In the fourth of his series of articles on process control in screen printing, Will Pearson, of Phantom
Screenprint, looks at the squeegee, floodbar and accurate diagnostics…
A good quality, triple durometer square profile blade is perfect for T-shirt
printing.
If you've been reading this series of articles, hopefully
you'll have found some information or advice that's of value, or
discovered a different angle on a problem, in each article. So far, I've
covered how to level, calibrate and 'parallelize' your press; how and
why to use high-tension screens; and how to create the perfect stencil.
So we're well on our way to achieving the sort of efficiency that creates
extra capacity and reduces needless demands on your labour force. However,
there's still one component that, if not brought under statistical and
measurable control, can jeopardise a lot of the hard work that has gone
beforehand. I'm talking about the squeegee and floodbar.
Squeegee and floodbar functions
Before I look at how to standardise the use of the squeegee and floodbar,
I'll summarise the job(s) they actually perform.
Remember our nightmare job from the February issue? Excessive
squeegee pressure required; slow and double strokes needed; and a
blurred image with heavy ink build up on successive screens. All of these
problems were evident on this job, and many of the problems were due to
the way the squeegee and floodbar had been set up. In this scenario, the
action of the squeegee and floodbar can be described as follows…
The floodbar drags the free ink in the screen back to the top of the image,
and the squeegee then forces the ink into the open areas of mesh,
and down onto the T-shirt below.
See anything wrong with this? The floodbar isn't really doing any work.
Whereas, in my opinion, the floodbar should be doing 60-70 percent of
the work of preparing the ink in the stencil ready for the ink to be deposited
on to the shirt.
The floodbar should be set up such that it drives the ink into the profile
of the stencil, so the ink lies on the print side of the mesh, and fills the
emulsion-over-mesh (EOM) profile that we talked about last month. The squeegee then only has to 'cut off' the ink in the stencil, and press the
mesh down enough to deposit the ink onto the shirt's surface, and then
clear the ink from the screen. When the ink transfer process is happening
in this way, it means that the mesh and the stencil profile determine the
amount of ink being deposited on the shirt. This becomes a measured,
precise amount. If you're having to 'smash' the ink through the mesh and
stencil and onto the shirt with excessive pressure, the ability to precisely
meter even, thin ink deposits is lost.
So why is achieving a mechanically efficient flood stroke such a challenge?
Firstly, high-tension, (tension of 35 Newton-metres plus), is a
pre-requisite for the optimum flood stroke, particularly with the heavier
plastisols, such as first down whites, high opacity yellows and shimmer/
glitter inks. With low-tension screens, you tend to find you need excessive
floodbar pressure; and a typical low-tension floodstroke results in a flood
that reaches into the print side of the stencil at the edges of the print,
and tends to 'pool' on top of the squeegee side of the stencil toward the
centre of the print.
The floodbar's profile and material are also partially responsible: most
floodbars are designed to sit at around a 90 degree angle to the mesh;
however, this is not the best set up for driving ink down into the stencil
profile, as ink is always deflected at 90 degrees to the flood or squeegee
driving it. Therefore, a floodbar set at 90 degrees is prone to spreading
the ink out across the top of the stencil, with very little downward dynamic
into the mesh. Now, setting the floodbar with anything less than
a 90 degree angle decreases your available stroke length, so that's not a
practical solution.
Usually, the problem is overcome by setting more flood bar pressure.
Newman
floodbars are
designed to
keep the ink
flowing, and to
inject the ink
into the stencil
apertures
for better ink
transfer
However, Stretch Devices, which developed the roller frame, produces an
interesting floodbar with a curve to the edge of the blade, and a curved
lip running about 12 mm up the blade on the ink side. This means that the ink is rolled in front of the floodbar and forced downward into the stencil.
The rolling action also keeps the ink moving, which helps improve its flow
characteristics. We've been trialling these floodbars recently and have
found an improved flood for less pressure. This, in turn, means that adequate
coverage can be produced with less squeegee pressure.
Squeegee characteristics
Now let's look at the squeegee. One of the most desirable characteristics
for a squeegee blade is the ability to maintain a sharp edge for a long
period of time. There are two main types of blade available:
1) Cut blade
2) Moulded blade
The moulded blade edge is more durable than the cut blade edge; however,
it is also quite a bit more expensive.
