PRINTING TECHNOLOGY

Printing can be defined as the localised application of dyes or pigments in a thickened form to a

substrate in order to generate a pattern or design. The printing paste contains colourants or dyes,

thickening agents, and other addittives. The thickening agents are mainly used to improve the

adhesion between the ink and the substrate as well as prevent the ink from spreading away from the

target position.

Most classes of dyes adaptable for printing but the choice depends on the type of fabric or fibre, the

end use of the products and the costs involved.

Fr cotton and other cellulosic fibres, VAT dyes are used. Silk is printed with acid dyes. Wool is usually

treated first with chlorine to make it more receptive and is then printed with acid or chrome dyes.

Most synthetics are printed with cationic dyes and disperse dyes.

Textile materials can be printed in different forms such slubbings, vigorous printing of yarns, warp

printing, piece goods, and garments. Fabrics to be printed need to undergo preparation prior to

printing mainly to remove any impurities. Bleaching, scouring and singeing are some of the

preparatory process given to the fabric prior to printing. The fabric may also be dyed before printing

if a coloured background is required.

Dyeing v/s Printing

The fibre, yarn or fabric is impregnated with the dye stuff while in printing the pattern is generally

imprinted on the fabric surface.

Dyeing gives solid shades all over the fibre while Printing is application of colour on the surface of the

cloth on selected areas only.

During dyeing aqueous solution or dye- bath is used while during printing process a thick paste of the

dye with thickeners is used

With dyed fabrics the outline of the design is the same on both sides the fabric while for printed fabrics

the outline of the design is sharply defined on the right side.

Printing styles or techniques

Direct printing

This is the commonest method of printing and can be done on an already dyed fabric or a white fabric.

During printing each colour has a specific place on the fabric such that there is no overlapping of

colours. Successful printing is dependent on the skill of the printer and the viscosity and rheology of

the print paste used. The dyes are usually dissolved in a limited amount of water together with

thickening agents to get the desired viscosity.

Over printing

This is a techniques where one or more colours are printed wet-on-wet on top of another colour that

has just been printed to create a fall-on effect. The colourist must be aware of the effect of the colour

beneath on the over printed shade.

Discharge printing

Here, the fabric is dyed and dried prior to printing. It is then printed with print paste containing a

chemical which will destroy or discharge the colour of the dyed ground shade within the printed area.

Resist printing

Bleached goods are first printed with a resist paste which is normally a resinous substance such as wax

or starch that cannot be penetrated when the fabric is subsequently immersed in a dye solution. The

dye affects only the parts that are not covered by the resist paste. The paste is then removed by

washing or ironing with a hot flat iron.

PRINTING METHODS

BLOCK PRINTING

It is the oldest method of printing that uses blocks where the design to be printed is curved in the

block out in relief against the background surface. It is a slow, laborious process and is not suitable

for high volume commercial use. It is a method still practised in the oriental countries where markets

exist for the types of printed fabrics produced.

Procedure

The design to be printed is curved on the face of a wooden r metallic block. If there are more than one

colour in the design, colour separation is done and each colour is traced out individually on a separate

block.

The table on which printing is to be done is protected with polyethylene paper and then covered with

newspapers or printing pads.

The fabric to be printed is spread carefully on top f the table and held in position with thumb nails.

Measurements of the printing block are taken and used t demarcate repeat registration and bright

coloured thread is used to demarcate the printing area according to the size of the blocks.

The print paste is applied to the design surface on the block and the block then pressed against the

fabric. The process is repeated with different designs and colours until the pattern is complete.

STENCIL PRINTING

This printing method is restricted for small jobs such as writing of banners. Commercially it is of little

value because it is labour intensive and time wasting, and accuracy is not guaranteed.

Most of the stencils are normally cut out of the hardened papers, plywood, metal or plastic. If more

than one colour is required, colour separation is necessary and a stencil for each colour is prepared.

The procedure for stencil printing is similar to that of block printing except that here, the design is cut

out in the stencil. The stencil is placed on the fabric and paste is applied using a squeegee or sponge

or brush.

ROLLER PRINTING

In roller printing, the colour is transferred continuously onto the fabric by engraved rollers. The repeat

of the design is governed by the roller circumference. Each roller applies only one colour on the design

while the thickened dye paste is held in the engraved roller area. It is the oldest mechanized method

for continuous printing and started way back in the 18th century. It represents only about 16% of print

production today, and is declining. Roller printing is capable of producing very sharp outlines to the

printed pattern which is especially important for small patterns. The number of rollers depends on the

number of colour ways involved. Each revolution of the roller makes one repeat of the design.

