K Mean Black

• Name: Printing Technology
• Branch: Printing Technology Diploma 6th Sem
• Published: May 23, 2025

Screen Printing

 

Unit – 5

Screen Printing

PRINCIPLES OF SCREEN PRINTING PROCESS:

 

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 (see Figure 4). 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.

Figure 4: Stencil printing

 

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.

 

 

ii. Screen Printing on Curved Surfaces

Almost anybody 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.

 

MAIN SECTIONS OF A FLATBED SCREEN PRINTING MACHINE

 

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.

 

(3) Screen Fabric:

The screen fabric is a woven material. It is a tightly stretched across the frame. This Screen fabric serves as a carrier for stencil. The selection of fabric for particular work plays a major role. Following are the types of fabrics.

 

a) Silk:

Silk is a natural fiber produced by the silk worm. Hand cut and indirect stencils adhere well to silk fabrics. However this silk is not dimensionally stable. Size variation can occur due to change in temperature and humidity. Therefore silk is unsuitable for jobs requiring critical registration.

 

(b) Polyesters:

Polyesters such as darcon, Terital and polylast are man-made synthetic materials containing cellulose, resins and hydrocarbons. Polyesters fabrics are woven very uniformly and possess good dimension stability. They are extremely strong and used for long runs. A major disadvantage is that indirect photographic stencil will not adhere so good as like in silk.

 

(c) Nylon:

Nylon is also a man-made synthetic material having uniformly woven fabrics. This fabric is strong and durable and can be used for long run jobs. Unlike polyesters, nylon fabrics lack dimensional stability. Nylon fabrics will go on stretched and react to temperature & humidity changes. So before mounting a nylon fabric on frames, it should be wet firstly and stretched very taut, to maintain the good registration.

 

d) Metal fabrics:

These types of fabrics are used for only special application. Unlike the synthetic fabric, it does not absorb moisture and is therefore unaffected by changes in humidity. Also it is unaffected by temperature. As it has very good dimensional stability it is used for very precision printing like printed circuit board or very specialized application. Usually “Stainless Steel wire” is used as a metal fabric.

 

Stainless steel will retain its tension almost indefinitely, whereas all synthetic meshes-show a tendency to loose tension with use. Also stainless steel mesh allow more volume of ink to pass through. As it is electrically conductive it can be used for printing thermoplastic inks. Stainless steel screen printing fabrics are more expensive than synthetic material.

 

(4) Squeegee: The squeegee performs a very important function in screen printing. It is used to force the ink through the screen mesh and stencil on to the printing stock below. Squeegee blades are made from high quality natural rubbers and synthetic material. Polyurethane squeegee blades are now- a-days used widely due to their resistance property to abrasion so there is no need for sharpening or reshaping.

 

Squeegeees are normally supplied in three grades: Hard, Medium, Soft. The hard and medium grades are used for printing thin film inks, the soft grade is used for printing on to non-absorbent materials such as metal & glass. During the printing action the squeegee is moved across the screen and force the ink to pass through the mesh opening.

 

IMAGE CARRIERS USED FOR SCREEN PRINTING

Negative and Positive making:

Line and halftone positives are needed to prepare photographic screens. These positives are obtained by photographing line and continuous tone copy. In every printing unit, whether the work is done by hand or by machine, they employ a screen as a means of holding the design to be printed. The screen consists of a wooden frame, metal frame or plastic frame. Metal meshes are used for high precision jobs. The stainless steel mesh is usally fitted in a metal frame with the use of vacuum pressure.

 

Preparation of a Screen Frame

The most common frame used is made up of soft, straight, grained, dried soft wood such as white pine or walnut or any other wood which is light in weight and strong. The thickness and width of the sides of the frame varies depending upon the size of the screen.

 

Attaching the Screen Fabric to the Frame

The screen fabric may be attached to the wooden frame by means of nails or stapplers to the underside of the screen. Care should be taken so that the threads of the fabric run parallel to the sides.

 

Metal Screens

Metal meshes are used when thousands of impressions are to be printed from a screen and for high precision work or where the ink employed is unsuited for fabric screens.

 

Ceramic ink or dye, could destroy synthetic fabrics after a relatively smaller number of impressions.

 

Metal meshes are made of very fine threads of uniform diameter and their strengths are also classified by their numbers. The numbers denote the mesh variety or number of openings per square inch. The most common metal screen used is stainless steel, although phospher bronze and copper meshes are also used. Wire or metal screen are very durable but they do bend and crack, whereas silk is resilient and gives more value than working with screens made of metal.

 

Screen Fabrics

The printing screen contains uniform mesh openings and blocking material or a masking medium applied to the fabric which provides the design to be printed.

 

The meshes or the bolting cloth must have uniform strong and fine threads. It must be durable and it must be woven in such a way that the threads are parallel and will not be mis-positioned. The mesh or the bolting cloth variety used for screen process work is identified by a number. Smaller the number the coarser the mesh, that is the larger the openings in the fabric. The numbers ordinarily employed vary from 120 mesh to 400 mesh, that is the number of weaves vary from 120 to 400. The mesh number is usually followed by one or more X's. It indicates the strength of the mesh or fabric, for example the 120 XXX is stronger than number 120XX, and the number 120 X is weaker than 120XX. 120 mesh can be used by the beginner for most purposes and the quality is recommended for quality works because it has more twisted fibres than the plain number 120. Coarser meshes give a heavy deposit of ink when printed and takes a longer drying period. In these meshes fine details cannot be obtained in printing.

 

New meshes should be washed with warm water (about boiling temperature). After it is attached to the frame, soap or detergent can be used for washing. Washing not only cleans the fabric for the photographic film to adhere properly, but also results in roughening of the fabric. Meshes are available in different widths and is generally sold by meters or yards and the cost varying with width, classification and quality of the fabric. All screens should be cleaned immediately after use with proper solvents. If ink is allowed to remain on the screen, it will dry and harden, thus shortening the life of the fabric.

 

VARIOUS METHODS OF PREPARING IMAGE CARRIERS FOR SCREEN PRINTING

There are several methods of preparing printing screens, most of which have become standardised. These methods enable the printer to reproduce any type of copy, including fine details in line drawing, single and multi colour halftone pictures for reproduction.

 

To a great extent, the versatility of screen printing is made posible by the varied printing screens which are used. These screens first of all must withstand enameling lacquers, synthetic inks, ceramic inks, water based inks, textile dyes etc., that are forced or pushed through the screens. In addition each screen must be resistant to normal handling and to atmospheric conditions. Variations are due to wear and tear in printing. It must withstand the cleaning solvents employed to clean the screens for future storage and use; and when necessary it should be possible to remove the screen completely from the screen fabric for future use.

