Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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182
GOOSSENS
3.4
Foam Coatings
Only three type of screen are applicable when coatings are applied by means of thermally
unstable foam (foam collapses in driedstenter) or stable foam.
1.
2.
3.
40 mesh
HX
[50% open area, 300 Fm thick, 91.4 cm (35.8 in.) repeat] for foam
coatings having a
dry
weight of
20-60
g/m2.
14 mesh
HX
[40% open area, 450 km thick, 91.4 cm (35.9 in.) repeat] for foam
coatings having a dry weight of 50-100 g/m2.
25 mesh
HX
(30% open area, 350 km thick, 91.4 cm (35.9 in.) repeat] for
average foam coatings.
Stable foam
is
applied by means of a squeegee blade 40 mm wide and
0.2
mm
thick.
End uses include black-out curtains or roller blinds, breathable coatings for anoraks
and rainwear, and flock adhesive on curtains and shower curtains.
Thermally unstable foam coatings are applied directly by means
of
the “closed”
system (see Fig. 2), via the foam processor and the closed squeegee through the screen
on the fabric. The wet application weight can be adjusted on the foam processor.
For example, to apply
30
g/m2
dry
of an acrylic
or
polyurethane binder (50% solids)
simply adjust the wet application weight on the foam processor at
60
g/m2.
End uses include velour backcoatings, automotive backcoatings, mattress ticking,
fixation of pile on pile fabrics, flame retardant and cigarette-proof coatings, imitation
suede, and antislip coatings.
4.0
ADVANTAGES
1. Since substrate, screen, and counterpressure roller all have the same speed,
coating is done without tension and friction. Thus, virtually all substrates can
Figure
3
Adaptation of screen coater for knife coating.
ROTARY SCREEN COATING
183
Figure
4
Scrccn
coating
line.
5.
6.
7.
8.
be processed on this system, including knitted fabrics, velours, nonwovens. and
shift-sensitive materials such as skiwear. mattress ticking, and Lycra fabrics.
The user has penetration control: penetration into the substrate can be completely
avoided or, if desired, controlled.
Thanks
to
the low system content, the coating method is clean, and fast changes
are possible.
Coatings are exactly reproducible. Since parameters, squeegee pressure, squee-
gee setting, mesh number, and viscosity can be measured and read off, any
given coating can easily be repeated.
Chemical savings (up to
20%
of the coating weight) are realized in two ways:
(a)
through accurately controllable application and because the screen follows
the web structure exactly (thus the textile character is maintained and an excess
of
paste, such as occurs with knife coating, is avoided) and (b) through great
accuracy, in lefdright and longitudinal directions, of the application amount.
Application is both tensionless and frictionless.
By means
of
the closed system, the user has total process control.
The knife coating option can be attached above the whisper blade roller, men-
tioned earlier.
This knife coating (see Fig.
3)
can be used
as
a knife-on-air system for paste or unstable
foam coatings and
in
the knife-over-roll coater made for foam applications. In both cases
the apparatus is fitted with a paste or foam distribution system over the
full
working width.
In this way
it
is possible
to
apply colored coatings with
a
totally even appearance.
The schematic diagram of a rotary screen coating line is shown in Figure
4.
This Page Intentionally Left Blank
20
Screen Printing
1
.O
INTRODUCTION
The Screen printing process is markedly different from most imaging processes generally
associated with the graphic arts. First, the printing plate is actually porous, formed by a
woven mesh of synthetic fabric threads or metal wire (or in at least one case. by a non-
woven, electroformed metal matrix), which is then combined with a selective masking
material, commonly called a stencil. Because the coating material flows under pressure
into and through this mesh or matrix before being deposited onto a substrate, the resulting
coating has a thickness far greater than that of a material printed onto the substrate by
offset lithography. gravure, tlexography, xerography, or inkjet printing.
For this and other reasons. the screen printing process has many practical applications
in industrial manufacturing areas in which other printing media have few or none.
The basic process steps are as follows (see also Fig.
