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| ELIMINATING 350 ON-PRESS PRINTING PROBLEMS WITH 21 SIMPLE TECHNIQUES - PART 1 |
THE NEGATIVE EFFECTS OF CENTERING AN IMAGE WITH CERTAIN CRUCIAL JOBS AND ISSUES CONCERNING SQUEEGEE/FLOODCOATER LENGTH WERE DISCUSSED IN DETAIL PREVIOUSLY. NOW WE SHALL COVER TECHNIQUES THAT MAKE LIFE MUCH ESIER TO REACH EXEPTIONAL PRINT PERFORMANCE BY REVIEVING FLOODCOATING BENEFITS, SQUEEGEE SNOWPLOUGH AND ZERO -PEEL (NO PEEL) FLOOD STROKE. |
Mike Young Imagetek Consulting International, USA.
Mr. Young has been a specialist in high-definition graphic and industrial screen printing for more than 30 years. He is a SGIA Fellow, a member of the Academy of Screen Printing Technology, recipient of the prestigious Swormstedt Award for technical writing. He is also a frequent contributing writer to trade publications, SGIA print judge, legal expert and a popular speaker at industry events. Mr Mike is the creator of the internationally known Troubleshooting Chart and published technical books on advance screen printing techniques, including The Register Guide about achieving print excellence. He reccently conducted a series of technical seminars at Screen Print India 2004 in Mumbai. He operates Imagetek Consulting International, a Connecticut USA-based consulting firm.
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FLOODCOATER 'BENEFIT'
Up to now in this series, the concerns of a
squeegee blade has been discussed as it
relates to obtaining good quality print results -
but what about the floodcoater (commonly
known also as a floodbar or scraper blade). It
may surprise many printers that the floodcoater
can be just as important as the squeegee or
even more significant in a number of situations.
The common trouble is that the floodcoater’s
role is often seen as nothing more than a tool
to scrape back the ink in readiness to the start
the next print cycle. Its real job is to “pre-fill”
or “prime” the screen with the correct amount
of ink for printing starts. This will reduce
squeegee stress and screen wear, thereby
making the ink-transferring phase of the
process easier to complete (refer back to PART
3 on the topic and Fig. 20). Simply put, the
purpose of the squeegee is to transfer ink out
of the screen. The purpose of the floodcoater is
to leave the correct amount of ink on the screen
without the need for the squeegee to crush
image integrity due to not enough ink left
behind.
Understanding the importance the
floodcoater’s influence, by its overall profile
(angle, pressure, speed, etc.), which determines
how much ink is left on the screen, then
consider the role its edge profile also plays. If a
heavy opaque coating were required, the
scenario shown in Figure 32a would produce
the most desirable results with the least amount
of effort and stress on the squeegee blade. On
the other hand, the most desirable profile to
reproduce fine lines and tones successfully is
shown in Figure 32c, while 32b would yield a
compromise between the two. Appreciating the
effective results of using a floodcoater’s seven
influential components (length, tip profile
(sharp, dull, rounded, etc), flood angle,
pressure, speed and print/flood or flood print
mode) is beneficial in any type of medium to
high end printing operation. This is because
pre-filling the screen correctly with the
floodcoater has a direct effect on image
resolution and final finishing quality - but
without creating registration problems if
adjusting the squeegee. Simply said, adjusting
the floodcoater in any way does not affect print
registration at all so, always consider the
floodcoater first to change deposit thickness or
other characteristics of image/finish
appearance.
FLOODCOATER 'BENEFIT'
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When a long squeegee is pressed hard against
the screen, the blade’s highest pressure points
naturally move towards the ends - the weakest
being in the middle (Fig. 33a), as previously
observed. Although not seen with a naked eye,
it creates a “sagging” effect in the middle of
the screen where the fabric is at its weakest
point. This is not a concern for the printer
because the fabric balances itself along the
squeegee when it comes into contact with the
substrate during the print stroke (Fig. 33b). The
problem with these two examples (33a and 33b)
is that the squeegee too long for the job. As we
now appreciate, once the correct length is
employed (Fig. 33c), better print integrity is
assured.
The trouble is if the floodcoater’s length is
not matched to size with the squeegee, the socalled
“sagging effect” becomes a problem
while printing - particularly if near the frame’s
inside edges. During the flooding part of the
print cycle, the floodcoater floods the ink along
the fabric while the screen is not in contact
with anything, therefore it cannot compensate
for the weak area of the middle of the screen.