 

The printing screens are prepared by hand or photographically. The actual printing is made possible by blocking out the unwanted parts of the screen or those areas that are not to print and keeping open only those parts in the screen that are to be printed, or areas through which the ink is to be squeegeed. There are many types of printing screens and each having its own methods of preparation. Four general types have been developed. They are

 

i. the knife cut printing screen,

ii. Photographic printing screen,

iii. The wash out or etched screen and

iv. The block art printing screen.

 

The first printing screens consisted of simple stencils which were attached to the screen fabric, screen printing has sometimes been reffered to as stencil printing due to this reason. Any printing screen can be used for single colour or multi-colour work. Regardless of the type of printing screen employed a screen has to be prepared for each colour that is to be printed.

 

Preparing the Screen by Knife-cut Stencil Method

The first printing screen used in the early days of screen printing consisted of knife cut or paper cut out or stencils representing the design or originals to be prited. These were adhered to the fabric with adhesives such as glue, shellac and paste or the cut outs were just held in place on the underside of the screen by the tackiness of the ink employed in printing. Then shellacked papers and lacqured papers were employed because they were easier to attach on the screens and the result was much better.

 

• The present day printer employs a synthetic film as a stencil.

• The method of cutting is by using a sharp knife blade.

• Placing the stencil film over the master drawing, the stencil is pasted on the four corners by adhesive tapes. The film side of the stencil is in contact with the design. The emulsion should face the user.

• The required areas are cut carefully.

• After completing the cutting, image areas are removed leaving only the non-image areas to block out the screen.

• Now the stencil is placed below a screen and solvent of the particular type mostly thinner is rubbed with a cotton waste from the top. This should be done slowly in all the areas of the stencil. First the thinner should be applied with one waste and rubbed on with another. This process should be repeated to all the areas of the stencil film.

• After drying for a few minutes, the backing film is peeled off.

• Now the screen is ready for blocking out the non-image areas and to carry out the printing.

 

I. PHOTOGRAPHIC METHODS OF MAKING SCREEN IMAGE CARRIERS

2.3.1. PREPARING THE SCREEN BY GELATINE PROCESS ("DIRECT" METHOD)

The photographic methods of making screens are greatly responsible for the tremendous growth of the industry. These have encouraged printers to step into fields which would have been impossible for them to enter in with handcut screens.

 

It is possible to print fine details, illustrations and to separate colours photographically from a coloured original and then print the colours to produce prints on varied surfaces. This enables to do one, two, three or four colour works by screen process printing. All proofs from engraving or from other printing processes can be used, enlarged, reduced and printed.

 

Screen process printing produces a more distinct and concentrated colour effect than it is possible to attain with photographic plates used in other printing processes. Although the method of photographic screen making is not difficult to carry out, it took a vast amount of experimentation and research by experts and suppliers to develop this phase of the graphic arts. The first photographic screen was made in United States.

 

Photographic screen process printing deals with the arts and processes employed in the production of photographic printing screens which are used for photography and screen printing as a combition of light energy or chemical energy to make the printing screens. It is based on the principle that substances such as gelatine, albumin, polyvinyl alcohol (PVA) or glue when coated or mixed with light sensitive salts such as potassium bichromate or ammonium bichromate harden upon exposures to light. Those parts of the screen which are covered (sensitized) so that no light strikes them during the period of exposure will not become hardened. The hardened or exposed parts will remain insoluble in water, while the unexposed parts can be washed or etched out in water. The substance or compound which makes the emulsion or coating sensitive to light is known as a sensitizer.

 

 

The ProcessIn the present day market the gelatine or gum is sold in commercial names such as

Silk coat, Red star etc.

• The method of preparing a sensitized emulsion is as follows

Emulsion - composed of polyvinyl chloride, a gelatin - based substance, Sensitizer 2% (Ammonium Bichromate), Few drops of liquor Ammonia (3 to 4 drops)

• The above proportion may be increased or decreased accordingly when larger or smaller quantities are required.

• The emulsion thus prepared is coated to the cleaned screen with a scale or a sharp edged squeegee in a dark room. The emulsion becomes light sensitive after the addition of Ammonium Bichromate.

• The coated screen is dried with a fan in the dark room.

• After drying the required positive is placed readable side in contact with the underside of the screen.

• The screen is then exposed to a light source, where light will go through the transparent parts of the positive but not through the opaque parts of the positive.

• Thus leaving some parts of the sensitized emulsion exposed and some parts where the light does not strike which will be washed away with the water when developed and produced as openings in the stencil.

• When the emulsion is dry, the screen is ready for printing.

 

2.3.2. SCREEN MAKING BY PHOTO SENSITIVE FILMS (5-STAR FILM) METHOD

(INDIRECT OR TRANSFER METHOD)

The photographic screen process printing is made from an emulsion which is coated on a strong translucent or tranparent backing sheet such as Vinylite (for perfect accuracy in large printing or small printing screens and when many colours are to be printed). The film with the plastic backing sheet will prove very effective especially in hot and humid conditions.

Contraction of the plastic backing sheet is negligible and therefore the registration of different colours is easier.

Usually the thin emulsion coating which is carefully applied on the backing sheet under cotrolled condition consists of the colliodal gelatine, pigment and plasticizer for imparting softness and flexibility to the coating.

The film should be stored according to the manufacturers directions. It is ordinary sold in tubes and may be left in these tubes in cool, dry places for a long time when not in use. The film should be stored in total darkness.

The technique of film cutting deserves careful consideration. Skill in cutting is developed through persistent practice.

 

The Process

Cut the five star film to required size and in excess of the positive’s size. Be sure that the hands are free from grease or perspiration. Keep the film well covered, especially after it is stripped to avoid dust or damage. Examine the cut film closely for ‘mistakes’, omissions and presence of foreign matter. Keep the film side in contact with the readable side of the positive.

• Then place it in a contact box so that light will pass through the positive and strike the five star film. Then expose it to sunlight or artificial light source. The exposure time varies from design to design, from 1 minute 10 minutes in some cases.

• After exposing remove the five star film from the contact box and place it in a tray, care should be taken so as not to expose it to actinic light. Then pour a diluted solution of Hydrogen peroxide, that is, one part of Hydrogen peroxide mixed with three parts of water. Develop the film for about one minute. Remove the Hydrogen peroxide solution from the tray and pour warm water, over the film.

Now the image areas wil open up. After all the image areas have been opened up, cool down the film by pouring cold water.

• Then adhere the developed film on the back side of the screen with the films emulsion side in contact with the screen.

• Keep the screen flat by placing it over some pile of papers. Then from the top place a blotting paper to blot out excess water. Allow the screen to dry either with a drier or allow it to dry naturally.

• When the screen is completly dry, peel off the backing transparent film of the 5 star film. Now the screen has a stencil which will allow the ink to pass through, only on the opened up areas.

• Cover the screen on the non-image areas. Bloack out unwanted areas with opaquing solutions like lacquer, gum, photographic opaque or any other blocking out medium recommeded.

• The screen is now ready for printing.

