SCREEN PRINTING

Screen-printing is the most flexible printing process. It can be used to print on a wide variety of substrates, including

paper,

paperboard,

plastics,

glass,

metals,

Fabrics

and many other materials including, nylon and cotton. Some common products from the screen-printing industry include posters, labels, decals, signage, and all types of textiles and electronic circuit boards. The advantage of screen-printing over other print processes is that the press can print on substrates of any shape, thickness and size.

An important characteristic of screen-printing is that a greater thickness of ink can be applied to the substrate as compared to the other printing techniques.

The major chemicals used in screen-printing include screen emulsions, inks, and solvents, surfactants, caustics and oxidizers used in screen reclamation.

Overview of the Screen Printing Process

Screen-printing consists of three elements

The screen which is the image carrier The squeegee Ink

The screen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Proper tension is essential for accurate color registration. The mesh is made of a porous fabric or stainless steel. A stencil is produced on the screen either manually or photo chemically. The stencil defines the image to be printed. In other printing technologies this is referred to as the image plate.

Screen printing ink is applied to the substrate by placing the screen over the material. Ink with a paint-like consistency is placed onto the top of the screen. Ink is then forced through the fine mesh openings using a squeegee that is drawn across the screen, applying pressure thereby forcing the ink through the open areas of the screen. Ink will

pass through only in areas where no stencil is applied, thus forming an image on the printing substrate. The diameter of the threads and the thread count of the mesh will determine how much ink is deposited onto the substrates. 

Many factors such as composition, size and form, angle, pressure, and speed of the blade (squeegee) determine the quality of the impression made by the squeegee. At one time most blades were made from rubber, which, however, had a tendency to warp and distort. While blades continue to be made from rubber such as neoprene, most of them are now made from polyurethane, which can produce as many as 25,000 impressions without significant degradation of the image.

If the item is to be printed on a manual or automatic screen press the printed product will be placed on a conveyor belt, which carries the item into the drying oven or through the UV curing system. Rotary screen presses feed the material through the drying or curing system automatically. Air-drying of certain inks, though rare in the industry, is still sometimes utilized.

The rate of screen-printing production was once dictated by the drying rate of the screen print inks. As a result of improvements and innovations in the printing technology, the production rate has greatly increased. Some specific innovations, which affected the production rate and have also increased screen press popularity include:

Development of automatic presses versus hand operated presses, which have comparatively slow production time.

Improved drying systems, which significantly improve production rate.

Development and improvement of U.V. curable ink technologies

Development of the rotary screen press, which allows continuous operation of the press. This is one of the recent technology developments.

Screen Preparation

Screen (or image transfer) preparation includes a number of steps. First the customer provides the screen printer with objects, photographs, text, ideas, or concepts of what they wish to have printed. The printer must then transfer a "picture" of the artwork to be printed into an image which can then be processed and eventually used to prepare the screen stencil.

Once the artwork is transferred to a positive image that will be chemically processed onto the screen fabric (applying the emulsion or stencil) and eventually mounted onto a screen frame that is then attached to the printing press and production begins.

FLAT BED SCREEN PRINTING:

A screen printing press comprising a main frame, a printing bed supported on the main frame, a rectangular printing frame having a rear end pivoted to said main frame for movement between a first printing position generally parallel to said printing table and a second flood position angled with respect to said printing table, said printing frame including elongated side members, a front member and a rear member, each of said side members defining an elongated track, a squeegee and flood bar carriage engaging said elongated tracks and movable along said side members, said carriage including a pair of spaced side plates, a pair of slides, each slide extending from one of said side plates and into a respective one of said tracks, a pair of pivot plates, each pivot plate pivoted to one of said side plates, a squeegee support member extending between said pivot plates and a flood bar support member extending between said pivot plates in spaced, parallel relationship, said squeegee and flood bar support members being engaged by said pivot plates so that pivoting of said pivot plates alternately raises and lowers said squeegee and flood bar support members.

pivot plate actuating means operatively connected to said main frame and engaging at least one of said pivot plates for automatically lowering said flood bar support member when said printing frame is raised to its flood position and for automatically lowering said squeegee support member when said printing frame is in its printing position, said pivot plate actuating means including an elongated, rigid actuating member pivoted to said printing frame and operatively connected to said main frame, said elongated actuating member extending along and parallel to one of said side members of said printing frame a pivot bracket supporting said actuating member, said pivot bracket being pivoted to said printing frame;

shifting means fixed to said pivot bracket and operatively engaging said main frame for pivoting said pivot bracket as said printing frame moves between said first and said second positions; and means operatively connecting said actuating member to one of said pivot plates.