As a result, a greater amount of ink is left in the
middle, which can easily be seen by press
operators, (Fig. 33d). Since there is now more
ink “pre-filled” in the middle than at the sides,
the squeegee correspondingly tries to transfer
more ink in the centre. While this may not be a
problem for many printing situations, it can be
a terrible problem printing fine lines and
halftone work as well as many threedimensional
requirements. The best rule to
ensure this problem does not occur is simply to
have the correct floodcoater length available
for each squeegee length, thereby creating a
“pair” as a set.
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SQEEGEE 'SNOWPLUOGH'
Originally devised to print and encapsulate soldermask over tracks and spaces on printed circuit boards, “snowploughing” has benefited many other applications that require full coating or surface encapsulation. To snowplough is simply the skewing of a
squeegee blade, at an oblique angle, a few degrees from the parallel direction of print travel. Do not confuse this with squeegee “print angle” during printing which is something else. Depending on the amount of skew and axis applied (rotated plus or minus from the parallel position), it helps to reduce or completely eliminate saw-tooth, moiré with process work and can prevent banding with dot graduations/vignettes (Fig. 34). |
The reason why such a feature works so well to eliminate those faults is that it actually distorts the image slightly in a ‘S’ shape manner, thus breaking up the linear alignmentof tone dots that causes the problems in the first place. Furthermore, it has proven to be very advantageous in many other aspects of quality screening, such as deposit uniformity, edge resolution and superior coating over uneven/textured surfaces. Having viewed the advantages of a snowplough squeegee, caution is stressed since using too much of a skew can potentiallydistort the image beyond registration control. |
PRODUCTIVITY vs QUALITY
It is sometimes seen as a strange relationship between production managers’ need to ship finished jobs out of the building to that of Quality Controls’ or Customer Service Reps’ quest to maintain in-house quality standards. The conflict is nothing more than one is meeting ‘numbers’ while the other meeting ‘specifications’. This is not to say production personnel are any less keen on quality, on the contrary, but getting finished prints out of the door is understandably their number one, two and three goal! This is the very reason why operations split responsibilities; simply because their ideals basically conflict with one another. One point in question is the seemingly harmless action of a squeegee “stroke length” to see what this could mean for operations seeking medium to high quality print finish. |
For productivity, it makes sense to run the squeegee not only at the fastest possible speed that produces desired results but also at the shortest travel length distance to reduce cycle time, thus increasing overall output rate. A great way to print if quality does not suffer. On the other hand, for crucial jobs, this could be a problem for quality, particularly when seeking threedimensional requirements - as often the case with many
industrial/electronic-type applications. |
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As most printers know, varying the
squeegee speed changes the amount of ink
deposited, which could have an adverse effect
with fine line image resolution and overall
deposit layer. How much is affected depends on the chemistry of the ink coating, mechanics
of the screen, substrate surface and the image
itself. A squeegee must start from a standing
position (zero point), speed-up, reach its predetermined
maintained speed, slowdown and
then stop, thereby creating three continuous
but different speed profiles during the same
print stroke (Fig. 35a). (Theoretical curve
shown will differ according to ones’ squeegee
drive system and pre-set speed.) As such,
speed variation from one end of the image to
the other will correspond three-dimensionally
to an uneven deposit.
If squeegee travel stroke length were
lengthened a little, say 7 - 10 cm beyond either
end of the image, clearance will then be enough
to increase and decrease the speed to take
place outside the image area without affecting
print integrity or deposit uniformity (Fig. 35b).
The squeegee would then be travelling at a
constant rate throughout the image itself. Such
benefit further adds credence to adopting a
healthy image-to-frame ratio to start with.
While it was not the intent to go any further
with this subject, since the solution regarding
uniformity clearly speaks for itself, it may create some opposing
views from some
experts; so a few
qualifiers are perhaps
justified here. First and
foremost, the writer
has personally
experienced on many
occasions immediate
improvements with
print definition when
squeegee travel
distance has been
extended - without changing anything else.
Second, the sum of increase/decrease curve
depends entirely on the squeegee drive system
used and speed setting as well as its direct/
indirect mechanical smoothness (non-jerking,
non-vibrating, etc). Actual curve illustrated is
normal but there are some sophisticated drives
that have very short speed-up and almost no
distance for slowdown to stop - yet extremely
smooth without promoting inertia. Conversely
though, there are drives used that are very
sluggish (not readily seen by an untrained eye)
while some work on an inverted ‘V’ principle.
The latter basically means travel does not
adopt a “maintain speed” line as shown in red
in Figure 35b. Here, the squeegee increases
speed gradually to the midway point in the
screen then. It then slows down to a stop at
the other end of the print stroke, thereby
forming the inverted ‘V’ curve. Great for
smoothness -
absolutely useless for
coating uniformity!