 

2.3.3. CHROMALINE FILM METHOD OF SCREEN MAKING (DIRECT/INDIRECT METHOD)

This film combines the advantages of the strength of gelatin method and the sharpness of the photographic method. Hence it is a hybrid film. With this type of film we can print fine details and halftone reproductions including colour separation work. This film can be used for long runs and are not easily damaged.

 

The method of preparing the screens are as follows:

• Prepare the gelatin or silkcoat solution and sensitize it with Ammonium Bichromate to 100 grams of silkcoat solution. Add 2% of Ammonium Bichromate. Thin the solution till it becomes like honey.

• Cut the chromaline film (dark blue in colour) to the required size.

• Place the screen over the chromaline film emulsion side. Pour the sensitized solution over the screen. Using a squeegee give an even coat of the solution over the film. Remove the excess solutions which appear on the sides of the film with a waste. Dry the screen under a fan. Carry out this process in a dark room.

• After the film is dry peel off the backing of the chromaline film.

• Place the positive’s readable side in contact with the emulsion of the chromaline film. The positive may be held rigidly by pasting cello tapes at the corners. Give sufficient backing on the printing side of the screen, so that the screen is slightly above the table level. Place a rigid glass on top of the screen.

• Now expose the screen to a light source. The exposure time varies from 30 seconds to 3 minutes for a bigger design. This condition is with a powerful carbon arc lamp. It may vary for other sources of light. The exposing may also be done with the use of a contact box in which the screen can be placed inside.

• Take out the screen, remove the positive and dip the screen in a tray of water; slightly agitate the screen. Now the image areas will open up.

• Instead of dipping in a tray of water it can be developed by placing it in a sink and spraying water with a tube with moderate pressure.

• The screen is dried in natural atmospheric conditions after blotting out the excess water.

 

Direct/indirect screen stencil process

 

5.1. MESH, SQUEEGEE SELECTION

5.1.1. MESH (WOVEN SCREEN PRINTING FABRIC)

The woven screen printing fabric serves two primary functions: the fabric supports the stencil system, and the fabric’s mesh permits ink to flow through the image area. The mesh plays a dominant role in metering the amount of ink that will flow onto the substrate. The earliest fabrics used for screen printing included silk, hence the former name for the process: silk screen printing. Today, monofilament polyester is the most common screen fabric, followed by multifilament polyester, nylon, wire mesh, and silk, in that order.

 

Nylon, for example, is often used for container printing where the fabric must conform to unusual surfaces during printing and then it return to its original shape afterwards.

 

Metalized polyester and stainless steel are commonly used when maximum stability is required and static is a by-product of the print action. The mesh may be grounded to relieve erratic print edges where the ink follows a conductive path onto the substrate.

 

MATERIALS USED FOR SCREEN P RINTING FABRICS

The two basic categories of fabrics commonly used in screen printing are multifilament and monofilament.

 

MULTIFILAMENT FABRICS

Multifilament Fabric is made up of many fine strands twisted together to form a single thread. The multifilament threads are woven together to form the screen mesh. Multifilament fabric is gauged by the double-X system. Used for many years for measuring silk bolt cloth, but not based on any real measurement, the double X is preceded by a number denoting mesh count. The higher the number the finer the mesh and the smaller the mesh openings. Multifilament fabrics commonly range from 6XX to 25XX. Most multifilament fabrics used for screen printing applications are either silk or polyester.

i. Silk

Silk, the original mesh fabric used in screen printing, is the strongest of all natural fibers. Each silk filament varies in width, causing irregular mesh apertures that can distort the printed image. Since silk is a multifilament mesh, it cannot be woven as fine as monofilaments. Therefore silk is only suitable for work where accurate registration and fine details are not required. Because silk has irregularities and a rough surface structure, ink particles tend to become lodged between the twisted strands, making silk difficult to clean. For these and other reasons, long-time users of silk have turned to multifilament polyester.

ii. P olyester

Multifilament Polyester is less expensive than domestic and imported silk. It has more uniform mesh apertures and doesn’t expand as much as silk during printing. As opposed to silk, polyester is not affected by strong chemicals used in cleaning or reclaiming the screen. The disadvantage of multifilament polyester is that the fibers tend to flatten considerably more than monofilament fibers at thread intersections. This results in a closing of mesh apertures that shows up in printing as saw-toothed image edges. Because of their construction, multifilament fabrics are thicker and have a rougher surface structure than monofilament. They adhere well to knife-cut stencils and are best suited for printing where heavy ink deposits are required. Multifilament fabrics are usually used to print textiles, large posters, and textured or contoured surfaces.

Fig. Multifilament mesh (top) and monofilament mesh (bottom)

 

MONOFILAMENT FABRICS

Monofilament fabrics are constructed of single strands of synthetic fiber woven together to form a porous mesh material. Monofilament fabrics have a smooth surface structure that produces uniform mesh apertures. These fabrics include polyester, nylon, wire mesh, and metalized polyester. Monofilament fibers can be woven finer than multifilament’s and still retain adequate open areas for easy ink passage. Unlike multifilament fibers, monofilament fibers are measured by actual mesh count per inch or centimeter. Therefore a #200 nylon mesh would contain 200 threads in one linear inch (tpi). Monofilament fibers are available in a wide variety of mesh counts ranging from approximately 38 to 420 threads per inch. Multifilament fibers, on the other hand, can be woven only to 25XX or 30XX, which roughly corresponds to 200 tpi.

iii. Nylon

Nylon, which is available only as a monofilament, has similar construction characteristics to monofilament polyester with the exception of stability. Nylon is a very elastic fiber, making it a favorite for printing irregularly shaped or contoured surfaces.

However, elasticity is an undesirable characteristic wherever critic registration is a necessity.

Nylon is also affected by temperature and humidity, making multicolor registration very difficult at times.

iv. Wire Mesh

Wire mesh, commonly called wire cloth, is commonly used with abrasive inks, such as those used to print on ceramics, or wherever extreme sharpness, close tolerance, and thick ink film deposits are required, as in printed circuit boards. Wire mesh is extremely stable and is available in very fine mesh counts up to approximately 635 tpi. Reclaiming these screens, the process of stripping the stencil from a screen so it can be reused, is comparatively easier than with nylon or polyester, and they can be reused many times. Wire mesh, however has a total lack of memory, i.e., it will not spring back if dented or grooved, as will nylon, polyester, or silk.

v. Metallized Mesh

Metallized mesh is a relatively new fabric developed for screen printing. It is composed of a monofilament synthetic fiber, either polyester or nylon, coated by an extremely thin layer of metal. In combining these elements, metalized polyester or nylon mesh has the advantages of both wire and monofilament synthetics. It will not dent or deform like wire nor does it repel indirect stencils without pretreatment as does polyester or nylon. The metal coating makes cleaning the screen easier than that of synthetic fibers. Metalized mesh has excellent dimensional stability and can be used for very long runs where close tolerances and exact register are a necessity.

