JANUARY/FEBRUARY 2002

PaperAge

Historically, forming fabric developments have been

focused on improved sheet quality. Although sheet qual-

ity improvements will always be a top priority, in recent

years some forming fabrics have been successfully engi-

neered and specifically designed to also maximize

machine efficiency and profitability.

Reducing chemical and energy costs, while improv-

ing overall machine runnability, have become top pri-

orities for today’s papermakers, along with premium

sheet quality. And since machine clothing usually rep-

resents less than 2% of a mill’s overall costs per ton of

paper produced (see Figure 1), it is imperative that

clothing design and application be focused on the

largest cost drivers.

Functions and Applications

Fundamentally, forming fabrics are expected to pro-

vide the following basic functions on a paper machine:

• Form a quality sheet

• Convey the sheet from the headbox to the press section

• Run for an economic life.

However, in addition to the above, forming fabrics

are often judged to be successful or unsuccessful in

numerous other ways:

• Retention

• Formation

• Drag load (power amps).

• Profile

• Sheet two-sidedness

• Sheet porosity (coating efficiency)

• Filler distribution

• Cleanliness

• Sheet release

• Guiding

• Abrasion resistance

• Ability to drain

• Mechanical stability

• Off-couch sheet solids.

Until recently, traditional fabric styles had rather

daunting application limitations.

Double-layer fabrics, for example, have significant

tradeoffs. Maximizing fiber support can severely retard

drainage by closing the “holes” at high mesh counts.

Non-uniform drainage hole sizes and relationships in

double-layer designs can mark the sheet, retard release,

and cause fiber carry (run dirty).

Relatively unstable

weaves can contribute to

unsatisfactory CMD pro-

file, especially on long

unsupported runs and gap

formers. And relatively

low stiffness factors can

cause excessive deflection

into drainage elements,

thereby increasing drag

loads.

Similarly, conventional triple-layer fabrics also have

their own set of negative attributes. The separate stitch

yarn can cause non-uniform drainage velocity, leading

to sheet defects (“dimple mark”). Also, having been

forced to limit the quantity of stitch yarns due to the

aforementioned mark potential, the stitch yarns cannot

effectively “bind” the two distinct top and bottom lay-

ers. These two layers can become unstable and produce

a sheet with poor profile, slowed overall drainage, or

can ultimately lead to catastrophic delamination (top

and bottom layers completely disintegrate).

Overall, the triple-layer’s inability to behave as a uni-

form, homogenous structure can make it run very dirty

by trapping fibers between the layers and in the stitch

yarn crossover points. This condition magnifies as the

fabric wears and the magnitude of “layer slop” increases

when the mechanical and hydraulic pressures begin to

break down the integrity of the fabric.

New Multi-Layer Developments

In the mid-to-late 1990s, forming fabric design turned

some dramatic developmental corners. For example,

during this period Weavexx and Huyck engineers began

Figure 1. Paper manufacturing costs