PRINCIPLES OF SCREEN PRINTING PROCESS

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PRINCIPLES OF SCREEN PRINTING PROCESS

23 September 2018


Screen Printing : In this type of printing, the image and non-image areas are carried
on a mesh (woven) screen, the image areas being open or ‘unblocked’ in the form of a
stencil. The non-image areas are formed by ‘blocking out’ the mesh by coating
The paper is placed under the screen. After the screen is lowered into contact with the
paper, ink is passed across the upper surface of the screen. Where the screen is open, ink
goes through to the paper beneath.
Screen printing is an example of the stencil printing process.

Screen printing is a process in which ink is forced through a screen. The screen
printing stencil serves as a printing plate. The screen is a fine fabric made of natural silk,
plastic, or metal fibers/threads. Plastic or metal fabric is generally used nowadays. Ink is
imprinted/transferred through the image-specific, open mesh that is not covered by the
stencil. The screen printing plate is therefore a combination of screen and stencil.
It is the material, the fineness of the screen (the number of screen threads per
centimeter of fabric length), the thickness of the screen, the distance between the top and
bottom sides of the screen, and the degree of opening of the screen (the degree of screen
opening areas as a percentage describes the ratio of the total of all mesh openings to the
entire surface of the fabric) that determine the printing properties and quality of the fabric
(screen). Fabrics can be obtained in levels of fineness from 10 to 200 fibers/cm. The most
frequently used fabrics are those between 90 and 120 fibers/cm.
The screen work and printing of very detailed illustrations necessitate the use of very
high levels of fabric fineness that are matched to the resolution requirements of print image
reproduction. For screen work, fabric fineness (threads/cm) should be around three to four
times greater than the screening of the print image (lines/cm) – therefore nine to sixteen
different screen dot area surfaces per screen cell.
The stencil on the fabric defines the actual print image. The stencil is on the side of
the screen opposite the side on which the squeegee (blade) works, to avoid damage and
wear to the stencil. Manual stencils, which can be produced as drawn or cut stencils and
transferred to the underside of the screen, are used for simple solid-area print work.

SCREEN PRINTING PROCESS
Screen printing (formerly called silk-screen printing) is a stencil process whereby ink
is transferred to the substrate through a stencil supported by a fine fabric mesh of silk,
synthetic fibres or metal threads stretched tightly on a frame. The pores of the mesh are
‘blocked-up’ in the non-image areas and left open in the image area. This image carrier is
called the screen.
During printing the frame is supplied with ink which is flooded over the screen. A
squeegee is then drawn across it, forcing the ink through the open pores of the screen. At
the same time the substrate is held in contact with the screen and the ink is transferred to it.
The principle is shown in Fig.
Because of their simplicity, screens can be produced cheaply and this makes it an
attractive process for short-run work. Furthermore, since the image is produced through a
screen rather than from a surface the impression pressure is very low. This makes it ideal for
printing on fragile boxes or awkward shapes.

Irrespective of the type of machine the printing procedure is generally the same. A
working supply of ink is placed at one end of the screen and the screen is then raised so that
the stock may be fed to register guides or grippers on a base. The screen is then lowered
and a rubber or plastic squeegee drawn across the stencil to produce the print. Ink
replenishment is undertaken as necessary.
On most flat-bed machines the base to which the substrate is applied is of a vacuum
type. This prevents the stock sticking to the screen and being lifted by tacky inks. To a
certain extent the thickness of the ink film printed can be controlled by the pressure,
sharpness and angle of the squeegee blade.
The more upright the blade the thinner the deposit of ink. Thus, in general, fine work
requires a more upright blade. However, the type of ink, stock and machine govern the blade
setting also.

Advantages of Screen Printing Process
One of the major advantages of the screen process is the ability to obtain prints on
non-flat objects. For example, printing on bottles or other cylindrical objects is achieved by
using a press of the cylinder type described above but the object to be printed is placed in
the machine where the impression cylinder is shown. After each impression the bottle is
removed and another unprinted one substituted. There are few limitations on size or shape.
Special screens and jigs are produced for printing on shaped objects such as cups with
handles or tapering cylinders, and screens with high elasticity combined with shaped
squeegees are used for conforming to irregular objects. Print heads can also be bolted to
automatic production lines, so that printing becomes a part of the total production process of
such objects as filled polythene bottles.