 

SELECTING A MESH (SCREEN PRINTING FABRIC)

Selecting a screen fabric is one of the most important decisions a screen printer must make. The type of material along with mesh count, substrate absorptivity and shape, nature of ink, type of stencil, squeegee composition and blade angle, the design characteristics, and the thickness of the printed ink deposit required are all factors considered prior to actual printing.

 

The following are general rules of thumb that can be used in deciding which screen fabric will best suit the printer’s needs.

• Monofilaments are more abrasion resistant, available in finer mesh counts, and offer easier cleaning and ink passage than multifilaments. The screen surface must be mechanically and chemically pretreated to allow indirect stencils to adhere.

• Multifilaments have a thicker and rougher surface than monofilaments and offer excellent adherence for knife-cut stencils along with heavy deposits of ink.

• The open area of a mesh is the area between threads; it allows the passage of ink. The larger the percentage of open mesh area, the greater the amount of ink deposited during printing.

• Each mesh opening should be at least three times larger than the average grain size in the pigment of the ink otherwise a screen will clog during printing.

• Mesh count varies according to thread diameter the smaller the thread diameter, the finer the mesh.

• Thread diameter is one factor that determines the thickness of the printed ink film—the thinner the thread, the thinner the printed ink deposit; conversely, the thicker the thread, the thicker the ink deposit.

• The finer the detail in the design, the finer the mesh needed to reproduce it.

 

For halftone and full-color printing, a mesh least three times finer than the screen ruling of the halftone is needed.

 

5.1.2. SQUEEGEE SELECTION

THE SQUEEGEE

The squeegee is a rubber or plastic blade, attached to a handle, use to force ink through the open areas of the stencil and mesh to the substrate. The functions of the squeegee are to control the spread of ink across the screen during printing to bring the ink filled screen into contact with the substrate and-to a certain extent-to determine the thickness of the printed ink film.

 

Ink is applied to one end of the screen. The squeegee blade should be slightly larger than the image area to ensure even ink coverage. The Width of the blade is a function of the image size. As much distance as possible between the blade and edges of the frame is recommended, but the squeegee needs to exceed the width of the image area by a inch or two on each side of the image width. The squeegee controls the spread of ink because it is used to draw the ink across the screen, causing it to penetrate the open area of the image carrier. This can be done either manually or by machine, depending upon the type of work, length or run, or availability of equipment.

 

The second function of the squeegee is to bring the ink-filled screen into contact with the substrate during off-contact printing. Screen printing can be done either on-contact or off contact with the substrate.

 

During off-contact printing, the screen is lowered to a point slightly above the substrate. The squeegee is drawn across the screen with downward pressure. Because of the elasticity of the screen, the pressure of the squeegee forces the stencil into contact with the substrate. As the squeegee passes, the stencil immediately separates or snaps off from the wet print. Off-contact printing generally produces sharper prints by eliminating image spread and smudging. The use of a vacuum base will prevent a flat, lightweight substrate from sticking to the underside of the screen when it is raised. In manual printing, off-contact can be established by taping cardboard shims to the underside corners of the frame. Automated Screen printing presses employ adjustable devices that control the amount of off-contact.

 

On-contact printing is done with the underside of the screen in full contact with the substrate. On-contact is used when heavy ink deposits are required. However, since image sharpness will decrease considerably, it is used only on substrates for which image sharpness is of little importance, e.g., textiles such as terry cloth or towels.

 

SQUEEGEEE SELECTION

I. Shapes of Squeegee blades

Squeegee blades are available in a variety of shapes. Different shaped blades are used to print on different substrates. The simplest and most common profile used in screen printing is a square 90° angle. The general shapes and uses for each blade angle are found in the following table.

Shape of Squeegee Blades
1	Square edge	For printing on flat objects
2	Square edge with rounded corners	For extra-heavy deposits. For printing light color on dark backgrounds or printing with fluoresent inks.
3	Single-sided bevel edge	For use mostly by glass or nameplate printers.
4	Double-sided bevel edge	For direct printing on uneven surfaces; bottles.
5	rounded edge	For printing heavy deposits of ink on containers and ceramics.
6	Double bevel edge	For printing textile designs.
7	Diamond edge	For container printing and applications

 

II. Squeegee Hardness

Squeegee blades are rated according to hardness, which is measured in values of durometer. Generally, soft, low-durometer, dull squeegees deposit more ink; while hard, high-durometer, sharp squeegees deposit less ink.

Hardness Categories of Squeegee Blades
Extra	45 - 50 durometer
Soft	50 - 60 durometer
Medium	60 - 70 durometer
Hard	70 - 90 durometer

 

III. Squeegee Materials

Squeegee blades are more commonly composed of synthetic materials rather than rubber, especially for printing runs over 200. Although rubber blades are easy to use, they tend to lose their shape and edge quickly. The introduction of plastic compounds, such as polyvinyl and polyurethane, has solved this problem. Synthetics tend to keep the desired edge throughout long print runs and will resist inks, solvents, and abrasion better than rubber.

 

Squeegee composition has evolved through the l990s. Notable Variations on the material used has resulted from manufacturers offering dual and triple durometer squeegees, fiberglass backing support, and the Combi™ which offers a more consistent printing edge. Dual durometer squeegees evolved as a reaction to the use of a metal backing blade. The backing blade was added to the squeegee holder assembly to provide rigidity support for the squeegee to reduce flexing during the print stroke. One layer of the squeegee has the specified durometer for printing while the second has a higher durometer.

 

Triple-durometer squeegees sandwich the higher durometer with two layers providing two printing edges: the blade is turned when the first side wears, offering more production time between sharpening.

 

The squeegee must be flexible, because there will be a measurable amount of bending in the squeegee as the force of the printing cycle occurs. Squeegees may be placed in the press at a predetermined angle. Nevertheless during the printing stroke both downward pressure and forward motion exert stress to the squeegee. If the material does not have sufficient resilience, the transferred ink may become distorted during printing. On the other hand the squeegee must be stable so a consistent printing edge will be presented stroke after stroke.

 

Squeegee profiles and durometer must be selected with respect to the material and image to be printed. A squeegee that is too soft or hard can distort the image or cause poor ink transfer. The following table provides guidelines to follow during squeegee selection.

 

Hardness Categories of Squeegee BladesGuidelines for Squeegee selection
Soft	Textitles, garments,irregular shapes	low resolution, large ink deposit
Medium Deposit	most products	good resolution, varied ink
Hard	flat surfaces	high-resolution graphics

 

FLOOD BAR

During the printing sequence the flood bar cycles to replenish ink in the mesh image areas. The flooding action of the cycle ensures that a continuous supply of ink is in place for the print stroke. Flooding also helps to prevent ink drying in the image areas when using conventional solvent inks. Flood bars are typically made of metal. Care should be taken to avoid nicking the flood bar, which can result in uneven ink flow and damage to the mesh.

