PAPER NEEDS OF XEROGRAPHIC MACHINES (a summary)

Charles Green, 23 Maryvale Dr., Webster, NY 14580 © 2003 techman@papercurl.com &

http://www.PaperCurl.com

INTRODUCTION

There may be no single set of paper properties that are optimum for all xerographic machines
(including the so called ‘laser’ printers). This article will summarize properties and quality
procedures important xerographic papers used in faster machines.

BASIS WEIGHT AND CALIPER

The usual basis weight of xerographic paper is 20 pounds (17” x 22” 500 sheets) or 75 grams/sq.
meter. Good manufacturing control should keep weight within 0.5-0.75 pounds of nominal 99% of
the time. The choice of a caliper specification depends on other factors such as the type of fuser
roll used in the copy machine. With hard roll fusers print quality depends on the paper being
sufficiently smooth. With such machines the nominal caliper should be 0.0042” or less. For
machines with soft roll fusers the caliper can be greater than 0.0042” to obtain better copy
machine performance from paper that is rougher and stiffer. Caliper should be controlled within
±0.00025” of nominal. The control of basis weight and caliper will improve control of other
properties such as smoothness and stiffness.

SMOOTHNESS

IBM publications recommend a range of 100-200 Sheffield for the 3827® Page Printer. Xerox
papers will vary depending on the grade. Smoothness in the range of 60-250 Sheffield will affect
print quality when the print fixing is done with a hard type fuser roll (the roll that touches the print
side). Rougher (higher Sheffield) is worse for solid area mottle, toner fix to the paper, and print
density. Higher toner concentrations and usage will be needed to help improve mottle and
density. With soft fuser rolls we should not have mottled solid areas, good density and fix over
the entire 60-250 range, with fix getting somewhat better at higher values.

Smoothness plays an important role in the separation of paper from the photoreceptor (where
toner is transferred to the paper). The smoother the paper the more difficult it is to separate
because of increased the electrostatic attraction. We should expect rougher papers to perform
better, to separate easier from the photoreceptor. Smoothness is also indirectly related to sheet
stiffness in that a rougher paper will also be thicker, and therefore stiffer (assuming same basis
weight). For this reason rougher paper is also more likely to have fewer jams.

In machines that have hard fuser rolls, we are limited to maintaining the sheet reasonably smooth
so that print quality is not compromised too much (includes the Xerox 3600®, 9200®, 9400® and
9700®). On the other hand, a paper targeted for soft roll fuser machines (Xerox 1065®, 1075®,
1090®, 8200®, 9210®, 9500®, 9700-mod 5® and 5090®; Eastman Kodak machines; IBM 3827
Page Printer® are examples) higher Sheffield values can be used to help decrease jam rates.

STIFFNESS

The normal minimum stiffness of 20 pound paper (0.7-0.8 CD Taber) should be sufficient for
xerography. On the other hand, with continued use and lack of maintenance copier machine
parts may become worn and out of specification. In this situation lower stiffness paper may not
be as tolerant and jam more frequently. If paper can be made to a 1.0-1.2 CD Taber stiffness it
will have a better chance to perform well.
ELECTRICAL RESISTIVITY

If resistivity is too high the prints may have image distortions because unwanted charges may
move the toner to where it is not supposed to be. If resistivity is too low there may be deletions
because the charge placed on the paper to transfer toner conducts away too rapidly. A
reasonable range for paper surface resistivity is 1011 to 1012 ohms at 5% moisture. Packaged
paper should be protected from moisture increases in high humidity conditions. In the 4-6%
moisture a one percent increase in moisture will decrease resistivity about one decade. If
resistivity is not in the desired range small amounts of salt(s) can be added to the surface sizing
to adjust it. Only small amounts are needed, usually in the order of tenths of one percent.
Laboratory surface sizing experiments may be a good way to determine the amount needed.