You can also buy blades in different durometers. The durometer of the
squeegee is the measure of the squeegee's hardness according to the
Shore A Scale (generally referred to a 'shore'). As a rule of thumb, a softer
durometer squeegee (lower shore measurement) gives a heavier ink
deposit than a harder durometer squeegee (one with a higher shore rating).
This is why some people use a softer durometer for their first down
whites, to help achieve good coverage.
You can also choose between squeegees that comprise a single durometer
or triple durometer blades. Single durometer squeegee blades are OK
for manual printing, but for automatics a triple blade is the best option.
Triple blades flex less for a given amount of squeegee pressure, which is
desirable as it means the pressure is more controllable.
At Phantom, we settled on a 70/90/70 triple blade, which is medium
shore. We've found we can tackle all types of image and ink types with
this one blade. This helps to simplify and standardise the squeegee part
of our process a great deal.
There are also several different types of squeegee section available:
square cut, bevel cut, double bevel and rounded. Here's a quick summary
of what each is used for:
Square cut: T-shirts and piece textile goods
Bevel cut: heavier deposits and greater coverage on T-shirts and piece goods
Double bevel: mainly used in printing cylindrical objects and glass
Rounded: often used for rough surfaces needing a heavy deposit, or printing water-based inks on beach trowels
As you can see, it's only the first two squeegee blade
sections that are relevant to T-shirt printers. I prefer to
use a square cut blade, to produce a good, sharp edged
print, and to control deposit and coverage with mesh
tension and stencil profile. The bevel edge has regained
some popularity recently for underbases, but my feeling
is that, with the correct set up, it really isn't necessary.
(Disagreements by email, please!)
Hopefully, these days there's nobody that never
change their blades; however, the question of how frequently
to change your blades still crops up regularly.
There are a few factors that will dictate a blade's useful
life…
1) Squeegee pressure Good process control will mean you always can use the minimum squeegee pressure, which will help increase blade life.
2) The amount of time the squeegee blade spends in solvent. Blades should never be left to soak in solvents; they should be cleaned quickly with a solvent rag and then polished with a dry rag.
3) The mesh count the squeegee is used on The higher the mesh count, the longer the blade will maintain a usable edge. Good process control will allow you to move on to higher mesh counts.
4) The quality of the squeegee blade material As for blade material, you 'pays yer money'...
Correct labelling and a timetable for blade changing will eliminate the risk of printing with blunt
blades.
We change our blades after about 6000 impressions, which is well
inside each blade's useable life. Bear in mind, a blade has four useable
sides, so we turn each blade four times before we're done with it. Incidentally,
we keep track of where we are by sticking masking tape along the
side of the squeegee holder currently in use and mark it "This side in use,
side 1 (2,3,4)".
Ensure new blades are seated level in the squeegee holder, and securely
tightened up.
When replacing a blade, make sure the channel in the squeegee holder
is clean and free of any dried ink or debris. Then, seat the blade firmly
and, pressing down on a glass surface, tighten the bolts working from the
centre to the outside; firstly 'snug' all the bolts, then torque them – the
same way you might replace a head gasket. The point of this routine is to
ensure the blade is seated correctly and completely level. The final check
is to hold the squeegee up to the light and place a straight edge along it,
checking for full contact all the way along the length of the blade. There is
absolutely no point in having a calibrated and parallel press if your squeegee
blades aren't seated square!
Corners of the blade should be sanded with a small radius to avoid
creating a stressed point on the mesh and stencil
Finally the corners of the blade need to be given a radius using some
sandpaper, to avoid a pressure point on the stencil and mesh. It need only
be a small radius – say, 4 mm or so.
When setting the squeegee angle for T-shirt printing, try starting in the
18-20 degree area, then try a few different angles to see which gives you
the optimum deposit in all situations.
At this point, I feel I should apologise to all you manual printers - I
haven't forgotten you! But you're all thinking about the move to automatics
anyway, right?