The design is engraved onto copper rollers, a separate roller for each colour.

The rollers are mounted against the large main cylinder, around which the fabric travels together with

a resilient blanket to prevent sideways movement and a protective back grey. The printing paste is

located in a trough. A transfer roller or furnishing roller runs partly immersed in the paste and in

contact with the engraved roller. It picks colour from the trough and transfers it to the printing roller.

A colour doctor blade, scraps away all of the paste except for that contained in the engraving. A

cleaning lint doctor blade on the other side scraps away any lint picked up from the fabric.

In its passage around the cylinder, the pressure of the engraved roller against the fabric causes the

design to be transferred. Each roller applies its colour and part of the design. Any excess paste which

is squeezed through the fabric is taken up by the back grey. This protects the blanket and prevents the

design from being smeared on the blanket. After leaving the cylinder, the cloth is dried often by

passage through a hot air drier and then over steam heated cylinders to help in setting f the dye.

Advantages and disadvantages

Roller printing is especially suited for printing large batches. Speeds can amount to approximately 100 meters per minute. Moreover, roller printing can be used for very fine printing. For small batches, however, the changing times between printings of the various batches are so considerable in the complete production process, the efficiency (cost effectiveness) in machine utilization can drop to 50%. The changing time is necessary for adjusting and preparing the machines for a new series. Another advantage is the crush effect. Applying several colours in one drawing is achieved by using several printing rollers. Each printing roller applies one colour. During the printing process, each colour will be “crushed” by the following rollers as many times as there are colours left to be applied.

Consequently, the colour will be pushed more and more through the fabric to be printed. Deep colours are hard to obtain, which benefits screen printing. There may be a reduction in colour strength of up to 50%. However, in roller printing, it is essential to apply the light colours before the darker ones because traces of the preceding colour can be carried forward in to the following colour. Engraving the printing rollers is an expensive operation which raises the price of the roller printing technique considerably. The cloth width to be handles is limited by the length of the printing roller. There are delays when changing patterns or colour wheels and reduced colour yields sue to printing pressure employed Roller printing faults

Faults commonly encountered are connected with the working properties of the colour paste and

their interactions with the doctor blade and the printing roller. They include;

Scratches and doctor streaks; Scratches are due to the surface of the printing roller getting scratched

by a hard practical in the paste while doctor streaks are caused by damage at the edge f the doctor

blade. Scratches are parallel to the selvedge while doctor streaks are zigzag due to the traversing

motion of the doctor blade.

These can be reduced by adding lubricants in the print paste and use of chromium plated print rollers.

Snaps and doctor lifts; these are caused by some foreign substances lifting the dpctor edge put of

contact with the surface of the roller. They appear as strong continuous double lines of colour running

up the fabric. The foreign substance which becomes trapped under the doctor may be a thread on the

cloth being printed or particle such as dry thickening from the print paste.

This can be avoided by washing up the doctor and the roller.

Scumming; Is due to inefficient cleaning of the surface of the printing roller so that finer unwanted

film of colour is actually being printed from plain parts of the roller. This an be a result of a rough

doctor blade, badly polished roller, defective paste which resists cleaning action of the doctor blades.

This is common in discharge printing. It can be reduced by pre washing r pre-pdding with dilute

solution of an oxidising agent

Application of varying pressure across the fabric width which results in variable colour depth.

SCREEN PRINTING

Hand screen printing

Screen printing is basically a form of stencil printing. It consists of a synthetic fabric or metal gauze

stretched taut over the frame. Part of the gauze has holes blocked off (non-printing area) and the

printing paste is forced through the open printing areas by a rubber or metal squeegee onto the fabric.

Each screen is required for each colour.

Procedures

The fabric is placed on a firm table covered with printing pad and newspapers.

Screens are moved along the print surface with the paste. A squeegee is employed in driving across

the screen unidirectional and the number of strokes may vary according to the individual preference.

After printing the fabric is left to dry.

The method has low production costs and suitable for short runs particularly order oriented. It is

labour intensive but cheap printing method. Since printing is done by hand, there are pressure

variations such that the repeat registrations are never perfect and it is not easy to obtain sharp images.

Screen preparation

Design selection

The design and fabric quality must be demand driven so as to command a market share. The designs

should suit end use for example small motifs are made for shirts while larger motiffs are made for

furnishings.

Repeat sketches

The individual design units must fit together perfectly in order to eliminate the introduction of

discontinuities. The croquis (the artist’s original design) is redrawn too give a modified version known

as a repeat sketch. During the preparation of a repeat sketch, the boundary of the repeat is usually

disguised as much as possible.

