Flash Curing Slows Production

flash

Flash curing slows me down.  Is there some way to eliminate flash
curing?

Almost all flash curing can be eliminated, and yes, you are right, flash curing
severely reduces productivity and profitability.

The typical screen printer says he or she can print 100-120 one color shirts
per hour when the ink color is dark and the shirt is light.  When printing
light colors on dark shirts, like white ink on a black shirt, the same screen
printer says they produce about 30 per hour.  That is a productivity difference
of 300-400%.  That's huge.

People printing low numbers like 30 per hour are printing, flash curing, and
printing.  In addition to low productivity, this process incurs other
risks.  A flash should be until the ink does not pick up on your finger,
but may be slightly tacky and certainly will be malleable.  That usually
takes 6-8 seconds, but most people leave the garment under the flash dryer
until the next garment has been printed and is ready to be flashed.  

Well that is going to be in excess of 30 seconds to load and print a shirt
one color.  On jobs where ink is being printed and flashed the first color
down is fully cured after the flashes total 60 seconds.  That is because
inks typically are fully cured after 60 seconds.   On multiple color
jobs the third and subsequent colors will not bond to colors that have been
fully cured.  When multiple layers of white ink are printed on a black
shirt, the top layers might also not bond.  The shirt will look great
leaving the screen printer's shop, but will also crack and fade from repeated
washings.

The purpose when printing on dark garments is to block
the color of the garment.  The ink film needs to lay on top of the shirt and be opaque to block the shirt
color.  How do we manage that?  The answer in one word is control.  Actually,
we need to control a number of printing variables.  If we can control
those variables, we can eliminate flash curing in most cases.

The first variable is screen tension.  The mesh needs to be very tight
and be just as tight at the end of the job, even if that is the 300th shirt
printed.  If mesh is not very tight, the best screen printer will drive
the ink into the garment and the color of the garment will show through the
ink film.  The problem will be greatest in the center of the screen where
the mesh deflects more than out by the sides of the frame where there is more
support.

The diameter of mesh threads varies with mesh count.  A 110 mesh might
be 80 microns in diameter and a 305 mesh might be 35 microns.  The greater
physical size of 80 microns allows stretching mesh to a higher tension than
35 microns.  Also, the larger physical size will hold tension better.
Regardless, all mesh will relax and lose tension. 

After the first tension the loss will probably be 25% of the tension.  Wait
two or more hours, and apply more tension to exceed the prior tension level,
and the tension will decline another 15%. Repeat the process and the next day
the tension will drop another 5% or more.  Each time we print we deflect
the mesh on each print stroke, and that causes mesh to lose tension.  You
might have noticed that new screens are not as tight at the end of a job as
they were at the beginning of the job.

If mesh has been re-tensioned after each job, and the total prints from the
various jobs exceeds 500, then we might get to work hardened status.  Mesh
is work hardened when the mesh is as tight at the end of the job as it was
at the beginning or within 1-2 newtons/cm.  Any frame, wood, aluminum
or steel, that does not provide for the mesh to be re-tensioned will not allow
white ink to be printed on black shirts without flash curing.  So the
first variable is retensionable frames and mesh that is work hardened.

The second variable is capillary film.  The film is laid against the
mesh forming a two-ply structure where the ink passes around the threads and
the ink film thickness is determined by the film thickness.  This is
fundamentally different from liquid emulsion which is in the mesh.  When
applying capillary film only water is used, and the film must not be squeegeed
into the mesh.  Multiple coatings of liquid emulsion will not produce
the uniform and precision coating thickness of capillary film.

Next, we need an opaque ink.  Inks are made with different percentages
of pigment.  A process ink, for example, has the lowest percentage.  That
ink is transparent.  You must print four color process on white garments,
because you can see the garment through the ink film, and the garment color
becomes one of the final colors.  An opaque ink has, by contrast, a high
pigment percentage so that the garment color is completely blocked.

Often opaque inks are stiff or like taffy, and therefore more difficult to
print.  The remedies are vigorous stirring, and blending with up to 20%
of a medium pigmented ink of the same color.  Do not add curable reducer.  Actually,
you can add 2-3 drops per screen to improve shear, but that is all.  Most
people add reducer like making the witch's brew.  They are converting
an ink to a soupy status.  Ink will then pass easily through the screen,
but also settle into the garment rather than stand up on the surface.  So
avoid curable reducer if at all possible, and add only 2-3 drops which must
be thoroughly mixed in to help the ink shear from the screen.

