Textile Printing Process, Type of Printing, and Printing Machinery

In printing, wooden blocks, stencils, engraved plates, rollers, or silkscreens are used to place colors on the fabric. Colorants used in printing contain dyes thickened to prevent the color from spreading by capillary attraction beyond the limits of the pattern or design.

Traditional textile printing techniques may be broadly categorized into four styles:

• Direct printing, in which colorants containing dyes, thickeners, and the mordants or substances necessary for fixing the color on the cloth are printed in the desired pattern.
• The printing of a mordant in the desired pattern prior to dyeing cloth; the color adheres only where the mordant was printed.
• Resist dyeing, in which a wax or other substance is printed onto fabric which is subsequently dyed. The waxed areas do not accept the dye, leaving uncolored patterns against the colored ground.
• Discharge printing, in which a bleaching agent is printed onto previously dyed fabrics to remove some or all of the color.

Resist and discharge techniques were particularly fashionable in the 19th century, as were combination techniques in which indigo resist was used to create blue backgrounds prior to block-printing of other colors. Most modern industrialized printing uses direct printing techniques.

Origin of Textile Printing

Woodblock printing is a technique for printing text, images or patterns used widely throughout East Asia and probably originated in China in antiquity as a method of printing on textiles and later paper. As a method of printing on cloth, the earliest surviving examples from China date to before 220, and from Egypt to the 4th century.

Textile printing was known in Europe, via the Islamic world, from about the 12th century, and was widely used. However, the European dyes tended to run, which restricted the use of printed patterns. Fairly large and ambitious designs were printed for decorative purposes such as wall-hangings and lectern-cloths, where this was less of a problem as they did not need washing. When paper became common, the technology was rapidly used on that for woodcut prints. The superior cloth was also imported from Islamic countries, but this was much more expensive.

The Incas of Peru, Chile, and Mexico also practiced textile printing previous to the Spanish Invasion in 1519; but, owing to the imperfect character of their records before that date, it is impossible to say whether they discovered the art for themselves, or, in some way, learned its principles from the Asiatics.

During the latter half of the 17th century the French brought directly by the sea, from their colonies on the east coast of India, samples of Indian blue and white resist prints, and along with them, particulars of the processes by which they had been produced, which produced washable fabrics.

Early Textile Printing Methods

The other forms of textile printing are stencil work, highly developed by Japanese artists, and block printing. In the latter method, a block of wood, copper, or other material bearing a design in intaglio with the dye paste applied to the surface is pressed on the fabric and struck with a mallet. A separate block is used for each color, and pitch pins at the corners guide the placing of the blocks to assure accurate repeating of the pattern.

• In-cylinder or roller printing, developed in 1785, the fabric is carried on a rotating central cylinder and pressed by a series of rollers each bearing one color. The design is engraved on the copper rollers by hand or machine pressure or etched by pantograph or photoengraving methods; the color paste is applied to the rollers through feed rollers rotating in a color box, the color being scraped off the smooth portion of the rollers with knives.

• More recent printing processes include screen printing. This is a hand method especially suitable for large patterns with soft outlines, in which screens, one for each color, are placed on the fabric. Then, the color paste is pressed through a wooden squeegee.

• Spray printing, in which a spray gun forces the color through a screen; and electrocoating, used to apply a patterned pile is the other latest printing process.

• In certain cases, the cloth is painted by using a pen with dyes and mordants. This method is known as kalamkari, a pen work. Printing the outline of the design and filling in the details with a kalam, a pen, combines the techniques of printing and kalamkari.

• Direct printing is practiced all over India where bleached cotton or silk fabric is printed with the help of carved wooden blocks.
  Another technique employed was printing with the use of mordants. Mordants are chemicals that absorb the dye. The cloth is first printed with mordants and then immersed in a dye bath. Only the sections that have absorbed the mordant absorb the dye. The cloth is then washed in flowing water and spread out to dry on the riverbank allowing the sun to develop the color. Then the untreated sections were bleached with local ingredients like goat droppings, etc. Recently, discharge printing with the use of chemicals has been developed. Here dyes when printed react on one another, either bleaching the background material or producing a different shade.

Types of Textile Printing

Discharge Printing

Discharge Printing is also called Extract Printing. This is a method of applying a design to dyed fabric by printing a color-destroying agent, such as chlorine or hydrosulfite, to bleach out a white or light pattern on the darker colored ground. In color-discharge printing, a dye impervious to the bleaching agent is combined with it, producing a colored design instead of white on the dyed ground.

Discharge printing has been around for decades. But only in the past 7-8 years screen printers in the industry have recognized it seriously. In the early years of discharge printing, the finished discharge print needed to be steamed during the drying process. This discouraged the use of discharge systems in the finished garment arena. The newly developed discharge ink systems are chemically reactive and don’t need to be steam-neutralized. This advancement opened the door to discharge printing for the average screen-printer.

Discharge printing has the ability to make bright, opaque colors on dark fabrics with a soft hand. Years ago the idea of opaque colors on dark fabrics and soft hands couldn’t co-exist.

Successful light-on-dark printing with plastisol relies on increased pigment loads, fillers, and other additives to block out the color of the garment. Discharge inks modify the garment color by removing the garment color and replacing it with the new ink color. In simple terms, the discharge ink “bleaches” out the dye in the garment, thus allowing the pigment in the ink to absorb into the shirt fibers.