Diagnostics
I'm sure that everyone is aware of the co-dependence of many factors in
the screen print process, and that all the variables in screen printing have
a close relationship with each other. A butterfly flaps its wings in Mexico
etcetera… With this in mind, it can be very hard to accurately and quickly
diagnose problems on press. Ever stood there with a specific problem on
press and fiddled with every single parameter before finding the culprit
was something completely different? With this in mind, I thought I'd finish off this month by isolating some of the common on-press problems and
defining possible solutions, as a kind of checklist for quick diagnosis…
Print isn't sharp, fine detail is missing and small dots or lines have 'gained' in size
Check that the blade is sharp and seated correctly, and check that the
squeegee pressure is set at the minimum required to produce a print.
Check exposure settings – if there are pinholes it's probably related to underexposure.
A quick check on press to see if your stencil is too thin and
causing problems is to run your finger across the image on the print side
of the screen (obviously with the screen unflooded!) You should be able
to feel the relief of the stencil on the mesh. If you can't feel this, it's either
underexposed or there's not enough depth of emulsion on the screen.
Check your coating technique and review if necessary.
Other causes include: the platen is out of parallel; ink has been adapted
and is too thin; or the mesh count is too low.
Shirt sticks to the screen on colours after flashing
This could be after-flash tack; try repositioning the screen so it isn't printing
in the station directly after the flash. Check that the colour before the
flash is getting fully flashed; if the colour before the flash is an excessively
heavy ink deposit, it can be very difficult to get adequate flashing
before printing top colours and then, because of excessive time under
the flash, the ink is so hot it remains tacky for some time, or worse still, it
cures ink in the screen and mesh on subsequent screens. You will need to
review your mesh count choice to stop this happening.
As a general rule of thumb, with a quartz flash set at 100 percent power,
you should be achieving a good flash in less than three seconds.
Colours on top of the underbase exhibit an orange-peel surface, or have pinpricks where the underbase shows through; increasing pressure results in loss of sharpness
This problem is down to your underbase. The underbase is rough, with
lots of peaks and valleys, which means your subsequent colours have a
tough job of covering this 'relief' completely or smoothly. You will need
to go over mesh tension, squeegee sharpness and stencil profile to begin
with, to start to see an improvement in your underbase. A 'quick fix'
that sometimes gets you out of a pinch is to take the white ink out of the
screen, replace it in the ink can and stir extremely well (use a drill with a
paint mixer on a slow setting). This will improve the flow of the white ink and allow you to reduce squeegee pressure. It may just be that the ink is too cold to flow during the print stroke. Another quick fix is to slow the
squeegee speed a bit.
Wet on wet prints exhibit excessive build up on the backs of screens, and butt-registered colours quickly blur into
each other.
Screen tension is the main culprit here. Higher tension means a thinner
deposit of ink and a much more dimensionally stable stencil – two prerequisites
for successful wet on wet printing. A sharp squeegee also helps in
depositing less ink and preventing blurring.
As you can see, squeegee condition and pressure are an intrinsic part of
our efficient printing model, and an area that can't be overlooked. These
days, it's a better bet to simply replace a blade with new and use all four
sides, rather than using squeegee grinders to redress the blade edge.
The price of blade has come down considerably in the last few years,
and grinding or cutting will never produce as even a blade edge as a new
blade. It also alters the stack height of the squeegee, which affects the
dynamic of your pressure/angle relationship and means that each time a
blade is sharpened, the barrel adjusters or pressure need to be adjusted.
Having reached this point, you should have developed a pretty good
system for producing consistent work fast. Next month, I'm going to look
at workflow and how to manage the extra capacity, and the labour savings
for ever-more profitable work.
IMAGES is published by: CN Publishing, 9a, Kings Road, Flitwick, Bedfordshire, MK45 1ED, England.
Tel: 01525 718890/717655
IMAGES website designed and hosted by Cyberscreen Internet Services
Will Pearson has been screen printing since working part time at school. He spent his first five years of employment working his way round various T-shirt and graphic screen printing shops, gaining experience, before starting Phantom Screenprint in 1995. The business quickly became recognised as one of the leading screen printers for independent music merchandise. Since then, Will has studied the screen process and commercial print management in great depth and currently provides a technical consultation service for other screen printers. He is currently writing a book entitled, Managing for Profit in Screen Process. You can email Will at phantomscreen@btconnect.com.
Most recent articles by Will Pearson:
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New bags and jackets from Regatta
Regatta’s schoolwear offering for 2010/11 goes from strength to
strength with the launch of a new range of school bags.
