Vol. 1, No. 3-Forming
ExperTips
How Fabric Properties Relate
to Machine Operation
and Sheet Quality
Often the need arises to quickly narrow down cause and effect
when troubleshooting or optimizing the wet end of the paper
machine. The objective of this article is to provide a tool for a
quick first approximation for relating sheet parameters and machine operation to fabric properties. The table below provides a
quick general reference as a starting point.
There are two areas to consider with respect to drainage:
Impingement
Fabric Property
Effects on Operation
Effects on Sheet Quality
Drainage (air perm + Drainage
Index, %DA, CD Support)
Power consumption
Draws
Formation
Consistency at couch
Crim differential
Fiber Support Index
Cleanliness
Wire mark
Sheet release
Porosity
Pin-holing
Retention (frame length)
MD, CD support
First pass retention
Clearer white water
Clearer running machine
Improved twosidedness and
linting
Better total retention (fines and
fillers)
Generally better paper properties: tensile, burst, porosity,
pin-holes
Impingement Drainage
For a given delivery geometry, the fabric’s air permeability will indicate the amount of
impingement flow.
Sheet Formation
b
β
Sheet Formation
AstenJohnson is a supplier of Paper
Machine Clothing, Equipment, Machine
Rebuilds, Specialty Fabrics and
Monofilaments.
Once the sheet starts to form, the flow resistance of the formed sheet is the governing
influence in drainage. Therefore, how the sheet is formed in relationship to the fabric will
control drainage. The fabric parameters used to predict this are % Drainage Area (open
area of the paper side) coupled with the fiber support characteristics, specifically the cross
direction support component.
A fabric design with the optimum parameters will control drainage on the machine to
optimize the drainage components’ action, which will impart the desired energy pulses to
create formation, reduce draw and optimize off couch consistency.
Fiber Support and Surface Topography: FSI
Crimp Differential
The manner in which the fabric supports the fibers in the sheet consolidation process will have an effect on machine cleanliness, sheet release, wire
marks and porosity (pin holes).
The fabric’s crimp differential is a measure of how much out of plane the
cross direction strands are relative to the machine direction. High crimp differentials can cause pin holes and aggravate wire marks.
The more the fabric supports the fibers, the less the fiber embedment or
bleed through. Fabric impressions, which can cause wire marks and pin
holes, are reduced in the sheet with high fiber support.
Paper side crimp differential
The measure of fiber support is the Fiber Support Index (FSI). FSI, developed
by R. Beran, provides a characterization of the fabric’s ability to support a
moderately machine-direction-oriented sheet.
• FSI indicates what degree of fiber embedment the fabric will permit.
Therefore, higher values of FSI imply reduced embedment.
• FSI is the average number of support points per square inch unique to
the weave pattern of the fabric and is expressed as follows:
FSI = 2/3 [aNu + 2bNc]
Where a, b = support indices related to the fabric design.
Nu = machine direction mesh count.
Nc = cross machine direction mesh count.
1. Extra support strand
2. Primary CD strand
3. Machine direction
strand
Retention
How the fabric structure retains fiber as the sheet is formed has a direct
influence on key aspects of sheet quality, such as linting, porosity, smoothness (two sidedness) and strength characteristics.
Fiber retention (for moderately MD-oriented flow) is inversely proportional
to the average frame length on the paper side of the fabric.
Single layer top surface has
elongated irregular frames.
Double layer top surface has
shorter regular frames.
Extra Support Double layer
top surface has shorter
rectangular frames.
Triple layer top surface
has short regular frames.
The prediction of retention is based on the calculation of frame length, which
is a function of the cross machine count and the fabric design. Knowledge of
the type of furnish, basis weight, machine speed and configuration should
be considered when choosing appropriate frame length.
The above information provides a quick reference as to how fundamental
fabric design characteristics relate to machine operation and sheet quality.
This can help the papermaker open up a meaningful dialogue with suppliers.
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