The real magic of discharge printing can be witnessed when printing the four-color process on black 100% cotton shirts. The print before curing appears very transparent. One can barely see the print until the garment exits the oven chamber, where the results can be quite remarkable: bright, vivid colors with a soft hand.

The graphics on the casino gaming tables are printed with discharge inks to avoid the interference of the printed line with the roll of the dice. If the ink on these tables were printed with plastisol, the ink film (because it is a surface print) would change the speed and direction of the dice, thus changing the way the dice land. Discharge ink, on the other hand, provides a dyed-in-the-fabric result, keeping the playing surface smooth.

The decrease in the production time is the biggest bonus of all. The fact that you can skip flash curing completely saves hours of production time and eliminates registration problems between the design’s colors and the white printer under base used in normal printing on blacks.

However, flash curing can be used in conjunction with discharge printing when printing discharge as an under the base.

Cleaner and more transparent inks can also be printed onto dark garments with the help of discharge additives. Early discharge additives were designed only for water-based inks, but plastisol additives are available.

Characteristics that indicate a garment will work with discharge ink

• The garment has to be made of natural fibers (100% cotton)
• The dye used in the garment must be dischargeable. The best results are achieved with garments that are 100% cotton and dyed with a reactive dye.
• The garment should not have been over-dyed (when the fabric is re-dyed to another color). This often happens because of a shortage of a certain fabric color or, in many cases, because quality control rejected the fabric color. These rejected colors are then dyed with a black dye, which will bring nightmares to life when trying to use discharge inks. The discharge ink might discharge the black dye – only to reveal a phantom color underneath.

Always test your garment to see if it is suitable for discharge printing. If you are a major printer doing large-volume printing, be aware that the shirts you order from the mill are tracked by lot numbers and it is possible that a completely different dye may be used from one lot to the next. Let your sales representative know that you are doing discharge printing and mention in writing that you need a dischargeable garment.

Discharge printing is frequently used for all-over prints because of its soft hand

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Direct Printing

A direct print is one in which the design is printed on a white cloth or a previously dyed fabric. The latter is called overprint and here the printed design must be of a darker color than the background design. This style is also called application printing because the design is directly printed over the fabric.

It is the most common approach to apply a color pattern on fabric. It can be done on white or colored fabric. If done on colored fabric, it is known as overprinting. The desired pattern is produced by imprinting dye on the fabric in a paste form. To prepare the print paste, a thickening agent is added to a limited amount of water, and dye is dissolved in it. Earlier corn starch was preferred as a thickening agent for cotton printing. Nowadays gums or alginates derived from seaweed are preferred because they are easier to wash out, do not themselves absorb any color, and allow better penetration of color. Most pigment printing is done without thickeners as the mixing up of resins, solvents, and water itself produces thickening.

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Resist Printing

Resist or reserve printing is related to discharge printing in that the end results are often indistinguishable. The resist style, however, offers the advantage that dyes of great chemical stability, which could not be discharged, can be resisted to give prints of high fastness standards.

Resist printing involves a two-step procedure:

• Printing a pattern design on fabric with a chemical or wax-like resinous substance that will prevent or resist the penetration of dyes.
• Piece dyeing the fabric

The justification for both styles lies in the aesthetic appeal of a white or colored pattern on colored grounds, an effect that very often could not be reproduced by any other technique. The difference, therefore, between discharge and resist printing is not one of appearance, but of process. In discharge printing, the discharging agent is applied to the fabric after it has been dyed and the dye in the printed areas is destroyed during subsequent processing.

In resist printing, the resisting agent is printed onto the undyed fabric and effectively prevents the fixation or development of the ground color, which is subsequently applied by an appropriate ‘dyeing’ technique, such as dyeing, padding, or overprinting. The result can be either a white resist or a colored resist, where a selected dye or pigment is added to the resist paste and becomes fixed to the fiber during subsequent processing.

Virtually every class of colorant is capable of being resisted, as is borne out by reference to older publications on textile printing. Many of the techniques they describe are too complex and time-consuming to be of commercial interest today, but they do illustrate the wide scope of the style, with the proviso that it has little application to synthetic-fiber fabrics.

The resisting agents employed, then as now, function either mechanically or chemically or, sometimes, in both ways. It is used where background colors in a fabric cannot be discharged. It is usually not possible to visually differentiate between the discharge and resist printed fabrics since both of them produce the same results.

The mechanical resisting agents include waxes, fats, resins, thickeners, and pigments, such as china clay, the oxides of zinc and titanium, and sulfates of lead and barium. Such mechanical resisting agents simply form a physical barrier between the fabric and the colorant. They are mainly used for the older, coarser, and, perhaps, more decorative styles in which breadth of effect and variety of tone in the resisted areas are of more importance than the sharp definition of the pattern.

A classical, and nowadays almost unique, example of a purely mechanical resist is to be found in the batik style, using wax applied in the molten state. In a true batik, the wax is applied by hand, but the process has been developed and mechanized for the production of those styles which now come under the general heading of ‘Africa prints’. It is not possible to apply an illuminating color with wax resist but, after removal of the wax, another color can be printed within the resisted area. A mechanical resist is usually used in conjunction with a chemical resist, so improving the overall effect.

Chemical-resisting agents include a wide variety of chemical compounds, such as acids, alkalis, various salts, and oxidizing and reducing agents. They prevent fixation or development of the ground color by chemically reacting with the dye or with the reagents necessary for its fixation or formation. The actual choice of chemical-resisting agent depends, therefore, on the chemistry of the dye being used and its fixation mechanism. Consequently, as in discharge printing, working knowledge of the relevant chemistry is necessary when choosing effective resisting agents.