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"ZERO PEEL" OR "NO PEEL FLOOD"
This operation
function is rather
interesting because
while a number of
knowledgeable press
operators are fully
aware that screen
printing tends to
deposit more ink at one
end than the other -
many are at a lost to
the cause. Discrepancy
in the variation is
naturally more
noticeable with larger
print sizes, as many
multi-sheet/billboard
printers will annoyingly confirm. Nevertheless, the results
can still represent a problem with small format
sizes that involve three-dimensional
requirements.
You may recall in PART 4 that while the
floodcoater returns ink back in readiness to
start the next print cycle, in essence, its real
purpose in life is to “pre-fill” the screen with
the correct amount of ink for the squeegee to
do its job properly. Many flatbed presses are
fitted with a peel-off device that work on the
principle of the screen lifting (peeling)
incrementally during the print stroke to
improve separation between mesh and
substrate. The screen peels from the same end
where the print stroke began (Fig. 36a).
With
some types of presses, particularly older
models, the screen does not go back to the
parallel position until the floodcoater returns
fully to its own original position, meaning peeloff
gradually deactivates incrementally only
during the flood stroke (Fig. 36b).
Let’s make one thing absolutely clear at this
point, with the majority of operations, flooding
while peel-off deactivates itself should not
pose a problem except perhaps for those
specializing in large format or critical
demanding work. The problem with the peel-off
functioning in this manner is that floodcoaters
typically push more ink through the screen at
the start of flooding - but gradually less
towards the other end. This is due to the extra
upward force the screen exerts on the flood
blade at the beginning of the flood stroke in
addition to certain angle profiles that may
exaggerate the problem further. In other words,
peel-off functioning in this way pre-fills the
screen with more ink at the beginning of
flooding than at the end. As a consequent, the
squeegee will try to deposit more ink, albeit, at
the innocent but guilty end.
Simply said, “zero-peel” flood is a fairly
recent technique equipment manufacturers
have adopted that automatically parallels the
screen immediately as the squeegee stroke
finishes - before flooding starts. It allows the
floodcoater to flood the screen uniformly while
it is completely level throughout the flood
stroke. |
STILL MORE INK AT ONE END?
In addition to the functional ways peel-off systems can negatively operate, another reason
for printing more ink at one end is caused by none other than high off-contact; an issue that
has been discussed several times throughout this series. While the subject of imbalance ink depositis complex and lengthy, it shall be briefly
reviewed here in a simplistic way.
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The first concern in this respect is the angle
of “squeegee-to-fabric” at the beginning of the
print stroke (not to be confused with the more
commonly understood “squeegee-to-table”
angle). While the preset mechanical angle stays
fixed to the print table the physical true print
angle (flex at the squeegee’s tip) increases
while actual squeegee-to-fabric angle (shown
in green) continuously decreases towards the
end of the print stroke (Fig. 37a). As such, more
ink is transferred at the end of the stroke due a
greater push-through-force has been created -
fabric aggressively deflecting tighter to the
blade as well as developing a greater true print
angle. |
Then compounding matters further, observe
at the start the relatively large “fabric-to-table”
separation angle behind the squeegee (shown
in blue, Fig. 37a) to that at the end of the print
stroke. Although illustrated out of portion in
relative size, it serves to demonstrate that ink
has some difficulty in getting deposited at the
beginning, due to fabric’s aggressive upward
force lifting off some of the transferred ink,
while too much is deposited at the other end
because of
sluggishness in screen
separation behind the
squeegee. Not shown
in the illustration is a
squeegee that tends to
exert more print angle
towards the end of the
stroke also because
the screen is being lifted (effectively increasing off-contact
continuously - which is precisely what the
peel-off function is suppose to do).
WHILE A NOMBER OF KNOWLEDGEABLE PRESS OPERATORS ARE FULLY AWARE THAT SCREEN PRINTING TENDS TO DEPOSIT MORE INK AT ONE END THAN THE OTHER MANY ARE AT A LAST TO THE CAUSE.
Skilled operators are aware this happens
because they often experience incomplete
impression at the start of print while the screen
remains in contact with the printed substrate long after the squeegee has finished its stroke.
Even if poor screen separation does not
deposit more ink, image integrity is
nevertheless compromised because the wet
printed image is pulled along by the fabric, then
suddenly, a large area of the screen snaps back
up at cycle’s end.