 

OPERATION OF SQUEEGEE AND FLOOD BAR

The squeegee and flood bar are generally fitted in a holder and then clamped in the press (except in the case of manual printers who have a multitude of handle shapes to choose from). Presses will have a clamping system that fits the supplied squeegee holder or can be fitted to holders supplied by other manufacturers. Other considerations include adjustments for squeegee and flood bar angle. This ability can assist in improving ink transfer on inks with different viscosities or different squeegee blade profiles. The holders should have control screws for adjusting pressure the best case is to have a pneumatic system to maintain consistent pressure during the print and flood strokes. During print setup the squeegee and flood bar pressures can be increased at staged intervals to optimize the best impression and flooding of ink.

 

The contribution of the squeegee and flood bar to the print is often overlooked or underestimated. Optimizing print performance is simple: use three control points—speed, angle, and pre sure. Whether the press is manual or automated these monitoring points must be addressed for properly controlled printing. speed directly linked to the ink’s thixotropic properties. The ink is formulated to move and shear when energy is applied by the squeegee. Careful observation and measurement will help identify the speed best suited for a particular operation. The angle of attack is critical in transferring the ink from the mesh to the substrate. Practice has shown that 70-75° is best for most applications. The flexing action of the squeegee will place the printing edge at approximately 45° with the edge at 90° to the substrate. A steeper angle may cause the ink to snowplow, resulting in poor ink transfer. A shallower angle will push the ink through the mesh, causing distortion in the image. Pressure must be controlled to provide sufficient transfer of the ink. To determine best pressure, begin with too little pressure and increase in incremental adjustments. When optimum printing is achieved, reduce pressure until the print breaks and then add pressure until the print is optimal. The best pressure may be determined by examining microline targets, tint patches, slur or resolution targets on both sides of the substrate (flat), or similar targets on containers. When targets cannot be placed on garments or similar products, pressure must be adjusted on setups and the targets blocked out for production. The targets should be checked at regular intervals to establish printing consistency.

 

5.2.1 SCREEN PRETREATMENT

After the fabric has been stretched and mounted to the frame, it must be properly prepared to receive the stencil. Generally synthetics tend to repel indirect stencils because of their smooth filament structure. For such stencils, the fabric must be lightly roughened to insure excellent adhesion. A fine abrasive powder is gently rubbed into the stencil side of a wet screen, then thoroughly rinsed.

 

Degreasing is the next step in screen preparation. Degreasing removes any grease or oil residue left in the screen from reclaiming chemicals. In the case of new screens, degreasing removes grit and hand perspiration deposited during the stretching procedure. Degreasing should be done to all screens, new and reclaimed, immediately prior to stencil application. This will ensure tight stencil adhesion and prevent stencil breakdown.

 

5.2.2. SCREEN STRETCHING / TENSIONING

BASIC STEPS IN SCREEN STRETCHING / TENSIONING

i. Inspect frame for any damage (nicks, old adhesive, etc.)

ii. Select and inspect specified fabric. The fabric should be properly sized to fit stretchning equipment and frame. Follow manufacturer’s recommendations.

iii. Be sure the fabric is square to the frame.

iv. Lock or secure the fabric to the frame or clamping system. Loosen corners to avoid stress if possible.

v. Begin tensioning the fabric incrementally; do not exceed manufacture’s maximum tension specifications immediately. Experiment with rapid tensioning as well. Keep in mind the objective: a finished screen with the fabric at the recommended tension.

vi. Measure the tension of the stretched screen and record the final tension level prior to placing the screen in inventory.

 

Fabric color is an important characteristic to consider, particularly as the color affects stencil exposure. Threads can be dyed to promote better stencil exposure factors; e.g., reducing light undercutting. Fabric colors available are typically red, yellow, and gold-orange.

The fabric color filters incident light from emerging out of a thread and exposing the stencil in an image area, hardening the emulsion, and preventing printing ink from passing through to the substrate.

 

STRETCHING THE SCREEN PRINTING FABRIC

Stretching and attaching the mesh material to a wooden or metal frame is a major factor in preparing the image carrier. Overstretching or understretching the fabric directly influences the quality of the printed image. Smudging, poor registration, and premature stencil wear can all be attributed to incorrect screen tension.

 

Manual Stretching

In many small shops, screen material is stretched by hand. A device that resembles rubber-tipped pliers is used to stretch the fabric over a wooden frame. Tacks, staples, or the groove-and-cord method are commonly used to attach the fabric to the frame. Handstretching is very time-consuming and usually will not produce uniform stretching or the high tensions needed for synthetic fabrics on large frames.

Uniform stretching assures even screen tension, which is required for accurate printing production. This, plus the need for timesaving procedures, has led most large shops to use mechanized stretching devices.

 

Machine Stretching

Most machines used for stretching are either mechanically or pneumatically controlled. In either system, the procedure is basically the same. The screen fabric is cut slightly larger than the frame to allow a series of grippers or stretcher bars to suspend it above and outside the frame edges. The mesh is stretched to a specific tension percentage which is dependent upon the type of fabric and mesh count.

 

A tension meter is a precision instrument used to measure the surface tension of the stretched screen fabric. Obtaining a specific tension level affects print sharpness, register, printing ink density, and stencil life. The tension meter consists of an indicator dial and a spring-loaded measuring bar supported by metal beams. When a tension meter is placed on the screen fabric, the tension meter’s measuring bar pushes into the fabric. As the screen tension increases, so does the pressure on the measuring bar, and the tension is indicated on the dial. The tension variation within a screen should not vary by more than +0.5 newton/centimeter (N/cm) for high-quality printing and +1.0 N/cm for an average-quality job.

 

The allowable variation between screens is just slightly higher: +1.0 N/cm for exact register, and +1.5 N/cm for average register.

 

Using a tension meter with a mechanized stretching device can make it possible to obtain the correct tension for screen printing and to duplicate tension accurately from screen to screen.

 

In addition to separate machines and devices used to stretch the fabric off the frame, some frames have a built-in mechanical stretching system. Basically, these devices are composed of a hollow aluminum frame with adjustable gripper bars housed inside that hold the mesh securely. A series of tension bolts, which are accessible from the outside frame edge, are tightened, causing the gripper bars to pull the mesh in a straight outward direction.

Fig. A Tension meter is used to ensure proper screen tension

 

 

Unit - 6

6.1. SCREEN PRINTING MACHINES

 

6.1.1. CONTAINER PRINTING MACHINES

These machines are designed on the cylinder-bed principle. The curved surface of the printing cylinder is replaced by the curved surface of the container, which is supported by two roller bearings. The printing action is exactly the same as on the cylinder press; the screen reciprocates over the rotating container while the stationary squeegee forces the ink through the screen. The machines are usually an integral part of the container making and filling process, though some pre-printed containers are still made. These machines are made in a range of sizes to print the smallest perfume or large oil drums.