Mechanization of hand screen process

Fabric glued to blanket

Screens rise and fall

Printing done while screen in down position

Rod or blade squeegee system

Up to four strokes possible

Slow process

All the screens for the design, one screen for each color are positioned accurately along the top of a long endless belt, known as a blanket. A machine intended to print traditional furnishing designs might have space for 15 or more screens. The width of the gap between the areas printed by any two adjacent screens must be a whole number of lengthways design repeats. This need not necessarily be the same as the lengthways screen repeat as there may be several design repeats per screen repeat; for example, where there are three design repeats per screen repeat, the gap between adjacent screens need only be onethird of a screen repeat.

The fabric is gummed to the blanket at the entry end and moves along with the blanket in an intermittent fashion, one screen-repeat distance at a time. All the colors in the design are printed simultaneously while the fabric is stationary; then the screens are lifted and the fabric and blanket move on. When the fabric approaches the turning point of the blanket, it is pulled off and passes into a dryer. The soiled blanket is washed and dried during its return passage on the underside of the machine.

Advantages

large repeats

Multiple strokes for pile fabrics

Disadvantages

Slow

No continuous patterns

PRINTING MACHINES

Machines used for printing the fabrics are explained in the forth coming section Fabric are printed conventionally using the table screen printing machines manually. More perfection and production can be achieved using as modern automatic flat bed screen printing machine.

Today’s most important development in printing machinery lies in rotary screen printing machines. Before we go into the details of the printing machinery, let us discuss about the preparation of the screens used for flat bed and rotary screen printing.

SCREEN PREPARATION

Flat Bed Screens

A piece of nylon bolting cloth or metal gauge (phosphobraze) is stretched and nailed to a strong wooden frame, strengthened by metal brackets at the corner. The frames are usually 26” x 55” (measured externally) and 23” x 52” (measured internally) for printing 45” wide cloth. When

metal frame is used for making the screen, the bolting cloth may be fixed to the frame by using a solution of polyvinyl acetate in a suitable solvent.

Photochemical method is the most widely used method for preparing screen. This is based on the principle that when a coating of solution ammonium dichromate – gelatine or ammonium dichromate – polyvinyl alcohol is dried and exposed to light, insolubilisation takes place. The photosensitive coating may be given to the bolting cloth, fixed to the screen frame, dried and exposed to light after keeping in contact with a positive film and after insolubilisation of exposed portion, the unexposed photosensitive coating may be given to the bolting cloth, fixed to the screen frame, dried and exposed to light after keeping in contact with a positive film and after insolubilisation of exposed portion, the unexposed photosensitive coating is washed out leading the blocking of the interstices of the cloth at the exposed portions and keeping them open (for forcing of the print paste later while printing) at the unexposed portion. The sensitizing solution may be prepared as follows:

Chrome – Gelatine Solution

Solution A

200 g pure gelatine

500 g boiling water

700 g

Solution B

70 g Ammonium dichromate

150 g boiling water

80 g liquor ammonia

300 g

Solution A and Solution B are mixed in dark room.

Chrome-polyvinyl alcohol solution

600 g polyvinyl alcohol (15% solution)

120 ml ammonium dichromate (33% solution)

240 ml cold water

1 litre with cold water

After applying the solution to the screen cloth, it is dried in the dark room at room temperature. The positive of the design is placed on a glass-topped table and the dried photosensitive screen is placed over it. The exposure is started from under the glass table using mercury vapor lamp or fluorescent tube lights with uniform intensity of light all over the screen. The screens are then washed in the dark, first with hot and then with cold water. The hardening of the insolubilised chrome-gelatine complex is done by placing the screen in a solution containing.

50 g chrome alum

50 g formaledehyde

25 g sodium dichromate

1 liter

For 5 minutes at room temperature, washed with cold water and dried. In the case of chrome-polyvinyl alcohol complex, the hardening is done in a solution containing,

50 ml acetaldehyde

50 ml Isobutyraldehyde

80 ml water

20 ml sulphuric acid (168 Tw)

1 litre with cold water

It is boiled at 15 to 25 degree C for 1 to 2 hours. It is then washed with cold water and dried.

TABLE SCREEN PRINTING

In Table Screen Printing the fabric is stuck to the printing table, which is covered with a resilient felt, wax cloth or rubber material. Each screen is placed on the fabric in turn, the paste applied to one end of the screen and the squeegee drawn by hand through the paste and across the screen, forcing it through the open mesh areas on to the fabric beneath, Guide rails along the edges of the table ensure each screen is applied in register. Although the highly skilled printer can produce good quality prints by a hand screen technique, the production rates are extremely slow.

