| Problem
| |
Solution |
TOO MOIST PAPER
This defect can be seen as a motting structure,
with the defect diameter on the area of 1.5min, It can
be observed most clearly on areas covered with multiple
toner layers, like the solid green. If the paper is
very moist the toner transfer efficiency and the print
density can be reduced to a very low level (lower image).
When printing two sided prints, the second side print
quality is better than on the side printed first.
| The paper is too moist and
because of this too conductive in the toner transfer
phase.
|
Keep the paper ream closed until
the paper is used. Try another, unopened ream. The
moisture level of the paper can be adjusted by the
paper manufacturer to fit the other paper properties
that effect the toner transfer efficiency.
Acclimatize the paper shipments in the printing room
conditions with out opening the moisture barrier packaging,
until the paper has reached room temperature.
With paper that is too moist it may be possible to
enhance the print quality of a duplex job by printing
the more critical side during the second pass, because
paper loses moisture in the fuser. |
| Problem
| |
Solution |
MOTTLING - TOO DRY PAPER
seen mores clearly on the areas covered with multiple
toner layers.
| The paper is too dry and
resistive in the toner transfer phase. |
Keep the paper reams closed until
the paper is used. In dry condition the paper can
loose too much moisture. Try another unopened ream.
With paper that is too dry it may be possible to
enhance the print quality of duplecx job by printing
the more critical side during the first pass. |
| Problem
| |
Solution |
MOTTLINNG - BAD PAPER FORMATION
Fomation relates to the degrees of uniformity of the
distribution of the fiber in the sheet. Bad formation
can be seen as a mottling pattern in the printed sheet.
with some machine this mottling pattern may look similar
to that created with paper that is too moist. The defect
can be seen more clearly on areas covered with multiple
solid color layers, like the solid green.
| Because a sheet of paper
that has a bad formation is clearly un - uniform, the
resistively characteristics vary from spot to spot over
the sheet. These local variation cause local difference
in transfer efficiency over the sheet seen as mottling
of specially solid tone areas. |
To examine the formation of the sheet,
hold it against a light and see if the sheet is uniform
and well formed. Incase the sheet is irregular, the
mottling can be related to bad formation. In this case
another paper type should be tested. If the moisture
layer of the paper is too high, the defects related
to bad formation become more severe. Because of this
it is necessary to keep the paper reams closed until
the paper is used. |
| Problem
| |
Solution |
MOTTLINNG - UNSUITABLE TRANSFER
CURRENT( WHEN ADJUSTABLE)
In some electrophotography machine it is possible
adjust the transfer current to suit the substrate
used. Unsuitable level of transfer current may cause
low transfer efficiency, or in some case toner already
placed on the substrate to be moved from the sheet
before the fusing phase. This type of problems can
then be seen as mottling in specially those area that
have multiple toner layers on top of each other, like
for example on solid green.
| The level of transfer current
effect the print quality, as it is the parameter used
in controlling the electric field applied to move the
toner to the substrate. |
Try to optimize the transfer current
by altering the transfer current in small steps and
making test prints. Select the starting point based
on a similar type and grammage paper printed earlier.
When the paper is moist ( conductivity) it requires
lower transfer current than when the same paper is
dry (resistive). Keep the paper reams close until
the paper is used.
Incase the paper is unsuitable for the process, for
example because of vary bad formation, the solid color
area print quality cannot be optimized to can excellent
level.
|
| Problem
| |
Solution |
LINES, STREAKS AND BANDS - CRACKING
The defect is visible as tiny ruptures at a fold. It
is most common on coated grade and on high grammage
papers. The defect is most visible when the folded area
is printed.
| The tensile strength of the fold decrease
with decreasing moisture content of the paper, as shown
in chart 5.1. Therefore cracking problem are more severe
when the paper is folede when two dry, or against the
fiber direction. |
Acclimatize printed sheets when possible
and crease heavy grammage paper before folding. High
grammage paper will be in most cases give problems
in critical print jobs, if folded without creasing,
specially if folded dry directly after printint.
If possible, the design of page layout should be
done so that the folds are in line with the grain
direction. Grain direction of paper can be determined
by bending and sequare piece of paper. The paper is
about twice as stiff against the grain than in the
grain direction, so sheets will resist the bending
more strongly when folded against the grain. |
| Problem
| |
Solution |
LINES, STREAKS AND BANDS - LOW
SCRATCH RESISTANCE
The toner layers have not fixed properly. The printed
image can have a very low scratch resistance.
| There has not been a sufficient amount
of fusing energy to melt the toner onto the paper.
This may happen specially when high grammage papers
are run with high process speeds.
Some paper grades may contain compounds negatively
affectig toner adhesion. In case these compounds migrate
to the surface of the paper, only a weak bond is formed
between the toner layer and paper sheet, creating
low scratch resistance. The surface energy of the
paper surface should match the toner used, to enhance
the adhesion.