6.1.2 THE FLATBED HINGED FRAME PRESS

Flatbed presses are primarily used for printing on flat substrates of various composition, size, and thickness. For example, flatbed presses can print on a wide range of substrate thicknesses, from very thin plastic and textiles to 1-inch. (25mm) board. Flatbed presses can be divided into three categories: hand-operated, semiautomatic, and fully automatic.

 

Hand-operated screen printing tables are still used in many commercial shops.

The frame is placed in clamp-type hinges, which allow the operator to lift the screen between print strokes to remove and replace the substrate. Improvements to hand table operation have increased speed and quality. Vacuum tables or beds which keep the substrate stationary during printing, improve print quality and multicolor registration. Counterweights and larger handles are attached to the squeegee to increase printing speed and to maintain a constant angle between the screen and squeegee.

 

Hand tables are often found alongside highly developed automated presses. They can be used for test runs of packages that will eventually be mass-produced either with automatic screen printing presses or an entirely different printing process.

 

Semiautomatic flatbed presses work on the same principle as hand tables except the hand operation of the squeegee and frame lift are mechanized. Vacuum beds are used to keep substrates in position during the printing operation. Feeding and delivery of the substrate can vary according to the manufacturer’s design or the printer’s needs. Some semiautomatic presses employ manual feed and delivery while others have manual feed but automatic delivery. Semiautomatic flatbed presses print the same substrates as the hand table; however, production and print quality improve because of the consistency maintained by mechanical squeegee stroke pressure and constant blade angle.

 

6.1.2 - AUTOMATIC FLATBED HINGED FRAME SCREEN PRESSES

The automatic flatbed hinged frame screen press is capable of printing on both flexible and rigid substrates—as thin as paper or as thick as 0.75-in. (18-mm) masonite.

 

During the printing cycle of an automatic flatbed press, the flat or sheet-like substrate is automatically fed and registered on a stationary vacuum flatbed. The screen is held in a carriage, which brings it into printing position above the sheet. Image transfer takes place as the mechanically controlled squeegee moves across the screen. After the impression is made, the carriage moves away from the bed and the squeegee returns to its starting position, coating the screen with a layer of ink called the flood coat. This is accomplished by a metal blade attached to the back of the squeegee that comes into screen contact after the impression stroke. The flood coat returns ink to the starting position but does not force ink through the image areas. This insures a proper ink supply to every part of the screen. Most automatic presses use the flood coating method. After the printed substrate is mechanically removed, the press repeats the printing cycle.

FLAT-BED HINGED FRAME

 

Flatbed press sizes vary enormously. Although the common press sizes range from

8.5x11 in. (215x279 mm) to 60x90 in. (1.5x2.3 m), presses especially for circuit printing are smaller than 8.5x11inches., and one standard flatbed press measures 78x156 inches (2x3.9m). Speeds range from over 2,000 impressions per hour (iph) on smaller presses to over 1,000 iph on larger presses.

 

There are many variations of the flatbed principle, some of which are used in printing T-shirts, textiles, wallpaper, and electronic circuits. Flatbed web presses, for example, are used to produce labels and decals at relatively high speeds (150 ft./min.). Whether the press has manual feed and delivery or automated devices in any combination, the basic flatbed principle exists for all variations.

 

 

6.1.3. THE ROTARY SCREEN PRESS

Compared to the flatbed and cylinder designs, the rotary screen press is a relatively new screen printing system, with the first rotary screen printing machine introduced in Holland in 1963. A fine-wire cylindrical screen containing a squeegee-like blade inside rotates over a continuous roll of paper. The rotary screen mesh is coated with a photosensitive emulsion and exposed in contact with a positive. It is then processed similarly to most photostencil materials. The squeegee, which remains stationary, forces ink through the rotating Screen as the web travels underneath. Ink is continuously pumped inside to maintain high printing speeds. The web, which varies from lightweight giftwraps and textiles to thin paperboard and wall covering vinyl’s, is capable of traveling through several printing stations at speeds of 200 ft./min. (61 m/min.). Each station has its own screen unit that may be printing one of several colors or a clear final coating. At the end of the printing cycle, the web is transferred to slitting and sheeting units. The slitter first splits the web vertically, and then the sheeter cuts the split web horizontally into sheets.

 

This machines are specially used for high volume production of printed textiles and floor and wall coverings. The functional principles are entirely different from conventional screen printing. Here the screen is in the form of seamless perforated cylinder, made of light metal foil. The squeegee is hollow and run inside the perforated cylinder. Through the hollow squeegee, ink is pumped to the screen.

 

As the screen (cylinder) rotates the ink is passed on the web (stock). The screens are made in various grades according to the ink thickness required on the stock. In this method stencil are formed by direct photoemulsion method, but it requires specialized coating and exposing technique.

 

Rotary Screen

 

6.1.4. CAROUSEL MACHINES

Based upon the hinged frame principle, these machines were originally designed for multi-color printing on to T-shirts and sportswear. They consist of multiple printing bases or ‘garment platens’ which can be rotated on a central pivot – hence the name ‘carousel’. Above each platen is a printing head (also rotational) consisting of a hinged frame carriage, squeegee and flo-coater; the latter being mechanically driven on the more sophisticated machines.

 

The printing cycle begins with a garment being slid over the platen. The first screen is then positioned over the platen for printing. After the first color has been printed the second screen is brought into register with the platen. The process is continued until all the colors have been printed. The garment is then removed from the platen for drying; usually by infrared radiation.

 

The carousel principle has been adapted for printing onto a wide range of substrates.

The garment platens are replaced by small vacuum bases, and intermittent UV curing heads are positioned between each printing station.

 

The machines are available in a variety of configurations, the standard being 6 or 8 stations, having 4-6 printing heads. The standard formats are 406 x 355 mm (16 x 14") and 558 x 457 mm (22" x 18"). Maximum speeds of 4800 iph or 700 printed pieces per hour have been claimed by some manufacturers.

6.2. SCREEN PRINTING INKS - TYPES, PROPERTIES

 

SCREEN PRINTING INKS

Inks and Substrates. As was mentioned several times above, screen printing is suitable for printing on an almost infinite variety of substrates. The most important consideration is ensuring that the ink used is suitable with the surface, both in terms of chemical compatibility and the facilitation of proper drying. Screen printing commonly requires paste inks that are thick and able to print sharply through the screen. They must also perform well under the action of a squeegee. The solvents used should also not be overly volatile, as excessively early evaporation would cause the remaining ink components to clog the screen.

 

Screen inks typically utilize a drying-oil vehicle, although ultraviolet-curing inks and other forms of fast-drying inks are making strong inroads in screen printing. Often, in the decoration of fabrics, glassware, and ceramics, heat transfer printing is utilized, which involves screen printing the design onto a decal (in one of a variety of ways; see Decal), then transferring the design (which is composed of sublimable dyes) to the desired end substrate by means of exposing the decal to increased heat and pressure. (See Heat Transfer Printing).

