Printing on Black without Flashing
Printing Flash
You want to increase your income and reduce your labor required by screen printing. One
way to achieve these objectives is to eliminate flash curing when printing
white or light colors on dark garments like black.
If white ink is printed on a black shirt, flash cured, and then printed again before
the shirt is removed from the press, a typical production rate is 30-40 shirts
per hour. There are screen printers who say they do more or less per
hour, but 30-40 is the number quoted by most.
When the same screen printers print a dark color ink on a light colored shirt, like
blue ink on a white shirt, they typically quote a production rate of 100-120
shirts per hour. Again, there will be some who say they do more or less
per hour. So printing without flashing results in production at a rate
300-400% higher than printing with a flash.
These higher production rates can be achieved with white ink on black shirts by following
these procedures.
- Use a very tight 110 mesh on a retensionable frame.
- Use 50 micron capillary film.
- Use an opaque white ink that shears easily.
110 Mesh on a Retensionable Frame Soft mesh results in ink being
driven down into the shirt, soaking the shirt with ink, and allowing the color
of the shirt to show through the ink film. The dull print then needs
to be flashed and printed again to achieve a bright, white image when white
ink is printed on black shirts, for example.
Mesh stretched to its ultimate breaking strength relaxes 20-25% within a few
hours. The loss of tension occurs more quickly with high mesh counts
like 230 and 305 than more coarse mesh counts. With more time,
the mesh will relax even more. The slack should be taken out of the
mesh bringing the tension back to its former level, plus 2-4 more N/cm2. Then
the mesh will relax again, but only 15%. The mesh should be retensioned
again to recover the lost 15%, and the tension should be ratcheted another
2 N/cm2 higher. By the next day, the mesh will only lose about 5% of
its tension. The screen printer with a tension meter will witness this
experience.
Mesh deflects more in the center of the screen than near the frame. Very
tight mesh is harder to deflect, holds registration better, and contributes
to the ink being cut, or sheared, more than mesh that moves laterally with
the force of the squeegee. Tight mesh deposits a more consistent thickness
of ink.
When asked, all screen printers respond saying they prefer to print with tight
screens rather than loose screens. The tightest screens are the best,
and optimal tightness comes from retensioning.
Capillary Film With liquid emulsion, the mesh count is
used to meter the amount of ink that passes through the image area of the screen
onto the garment. That function is fundamentally changed with capillary
film.
Capillary film is purchased as a liquid emulsion that has been coated to a
polyester sheet at a precise thickness from 10 to 700 microns thick. That
film can be adhered to mesh with plain water, and after drying, the polyester
sheet is peeled off and discarded.
Now ink passes around the threads of the mesh and into the image area like
a mold without the influence of the mesh on the edges of the image. More
importantly, the thickness of the ink deposit is determined by the thickness
of stencil. Thickness is a determinant of opacity as long as the mesh
is very tight so that the ink is on the surface of the garment rather than
driven into the garment.
Thicker stencils result in heavier ink deposits and more “hand” or
feel to the print. Most consumers want to see the image, but not feel
a lot of image. So the objective is to print the maximum thickness of
ink that creates the acceptable level of opacity or brightness to the image. If
you have never used capillary film, most manufacturers and distributors will
provide a free sample with instructions, or you can contact R Jennings Manufacturing
for a free sample with instructions.
When the optimal printing technique (squeegee angle, pressure, speed) are
used with a tight screen, capillary film properly applied, and an opaque ink
that shears easily, that capillary film thickness is 50. If a person
is new to screen printing and has not developed their technique, then 70-80
microns might be more appropriate. A thicker stencil like 70-80 can be
created by coating the screen with liquid emulsion first before laying the
capillary film on the mesh. The liquid emulsion fills the mesh and prevents
the film from being drawn into the mesh. Similarly, capillary film should
never be squeegeed into the mesh.
Opaque White Ink that Shears Easily All inks should be
thoroughly stirred in the manufacturer’s container. An electric
drill with a bent rod or Allen wrench in lieu of a drill bit will do the job
after about 3 minutes in a quart and 5 minutes in a gallon. Then pull
the rod straight out of the ink. Observe. Does the ink drop off
the rod immediately like pudding from a spoon, or stay attached when the rod
is 6” above the container of ink?
Ink should drop off the rod like pudding for the ink to shear easily from
the mesh when printing. All experienced printers print off-contact, i.e.
with the mesh 1/32 – 1/16” above the T-shirt, and more for compressible
materials like sweatshirts. When pressure is applied to the squeegee,
the mesh is deflected so that the mesh makes momentary contact with the shirt. As
the squeegee passes over the image, the squeegee leaving a portion of the image
area releases the tight mesh which springs back (or peels off) to its original
position.
If ink is adhesive, it will draw out and continue to connect the mesh and
ink on the garment rather than releasing from the mesh. By contrast,
ink that shears properly will appear to drop out of the image and be entirely
on the garment. There will be no ink in the image area of the screen
or, when using very tight screens, built up around the edges of the image.
If ink does not shear properly, the choices are to change to a brand that
does shear like pudding or modify the ink to shear as described. Ink
can be modified by mixing with the same color ink that shears more easily. Another
option is to add curable reducer. There are risks with curable reducer.
The risks are: (1) Too much curable reducer can result in the ink never curing
properly regardless of curing temperature or time. The ink will wash
off the shirt. (2) Shirts with man-made fibers like polyester, e.g. 50-50’s,
will show dye migration. White ink on maroon shirts will turn pink. (3)
Ink will absorb into the garment rather than stand up on the surface. Ink
that absorbs into the shirt will need to be flashed and printed a second time.
Curable reducer should never be freehand poured into ink. It must be
measured in with a scale. The appropriate amount will be some increment
of 2% with the maximum of 10% by weight. After each 2% is added, the
ink should be whipped again with the electric drill, and then the rod pulled
from the ink to see if the ink drops off like pudding.
Conclusion The mechanics of 110 mesh on a retensionable
frame, 50 micron capillary film and opaque ink that shears easily are easy
to achieve. This combination duplicates an ink film thickness equal to
printing, flashing and printing, but without flashing.
The challenge for most people will be finding the optimal printing technique. For
many, that means slowing the print stroke, using less squeegee pressure, and
maintaining a constant squeegee angle of 45 degrees. Until that technique
is perfected, pulling the squeegee twice without lifting the screen or flashing
curing is a much faster way to print than using the old print-flash-print method.
- How to: