When we talk about screen printing, we usually mean silkscreen printing. However, screen printing is somewhat different. Although it is not the subject of today's article, we will briefly go over the basics of screen printing technology, as this will help us understand the principles of silk-screen printing.
Screen printing
A stencil is a device usually used to apply a large number of identical characters, such as letters, numbers, a variety of figures and images of varying degrees of complexity, to various surfaces.
Stencil is a small sheet of paper or transparent foil in which several segments composing the initial image are cut out. When making a stencil, the picture is divided into segments so that the stencil does not tear when drawing.
The elements of the image are cut out with the right tool for the particular case. Usually it is a sharp knife, scalpel, razor or something similar.
When the stencil is ready, it is placed on the surface to be painted and rolled with a paint roller, a sponge or a spray gun. The paint flows freely through the holes cut in the stencil, thus creating a segmented image on the painted surface.
Screen printing is actively used in small and medium runs of fabrics, in various cottage industries, street graphics, etc.
One of the varieties of screen printing is risography, which is considered to be a method of rapid printing. Risography is printing using a printing plate that is made by burning micro holes in the plate material (master film) with a thermal head to form printing elements through which the ink is subsequently pressed.
Ripography is used for rapid reproduction of one-color or multicolor (full-color printing is not possible) printed products on paper in small print runs (from 20 to 1,000 copies). Under high magnification you can see that the strokes on the print consist of dots and resemble dotted lines. Paper documents or files can be used as originals.
Silkscreening
Silkscreen printing is a modern variation of screen printing that uses the same principles. But if in the stencil the overall integrity of the image is preserved by the bridges, in silk screen printing this function is performed by the mesh. The size of the stencil form is limited only by the width of the mesh produced and the size of the printing frame.
This method received its name "silk-screen printing" due to the patent for the process of screen printing issued in 1907 under the name "Silk screen printing".
Silk screen printing is probably the most affordable printing method today. On the world market you can see a lot of products printed exactly by silk-screen printing: packaging, stickers, labels, various advertising and souvenir products, etc. Nowadays silk-screen printing is applied not only in polygraphy, but also in textile, electronic, automotive, glass, ceramic and other industries.
Technical progress of silk-screen printing is observed in modern Russia. This is primarily due to appearance of foreign technologies and materials in the domestic market. Silk-screen equipment has become more accessible, and domestic producers have appeared.
Silkscreen printing is not a single technology, but a combination of several applied technologies: technology of plate frame tension, technology of choosing the most suitable screen fabric, emulsification of printing plates, use of many printing possibilities, ink selection, drying by special method and many others.
Many inks are used in modern printing, but it is in silkscreen printing that they are very widely used, revealing all their possibilities.
Silkscreen printing has a good performance at low cost and price of the finished product, which attracts both manufacturers and buyers of finished products.
There are two ways of applying images in silk-screening. The first - contact, the second - non-contact. We will tell in more details about the first (contact) way of silk-screen printing because it is used much more often. Nevertheless let us tell a few words about the non-contact method of screen printing: when using it the material does not touch the screen but the ink is transferred from the form with the help of electrostatic forces. Accordingly, in the contact method, the screen is in contact with the printed material and the ink is transferred with the help of a squeegee.
With manual silkscreen printing there is no need to buy expensive printing machines, which are required only for large print runs.
A peculiarity of the silk-screening technique is that it is possible to use a wide range of printing inks with different binders which allows to work on any materials: paper, plastic, glass, fabric, etc. The main condition for the ink is that it must not destroy the stencil material and must not dry faster than necessary for the normal printing process.
Silkscreening is especially appreciated for the thickness of the ink that can be applied to the material. This thickness is 10 and sometimes even 30 times that of offset or gravure printing, and it gives the silk screen printer the ability to create an embossed colorful image. How thick the ink will be on the material depends on the inkjet applicator, its angle of inclination, as well as the grid. In addition, the quality of screen printing is affected by the contact between the fabric and the screen, as well as the contact between the screen and the material to be printed. The contact of the fabric with the screen should be constant and uniform along the entire length of the screen.
Grid Preparation. Regardless of whether or not the stretched mesh has been treated with solutions to roughen it, degreasing is mandatory. This operation precedes the direct application of the photographic solution. Particular attention should be paid to secondary used screens which may contain not only fatty substances, but also paint residues from previous silkscreening. Degreasing should be carried out immediately before the application of the photo solution.
Chemical degreasing. Sodium hydroxide (NaOH) breaks down fats and forms compounds with them that can be easily washed out with water. An aqueous solution of caustic soda (caustic soda (caustic soda) - 20 g, water - 80 ml) using a synthetic brush is applied to the surface of the grid. This is an old and reliable way to remove greasy dirt. Since alkali reacts with aluminum, it is necessary to degrease the mesh with detergents when using aluminum frames.
After alkaline treatment, the mesh is washed with water and additionally neutralized with a 5% aqueous solution of acetic acid.
Degreasing with detergents. Surface-active agents, which include household detergents, are quite good at removing grease contaminants and can exceptionally be used to degrease stencil nets. But it should be borne in mind that water-softening agents are often added to detergents. Such additives, deposited on the surface of the mesh, are poorly washed off by water and will interfere with the strong adhesion of the photographic solution. The simpler the detergent composition, the fewer problems can occur.
When rinsing it is important to keep in mind that the salts contained in tap water may or may not interfere with the fabric's firm contact with the photographic solution, as they may or may not coat it with a thin film. But it is still better to wash in water with minimum salt content.
Making a stencil. There are many ways of stencil making - from cutting out the required elements with scissors and gluing them on the grid to burning out the holes in the metal sheet with the laser.
The simplest way to stencil is to cut the stencil elements out of paper and then glue them to the back side of the grid.
Various films can also be used instead of paper, including textured films that give the borders of pattern elements with halftones on the impression.
One of the ways of making a grid stencil is to apply a liquid copying solution directly on the grid by hand. Depending on the creative idea, a transparencies or negatives are exposed on the dried photolayer by contact method. The negative or positive can be an image painted by the artist on the transparent film with opaque black paint. The manifestation consists in the complete washing away of the unstitched layer from the mesh cells.
This is a fairly simple and cheap method, which, however, requires some skill and practice, which is necessary in any technique. The light-sensitive solution can be prepared and its quality controlled by yourself.
It is necessary to keep in mind that liquid photo-solutions decrease in volume after drying, and that those applied to the grained surface of the screen cloth repeat its entire structure. The uneven surface of the layer on the printed side does not allow the transparencies to be pressed tightly during copying and the stencil to the printed plane, which leads to the deterioration of image clarity.
To minimize this problem, several photo layers are applied to the outer side, which increases the thickness of the stencil, and consequently, the paint layer is thicker.
In addition, dust should be excluded, as its particles can get on the grid or on the already applied, but not dried, layer of solution and create, after developing, dotted elements, which, when coinciding with the thin elements of the pattern, can lead to hard-to-remove defects.
To get a high-quality straight stencil a uniform light-sensitive coating on the area of the sieve where the image is supposed to be copied is necessary. For this purpose, a rakelotok is used. But you can also try applying the solution with a brush.
In spite of the fact that photosolutions in liquid state are not sensitive to light, you should be careful to avoid bright and especially sunlight. The first layer can be applied in dim light, but it is better to conduct all work on the manufacture of stencils based on dichromats in orange or red light.
The working frame is set in a fixed vertical position. Take a tray, which should be wider than the future image, and pour the photographic solution. Preheat the gelatin solution in a water bath to a temperature of 50 ° C. The tray is pressed to the grid on the side which will be in contact with the surface to be sealed (outer side), tilted until it is wet with the solution and smoothly cover the required area from bottom to top. The tray is deflected to its original position until contact of the solution with the mesh stops and is moved away from the mesh. The whole operation is repeated once more. Now move on to the racetrack side of the frame and also apply the photographic solution 23 times.
The frame is dried in a dark place in a horizontal position with the outer side of the grid down. The temperature is kept within 2030 °C. Since the dried layer becomes sensitive to light, a second application of the photo solution is also mandatory under orange or red light.
After drying, the printed side is additionally coated with the photoresolution in 23 steps.