Of course, the squeegee must have a sharp edge to cut the ink film cleanly.  We
sharpen, or I should say, polish our blades before each and every print run.  The
barber polishes his razor on a leather strap before shaving each and every
customer, because a sharp blade works better.  Dull blades will not hold
the ink film thickness we need for opacity.

One of the most important variables is squeegee pressure.  When printing,
the blade must never bend.  All we want to do is to close the off contact
distance which will typically be one to three 32nds of an inch.  If the
blade bends, that means there is back pressure from the platen, and the ink
is being splattered rather than cut to the specific dimension of your image.  Look
at some CDs under magnification, and you will find some have splattered images.  That
spreads the ink to a larger dimension and the ink film thickness and opacity
are reduced.

100% cotton is easier to print than blends, because there is a tooth to the
surface that will anchor the ink film being transferred from the screen to
the shirt.  That anchor will hold the ink to the shirt when the tight
mesh snaps off the shirt as the squeegee passes over the image area.  The
capillary film is very smooth and slick and only touches the ink at the edge
of the image.  Similarly, the polyester mesh is also slick.  So
the cotton fibers will pull the ink out of the screen.  The challenge
is to pull the ink out of the screen without also driving the ink down into
the shirt.

A stiff blade like 75-95-75 is easier to control than a medium hardness like
70 durometer.  An aluminum handle is better than wood for this application,
because the handle compresses the blade the entire length of the blade rather
than at points where there are bolts in a wood handle.

All screen printers know that the angle of the squeegee blade to the mesh
affects the amount of ink deposited.  A lower, or more acute, angle will
deflect more ink down through the mesh and on to the garment.  For manual
printing, we find that the optimal angle is 40-45 degrees.  The angle
can be measured with a 25 cent plastic protractor from 8th grade math class.  Just
don't rotate your wrists when printing, and be very sensitive to locking  your
wrists at the optimal angle.

Speed of the print stroke is important also.  A faster stroke moves more
ink across the screen rather than depositing the ink on the garment.  So
to print white ink on black shirts without flash curing, the stroke speed should
be slower than normal.  A good way to produce a consistently slower speed
is to set the squeegee angle and pressure, and then step back holding the squeegee
rather than pulling the arms back.  The movement will be slower and more
consistent.

One of the simplest steps a screen printer can take is to set up using 1/8" plexiglas
on the platen. The screens should be checked for flatness before being coated
with stencil.  Plate glass like on the exposure unit is typically flat,
but should be checked with a metal straight edge to make sure there is no bow
to the glass.  The flat screen is put in the press and brought down on
top of the plexiglas sheet which is on the platen.  The color arm of the
press needs to be resting on the off-contact bolt. If a press has more than
one registration gate, then all off-contact bolts must be set at exactly the
same height.  

The platen should be checked for flatness also.  Just run a straight
edge over the platen front to back and side to side looking for space between
the straight edge and platen.  Excessive heat on platens from flash curing
will warp platens.  To print without flash curing, we must have flat platens.  Of
course the platen cannot move when printing, or the thickness of the ink film
would change.

When the flat screen comes down on the sheet of plexiglas and flat platen,
touch the mesh at the four corners of the platen.  Does the mesh give,
or is the mesh solidly on-contact?  Most likely, the first time a screen
printer applies this test to the press the revelation will be that the press
is out of adjustment.  Once apparently adjusted, remove the plexiglas
and bring the screen down to the press. The color arm of the press should rest
on the off-contact bolt before the screen touches the platen. Touch the mesh
again in the four corners.  Does the mesh appear to deflect the same amount
regardless of where you push down on the mesh?  Does the deflection appear
to be 1/8"?  If not, more work adjusting the press will be required.

The one problem with printing with retensionable frames is the mesh is so
tight that when you push down with the squeegee the mesh will make the aluminum
frame bow down, and the ink will not release properly from the screen.  So
take a box cutter, and cut a piece of cardboard maybe 3" x 3" and
use packing tape to hold the cardboard to the neck of the platen.  You
could, instead, tape the cardboard to the bottom of the screen.  The object
is to support the frame so only the mesh deflects. More ink will be released
by using the cardboard, and the image will be more opaque.

If a screen or platen is warped, the off-contact distance will vary across
the screen, and the amount of ink transferred to the shirt will vary.  We
want to control that transfer and pre-determine the exact amount of ink that
is deposited on the shirt.

So there are many variables to control the exact transfer of ink to lay on
top of a garment, and the requirement to flash and print again depends on how
well we can block the garment color in one pass of the squeegee.