Colour separation

This is done by reproducing the design areas of each colour separately on a clear transluscent

dimensionally stable film.

The colour separations (dipositives) usually consist of positives painted by hand using opaque ink or

paint. Of recent, programmed photoelectric scanners are used. Automated colour separations has so

far been successful from most designs.

The photo chemical process

Some polymers crosslink when exposed to UV light and become insoluble in solvents. The most

commonly used polymers are geratine and polyvinyl acetate. Both are sensitized with a dichromate

or diazole compound.

Procedure

A screen is coated with a light sensitive polymer, by standing it against a wall and applying the layer

of the viscous polymer solution. The polymer solution is applied using a smooth, straight aged trough.

The trough is tilted and moved from bottom to top of the screen applying the polymer.

The coated screen is dried in a dark cardboard ideally on horizontal shelves with a drought of cool dust

free air to shorten the drying time

Once the coated screen is dry, it is sensitive to light so photographic dark room conditions with safe

light are required during exposure and developing stages.

A dipositve is placed on top of the coated screen upside down and held in place with a 1mm thick

glass. For flat screens, the inside of the screen is supported with blocks which can fit properly. The

glass is covered using an opaque material and proceed to the area of light or under direct sunlight.

The time of exposure depends on the distance from the light source. After exposure the screen and

dipositvie is taken bark to the dark room.

Everything is removed from the screen and it is then immersed fully in water to allow development of

the design. Now the design is apparently seen on the screen, the design lines are open using highly

pressurised jets of water.

After opening, pinholes that are not opened are checked for on the screen before it is left to dry in

the dark room or dried immediately using hot air.

Rotary Screen Printing

Rotary screen printing uses CAD and roller squeegees. One roller is used for each colour. This is a very

fast process used in the continuous printing of furnishing and clothing fabrics.

The machine uses seamless cylindrical screens which are composed of a nickel mesh with end rings

stack at one end to tension the cylinder and prevent collapse. Each rotary screen is positioned across

the fabric and is independently driven at one end. As the screen rotates, it is fed internally with print

paste which is forced through the open mesh area by a stationery squeegee at the base of the screen

and onto the fabric being carried by a continuous moving conveyor. The fabric is then dried by passing

through a hot air drier.

The following are achievable on a rotary screen printer.

The squeegee blade is flexible to accommodate any variations in pressure required to force the paste

evenly through the mesh of the screen across the width of the fabric. In some models the squeegee is

replaced by a metal rod held in position by a magnetic field. This is more suited to heavier fabrics,

since the mechanism imposes a higher limit on the minimum amount of paste that can be delivered.

In general the rotary-screen printing machine requires a lower pressure between roller and fabric than

is used with engraved rollers.

Continuous rotation of cylindrical screens while in contact with the fabric leading to continuous

printing. The print paste is fed into the inside of the screen and during printing is forced out through

the design areas by a stationary squeegee.

Blanket washing and drying is effected from underneath during the return passage.

Provision for the use of adhesive with a curved heating plate to heat the fabric before it is placed into

the blanket. It is generally shorter for a given number of colours because cylindrical screens can be

much closer together than is possible with flat screen.

The fabric drier must be longer to enable the printed fabric to be adequately dried at faster running

speeds.

Speeds of 30-50m/min are used depending on the design and fabric quality. It is possible to run the

machines faster but limitations often being the length of the cloth and blanket driers. The difficulty of

observing printing faults at high speeds renders the movement of the cloth to be at a lower speed.

As with all screen printing, some control of paste delivery can be obtained through variation in the

mesh size, a large mesh being appropriate for fabrics made from coarser fibres or areas of solid colour,

whilst a finer mesh is better for producing fine detail or for fabrics made from fine fibres.

The advantages of rotary-screen printing machines include; faster production rates, greater ease of

setting up and a lower dependence on experience for successful operation. Computer aided design

techniques for printing screens are now increasingly widely used. Rotary screen machines are more

compact than flat screen machines for the same number of colours in the pattern. Therefore, they use

less plant floor space

The principle disadvantage of rotary screen printing is the high fixed cost of the equipment. The

machines are generally not profitable for short yardages of widely varying patterns, because of the

clean-up and machine down time when changing patterns. However, rotary machines are used for

carpet and other types of pile fabrics.

Most knit fabric is printed by the rotary screen method, because it does not stress (pull or stretch) the

fabric during the process. Flat screen printing is much more suitable for high pile fabrics, because only

one squeegee pass is available with rotary screen.

Flat Bed Screen Printing

In flat bed screen printing, this process is an automated version of the older hand operated silk screen

printing. For each colour in the print design, a separate screen must be constructed or engraved.