During repeated drying, the frame is placed in a dark drying cabinet, but already upside down with the printed (outer) side of the grid, and dried at 2030 ° C. Photolayers tend to self-polymerize after drying without access to light (so-called dark tanning). At high temperature and high relative humidity, this process is accelerated. Therefore, it is necessary to minimize the time between the photo layer and the copying process and to keep the room at a temperature of about 20°C and a relative humidity of 5565%. Do not perform drying with electric reflectors which blot the photolayer with direct radiation of infrared rays.
The appearance of sawtooth edges in a picture or the merging of closely spaced lines and fine details in printing is usually due to the fact that an insufficiently thick copy layer has been applied. This layer follows the grain structure of the grid on the printed side of the stencil and does not adjoin tightly to the printed surface. This waviness allows the ink to penetrate unhindered into the blank spaces. The dependence of print quality on the thickness of the photo layer applied is the more pronounced, the larger the cells and the thicker the threads of the mesh web.
If a very thick copy layer is applied at the edges of printed elements, a thicker layer of ink is deposited on the print. If the viscosity of the ink is insufficient, the outlines of such a layer begin to blur, impairing the clarity of the design.
Copying. The diapositive is mounted on the grid on the outer side with adhesive tape. The time of illumination is determined experimentally.
When making the diapositive, one should take into consideration that in order to obtain sharp image elements, the emulsion side should be mounted to the photolayer. That is, the image should be in maximum contact with the photographic layer during exposure. Making drawings on transparent film reduces exposure, and matte tracing requires much more time.
For copying, you can use the following homemade device. A box is made, on the bottom of which a single light source or a pack of fluorescent lamps is mounted. On top of everything is covered with organic glass, which allows ultraviolet light to pass through. A slide or transparent tracing paper with a drawn image is placed on top of the glass.
A stencil frame with a copying layer is pressed against the diapositive in various ways. Usually some kind of weight is used.
Screening. In order to allow the unexposed layer to swell, it is treated with water whose temperature should be 2025 ° C. The water is then heated to 40 °C to dissolve the swollen gelatin. The rinsing of the matrix lasts about 5 min.
Before the developed stencil is dried, it is desirable to remove the excess moisture from it with filter paper. The stencil is dried at room temperature.
All open fields of the grid, which should be closed, are covered with the copying solution with a small doctor blade or brush. If necessary, use a thin brush to retouch pinholes and details of the pattern.
The dried proofreading is illuminated with an appropriate light source.
A stencil designed for printing with water-soluble binder paint requires additional fixation from swelling in water. Chemical tanning is carried out in the cell with a small sieve size, or set the frame in a special tray with a recess to collect the solution and pour it over the stencil within 45 minutes. Tanning is carried out with 5% solution of alum alum at a temperature of 2530 ° C.
The copying process is based on the effect of light on the photoemulsion.
As a result, the photolayer should consist only of exposed and unexposed areas. That is why it is so important to be as careful as possible in preparing the image to be copied directly onto the photolayer and to protect the copy layer from light.
The duration and intensity of the light flux during the exposure of the stencil requires the image to have sufficient density and contrast. Depending on the printing conditions a diapositive or negative on photographic film, tracing paper and other transparent films is made for copying; the main thing is that the material should let in UV light which leads to the tanning of the photolayer. For precise matching of contours in multi-color printing, the fields of transparencies are marked with identical marks in the form of drive crosses. Pre-adjustment of stencils and subsequent control during printing is carried out using these marks.
Removal of the old gelatin layer. The stencil is thoroughly washed from the remnants of paint and for the second use of the grid it is treated with a destructive solution.
Silkscreen printing. Various configurations of printing equipment are used in silkscreen printing. The alignment of the printing plate with the material to be printed occurs in a variety of ways. In some cases the stencil with the frame is lifted up, in other cases the working table is tilted down.
The simplest and most frequently used method in manual screen printing is that the stencil frame on one side is fixed to the hinges and lifted at a slight angle in order to remove the finished print and put a new product. Sometimes the hinges are provided with clamps for attaching them to the table, which is convenient in cases where stenciling is done occasionally.
In some cases it is necessary to leave a gap between the grid and the print plane. For printing format 30S40 cm set a gap of 13 mm, and for format 80S100 cm increase to 35 mm. The contact of the stencil with the surface occurs with the movement of the doctor blade.