Methods Of Printing

There are several methods of printing namely:

• Block printing
• Roller printing
• Screen printing
• Heat transfer printing

Block Printing

Block printing is a special form of printing first developed in China. The earliest known example with an actual date is a copy of the Diamond Sutra from 868 A.D (currently in the British Museum), though the practice of block printing is probably about two thousand years old.

The first step in block printing is the production of the original document. This is laid on a large, smooth wooden block and fixed into place, reversed. Next, craftsmen of various skill levels, ranging from master carvers for the fine work to less talented artisans for cheaper blocks or less important sections, carve the original painted, drawn or written image into the block of wood. The block can now be covered with ink and used in a press to create duplicates of the original.

The color paste must be applied to the block surface in a controlled manner, and this was achieved by using a ‘sieve’. A small tub was nearly filled with a starch paste and a waterproof fabric, stretched on a frame, rested on the paste. A piece of woolen fabric was stretched on a slightly smaller frame and fastened to make the sieve. The sieve as saturated with color paste and placed on the waterproof fabric.

For each impression, the ‘tiered’ (a boy) spread the color paste on the top surface of the woolen sieve with a large brush and the printer charged the block by pressing it on the wool. The block was then carefully positioned on the fabric, using the pitch pins as guides, and struck with a mallet. After printing a table length with the first block, the second was printed, and then any others were required to complete the design. The fabric was then transferred to a few elevated rollers or rods and allowed to dry, while the next table length was printed.

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Roller Printing

It is a high-speed process capable of producing 6000 yards of printed fabric per hour. It is also called machine printing. Printing, originally a handcraft, developed into an industrial art requiring the contributions of a range of specialists, coordinated by someone with a clear vision of the desired end-product.

The first of the specialists is the creator of the original design, which may already be suitable for reproduction but usually is not, and is sometimes little more than an idea. Another specialist is the engraver whose task is to convert the original design into a set of engraved rollers that will enable a printer to achieve an effective reproduction of the design on fabric. Sensitivity to the original design objectives and awareness of the printer’s requirements is important as well as skill and accuracy.

An original design must be put into repeat and the dimensions adjusted so that one or more repeats will fit accurately around the roller circumference. If the repeat is small it may be that the mill-engraving method should be used. This starts with the hand engraving of a few repeats on a small soft-steel cylinder, which is then hardened. The design is then obtained in relief by running the first cylinder (the die) under pressure in contact with a second soft-steel roller. This relief roller (the mill) is hardened and run in contact with the copper cylinder to obtain the desired depth of impression, and this is repeated across the cylinder until the full width is engraved. The raised copper around each groove must then be polished off.

In the roller printing process, the print paste is applied over an engraved roller and the fabric is guided between this roller and a central cylinder. The pressure of the roller and the central cylinder forces the paste into the fabric. Approximately 26% of printed goods are printed using engraved roller printing.

Advantages

• High design capability
• Fine detail
• Multiple tones

Disadvantages

• copper cylinders are very expensive
• not economical for short runs
• requires highly skilled workers

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Screen Printing

Screen printing is the most flexible printing process. It can be used to print on a wide variety of substrates, including paper, paperboard, plastics, glass, metals, fabrics, and many other materials including paper, plastics, glass, metals, nylon, and cotton. Some common products from the screen-printing industry include posters, labels, decals, signage, and all types of textiles and electronic circuit boards. The advantage of screen-printing over other print processes is that the press can print on substrates of any shape, thickness, and size.

An important characteristic of screen printing is that a greater thickness of the ink can be applied to the substrate as compared to the other printing techniques. This allows for various interesting effects that cannot be achieved through the other printing methods. Because of the simplicity of the application process, a wider range of inks and dyes are available for use in screen printing than for use in any other printing process.

The utilization of screen printing presses has begun to increase because production rates have improved. This has been a result of the development of the automated and rotary screen printing press, improved dryers, and U.V. curable ink. The major chemicals used in screen-printing include screen emulsions, inks, solvents, surfactants, caustics, and oxidizers used in screen reclamation.

Overview of the Screen Printing Process

Screen-printing consists of three elements

• The screen which is the image carrier
• The squeegee
• Ink

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

Screen printing ink is applied to the substrate by placing the screen over the material. Ink with a paint-like consistency is placed on the top of the screen. Ink is then forced through the fine mesh openings using a squeegee that is drawn across the screen, applying pressure thereby forcing the ink through the open areas of the screen. Ink will pass through only in areas where no stencil is applied, thus forming an image on the printing substrate. The diameter of the threads and the thread count of the mesh will determine how much ink is deposited onto the substrates.

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

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

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

• Development of automatic presses versus hand-operated presses, which have comparatively slow production time.
• Improved drying systems, which significantly improve production rate.
• Development and improvement of U.V. curable ink technologies
• Development of the rotary screen press, which allows continuous operation of the press. This is one of the recent technological developments.

