Textile Printing Process, Type of Printing, and Printing Machinery

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