1).
The woven mesh (or matrix)
is affixed
to
a rigid framework of aluminum or steel. In most applications, this framework
forms
a
rectangular plane. However, variations are possible, including the cylindrical
screen, which is affixed and sealed at both ends. In the case
of
mesh, whether of synthetic
polyester monofilaments or stainless steel wire, tension is applied simultaneously in oppos-
ing directions to obtain a semirigid planar surface. This stretched printing screen then
performs three distinct functions: (a) to meter the fluid coating (or ink) that flows through
it under pressure (b)
to
provide a surface
for
shearing the viscous columns of coating
material. which form during transfer
to
the substrate and (c)
to
provide support for the
imaging elements (the stencil).
Ink or coating transfer is initiated by the imposition of pressure on the screen by
means
of
a flexible plastic blade, the squeegee. Because of the flexibility
of
the blade
material and its physical profile.
a
hydraulic action is caused by force exerted in two
directions. The blade presses into the screen, and its inherent flexibility enables it to be
185
z
0
SUBSTRATE
Figure
1
Ink
transfer
by
screen
printing.
s
m
rn
SCREEN PRINTING
187
put into direct contact with the substrate. thus effecting ink transfer. The blade also sweeps
in a horizontal direction, thus applying the ink or coating
as
it moves, and causing the
columns of material to shear as the printing screen rebounds after the squeegee has passed.
It is the combination of the horizontal print stroke and the vertical (downward)
pressure, effected by the squeegee glade, that forces the ink
of
coating into the ink “wells”
created by the areas of intersection
of
woven mesh. (Similar wells, though shaped differ-
ently, are produced within the matrix
of
electroformed metal printing screens.) The elastic-
ity of the printing screen then allows temporary contact with the substrate, and subsequent
ink or coating transfer.
2.0
GEOMETRY
OF
THE PRINTING SCREEN
Due to a variety of materials and manufacturing methods, the geometry of the printing
screen can vary considerably. Of most common use in industrial applications is the woven
mesh, consisting of extruded polyester threads that have been woven into a precise and
regular pattern known
as
“plain weave.” For a given linear measurement, the number of
thread crossings or intersections. called the mesh count, may be expressed, as produced,
in either metric (European) or British (Japanese) increments. At 130 intersections (meshes)
per centimeter and higher, it is common to weave the fabric in a twill weave. where one
x-axis fiber crosses under and over every
other
y-axis fiber (instead of
every
y-axis fiber
in the plain weave). Weaving produces thousands of roughly cubical apertures formed by
fiber intersections at each four corners. It is the length, width, and depth of these apertures
that form the ink wells and exert the primary influence
on
amounts of coating deposit.
Thread diameter though consistent within each woven fabric, is a variable that allows
a
wide choice of fabric thicknesses and thus substantial control over applied coating thick-
nesses. Threads available range from over 700
km
down to just over 30 km
(0.0275-0.0012 in.) in diameter. The fabric thickness, and therefore the ink well depth.
is approximately
1.85
times thread diameter.
The woven mesh is tensioned to predetermined levels and affixed
to
a frame. For
synthetic fabrics. the effects
of
tensioning can also reduce aperture depth slightly by
reducing fabric thickness by up to 3%. For steel wire, however, there is no such reduction.
A
stencil material, when present blocks the aperture either partially or totally, thus
diminishing the amount
of
deposit, while adding depth to the walls of the aperture, thus
increasing coating deposit.
The total volume
of
ink or coating deposited is thereby determined by the fabric
thickness (the area of the screedstencil that encompasses the image) minus the volume
of
the threads (or wires) within the image area.
The coating flow through the mesh aperture is also affected by its viscosity and the
operation of the squeegee.
2.1
The Rotary Screen
In principle, the action of the cylindrical screen used in rotary screen printing is similar
to that of flat screens,
in
that the coating material flows through open areas perforating
a
thin printing “plate.” This differs, however, in the basic configuration, which is cylindrical
rather than flat. The ink or coating is pumped into the interior of the cylinder. which is
sealed at either end by printing heads. Also positioned within the cylinder is the squeegee.