Screen Preparation

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

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

Hand Screen Printing

The silkscreen is a wooden or aluminum frame with a fine nylon or silk mesh stretched over it. The mesh is coated with a light-sensitive emulsion or film, which – when dry – will block the holes in the mesh. The image that needs to be printed is output to film either by camera or image-setter. This film positive and the mesh on the screen are sandwiched together and exposed to ultraviolet light in a device called a print-down frame. The screen is then washed with a jet of water which washes away all the light-sensitive emulsion that has not been hardened by the ultraviolet light. This leaves you with an open stencil that corresponds exactly to the image that was supplied on the film. Next, the fabric to be printed is pinned on a wooden table so that it is evenly stretched and there are no ripples.

Alternatively, a wax table is used. The surface of the table is covered with wax. Below there is a network of pipes through which steam is passed. This causes the wax to soften and the fabric is just firmly pressed onto the table. The wooden frame of the screen is fitted with metal handles that will fit onto corresponding wooden protrusions on the table. This is to aid placement when two or more colors are being used. The dye is poured on the screen A rubber blade with a wooden handle is firmly pulled across the top of the screen; it pushes the ink through the mesh onto the surface of the fabric which is being printed. Another person stands at the other side of the table. He takes hold of the rubber blade and repeats the process.

Since the nonprinting areas on the silkscreen are blocked out, the ink is pushed through only the porous areas corresponding to the design and is thus transferred to the fabric. If more colors are used, the process is repeated with another screen and color. The screen is always washed with a lot of water immediately after use. If this is not done, the dye dries on the screen and clogs up the design.

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Flat Bed Screen Printing

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

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

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

• Mechanization of hand screen process
• Fabric glued to blanket
• Screens rise and fall
• Printing is done while the screen is in the down position
• Rod or blade squeegee system
• Up to four strokes possible
• Slow process

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

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

Advantages

• large repeats
• Multiple strokes for pile fabrics

Disadvantages

• Slow
• No continuous patterns

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Rotary Screen Printing

In rotary-screen printing, continuous rotation of a cylindrical screen while in contact with the fabric ensures genuinely continuous printing. The print paste is fed into the inside of the screen, and during printing is forced out through the design areas with the aid of a stationary squeegee.

The design of most machines follow the pattern established for fully automatic flat-screen machines: an endless driven blanket, screen positions along the top, and blanket washing and drying effected underneath during the return passage. Provision for the use of a thermoplastic adhesive is common on rotary machines, with a curved-surface heating plate to heat the fabric before it is pressed onto the blanket.

The cylindrical screens can be much closer together than is possible with flat screens and so the blanket is shorter (for a given number of colors). The fabric dryer, however, must be longer to enable the printed fabric to be adequately dried at higher running speeds. Typically, speeds of 30–70 m min–1 are used depending on the design and the fabric quality. It is quite possible to run the machine faster than this, the limitations often being the length and efficiency of the cloth and blanket dryers and the difficulty of observing

Printing faults at high running speeds. The print paste is often poured into flat screens by hand, even in fully automatic machines, but the continuous movement of a cylindrical screen and the restricted access necessitates automation of this operation. The print paste is pumped into the screen through a flexible pipe from a container at the side of the machine; inside the screen, the paste pipe has a rigid structure as it also acts as a support for the squeegee.

Holes in the pipe allow the paste to run down into the bottom of the screen; since the paste is pumped in from one end, the holes need to be larger at the end furthest from the pump to achieve an even spread across the full width of the screen. A sensor (level control) actuates the pump when the paste level falls below a preset height.

• The continuous screen printing process
• Fabric glued to blanket
• Fabric moves under rotating screens
• Rod or blade squeegee system
• Fine adjustments easily made
• Speeds up to 100 yds per min

Advantages

• Fast
• Quick changeover of patterns
• Continuous patterns

Disadvantages

• Design limitations
• Small repeats

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Heat Transfer Printing

Transfer printing is the term used to describe textile and related printing processes in which the design is first printed onto a flexible nontextile substrate and later transferred by a separate process to a textile. It may be asked why this devious route should be chosen instead of directly printing the fabric. The reasons are largely commercial but, on occasion, technical as well and are based on the following considerations:

1. Designs may be printed and stored on a relatively cheap and non-bulky substrate such as paper and printed onto the more expensive textile with rapid response to sales demand.
2. The production of short-run repeat orders is much easier by transfer processes than it is by direct printing.
3. The design may be applied to the textile with relatively low skill input and low reject rates.
4. Stock volume and storage costs are lower when designs are held on paper rather than on printed textiles.
5. Certain designs and effects can be produced only by the use of transfers (particularly on garments or garment panels).
6. Many complex designs can be produced more easily and accurately on paper than on textiles.
7. Most transfer-printing processes enable textile printing to be carried out using simple, relatively inexpensive equipment with modest space requirements, without effluent production or any need for washing-off.

The design on paper is transferred to fabric by vaporization. There are two main processes for this- Dry Heat Transfer Printing and Wet Heat Transfer Printing. In Conventional Heat Transfer Printing, an electrically heated cylinder is used that presses a fabric against a printed paper placed on a heat-resistant blanket.

In Infrared Heat Vacuum Transfer Printing, the transfer paper and fabric are passed between infrared heaters and a perforated cylinder which are protected from excessive heat by a shield. The Wet Heat Transfer Printing uses heat in a wet atmosphere for vaporizing the dye pattern from paper to fabric.