While not perfect by any means, just by lowering off-contact helps to equalize both “print” and “separation” angles to a more
acceptable level (Fig. 37b). Although certain facts
and techniques discussed throughout this serieshave been frequently revisited for good reasons,
once again, the need to reduce off-contact (which
is directly related to using good tension levels) is often seen as a major key to success in many aspects of screen printing.
IN THE FINAL PART
Now that we have seen how influential
techniques are making life easier to reach
exceptional print performance, we shall
review other techniques in the this final
part that make life just that much easier to
reach exceptional print performance. Dot
banding is covered, as too the differences
in print finish compared from that of a
cylinder press to a flatbed, dryer
optimisation and the influential impact of
screen tension on overall print quality. |
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Coming in PART 6 of the series:
Now that issues related to overall image
integrity, squeegee and floodcoater lengths
have been tackled, we shall move on to
the final segment of the this series, PART 5.
There, we will review various on-press
troubleshooting techniques, such as the
effects of squeegee print stroke length, “nopeel”
flood, causes of dot banding, heatedair
dryer optimisation (shrinkage/substrate
instability) and the “natural” difference
between print quality performance from a
cylinder press to that of a flatbed. Then we
shall the last word on screen tension.
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KEY TO ON-PRESS PROBLEMS
To follow the effects of various aspects of printing, each illustration contains a row of letters to denote the problem that the scenario is likely to cause. For example, the matrix in figure #2 (illustrating the ideal maximum range of image-to-frame size ratios), contains the letters a, b, c, f, g, h, j, k, n, s, u & v. From the Key List below, it can be seen that working outside these ratios will potentially cause one or more of the following problems accordingly: image distortion, uneven ink deposit, image elongation, smearing, tone loss or gain, loss of edge definition (sharpness) and detail, die-cutting issues with screen-stretch, in spreading, voids and colour-shift. It is always possible that it will create other problems too but those noted here are typically the ones expected to occur in most printing environments under normal everyday conditions. As only "on-press" issues are discussed here, other problems may also occur because of poor screen making (unsuitable fabric grade, under tension, improper processing techniques, etc). Drawings shown without key letters are for information purposes only.
Lastly, what does "high-quality" or "high-definition" screen printing mean, particularly since it forms part of the article's title? Although every company have their own ideas as to what "definition" represents, it usually refers to jobs that require controllable accuracy or other demanding concerns to provide a high-level of quality for the markets served. Essentially, it would mean operations that specialize at medium-to-high quality graphics, POP/POS, up-market posters, decals, transfers and electronic applications (membranes, overlays, instrumentation, trim, printed circuits, dial/guages, etc) as well as other critical industrial applications. Examples of crucial work would be considered for close-tolerance, repeatable registration, fine lines/characters, tonal bleeds, precise deposit uniformity, multiple images (for die-cutting), circuitry, clear/tints, blemish-free requirements, printing on difficult surfaces and/or unstable substrates and, otherwise, meeting exacting mechanical specifications beyond those traditionally associated with quality printing.
KEY LIST OF ON-PRESS PROBLEMS
Over 350 causes and effects are covered in the series for each illustration with the repetitive quantity amounts shown. As with PART 1, to save needless repitition, there are letters at the bottom left corner in each illustration that denotes what problems are most likely to be created by the scenario to be portrayed. It should also be a reminder to readers that many illustrations are drawn out of scale for clarity and emphasis.
a) X 20 Causes of image distortion in all direction
b) x 25 Reasons for uneven ink deposit
c) x
17 Reasons that creates image elongation (in print direction)
d) x 12 Rationales of sporadic misregistration
e) x 16 Causes of streaks in squeegee travel direction
f) x 19 Reasons for smearing
g) x 25 Typical causes for tone-loss (mono or 4-color process)
h) x 15 Causes for tone-gain (not opposing reasons for 'loss' above)
i) x 10 Reasons for creating ghosting/double edge appearance
j) X 25 Factors creating loss of edge definition, sharpness
k) x 27 Reasons for detail loss (image, lines, characters, etc)
l)
x 8 Causes of tonal banding with blends/ vignettes
m) x 8 Reasons for streaks perpendicular to squeegee travel
n) x 18 Causes for 'image' or 'screen-stretch' when die-cutting
o) x 5 Factors for productivity loss with presence of static
p) x 16 Causes of pinholes and fish-eyes
q) x 3 Rationales creating moire
r) x 9 Reasons for orange-peel
s) x 15 Typical causes for ink spread (print beyond desired area)
t) x 12 Causes creating saw-tooth appearance
u) x 25 Reasons for causes voids
v) x 23 Colour-shift during production
Extra Print quality from a flatbed vs. cylinder presses
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