188
MCSWEENEY
The screen itself is composed (in one variation) of a seamless electroformed nickel
mesh tube, which then receives an emulsion coating for producing the patterned stencil.
Other methods for producing rotary printing screens achieve similar results.
Rotary screen printing has the advantage of continuous operation, without the inter-
mittent sweep and return of the printing squeegee as
in
flat printing. Thus, the opportunity
to
increase speed of production is presented, particularly for substrates printed on a web.'
3.0
THE STENCIL
The masking of open mesh, known generically
as
the stencil, serves to control the pattern
or shape
of
coating deposit (and
to
a lesser degree, the thickness of deposit). A variety
of masking materials and applications are available. from CAD-generated cut films to
photosensitive polymers and emulsions. Compatibility between the masking material and
the ink or coating chemistry is necessary to preserve the stencil during printing.
Film stencils are adhered to the substrate side of the printing screen, while liquid
emulsions are coated onto both sides. Positive-image photo
film
masters are then positioned
on
the screen prior
to
exposure of photosensitized masks. Exposed areas harden to form
the mask, while unexposed portions are washed away in the development procedure.
Where the stencil blocks a mesh aperture, the ink or coating material is prevented
from tlow-through
to
the substrate, whereas partial blocking only restricts deposition. In
this way, imaging is achieved.
4.0
DYNAMICS
OF
THE SQUEEGEE
The printing tool in screen printing is the squeegee. an instrument bearing some resem-
blance to the doctor blade used in gravure. However, the squeegee blade is much more
flexible, consisting
in
most cases
of
a
high density polyurethane elastomer, chosen for
characteristic durability and solvent resistance. Other elastomers and rubber can
also
be
used, depending on the application.
In flat screen printing, the squeegee makes intermittent and repetitive sweeps across
the printing screen, thus forcing the fluid coating into the empty apertures of the mesh,
and then into contact with the substrate (with the aid of the flexing
of
the screen). This
action necessarily involves the application
of
force. applied in vertical (typically down-
ward) and horizontal directions simultaneously; thus, the friction involved calls for wear
resistance on the part
of
the blade.
Since the blade is made of a flexible elastomer,
it
also flexes. which produces an
angle of attack with the mesh, typically about
80"
from horizontal.
The properly tensioned screen itself has
a
force of several pounds per inch, more
commonly expressed in newtons per centimeter, which resists the force of the squeegee.
The ink or coating has a lesser force, dependent
on
its viscosity, which contributes to the
hydraulic activity. Immediately following passage of the squeegee
in
the horizontal direc-
tion. the screen begins to separate from the substrate. and the coating shears away from
the screen flowing into deposition on the substrate.
An additional tool, known as the tloodbar, is a common component of automated
flatbed screen printing presses. This thin, wide. and relatively flat metal bar passes in
contact across the printing screen between squeegee print strokes, filling the open mesh
with coating material. The flood stroke does not, however, exert sufficient pressure to
cause the screen to flex
into
contact with the substrate. Therefore, no coating transfer out
SCREEN
PRINTING
189
of the Screen will take place. The flood stroke helps to ensure unifomlity of the printing
or coating application.
5.0
COATING TRANSFER
As
a
result of the application of the squeegee force, the mesh forms an area
of
contact
with the substrate, and part of the ink or coating that
fills
the mesh aperture now adheres
to the substrate. Cohesive forces within the viscous fluid (now in “columns”) tend to
hold the coating material together while it flows. As the squeegee passes, the flexing mesh
separates from the substrate, and shearing forces cause the fluid to separate at the mesh.
Even though the coating adheres
to
the aperture surfaces
of
the mesh, only a relatively
small layer of material actually remains within the mesh, as the coating separates from
itself in obedience to shearing forces thus the relatively thick deposit
of
coating material
achieved by the screen printing process.