The process of transfer printing to a substrate comprising applying onto temporary transfer support in the desired pattern a heat transfer printing ink composition, bringing said transfer support containing said ink into close contact with said substrate, the ink being in direct contact with the substrate, thereby temporarily transferring a portion of said ink to the said substrate as a result of the direct contact, and permanently transferring said ink to said substrate by applying heat and, optionally, pressure, wherein said ink is applied to said transfer support by the following computerized method inputting an image of the desired pattern into a computer central processing unit having a keyboard and a peripheral video display terminal and printer by means of an optical character reader, employing in said printer said heat transfer printing ink composition, and printing said the desired pattern onto conventional computer paper, said computer paper being said temporary transfer support.

Advantages

• Easier handling of units
• Easier training of operators
• Better registration and clarity
• Fewer seconds
• Inventory in paper
• Pollution-free

Disadvantages

• Slow
• Limited to synthetic fibers, mainly polyester

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Pigment Printing

70% of all printed fabrics in U.S. are printed with pigments. Since the pigments used contain significant amounts of dispersing agent, and since the polymeric binder is also a dispersion with its own dispersing agent, the printed deposit is readily redispersed in water after a short drying process.

Only when the printed fabric has been raised to an adequate temperature is the binder given enough energy to form a continuous film that incorporates the pigment particles and adheres satisfactorily to the fibre surfaces. At the same time, crosslinking of the binder molecules is completed if appropriate monomers or agents have been incorporated, and the required pH and temperature achieved.

Composition of print paste

• Pigment
• Thickener(emulsion or all aqueous )
• Binder (acrylic polymer )
• Low crock binder
• Softener

Process

1. Print
2. Dry
3. Care

Advantages

• All fibers and fiber blends
• No after wash

Disadvantages

• Poor crock fastness
• Harsh hard

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Other Methods Of Printing

Duplex Printing

Printing is done on both sides of the fabric either through a roller printing machine in two operations or a duplex printing machine in a single operation.

Stencil Printing

The design is first cut in cardboard, wood, or metal. The stencils may have fine delicate designs or large spaces through which color is applied on the fabric. Its use is limited due to the high costs involved.

Transfer Printing

The design on paper is transferred to fabric by vaporization. There are two main processes for this- Dry Heat Transfer Printing and Wet Heat Transfer Printing. In Conventional Heat Transfer Printing, an electrically heated cylinder is used that presses a fabric against a printed paper placed on a heat-resistant blanket.

In Infrared Heat Vacuum Transfer Printing, the transfer paper and fabric are passed between infrared heaters and a perforated cylinder which are protected from excessive heat by a shield. The Wet Heat Transfer Printing uses heat in a wet atmosphere for vaporizing the dye pattern from paper to fabric.

Blotch Printing

It is a direct printing technique where the background color and the design are both printed onto a white fabric usually in a one operation. Any of the methods like block, roller, or screen may be used.

Airbrush (Spray) Painting

Designs may be hand-painted on fabric or the dye may be applied with a mechanized airbrush which blows or sprays color on the fabric.

Electrostatic Printing

A dye-resin mixture is spread on a screen bearing the design and the fabric is passed into an electrostatic field under the screen. The dye-resin mixture is pulled by the electrostatic field through the pattern area onto the fabric.

Photo Printing

The fabric is coated with a chemical that is sensitive to light and then any photograph may be printed on it.

Differential Printing

It is a technique of printing tufted material made of yarns having different dyeing properties such as carpets. Up to a ten-color effect is possible by careful selection of yarns, dyestuffs, and pattern.

Warp Printing

It is roller printing applied to warp yarns before they are woven into fabric.

Tie-Dyeing

Firm knots are tied in the cloth before it is immersed in a dye. The outside of the immersed portion is dyed but the inside is not penetrated. There are various forms of Tie-dyeing like Ikat Dyeing where bundles of warp and/ or weft yarns are tie-dyed prior to their weaving. In Pelangi Dyeing the gathered, folded, or rolled fabric is usually held with stitching to form specific patterns.

Batik Dyeing

It is a resist dyeing process. Designs are made with wax on a fabric which is then immersed in a dye. The unwaxed portion absorbs the color.

Jet Spray Printing

Designs are imparted to fabrics by spraying colors in a controlled manner through nozzles.

Digital printing

In this form of printing micro-sized droplets of dye are placed onto the fabric through an inkjet print head. The print system software interprets the data supplied by a cademic_Textiledigital image file. The digital image file has the data to control the droplet output so that the image quality and color control may be achieved. This is the latest development in textile printing and is expanding very fast. Digital Textile Printing

Steps in Printing Process

1. Preparation of the print paste
2. Printing of fabric
3. Drying
4. Fixation of dyestuff
5. Washing – off

Preparation Of Printing Paste

The type of specific formulation used depends on the fiber, the colorant system used, and to some extent the type of printing machine.

Typical ingredients used include:

• Dyes or pigments
• Thickeners
• Binders, cross-linking agents
• Sequestering agents
• Dispersing agents
• Water retaining agents
• Adhesion promoters
• Defoamers
• Catalysts
• Hand modifiers

Dyestuff or pigments

Depending on the nature of the fiber on which the printing is done, suitable dyes or pigments are selected. Pigment color can be used for printing on all types of fibers. Reactive, vat, or azoic colors are used for cotton; disperse dyes for polyester and acid dyes and basic dyes for wool and silk.

Thickener

To make a viscous paste of dyes in water, a thickener is used. For example emulsion thickener, sodium alginate, starch, etc. the thickener will be dependent on the class of dyes to be printed and the style of printing.

Wetting agent

It helps in obtaining a smooth paste of dyes without any lumps, for example, TRO and ethylene oxide condensation.