The new bags include the TRB035 Kids Gymsac, TRB036 Book Bag,
TRB039 School Backpack 20l, TRB008 Highschool Rucksack 10l,
TRB029 Scholar Backpack 20l, and TRB058 Sports Bag 30l. All feature
ample areas for branding and will be available in a range of colours
that co-ordinate with the Regatta schoolwear range.
The brand’s school jackets include the TRW432 Fresher – a lightweight
water-repellent and windproof fleece-lined jacket, in six school
inspired colours.
The TRA652 Kids Uproar softshell is Regatta’s first kid’s softshell for
the school uniform market, and provides a sporty, stylish alternative
to conventional school outerwear. Made from resilient Regatta Softshell
fabric with water-repellent finish, it features reflective trim and a
printed name label. "This jacket is ready for the rough and tumble of
the playground,” Regatta advises.
The TRW418 Kids Dover Jacket, with its snug fleece lining and Thermoguard
insulation, is the children’s version of the popular adults’
waterproof Dover style. Made from Hydrafort fabric, the jacket’s waterproof
protection is guaranteed, states Regatta.
A staple of the Regatta schoolwear range, the TRA900 Kids Term
Time waterproof reversible jacket is made from peached polyester
with a reversible fleece inner, making it a versatile choice for the winter
term. It can be worn as either a warm lined waterproof jacket, or
reversed and worn as a fleece with waterproof lining.
The Regatta TRF542 Kids Thor III Fleece is made from 250 series
anti-pill Symmetry fleece. It has two lower pockets, an adjustable
shockcord hem on sizes 7 years and upwards, and is available in five
traditional school colours. The Kids Thor III is, "A hardwearing and
smart addition to any school uniform,” according to Regatta.
The brand also offers the KW943 Kids Breathable Packaway II Jacket
and KW944 Overtrousers, and the W908 Kids Stormbreak Jacket and
W808 Overtrousers. "These continue to be great sellers in the schoolwear
market," says Regatta.
www.regattacorporatewear.com.
New bags and jackets from Regatta
Regatta’s schoolwear offering for 2010/11 goes from strength to
strength with the launch of a new range of school bags.
The new bags include the TRB035 Kids Gymsac, TRB036 Book Bag,
TRB039 School Backpack 20l, TRB008 Highschool Rucksack 10l,
TRB029 Scholar Backpack 20l, and TRB058 Sports Bag 30l. All feature
ample areas for branding and will be available in a range of colours
that co-ordinate with the Regatta schoolwear range.
The brand’s school jackets include the TRW432 Fresher – a lightweight
water-repellent and windproof fleece-lined jacket, in six school
inspired colours.
The TRA652 Kids Uproar softshell is Regatta’s first kid’s softshell for
the school uniform market, and provides a sporty, stylish alternative
to conventional school outerwear. Made from resilient Regatta Softshell
fabric with water-repellent finish, it features reflective trim and a
printed name label. "This jacket is ready for the rough and tumble of
the playground,” Regatta advises.
The TRW418 Kids Dover Jacket, with its snug fleece lining and Thermoguard
insulation, is the children’s version of the popular adults’
waterproof Dover style. Made from Hydrafort fabric, the jacket’s waterproof
protection is guaranteed, states Regatta.
A staple of the Regatta schoolwear range, the TRA900 Kids Term
Time waterproof reversible jacket is made from peached polyester
with a reversible fleece inner, making it a versatile choice for the winter
term. It can be worn as either a warm lined waterproof jacket, or
reversed and worn as a fleece with waterproof lining.
The Regatta TRF542 Kids Thor III Fleece is made from 250 series
anti-pill Symmetry fleece. It has two lower pockets, an adjustable
shockcord hem on sizes 7 years and upwards, and is available in five
traditional school colours. The Kids Thor III is, "A hardwearing and
smart addition to any school uniform,” according to Regatta.
The brand also offers the KW943 Kids Breathable Packaway II Jacket
and KW944 Overtrousers, and the W908 Kids Stormbreak Jacket and
W808 Overtrousers. "These continue to be great sellers in the schoolwear
market," says Regatta.
www.regattacorporatewear.com.