SHEET TO SHEET COEFFICIENT OF FRICTION

When variation in sheet to sheet friction becomes too high, friction feeders will feed more than
one sheet (a multi-feed). A multi-feed can cause a jam or blank copies. Variation in friction
between sheets should be less than 0.1. If it is more than that, depending on the machine, multi-
feeds will occur. An average sheet to sheet friction between 0.4 and 0.55 should be satisfactory.
Friction values higher than 0.6 could cause misfeeds.

One of the frequent causes of multi-feeds is a low friction sheet pair at the interface of two reams
loaded in a feeder. The outer sheets of finished reams are often contaminated with friction
lowering materials used on the packaging line or from materials (such as waxes) in the wrapper.

SHEET TO FEEDROLL (OR FEEDBELT) FRICTION

Initially, a feedroll or feedbelt material has sufficient friction to accelerate a sheet from a pile of
paper. However, if the paper being used has poor bonding of the filler material within its
structure, the filler will contaminate the feeder surface and lower its friction against paper. The
result is a misfeed and a machine shutdown. The appearance of problems with contamination
may take up to several thousand feeds.

The effect of paper contamination can be tested. A single spot a feedbelt or feedroll is pulled
across 20-30 inches of paper and the decline in friction is measured. The test has to be carefully
controlled by comparing results with a paper that is known to be non-contaminating because of
the variability of feeder materials. The degree of degradation that can cause problems will
depend on the copy machine.

Bonding sufficient to prevent contamination is usually obtained by a good coverage of surface

size. Maintaining filler content at a reasonable level is also desirable. The felt side is almost
always more contaminating. In machines with friction-retard feeders (like the Xerox machines
mentioned in the section on smoothness) we can have two problems from felt side contamination.
If the felt side touches the feedbelt, its contamination will cause misfeeds. If the felt side touches
the retard surface, its contamination will lead to multi-feeds because it will no longer be able to
hold back the sheets that follow the top one.

CURL

Two important aspects of curl are as packaged curl and output curl (produced by the fuser). As
packaged curl can be adjusted by drying and decurling. Curl should be slightly away from the first
printed side. Curl toward the printed side is likely to increase feeding and stripping jams
significantly. Curl produced in the fuser is the result of an interaction of the heating in the fuser
with the paper’s structure and moisture content. The structural properties that affect curl include
wire vs. felt side (W/F) moisture expansivity (and the differential internal strain it produces within
the sheet) and average internal strain (how much a sheet shrinks by wetting and redrying without
constraining shrinkage). The most important driver of W/F moisture expansivity is W/F fiber
orientation. W/F moisture diffusivity (the rate at which moisture can diffuse from the surface) is
another potentially important property, but methods of measurement and interpretation need to be
developed.

Moisture is probably the most important variable because it defines whether curl is toward the
print or away from it. At low moistures (4-5%) curl is toward the print. At higher moistures (5.7
and higher depending on the paper and copy machine) curl is away from the print. The amount
of curl at low or high moisture will depend on things like W/F moisture expansivity and average
internal strain. More fibers oriented on the wire side will tend to increase toward print curl at low
moisture for wire side printing. It will also tend to decrease the moisture content at which curl
moves away from the printed side.

When there is more internal strain, curl will be higher toward the print at low moisture and higher
away from the print at high moisture. Internal strain can be decreased by reducing web tensions
and dryer felt tensions. This strategy is limited by any cockling and print deletions that are
produced.

The curl axis (whether machine or cross direction) will depend on how W/F fiber orienta-tion
affects moisture expansivity. When the wire side has more fibers oriented in the MD than on the
felt side, curl is to the wire side, MD axis (of the curl cylinder) will be more likely. If curl is to the
felt side, CD axis curl is more prevalent. When adjusting fiber orientation to change curl, the
sheet should remain slightly wire side oriented so that you do not get CD axis curl printed wire
side first. This kind of curl could cause stacking problems in the duplex tray. For most papers,
printing wire side first is probably best because printing felt side first could result in away from
print curl at lower moisture content.