Defoaming agent

Formation of foam during print paste preparation and application is quite common but should be avoided. The foam may produce specky dyeing. The antifoaming agents help in foam generation.

Acid or alkali

Depending on the types of dyes used in printing, acid or alkali is used in the print paste. An acid liberating salt is commonly used, for example, ammonium chloride and diammonium hydrogen phosphate. For reactive printing on cotton, sodium carbonate or sodium bicarbonate are used.

The oxidizing or reducing agent

They are used in printing with solubilized vat colors and also in discharge and resist printing. Discharging agents such as Sodium sulphoxylate formaldehyde (Rongalite) are used in the discharge printing.

Hygroscopic agents

The function of hygroscopic agents is to take up a sufficient amount of water (moisture) during steaming to give mobility to the dyes to move into the fiber. Eg. Urea and Glycerin.

Dispersing Agent

Dispersing agents are necessary for the print paste to prevent aggregation of the dyestuff in the highly concentrated pastes. E.g. Diethylene glycol

Preservatives

Preservatives are used to prevent the action of bacteria and fungus to make it dilute. Eg. Salicylic acid.

Binders

Binders are used in pigment printing as a thin film-forming polymer. Eg.  Melamine formaldehyde resin.

Fixation methods

Atmospheric steam

• treatment at 212 degrees f with saturated steam
• used with
  • Direct dyes
  • Vat dyes
  • Naphthol dyes
  • Acid dyes
  • Cationic dyes
  • Reactive dyes
• festoon steamer most common equipment
• Acid agar for acid dyes

Pressure steam

• treatment at 230 degrees f under pressure
• used with disperse dyes
• Turbo-autoclave most common equipment.

High-temperature steam

• treatment with superheated steam at temperature up to 420 degrees f
• used with disperse dyes and pigments
• can also be used as an atmospheric steamer

Dry heat

• treatment with dry heat at temperatures up to 420 degrees
• used with dispersed dyes and reactive dyes.

Textile Printing Machines

Machines used for printing the fabrics are explained in the forthcoming section Fabric are printed conventionally using the table screen printing machines manually.  More perfection and production can be achieved using a modern automatic flatbed screen printing machine.

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

Screen Preparation

Flat Bed Screens

A piece of nylon bolting cloth or metal gauge (phosphobraze) is stretched and nailed to a strong wooden frame, strengthened by metal brackets at the corner.  The frames are usually 26” x 55” (measured externally) and 23” x 52” (measured internally) for printing 45” wide cloth.  When the metal frame is used for making the screen, the bolting cloth may be fixed to the frame by using a solution of polyvinyl acetate in a suitable solvent.

Photochemical method is the most widely used method for preparing screen.  This is based on the principle that when a coating of solution ammonium dichromate – gelatine or ammonium dichromate – polyvinyl alcohol is dried and exposed to light, insolubilisation takes place.

The photosensitive coating may be given to the bolting cloth, fixed to the screen frame, dried, and exposed to light after keeping in contact with a positive film, and after insolubilisation of the exposed portion, the unexposed photosensitive coating may be given to the bolting cloth, fixed to the screen frame, dried and exposed to light after keeping in contact with a positive film and after insolubilisation of exposed portion, the unexposed photosensitive coating is washed out leading the blocking of the interstices of the cloth at the exposed portions and keeping them open (for forcing of the print paste later while printing) at the unexposed portion. The sensitizing solution may be prepared as follows:

Chrome – Gelatine Solution

• Solution A     – 700 g
• 200 g pure gelatine
• 500 g boiling water

Solution B     – 300 g

• 70 g Ammonium dichromate
• 150 g boiling water
• 80 g liquor ammonia

Solution A and Solution B are mixed in the darkroom.

Chrome-polyvinyl alcohol solution

600 g              polyvinyl alcohol (15% solution)

120 ml            ammonium dichromate (33% solution)

240 ml            cold water

1 litre              with cold water

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

• 50 g chrome alum
• 50 g formaledehyde
• 25 g sodium dichromate
•  1 liter

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

• 50 ml                          acetaldehyde
• 50 ml                          Isobutyraldehyde
• 80 ml                          water
• 20 ml                          sulphuric acid (168 Tw)
• 1 litre                          with cold water

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

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Table Screen Printing

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

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Flat Bed Screen Printing Machine

The printing operation in a flatbed screen printing machine can be made fully automatic by standardizing the stages of preparation and producing the print including screen holding, the addition of print paste, pressing the paste onto the cloth, lifting the screen, and carrying the cloth forward to the next screen printing screen position.  The automation of these stages makes the reproduction of printing results quite easy.

The fabric is suitably fed to the machine in a crease-free manner.  The print pattern is registered on the fabric by pressing the printing paste through the specially engraved portions on the flat screens.  There are as many screens as the number of colors in the print pattern.  The fabric to be printed is conveyed, i.e. carried forward, with each color, register by register, while the flat engraved screens continuously rise and come down,  at each repeat of the color pattern are printed at the same time, but on differed printing places of the cloth.  The entire colored pattern will be printed only when the far end screen completes its printing operation.

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

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Rotary Screen Printing Machine

This system with the matrix consists of the required number of protrusions per square inch (or square cm) Metallic nickel is electro-deposited in the hexagonal gaps in between the protrusions.  The most common mesh sizes used are 60, 80, 100, and 120.  Needless to say, the finer the mesh number, the finger is the pattern printed through the particular mesh.  The cylindrical perforated rotary screens are formed in the forcing trimmer.

