive to small changes in the slice setup and is apt to vary across the width of the jet resulting in undesirable CD formation and property variation.
Various studies have shown that the fines content in the white water directly behind the breast roll will increase with increasing speed and a steeper angle of impingement. It is expected then that poorer formation would result from a steeper jet angle, especially in highly filled papers. The wire side of the
sheet would be very rough and probably a decided wire mark would show.

Manson [TAPPI 46(8):472(1963)], using a QNSM formation tester conducted studies on an old machine operating with a double slice. By varying the L/b ratio (slice opening to lip extension) he found there was an optimum slice setting where the formation was best. With an increasing breast roll discharge, formation deteriorated and there was more CD variability. Manson also found that flattening the slice jet from the normal setting caused poorer formation to result, however, this was probably more an artifact of a faulty distribution system.

Headbox
Rectifier rolls, lump breakers, or holey rolls keep fibers dispersed by breaking up any stock lumps and flocs that might have formed. These various perforated rolls must be turning! Changing the speed of the perforated rolls has not always shown a significant effect on formation.

However, the height of the pond level and the clearances and position of the perforated rolls relative to the headbox floor and slice discharge are most important. These settings are often grade and/or machine dependent.

To compensate for changes in the condition of the forming fabric (e.g., type, age, wear, cleanliness, etc.), some adjusting of the jet-to-wire ratio is usually needed. The bottom slice lip is usually moved in or out to change the stock delivery and produce what is known as velocity (drag) or pressure (rush)
forming .