Formation notes

The following deals with formation on the paper machine with a brief description of some of the factors involved. Our general understanding of formation is often confounded by the diversity of manufacturing methods, furnish components used, equipment types and designs, the broad range of
grades and weights produced, and the speed of the machines involved.

With prevailing high speed operations, dewatering the stock for proper formation is a problem that entails using special fabrics, various types of forming boards along with a variety of drainage elements.

To solve formation problems, one must appreciate the many factors affecting web formation on the forming fabric. For years, formation was estimated visually (look-through) and was a qualitative, relative measurement. With new formation testers, which assign numbers to irregularities in the sheet, we have been able to improve our control over and attain a better understanding of formation.
The way that fibers are arranged in a sheet ultimately controls the formation so we need to first review our present understanding of sheet formation. By studying both the sheet network structure and formation, we then need to relate these two to the forming process through appropriate testing
and sheet sectioning.

The main areas of concern regarding formation cover stock preparation to the couch roll. It is only within these limits that we can control the formation of the paper if two assumptions are made:

(1) the stock is properly delignified and washed

(2) the operations beyond the couch are run so as to not destroy existing formation.

All fourdrinier made paper exhibits some preferred fiber orientation in the machine direction. This, along with tensions imposed on the sheet during drying, causes most paper to be stronger in one direction than the other.

The tensile strength, rigidity, and fold endurance are all usually greater in the
machine direction (MD) than the cross-machine direction (CD).

Burst is quite dependent on fiber orientation in that it is affected by stretch which is least in the stronger MD; therefore, the line of burst is usually in the CD.

Tear strength and stretch are always greatest in the cross direction for conventional flat papers. There is no universal consensus on the influence of fiber orientation on formation.

Some workers have reported that formation improves with less fiber orientation; others claim that the formation improves with more fiber orientation since flocculation is reduced when the fibers are in a dispersed state.
Due to the inherent fiber orientation in machine made papers, there is virtually always a difference in paper strength in the MD and CD and this phenomenon has been loosely related to formation. For well formed sheets, the observed MD:CD tensile ratio is about 1.3 while an MD:CD ratio of 1.6 usually results in a cloudy/wild formation.