The copied designs are transferred to the rotary screen by using a photo emulsion of suitable sensitivity.  The exposed portions retain the emulsion. The emulsion from the unexposed portion is washed away, thus forming the design as far as the color which is to be passed on to the cloth from that particular screen. For each color one separate cylindrical perforated screen is used. The entire pattern is formed on the fabric when the rotary screens pertaining to all the colors in the pattern have been printed on the fabric through the respective color screens.

The chosen design is engraved by one of the methods mentioned earlier.  The rotary engraved screens belonging to the desired pattern are placed and fixed in their proper positions in a predetermined order.  Although patterns with 12 to 14 colors are sometimes printed, only 5 or 6 colored designs are usually most commonly used. The printing paste is introduced in the center of the rotary screen by means of the colour pump and the colour distribution system.  The colour paste is well distributed all over the interior of the rotary screen and is pressed onto the fabric by means of a well-designed squeegee system.

The substrate, i.e., the cloth, travels over and along with the endless printing conveyor blanket to the end of the printing tabletop.  The printing process is completed at this stage, and the printed cloth is then led to the continuous dryer.  The dryer, having an adequate evaporation capacity is suitably designed.  After drying the fabric is passed on t the plaiting down arrangement. The rotary screens run in continuous pressurized contact with the fabric which is to be printed and also the endless printing conveyor belt.

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Curing Process

Curing Chamber

In case of pigment printing, the printed fabric has been raised to an adequate temperature for giving the binder enough energy to form a continuous film.  This binds the pigment particles and achieves satisfactory adhesion to the fibre surface.  At the same time, cross-linking of binder molecules is achieved if suitable monomers or agents have been used, and the required pH and the temperature are achieved.  The equipment for this purpose is the roller baker “curing oven”. It has an arrangement for carrying woven fabrics through hot air, with as many fabric transport rollers as are required to provide sufficient exposure time and fabric speed desired.  Treatment time varies from 5 to 8 minutes and the temperature of the treatment ranges from 140 to 160 C.

Loop Ager

Loop ager is continuous process equipment in which the cloth is handled in loop form with the least contact and transported in a tensionless manner.  It is a versatile machine by which hot air curing, saturated steaming water injection along with steam is involved.  Before starting, the inner parts are cleaned and made free of water.  The air inside can be expelled by slow injection and closing the manhole door. According to process temperature, heating value lines are kept open and temperature is set.  The speed of the machine is set as per process time and loop length.  Pieces of printed fabric (single or double web) are stitched to the leader fabric and fed into the chamber.

After the process, the cloth is drawn out from the chamber and is carried to the plaiter through top guide rolls.

Advantages

• The continuous process leads to consistency in fixation.
• Higher productivity and facility of process double webs.
• Better temperature control and less scope for variations.

Disadvantages

• Not economical for short runs.
• Consumes time and energy for heating and cooling.
• Risk of loop rods struck up falling and damage to the fabric.

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Flat-Screen Printing

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

Screen printing with an automatic squeegee system

 Flat-screen printing machines can be

• Manual
• Semi-automatic
• Completely automatic.

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

• • The fabric is first glued to a moving endless belt.
  • A stationary screen at the front of the machine is lowered onto the area that has to be printed and the printing paste is wiped with a squeegee.
  • Afterward, the belt, with the fabric glued on it, is advanced to the pattern-repeat point and the screen is lowered again.
  • The printed fabric moves forward step by step and passes through a dryer. The machine prints only one color at a time.
  • When the first color is printed on the whole length of the fabric, the dried fabric is ready for the second cycle and so on until the pattern is completed.

Flat-Screen Printing Machine

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

Mechanized Screen Printing Machine with Stationary Screens Mounted in a Frame

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

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

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

Features

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

• Pneumatically drive.
• Low noise
• Printing table with X, Y, and Rotation for accurate registration.
• Printing table with powerful Vacuum to hold the substrate on to its position while printing.
• Maximum Substrate height 40 mm.
• Squeegee /coater pressure with fine adjustment.
• The angle can be set as per requirement.

Applications

• Letterhead, wedding cards printing
• Stickers printing
• PCB printing
• Metal sheet printing
• Glass printing
• Scale printing
• Printing on Plastic plates
• V Cabinet
• Washing machine panels
• Industrial control Panels
• Membrane switches
• Battery containers
• Umbrella panels
• T-shirts
• Coaster printing
• Container lids
• Tube light fittings
• Speedometer dials
• Car stickers

Rotary-Screen Printing

Rotary-screen printing machines use the principle in which, the colour is transferred to the fabric through lightweight metal foil screens, which are made in the form of cylinder rollers. The fabric moves along in continuous mode under a set of cylinder screens while at each position the print paste is automatically fed to the inside of the screen from a tank and is then pressed through onto the fabric. A separate cylinder roller is required for each colour in the design.

A Rotary-Screen Printing Machine

A conventional paste feeding system for rotary-screen printing machines is as follows.

• A suction pipe leads from the paste vat to a pump, from where a printing hose leads to the squeegee (dye pipe with a squeegee). From here the paste is directed inside the cylinder roller.
• The fill volume of this so-called printing paste input system is quite high and as a consequence, the amount of paste residue that has to be removed at each colour change is also fairly significant.
• Various systems have been introduced in order to lower the volume configuration of this equipment, which also reduces the amount of such wastes.
• Another possibility, which has also already been implemented in some companies, is to recover and re-use these residues for making up new recipes.