 

COMPONENTS OF INKS AND INK SYSTEMS

The principle components of a printing ink are pigments or dyes (colorants), vehicles, and additives. An ink can be opaque or transparent, depending on the ink’s components.

 

COLORANTS

All printing inks consist of a colorant, almost always a pigment but occasionally a dye. Dyes are soluble in a solvent or vehicle, while pigments are insoluble.

Pigments are finely ground solid materials that impart colors to inks. The nature and amount of pigment that an ink contains, as well as the type of vehicle, contribute to the ink’s body and working properties.

 

Pigments can be organic or inorganic. Organic pigments tend to produce transparent inks, which are used for process-color printing, while inorganic pigments tend to produce opaque inks. The term organic means “derived from living organisms.” Organic pigments are made from petroleum products: blacks by burning gas or oil, other colors be reacting organic chemicals derived from petroleum. The most common black pigment, furnace black, is made by burning atomized mineral oil in brick-lined furnaces with a carefully controlled supply of air. The products of combustion are cooled, and the pigment is collected with electronic precipitators or in bag filters.

 

Inorganic pigments are formed by precipitation—that is, by mixing chemicals that react to form the insoluble pigment, which then precipitates, or settles out. The most common white inorganic pigment is titanium dioxide. Inks made from titanium oxide are very opaque and have excellent colorfastness.

 

VEHICLES

The vehicle carries the pigment and adheres it to the substrate. In addition, it gives an ink its consistency. The vehicle is composed mostly of a varnish, which is a solvent plus resin and/or drying oil, along with waxes, driers, and other additives. The vehicle carries the pigment, controls the flow of the ink or varnish on the press, and, after drying, binds the pigment to the substrate. Vehicles also control the film properties of dried ink, such as gloss and rub resistance. The resins are formulated to optimize the ink’s ability to adhere to a substrate.

 

The solvent serves to maintain the vehicle’s flow until curing. The solvent is carefully selected for its compatibility with the vehicle and the substrate. Different ink systems use specific solvents to enable the ink to function properly.

 

The solvent in the ink can flash off during curing. The solvent products are termed volatile organic compounds (VOCs) and are regulated by government agencies. Most ink systems producing VOCs contain petroleum-derived solvents. Consult the Material Safety Data Sheets (MSDS) for the content of the ink system. Note handling and disposal instructions as well.

 

ADDITIVES

Most ink systems offer greater versatility through additives, which change an ink’s out-of-the-can personality. Toners will provide greater color strength while mixing, and halftone bases reduce color strength. Thinners will change viscosity, and adhesion promoters improve adhesion. The printer must consult with the ink supplier concerning the use of additives. Improper use typically results in poor ink performance.

 

6.2.1 - TYPES OF SCREEN PRINTING INKS (FOR SPECIFIC APPLICATIONS)

i. INKS FOR DECALCOMANIAS

Pressure-sensitive decals or waterslide decal transfers are usually printed by screen because the process delivers thick, opaque films with enough flexibility to withstand the movement of the carrier paper while they are being transferred. These inks usually require good light resistance. UV inks have been used to successfully screen-print pressure sensitive decals.

 

ii. INKS FOR CIRCUIT BOARDS

When a thick film is required on a printed circuit, the screen printing process is often the best way to print it. The ink must adhere to clean copper and resist the chemicals used in etching the copper to produce the circuit. If it is necessary that the ink be removed with a solvent or alkali after etching, the ink must be sensitive to solvent or alkali.

 

iii. POSTER INKS

Posters are printed with poster inks on a variety of board and paper stocks Non-oxidizing resins and oxidative drying inks are used the most. Overprinting with a gloss varnish extends the life of the print.

 

iv. ENAMEL INKS FOR METALS

Enamel inks formulated from oil-based alkyds modified with melamine or urea formaldehyde, cellulose lacquer, epoxies, and other synthetic resins yield attractive signs for outdoor use.

 

The metal surface must be thoroughly degreased; aluminum is often anodized or given a nitrocellulose wash for the ink to adhere well. Baking enamels yields a product that is tough and has good resistance to aging, light, and weather.

 

Even more permanent are vitreous-enameled aluminum and steel products. Vitreous enamels are glasslike material or frit ground together with oxide colorants, clay, and water. After degreasing and surface treatment of the aluminum, it may be screen-printed with enamel based on borosilicates and immediately fired (without drying) at very high temperatures.

 

v. INKS FOR PLASTICS

Pigments for plastic printing must not migrate or bleed into the plastic. The solvent must be able to etch the plastic enough to improve adhesion without causing crazing (stress cracking of the plastic surface). Thermoplastic adhesives or binders are helpful if the plastic is to be vacuum-formed after printing.

 

The awkward shapes of polyethylene bottles are readily screen printed, and the thick ink deposits provide glossy and bright colors.

 

Screen Printing Inks for Special Applications
Poster inks	Used for general graphics printing.
Plastisol inks	Primarily used in garment printing (t-shirts). May include additives which puff when ink is cured.
Textile inks	Also used in garment printing and on large textile presses.
Decal inks	Suited for labeling and long lasting/weatherability properties.
Pastes	Found in industrial electronics printing.
Ceramic inks	Inks may be printed on decals then transffered to product for firing. Ceramic inks are permanent.
Epoxy inks	A two-part ink cures by oxidation with the hardner. Excellent durability.
Vinyl inks	Formulated to work with vinyl. Surface tension issues and off-gassing of the vinyl are primary concerns.
Speciality inks	Inks may be specially formulated to print with a particular material such as polycarbonates, polyethylene, or other types of plastics.

 

vi. INKS FOR GLASS

Inks for glass are either enamels or frits that are fired at high temperatures, or epoxy or other plastics that are baked at lower temperatures. Oil-based and synthetic resin-solvent based inks are used to print items like dials, mirrors, and glass signs. UV inks are used to decorate mirrors.

 

Special inks may be used for windshield applications in the automotive and aviation industries. Glass containers may have graphics applied where the ink provides a graphic design. The ink may also act as a resist for etching the image with acid.

 

vii. PLASTISOLS AND EMULSIONS FOR TEXTILES AND GARMENTS

Plastisols and emulsions are the two kinds of inks commonly used to print textiles and garments. Inks based on an acrylic emulsion are suitable for all types of fabric and are printed directly onto it. They will dry at room temperature, but to achieve resistance to laundering, they must be cured 2 or 3 min. at 320°F (160°C). A plastisol is a (dry) vinyl resin dispersed in a plasticizer; there is no solvent. The plastisol is pigmented and printed on the fabric. When heated above 300°F (149°C), the plasticizer “fuses with” the resin and a film is formed. Since the plastisol penetrates the fabric, the film formed on heating incorporates the fabric, producing an excellent bond. Plastisols can also be printed onto release paper, partially cured, transferred to the fabric, and then cured completely.