Silkscreen printing on textiles is carried out without a gap. The stencil mesh presses tightly against the fabric and reduces the possibility of its displacement when the stencil is moved. The frame is lifted slowly, because during the contact it takes some time for the paint to wet the grainy uneven surface of the fabric. In general, silkscreen printing is probably the most versatile type of printing in terms of printable materials: in addition to the usual paper and cardboard you can use a variety of plastics and films (while for offset printing only some plastic materials specially designed for this purpose are suitable). This leads to the obvious conclusion that there is a fairly wide range of coatings that can be applied by silkscreening.
Inks
Silkscreen printing allows the application of colorful layers of varying thicknesses to the printed material, which depends on the structure of the mesh used as the plate. In offset, the ink layer thickness can also be adjusted, but only to a small extent. Since the layer thickness in silkscreen is many times greater than in offset, the ink intensity on the print will be significantly higher. Moreover, as a rule, hiding colors are used in silk-screen printing, which means that one can easily, for example, print yellow ink on blue paper. And the yellow stays yellow, and doesn't turn into a barely noticeable green as it would in offset printing.
Very interesting graphic effects are obtained by using a covering white ink and the silkscreen method of application. You can, for example, put it on a dark base (tinted paper, gray cardboard, etc.) and print an image on top of it on a conventional offset press - naturally, after the silkscreen paint has dried. Covering whitewash can also be used for offset printing, but it is not possible to obtain such coverage quality because the ink layer in offset is much thinner.
The results of printing with bleached whites on transparent plastic are no less interesting. For example, if you offset a grayscale image on laminate and stencil a white opaque paint on top, you get a bright, saturated image that looks through the laminate. In this way you can make, for example, transparent labels or promotional materials for the design of outlets.
Gold, silver, metallized paints. In principle, both gold and silver can be applied with the usual offset method. However, as already mentioned, due to the greater thickness of the ink layer in screen printing, the metallic effect will be significantly higher. Moreover, it has been observed that metallic inks applied by offset take longer to dry. Therefore, if promptness is important, screen-printed metallization may be the best solution.
Fluorescent inks. There are quite a few types of these inks, but only with the use of screen printing can you get a really bright, saturated, fluorescent image - again, due to the greater thickness of the paint layer.
It should be kept in mind that all of the above applies to a variety of printable materials, among which are both quite common and quite exotic: glass, metal, leather, wood, various plastics, etc., for each of which there are special ink formulations. This means that the number of options for these very types of paints is truly enormous.
Obviously, it is simply impossible to review each type of paint or printing medium in detail. Therefore, we will describe only the basic components for making inks used in screen printing, as well as the inks and solvents used in modern printing.
Inks for screen printing are produced taking into account certain specifications for printing and finishing of products.
The ink must be liquid enough to pass freely through the stencil mesh without clogging the small mesh holes. In addition, it must have certain flow (rheology) properties that allow it to pass freely through the stencil and form an even, continuous film (strip) on the surface of the printing material. If the ink is too thick, it may not flow sufficiently after printing, leaving conspicuous grid marks on the surface of the fixed film.
However, if the ink is allowed to flow freely, it will not have enough adhesion to form a solid film with a distinct structure on the material. Paint that does not have the proper consistency tends to flow around the edges of the film. This phenomenon is exacerbated by the absorbent properties of the material being printed. The ink must adhere permanently and firmly to the material to be printed, that is, it must be adhesive.
Since many surfaces, especially those made of certain types of plastic, do not match well with conventional inks for printing on paper and cardboard, there is a wide variety of special inks.
After the paint reaches the surface, it should form a solid, dry film. Most paints are required to be fast on the material but remain liquid on the stencil. Depending on the composition, it may have a matte, semi-gloss, or high-gloss finish. The fixed film must be scratch and abrasion resistant, and it must be able to withstand subsequent printing and finishing processes without cracking.
All inks include a pigment, which should provide deeply saturated colors that are also highly resistant to light and weather conditions. Paints based on a single pigment provide the most impeccable (with minimal attenuation) colors.