For proper ink or coating release from the screen, the rate
of
ink shearing must equal
the speed
of
the printing squeegee and the rate of ink release from the mesh. Following
transfer
to
the substrate, the ink or coating that has been released has
a
further tendency
to
flow, from
a
series of “column-shaped” deposits into
a
more uniform layer. This
spreading phenomenon is largely dependent
on
the viscosity of the material that has been
printed, the amounts
of
coating material deposited, and the time that elapses until the
coating begins to dry.’
6.0
CONVERTING THE APPLIED COATING
Since the majority
of
inks and coatings applied by the screen printing process are liquid
or at least in
a
semifluid state, complete conversion of the end product requires a drying
or curing process. Among the methods used
to
achieve
a
solid film state are (from the
slowest to the most rapid) evaporation, oxidation, catalytic curing, infrared radiation, ultra-
violet radiation, and electron beam radiation. Each of these processes is most appropriate
to specific coatings chemistries. In web systems and nonweb conveyorized systems, the
speed of production depends not only on the ink or coating chemistry, but
also
on the
stability
of
the substrate when under temperature (and other) conditions imposed by the
conversion process,
as
well
as
the thickness of the applied coating film. Factors such
as
surface reflectivity, ambient humidity, and head absorption can
also
come into play.
7.0
CONCLUSION
The screen printing process is
a
unique method used for the application
of
inks or liquid
coatings to
a
wide variety of substrate types, shapes, materials and surfaces. It is easily
adapted to the problem at hand, capable
of
depositing relatively thick wet films in
a
short
time in either
a
repetitive or continuous fashion. The process can be integrated with other
production processes, whether specifically related to graphic arts or otherwise. Applica-
tions may be practical (e.g., conductive printed circuitry) or decorative, pigmented or
transparent, finely detailed or broad in coverage. A comprehensive industry support system
draws from the fields of photography, chemistry, industrial engineering, and manufacturing
190
MCSWEENEY
technology to provide
a
vast
array
of
process variations and potential uses for this most
flexible
of
printing techniques.
REFERENCES
1,
M.
Taylor, “Automatic printing
of
textile yard
goods,”
Technicctl
Guidrhook
o/‘
tlw
Screen
Printing
Itldustn
N4.
Screen Printing Assoclation International, Fairfax, Virginia,
198
1.
2.
E.
Messerschmitt,
Screen
Prim.
73
IO)
(1982).
Flexography
1
.O
INTRODUCTION
Throughout the printing industry, flexography. or flexo, has established its reputation as
a
quality printing process bearing comparison with letterpress, gravure. and offset. which
have been used industrially for many years. Today, the whole packaging sector and other
areas of the printing industry would be unthinkable without this highly economical quality
printing process. This is attributable primarily to the high flexibility flexo offers. its qualifi-
cation for
a
wide range
of
materials, the large and variable range
of
print repeat lengths,
the different press widths available, and the quite extraordinarily high production speeds.
Other advantages include the highly diversified flexo press specifications and the possibil-
ity
of
using flexo
in
line with other printing techniques and processing operations.
Finally, the developments and improvements achieved in the field of press engineer-
ing, flexo printing plates, and flexo inks have recently contributed quite decisively
to
the
position the process holds today.
1
.l
Historical Development
of
the Aniline and
or
Flexographic
Printing Process
Today’s flexo process is far more than 100 years old. According to historians, extremely
primitive aniline work was produced in the United States
as
far back as 1860.
The original name
of
this letterpress process, “aniline printing.” is traceable
to
the
aniline dyes used in the midnineteenth century. which were diluted
in
alcohol and had
been used
in
printing for many years. This rubber printing process-until 1970, rubber
printing plates were used exclusively-was initially employed for the printing of wrapping
papers. The first aniline printing apparatuses are said to have been used in England and
Germany beginning
in
1890. From about 1910. some European machine manufacturers
started to supply aniline printers in combination with paper bag machines
to
permit printed
191