Plastisol inks must be durable in order to withstand washing and drying of the garment. The cured ink film must remain flexible and adhered to the garment through repeated washings.

 

QUALITY CONTROL OF SCREEN PRINTING INKS

Because of the exceptionally broad variety of products produced by the screen printing process, a complete discussion of quality control is impractical. As with all inks, color match, color strength, and fineness of grind are important. Adhesion to the substrate, and compatibility with screen, squeegee, and stencil material should be checked. Suitability for the proposed end use is always important.

 

This is often determined with tests for light resistance, product resistance, weathering resistance, laundering, and the like. Some quality control tests appropriate for screen printing inks are listed in the following table.

 

Appropriate Quality Control Tests for Screen Printing Inks
Wet ink film tests	Dried ink film tests
Color	Color
Viscosity	Opacity/Hiding Power
Masstone	Rub Resistance
Length	Scuff Resistance
Fineness of Grind	Glass
Density/Specific Gravity	Adhesion
Tinctorial Strength	Flash Point
Tack	Drying Rate
	Flexibility
	Lightfastness

 

6.3. SCREEN PRINTING APPLICATIONS

 

PRIMARY MARKET SEGMENTS

Screen printing may be easily classified by the wide range of market segments that it serves, including the following:

• Garments and textiles: T-shirts, coats, sheets, towels, and fabrics

• Home products: wall coverings, linoleum, simulated wood grains.

• Product marking: appliances, dashboards, in-line applications

• Large-format printing: billboards, displays, fleet marking

• Electronic printing: circuit boards, membrane switches, display coatings

• Coating market: UV applications

• Fine art printing: collectable prints, fine art reproductions

• Poster printing: low-volume displays

 

These market segments point once again to the diverse applications for graphic communication and manufacturing that screen printing affords.

 

Applications

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 (examples are given in sec. 2.4.3).

 

Surface Finishing. Transparent varnish can also be applied using screen printing technology (for spot varnishing, in particular) to finish the printed product.

 

Screen Printing on Curved Surfaces

Almost anybody 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.

 

PRODUCTS PRINTED BY SCREEN

Decalcomanias. Pressure-sensitive decals or waterslide decal transfers are usually printed by screen because the process delivers thick, opaque films with enough flexibility to with stand the movement of the carrier paper while they are being transferred. These inks usually require good light resistance UV inks have been used to successfully screen-print pressure sensitive decals.

 

Circuit boards. When a thick film is required on a printed circuit, screen process is often the best way to print it. The ink must adhere to clean copper and resist the chemicals used in etching the copper to produce the circuit. If it is necessary that the ink be removed with a solvent or alkali after etching, the ink must be sensitive to solvent or alkali.

 

Posters. Posters are printed with poster inks on a variety board and paper stocks; they may be clay-coated, patent-coated, or liner. Clay-coated board is well suited for photographic halftone printing. Nonoxidizing resins and oxidative drying inks are used the most. Overprinting with a gloss varnish extends the life of the print.

 

Metals. Enamel inks formulated from oil-based alkyds modified with melamine or urea formaldehyde, cellulose lacquer, epoxies, and other synthetic resins yield attractive signs for outdoor use.

 

The metal surface must be thoroughly degreased; aluminum is often anodized or given a nitrocellulose wash to improve ink adhesion. Baking enamel inks yields a product that is tough and has good resistance to aging, light, and weather. Even more permanent are vitreous-enameled aluminum and steel products.

 

Vitreous enamels are glasslike material or frit ground together with oxide colorants, clay, and water. After degreasing and surface treatment of the aluminum, it may be screen-printed with enamel based on borosilicates and immediately fired (without drying) at very high temperatures.

 

Plastics. Thermoplastic adhesives or binders for inks are helpful if the plastic is to be vacuum-formed after printing. Pigments for plastic printing must not migrate or bleed into the plastic. The solvent must be able to etch the plastic enough to improve adhesion without causing crazing (stress cracking of the plastic surface). The awkward shapes of plastic bottles are readily screen printed, and the thick ink deposits provide bright, glossy colors.

 

Glass. Inks for glass are either enamels or frits that are fired at high temperatures, or epoxy or other plastics that are baked at lower temperatures. Oil-based and synthetic resinsolvent-based inks are used to print items like dials, mirrors, and glass signs. W inks are used to decorate mirrors.

 

Textiles. Plastisols and emulsions are the two kinds of inks commonly used to print textiles. Inks based on an acrylic emulsion are suitable for all types of fabric and are printed directly onto it. They will dry at room temperature, but to achieve resistance to laundering, they must be cured 2 or 3 min. at 320°F (160°C). A plastisol is a (dry) vinyl resin dispersed in a plasticizer; there is no solvent. The plastisol is pigmented and printed on the fabric. When heated above 300°F (150°C), the plasticizer “fuses with” the resin and a film is formed. Since the plastisol penetrates the fabric, the film formed on heating incorporates the fabric, producing an excellent bond. Plastisols can also be printed onto release paper, partially cured, transferred to the fabric, and then cured completely.

 

Membrane touch switches. These switches, seen at checkout counters at fast-food restaurants and supermarkets, on electronic games, on appliances, medical equipment, and many other electronic devices, provide a major market for screen printing. They are made of three layers of synthetic film the circuit layer, a spacer, and a graphic overlay. The circuit is usually applied to a polyester (PET) film by screen printing with conductive inks, which are typically blends of nonconductive, organic binders and conductive particles, usually silver or graphite. The ink is cured at temperatures around 300°F (160°C) to achieve the desired conductivity. Polyester is preferred because of its chemical resistance and its ability to withstand the high curing temperature. Polyester shows good resistance to flex fatigue, cracking, and deformation, and conductive inks work well on polyester.

 

The space layer (or spacer) holds the circuit layer and overlay sheets in register and keeps them apart except when the switch is depressed. The film has pressure-sensitive adhesive on both sides and is diecut at the locations of each switch. To assure that this layer expands and contracts at the same rate as the other layers, it is typically made of PET film. The graphic overlay is usually made of PET, but polycarbonate is sometimes used. It must withstand the flexing for the designed life of the device, and PET is reported to withstand 100 million switchings. This layer is decorated with screen inks, often on the back side (or second surface), which protects the print from scratching and abrasion. This structure is visually summarized in figure 6-1.

 

Trouble- shooting

For the ink manufacturer to help solve ink problems, the printer should be ready to supply the following information:

• Screen material (e.g., monofilament, nylon)

• Mesh size (e.g., 160)

• Stencil material (e.g., blackout materials, diazo-sensitized photo screens)

• Squeegee composition (e.g., polyurethane)

• Squeegee hardness and durometer (e.g., hard 66-75 Shore A).