Printing-Paste Feeding System For A Rotary-Screen Printing Machine

Rotary-screen printing machines are equipped with both gluing and washing devices analogous to those described earlier for flat-screen printing. The belt is washed in order to remove the residues of paste and adhesive. Not only the belt, but also the screens and the paste input systems (hoses, pipes, pumps, squeegees, etc.) have to be cleaned up at each colour change.

Rotary Screen Printing Machine

Rotary Screen Printing Machines are widely used in a large number of industries for faster and finer printing solutions. These printing machines are specially designed keeping printing precision and clarity in mind. These printing machines are apt for medium to high volume printing. These machines are used for both industrial and graphical applications.

Salient Features

• Variable screen angle adjustment.
• High grade aluminum castings.
• Robust and sturdy, one-piece tubular steel base.
• High precision tapered roller bearings for rotary sections.

Applications of Rotary Screen Printing Machine

Rotary screen printing machines are used in almost every industry as they provide quick, easy, and high-quality printing solutions.

• Specialty Advertising Industry
• Pharmaceutical Industry
• Cosmetic Industry
• Automobile Industry
• Membrane Switches
• Metal or plastic Sheets
• Glass and Ceramics

Roller Printing Machines

Roller printing machines work as follows.

• In roller printing, the print paste is supplied from reservoirs to rotating copper rollers, which are engraved with the desired design. These rollers contact the main cylinder roller that transports the fabric.
• By contacting the rollers and the fabric the design is transferred to the fabric. As many as 16 rollers can be available per print machine; each roller imprints one repeat of the design.
• As the roller spins, a doctor blade in continuous mode scrapes the excess of paste back to the colour trough.
• At the end of each batch, the paste reservoirs are manually emptied into appropriate printing paste batch containers and squeezed out.
• The belt and the printing gear (roller brushes or doctor blades, squeegees, and ladles) are cleaned up with water.

 

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Jet Printing

Jet printing is a non-contact application system originally developed for printing carpets, but now increasingly used in the textile sector.

The first commercial jet printing machine for carpets was the Elektrocolor, followed by the first Millitron machine. The Millitron printing system works, as follows.

• The injection of the dye into the substrate is accomplished by switching on and off a dye jet by means of a controlled air stream.
• As the carpet moves along, no parts of the machine are in contact with the face of the substrate.
• Air streams are used to keep continuously flowing dye jets, deflected into a catcher or drain tray. This dye is drained back to the surge tank, filtered and re-circulated.
• When a jet is requested to fire, the air jet is momentarily switched off, allowing the correct amount of dye to be injected into the textile substrate.
• The dye is supplied in continuous mode to the main storage tank to compensate for the amount of dye consumed.

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Millitron System

Spray printing systems and first-generation jet printing methods cannot be controlled to produce a pre-specified pattern. Thus the equipment must first be employed to produce a wide range of effects and only then can selections be made from these by the designer or marketing staff.

Digital Carpet Printing Machines

An early improvement was made by the first digital carpet printers (Chromotronic and Titan by Zimmer and Tybar Engineering, respectively). These machines are based on the so-called drop-on-demand principle, namely the use of switch-able electromagnetic valves placed in the dye liquor feed tubes to allow the jetting of discrete drops of dye liquor in a predetermined sequence according to the desired pattern. The features of this machine are as follows.

• In these machines, although the amount of dye applied can be digitally controlled at each point of the substrate, further penetration of the dye into the substrate is still dependent on the capillary action of the fiber and fiber surface wetting forces.
• This can lead to problems of reproducibility (e.g. when the substrate is too wet) and means that it is still necessary to use thickeners to control the rheology of the dye liquor.

Some latest jet printing machines are:

• The latest improvement in jet printing of carpet and bulky fabrics is the development of machines in which the colour is injected with surgical precision deep into the face of the fabric without any machine parts touching the substrate. Here, the control of the quantity of liquor applied to the substrate (which may vary for example from lightweight articles to heavy quality fabrics) is achieved by varying the firing time and the pumping pressure.
• Another digital jet printing machine commercially available is Zimmer’s Chromo jet. In the Chromo jet system, the printing head is equipped with 512 nozzles. These are magnetically controlled and can open and close up to 400 times a second. The carpet is accumulated into a J-box, and is then steamed and brushed. When it reaches the printing table it is stopped. The jets are mounted on a sliding frame that can itself be moved in the direction of the warp while the carpet remains stationary during the printing process.
• Ink-jet printing is another digital printing technique with its origins in paper printing technology that is now also increasingly used in the textile industry. In ink-jet printing, colour is applied to the surface of the substrate without variation in firing time, pressure or velocity. For this reason it can only be applied for flat light fabrics, especially silk.
• The TAK printing system can still be found in the carpet industry. With this technique irregular patterns can be produced. The carpet, previously dyed with a ground shade, is provided with coloured spots through dripping. The size and the frequency of the coloured spots can be varied by adjusting the overflow groove placed along the carpet width.

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Hot Stamping Machine For Flat Objects

A wide range of hot stamping machines is used for flat objects. These machines are widely acknowledged for their high Accuracy and smooth movement. These are ideally used for hot stamping on Plastic Panel, Clock, Paper Box, Garments, Books, Leather Goods, Wood, and Paper Products.

Specifications