In the liquid toner processes good adhesion is achieved
when the chemical properties of the paper surface
matches those of the liquid toner. |
Check the machine manufacturer specifications
on grammages that should be run from by pass tray and
or with lower process speeds. In case where the defect
is related to poor or unsuitable surface characteristics
of the paper, another paper type should be used.
|
| Problem
| |
Solution |
BACKGROUND NOISE
Single toner particles appear on unprinted areas, in
quantities high enough to cause an overall brightness
decrease of unprinted areas. Single particles can easily
be seen with a loup
| |
Check the developer age and
the service interval for the color units. For example,
if the developers are overaged there can be unacceptably
high level of backround toner. |
| Problem
| |
Solution |
SPOTS, BLISTERING - BLISTERING
OF THE PRINTED SURFACE
Small blisters can appear on coated grades on heavy
toner coverage areas. On a multicolor plane, the underlying
process colors may be seen. The blistered toner surface
has a reduced gloss level and a rough surface.
| This defect is caused by the heat applied
to the paper in the fuser. The water inside the paper
evaporates too quickly through the coating and toner
layers, causing the small blisters. Therefore this phenomenon
is more likely to appear on heavy grammage multi coated
papers that have very closed surface. |
The paper must be run with a lower
process speed, so that the water doesnot evaporate
too rapidly in the fusing phase. To avoid paper gaining
moisture before printing, donot open the reams until
the paper will be used. Incase the printing machine
has options to either dry the paper before the toner
transfer or to reduce the fusing temperature, these
adjustments reduce blistering.
Try to avoid unnecessarily high toner coverages,
for example by using under color removal and raplacing
three and four color blacks with one color black when
possible. |
| Problem
| |
Solution |
PAPER PROPERTIES AND PRINTING -
CURLING
Severe curling of unprinted paper can deteriorate the
runnability, especially in duplex printing. In some
machines curling can also cause registration and other
image quality problems. Curling of the printed sheets
affects the visual appearance and hinders the finishing.
| The change in humidity releases internal
stresses in the paper and the result can be curling.
This type of curling appears especially when the internal
stress distribution of the paper is not even.
Curl of printed sheets can occur when on one side
of the paper a large, high coverage toner layer is
printed, and printed paper is left in an a moist environment.
Paper sheet dimensions expand after printing as the
sheets regains moisture from the air, this may cause
curl when one side of the sheet is bound with the
toner layer.
|
When paper related curling tendency
is a continuous problem,another paper type should be
tried. The paper should not be left unwrapped, to avoid
the moisture change before printing. If possible, try
to avoid high toner coverages on one side of the sheet
with low grammage papers. The one - sided toner coverage
related curl reduces in low humidity environment. |
| Problem
| |
Solution |
PAPER MANUFACTURING AND CONVERTING - WAVINESS AND SLACK
EDGES
The edges of the pile of paper are wavy.
| Wavy edges are generally caused by moisture
pick up of paper exposed to ambient air containig high
relative moisture. The edges gain moisture and expand
more rapidly than the center of the pile. This problem
can occur with unprinted paper, it is more likely to
happen with a pile of printed paper. After the paper
has dried in the electrophotographic printing process,
the moisture difference between the air and the paper
pile has increased. |
Unwrap the paper just before printing,
and follow the storage and handling guidelines of
your paper supplier.
The waviness of the printed sheets relaxes if it
is possible to keep the printed paper in the room
conditions until the moisture level has reached the
equilibrium with the surrounding air. Unfortunately
this may take a long time, in some cases even days.
This process can be speeded up by dividing the printed
batches in to smaller piles. |
| Problem
| |
Solution |
PAPER MANUFACTURING AND CONVERTING
- PAPER DUSTING
As an offset printing also in electrophotographic
processes the dusting (linting) of paper is an important
quality criteria. In dry toner electrophotoghaphy,
even if there is no tacky inks stressing paper, the
static electricity and electrical charges applied
are dominat for generaing dusting from paper.
The dusting will accumulate on the transfer and photoconductive
belts causing print quality defects. Dusting may then
shorten the problem free lifetime of these senstive
components and increase the service costs.
To reduce these effects it is important to use papers
with lower dusting influence. Since it is almost impossible
to know before usage of paper which products are good
in terms of dust and which are not, the printers should
require from the supplier from dust quality evidence
about the products to be used. The paper producer
should have in the quality management procedure calibrated
method to evaluate potential dust (in ream dust) and
generated dust (process dust).
| Grater roller. |
Install grater roller with sharper points.
Increase pigment strength of ink so as to decrease in
ink film thickness carried on web. |
| Web bustle wheels. |
Move wheels so as to contact non image
areas only. If moving wheels is not possible, apply
teflon tape to lessen abrasiveness of wheel surface. |
| Improper air flow through oven causing
web to flutter and make contact with oven apparatus. |
Consult oven manufacturer for proper
adjustment or adjust web tension. |
| Chill role no cool enough. |
Reduce chill role temperature so that
post chill role web temperature is 900 F or less. Recommended
chill role water temperature is 72 - 750 F. Caution
: If chill roll are too cold, condensation can occur.
Reduce oven temperature to minimum necessary to get
the web through chilling and press folding delivery
operation. |
| Web temperature reached in the oven
is not high enough to drive off sufficient ink solvents. |
If flame impingement drier used adjust
flame tips to impinge properly. If high velocity forced
air drier used, raise circulating air temperature in
oven until making is eliminated. Caution : Remember
paper will burn it 4510 F and can scorch at lesser temperatures.
Decrease press speed. Increase air velocity through
oven as to remove air and vapor layer clinging to moving
web. Consult ink manufacturer to obtain faster drying
ink. |
| Dryer does not have capacity to properly
handle press speeds reached or type of work printed. |
Consult dryer manufacturer. |
| Paper is very absorbent, very little
ink penetration occurs to provide basic for binding
to paper. |
Consult paper manufacturer, change paper. |
| Ink film thickness is too great. |
Raise air temperature circulating in
dryer. Increase air velocity circulating to help remove
solvent vapor layer. Consult ink manufacturer, change
to faster drying inks. |
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