The pigment provides the color and, in some cases, the hiding power (optical density) of the paint. It is derived from organic and inorganic materials. Organic pigments are naturally stable, while inorganic pigments are less expensive and reproduce deeply saturated hiding colors. It should be noted that pigment is usually the most expensive element among the main components of paint.
The paint manufacturing process takes into account established safety rules and regulations. All paint cans should have labels indicating whether the paint contains hazardous substances.
The use of lead and chromium-based pigments is prohibited, especially in paints used in food packaging, toy trim, etc.
The most common solvent-based inks contain a film forming agent, a fast setting siccative, a thinner and a modifier.
Adhesion and bonding ability of inks are provided by binder which ensures its penetration through the stencil and promotes adhesion with the material to be printed. The binder consists of a viscous varnish-like substance that shrinks in the solvent and forms a continuous hard and flexible film after curing. Many kinds of resins are used in the production of screen printing inks. Their choice is mainly determined by the type of substrate and the finishing requirements. For example, paper and cardboard inks can be made on a resin basis, while acrylic or vinyl inks contain an acrylic or vinyl resin system. Special resin systems that can be chemically cured with UV light are used in the production of UV-curing inks.
Most printing inks are fixed by the evaporation of a solvent. It is often used for a specific purpose - for example, to increase the fluidity of the ink. However, it can be difficult to cure because the solvent in the resin system can interfere with evaporation. It may therefore be necessary to add another solvent in order to modify the properties by increasing the degree of evaporation, giving the opposite effect. However, if a solvent with a high evaporation rate is used in the paint without any modification, the paint may very quickly set not only on the substrate but also on the stencil, clogging it immediately and therefore making the overall production process more difficult.
Solvents are used to reduce the viscosity of the paint and to dissolve its resin components so that they can mix more easily with the other ingredients. In some inks, solvents promote adhesion, such as in vinyl inks, where the solvent "dilutes" the printing surface, resulting in a strong adhesive bond.
Fixatives are a combination of metallic salts of calcium, lead and cobalt. They are added to paints that are cured by oxidation. Most of these paints are made with alkyd resin and form a hard and flexible film after curing. It usually takes 68 hours to cure. This process can be accelerated by increasing the temperature (through use of a drying cabinet) to 80120 °C. At this temperature, curing takes 515 min.
In order to increase the content of non-vaporizable ingredients in the paint and to improve its rheological properties, a filler is added to it. As mentioned above, the pigment is the most expensive component. Adding colorless, non-vaporizing inert ingredients, such as calcium carbonate, increases the volume of the ink without changing the printability of the ink. The filler can also increase the hiding power (optical density) of the ink. In addition, there are fillers that can be used in transparent inks to increase their volume without compromising their ability to transmit light.
The whole spectrum of additives also includes modifiers that can be added to inks to enhance their printing and finishing characteristics.
Some screen inks, such as alkyd, vinyl, or epoxy-based inks, tend to bubble excessively during printing. This is caused by air entering the ink as it is pushed back and forth along the stencil. Normally, the bubbles formed will burst as the ink passes through the stencil and the ink film spreads out to form an even, smooth surface. In some inks, however, these bubbles disappear slowly, causing unevenness and mottling on the paint film surface that is already fixed. This problem can be solved by adding a liquid substance, such as silicone. Silicone reduces the tension of the paint surface and allows trapped air to escape while the film is still attached.
Anti-settling additives are added to paints that contain thick, high viscosity pigments and thinners. These additives prevent solids from settling from suspension to the bottom of the paint container, thereby extending the shelf life of the paint.
Wax adhesives are added to screen printing inks to improve their resistance to abrasion. However their addition, as all other modifiers, should be strictly dosed. Incorrect application of modifiers may cause decrease of gloss or gloss, as well as interlayer adhesion of inks of different colors.
In some types of paints, plasticizers are added to increase the flexibility of the anchored paint film. They are an important component in plastisol inks used in garment printing and inks for transfers and decals, especially when the ink film has to be resistant to excessive physical pressure during the transfer process. But plasticizers are also chosen with caution, since they can cause adhesion problems due to delayed migration, leading to cracks in the paint film and worsening the interlayer adhesion. And all of this can only be discovered after some time has passed since printing.