MOISTURE

Moisture content is of prime importance in keeping curl toward the first pass printed side. JCP 0-
60 targets moisture at 4.7%. IBM literature suggests moisture be kept within the range 3.4-5.5%.
By controlling to a nominal 4.5% we should be able to produce paper that performs well as long
as it meets other requirements set out for curl and resistivity.

SHEET FLATNESS

Sheet flatness is important for preventing print deletions (sometimes called voids). If parts of the
sheet are out of plane, the air gap prevents transfer of toner to the sheet. Deletions are not seen
on machines that have bias roll transfer because the pressure on the sheet to flattens it (Xerox
8200®, 9200®, 9400®, 9500®, 9700®). Machines which may have deletions from unflat paper
include IBM, Kodak, and Xerox 1065®, 1075®, 1090®, and 5090® (note: machine upgrades
have been made to reduce or eliminate deletions). Machines that use low voltage transfer
systems are also less likely to have deletions.

Sheet flatness can be caused by uneven drying. An important thing to remember that unflat
paper may be found in only some of the reams of a carton, because unflat areas may occur in
only parts of the deckle width. Sometimes deletions only show up when the second side is
printed. Second pass deletions observed evenly distributed across the deckle width is another
aspect of the problem. These can be reduced by tightening draws and drier felt tensions and
reducing paper moisture.

MILLWORK

It is probably self-evident that paper needs to be free of padding (edge welds), foldovers and
other finishing defects. The edge cut quality is important in preventing padding and reducing the
amount of paper dust. The wrapper should be of the polyethylene laminate type. Pallets should
be wrapped with a plastic wrap to prevent changes in moisture content. Paper should be free of
flat flaky material such as starch that may have dried on heated drying surfaces. These can
attach to the photoreceptor electrostatically and print out black spots on the copy. Flakes can be
detected by shaking stacks of paper above a sheet of plastic.

POROSITY

The IBM guideline for porosity is 10 sec./100 ml. Gurley minimum. Xerox has a similar
specification. Along with the absence of edge welds, a Gurley above the minimum is needed to
prevent multi-feeds and jams in machines with vacuum pickup feeders (such as the 3600I®). In
addition there is some indication that paper which is too porous may deplete oil applied to fuser
rolls cause and adhering and jamming.

COMPOSITION

Paper for xerography should be made free of talc, calcined clay, surface sizing containing
synthetic materials, and photoreceptor poisons such as ammonium and nitrate com-pounds.
Some IBM publications have recommended a maximum of 18% ash content.

MACHINE TESTING

Machine testing is an important part of xerographic paper quality assessment. Jamming

problems from friction, stiffness, curl, smoothness and resistivity can be uncovered. Print quality
can be checked for things like mottle, fix, and toner disturbances (unwanted toner movement on
copy). Several thousand sheets should be run to determine long term effects of friction
degradation of feeding surfaces. JCP 0-60 requires that there be less than one jam/5000 to meet
specifications.

SUMMARY

This article has briefly reviewed the paper property needs of medium and high speed xerographic
machines, which includes the so called ‘laser’ printers. These printers have the same machine
parts as their copier counterparts. The only difference is that the image is placed on the
photoreceptor by lasers. It is remotely possible that stripping from the photoreceptor could be
different, however, no differences have been reported.

Several manufacturers have made 24 pound grades for ‘laser’ printers that are very smooth.
Very smooth paper would be needed for high quality prints for machines using hard roll fusers
that are used in some low speed printers. The need for smooth paper with sufficient stiffness
seems to be met by the move to 24 pound paper. However, feeders in these machines should be
carefully evaluated to be sure they are able to handle higher stiffness papers.

For ‘laser’ printers based on medium and high speed copiers and duplicators that are
manufactured by Xerox, IBM and Kodak, the paper requirements should be essentially the same
as those for the base machines themselves. Enhancements in brightness and opacity would be a
marketing need.