FLAT BED SCREEN PRINTING MACHINE

The printing operation in a flat bed screen printing machine can be made fully automatic by standardizing the stages of preparation and producing the print including screen holding, addition of print paste, pressing the paste on to the cloth, lifting the screen and carrying the cloth forward to the next screen printing screen position. The automation of these stages makes the reproduction of printing results quite easy. The fabric is suitably fed to the machine in a crease free manner. The print pattern is registered on the fabric by pressing the printing paste through the specially engraved portions on the flat screens. There are as many number of screens as the number of colors in the print pattern. The fabric to be printed is conveyed, i.e. carried forward, with each color, register by register, while the flat engraved screens continuously rise and come down, at each repeat of the color pattern are printed at the same time, but on differed printing places of the cloth. The entire colored pattern will be printed only when the far end screen completes its printing operation.

Good quality nylon gauzes with superfine construction fulfill all the demands made on the screens by the automatic screen printing machines. These demands can be summarized as follows: Very thin application of the dyestuff paste on the screen; Optimum color permeability in relation to the fabric; Minimum number of squeegee strokes; Maximum mechanical resistance on the part of the screen gauze to the high squeegee strokes.

MACHINERY

Flat-Screen Printing

Flat-screen and rotary-screen printing are both characterized by the fact that the printing paste is transferred to the fabric through openings in specially designed screens. The openings on each screen correspond to a pattern and when the printing paste is forced through by means of a squeegee, the desired pattern is reproduced on the fabric. A separate screen is made for each colour in the pattern.

Screen printing with automatic squeegee system

Flat-screen printing machines can be

Manual

Semi-automatic Completely automatic.

One type of machine, which is still commonly found in printing houses, can be described as follows.

The fabric is first glued to a moving endless belt.

A stationary screen at the front of the machine is lowered onto the area that has to be printed and the printing paste is wiped with a squeegee.

Afterwards the belt, with the fabric glued on it, is advanced to the pattern-repeat point and the screen is lowered again.

The printed fabric moves forward step by step and passes through a dryer. The machine prints only one colour at a time.

When the first colour is printed on the whole length of the fabric, the dried fabric is ready for the second cycle and so on until the pattern is completed.

 

flat-screen printing machine

In other fully mechanized machines all the colours are printed at the same time. A number of stationary screens (from 8 to 12, but some machines are equipped with up to 24 different screens) are placed along the printing machine. The screens are simultaneously lifted, while the textile, which is glued to a moving endless rubber belt, is advanced to the pattern-repeat point. Then the screens are lowered again and the paste is squeezed through the screens onto the fabric. The printed material moves forward one frame at each application and as it leaves the last frame it is finally dried and it is ready for fixation.

Mechanized screen printing machine with stationary screens mounted in a frame

In both machines the continuous rubber belt, after pulling away the fabric, is moved downward in continuous mode over a guide roller and washed with water and rotating brushes to remove the printing paste residues and the glue, if necessary. After this, the belt is sent back to the gluing device.

In some cases the glue is applied in liquid form by a squeegee, while in other machines the belts are pre-coated with thermoplastic glues. In this case the textile is heated and then it is squeezed by a roller or simply pressed against the rubber-coated belt, causing the glue to soften and instantly adhere.

After printing, the screens and the application system are washed out. It is common practice to squeeze the color from the screens back into the printing paste mixing containers before washing them. Specially developed Screen printing machine for Flat printing application

Features

These flat screen printing machines are extensively used for printing on flat surfaces like PVC, Metal, Glass, Paper, Board and PCB. These are highly cost effective as these require less power. The salient features are as below: 

Pneumatically drive. Low noise

Printing table with X, Y and Rotation for accurate registration.

Printing table with powerful Vacuum to hold the substrate on to its position while printing.

Maximum Substrate height 40 mm.

Squeegee /coater pressure with fine adjustment.

Angle can be set as per requirement.

Applications

Letterhead, wedding cards printing

Stickers printing

PCB printing

Metal sheet printing

Glass printing 

Scale printing

Printing on Plastic plates

T.V Cabinet

Washing machine panels

Industrial control Panels

Membrane switches

Battery containers

Umbrella panels

T-shirts

Coaster printing

Container lids

Tube light fittings

Speedometer dials

Car stickers