APPLICATIONS OF SCREEN PRINTING

i. Screen Printing on Flat Surfaces
Posters and Graphics Printing in Short Print Runs.
Large-format posters in particular can be produced relatively conveniently in fairly
small print runs. The quite thick ink film produces coloring that is very brilliant and resistant
even with halftone color impressions.
Traffic Routing Systems and Signs. Large printing surfaces for high resistance
inks are found with traffic signs and routing systems. The requirements they impose are best
met using screen printing.
Vehicle Fittings and Instrument Dials. With vehicle fittings a narrow tolerance
range of the translucency of the impression is required in addition to its precision. For
example, it must be possible for control lights to light up in precisely defined colors.
Printed Circuit Boards for Electronics. Due to its simplicity and flexibility, screen
printing is an important process during the development of printed circuit boards for
electronic circuits. Accurate printing onto copper-laminated hard paper or glass-fiber
reinforced epoxy board with etching allowance, solder resist, or assembly designations in the
necessary coating thickness is only possible in large quantities with screen printing.
Restrictions are, however, imposed on the latter as a result of the extreme miniaturization of
components and printed circuit boards.
Photovoltaic. Special conductive pastes are used to print on photo resistors and
solar cells, which serve as the contact points for current transfer. In doing so, particular
importance is placed on high coating thickness in areas that are, at the same time, extremely
small and covered with printed conductors, in order to optimize the efficiency of the energy
production with the solar cells as fully as possible.

Compact Discs (CD). Screen printing is one of the major processes for printing on
CDs. Pad printing and more recently even offset printing are also used.
Textiles. The depth of the ink absorption in textiles calls for a large volume of ink to
be supplied and screen printing is the preferable process for applying it. Clothing, canvas
shopping bags, webs of material, and so on, can be printed in both flatbed and rotary
screen printing.
Transfer Images. Screen printing is frequently used to produce transfer images for
ceramic decoration. These images are put together from ceramic pigments for firing. The
pigment’s grain size necessitates the use of a screen mesh that is not too fine. After
detachment the images are removed from the base material and placed on the preburned
bodies by hand. A recognizable feature of these ceramic products is the thick layer of ink.
The images can be placed above or below the glazing.
Decorative Products, Labels, Wallpapers. Seamless decorations such as textile
webs, wallpaper, and other decorative products, as well as labels often require rotary
printing combined with reel material. Special machines are designed for this. Rotary screen
printing with sheet material is used primarily for higher print runs.
Surface Finishing. Transparent varnish can also be applied using screen printing
technology (for spot varnishing, in particular) to finish the printed product as add on value to
attract the customers.

Screen Printing on Curved Surfaces
Almost any body that has an even, convex and concave (to a limited extent) not too
structured surface can be printed using screen printing. There are virtually no restrictions
with regard to the material of the body to be printed on. Ceramics can be printed directly
with screen printing. Ceramic pigment inks can be used for subsequent baking or just a low
durability varnish applied to the glazed product. It is not always possible to print directly onto
plastic components. Surface treatment, for example involving flame treatment, corona
charging, or the application of primer is often necessary to ensure that the ink adheres.
Bottles. Glass bottles with a baked finish or pretreated plastic bottles for the food
and domestic products sector are printed using the screen printing process.
Toys.Toys, such as balls, and so forth, can be printed in full in several operational
steps.
Glasses. The screen printing process is often used for drinking glass decoration,
with thick coatings of all inks and also gold being applied.
Advertising Media. The type of advertising medium that can be decorated or
provided with some other overprinting by the screen printing process ranges from cigarette
lighters or ballpoint pens to pocket knives and pocket calculators.
Following are the parts of a screen printing press of hand operated one:
1. Frame
2. Base
3. Screen fabric
4. squeegee

(1) Frame:
The frame serves as a support for the screen fabric. It can be made from wood,
metal or any other rigid material.
a) Wooden frame:
Wood used for screen printing should be soft, straight, grained and should resist the
moisture and temperature. Wooden frame are easy to handle and assemble. The cost of the
wooden frame is less than metal frame. Leveling is also important for wooden frames.
Coating the wooden frame by a two-component lacquer protects the wood from water and
solvent. Pine or popular wood is usually used for making frames. Before making a frame,
wood is seasoned. The corners of the frame is joined by miter, end lap, or spline joints.
Angle and corner irons are sometimes used to reinforce the corners of a large screen
printing frame.
(b) Metal frames:
Steel is used for screen frames as its rigidity, life is more when comparing the
wooden frames. For corrosion protection, steel frames are galvanized or coated with
lacquer, sometime with stored varnish. These steel frames are available as rectangular or
square section. For easier handling of large frames, steel is replaced by aluminium alloy, but
care must be taken in providing rigidity. Also aluminium frames are corrosion – proof when
comparing steel frames. Leveling of metal frame is very important. This leveling is done on a
special leveling slab. Before mounting the fabric, sharp edges and pointed corners should be
well rounded to avoid the tearing of fabric.
(2) Base :
This is the surface upon which the substrate to be printed is positioned and held. It is
usually made from a thin sheet of plywood or hardboard or table. This is longer than the
frame used. Loose-pin built hinges serve to hold the frame and base together