If the design has four colours, then four separate screens must be engraved. The modern flat-bed

screen-printing machine consists of an in-feed device, a glue trough, a rotating continuous flat rubber

blanket, flat-bed print table harnesses to lift and lower the flat screens, and a double-blade squeegee

trough. The in-feed device allows for precise straight feeding of the textile fabric onto the rubber

blanket.

As the cloth is fed to the machine, it is lightly glued to the blanket to prevent any shifting of fabric or

distortion during the printing process. The blanket carries the fabric under the screens, which are in

the raised position. Once under the screens, the fabric stops, the screens are lowered, and an

automatic squeegee trough moves across each screen, pushing print paste through the design or open

areas of the screens. The screens are raised, the blanket precisely moves the fabric to the next colour,

and the process is repeated. Once each colour has been applied, the fabric is removed from the

blanket and then processed through the required fixation process. The rubber blanket is continuously

washed, dried, and rotated back to the fabric in-feed area. The flat-bed screen process is a semi-

continuous, start-stop operation. Flat screen machines are used today mostly in printing terry towels.

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, is prone to wear and edge nicks and has a tendency to warp and distort.

While blades continue to be made from rubbers such as neoprene, most are now made from

polyurethane which can produce as many as 25,000 impressions without significant degradation of

the image.

There are two types of squeegees; Conventional squeegee which is driven forward and backwards

across the screen by a motor. The other is a small diameter roller bar which is attracted and rolled

over the screen by a powerful electromagnet moving beneath the printing blanket.

Control of paste

The amount of paste passing through the screens in both hand and flatbed screen printing can be

controlled in the following ways

The mesh (thread/cm of the screen); generally a coarse mesh allows a predetermined amount of paste

to pass through compared to an open mesh. The fraction of the open area in the fabric depends not

only on the mesh but on the yarn diameter.

The hardness and cross section of the squeegee blade; A harder rubber squeegee with a sharp cross

section is suitable for outlines where a soft rounded blade applies more paste and is suitable for big

blotches or motifs.

Print paste Viscosity; the viscosity can be varied where thinner pastes passing through through the

screens pours more readily than thicker paste

The number of squeegee strokes; 2 to 4 strokes are applied for a better amount of paste to be applied.

The squeegee angle and pressure; the recommended angle is 65º to avoid slapping. The pressure must

be appropriate because too much pressure implies too much paste on the fabric.

Advantages and disadvantages of the method

From a productivity standpoint, the process is slow with production speeds in the range of 15-25 yards

per minute. Additionally, the method has obvious design limits. The design repeat size is limited to the

width and length dimensions of the flat screen. Also, no continuous patterns such as linear stripes are

possible with this method.

However, this method offers a number of advantages. Very wide machines can be constructed to

accommodate fabrics such as sheets, blankets, bedspreads, carpets, or upholstery. Also, this

technique allows for multiple passes or strokes of the squeegee so that large amounts of print paste

can be applied to penetrate pile fabrics such as blankets or towels. Currently, approximately 15-18%

of printed fabric production worldwide is done on flat-bed screen machines.

Screen printing faults

Most printing faults are registration faults i.e. misfitting of colours in the design r it may be associated

with screen frames falling n wet areas of the printed fabric.

Registration faults; accurate movement of the blanket on screen repeat distance each time it is

advanced. In adequate adhesion of the fabric to the blanket will cause local misfitting. Screen

distribution due to excessive drag by the squeegee especially rubber squeegees may lead to poor

paste application and poor registration

Frame marks; when printing consecutive screen pints, the screen frame inevitably falls on part of the

area most recently printed and may leave an impression. It is also common in hand screen printing

when moving the screen back to fill in a gap left in the previously printed motiff.

The technique used to counteract this problem is contact printing where the screen frame is lowered

to a print above the blanket, such that the gap is small. During passage of the squeegee, the screen is

stretched and makes contact with the fabric.

Splashing; this is the falling back of the print paste beneath the screen back into the cloth on lifting

the screen. It is reduced by lifting the screen first on one side and then the other.

TRANSFER PRINTING

Transfer printing is the term used to describe textile and related printing processes in which the design

is first printed on to a flexible non textile 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 on to 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.

Against these advantages, may be set the relative lack of flexibility inherent in transfer printing: no

single transfer-printing method is universally applicable to a wide range of textile fibres. While a

printer with a conventional rotary-screen printing set-up can proceed to print cotton, polyester,

blends and so forth without doing a great deal beyond changing the printing ink used, the transfer

printer hoping to have the same flexibility would need to have available a range of equipment suited

to the variety of systems that have to be used for different dyes and substrates using transfer

technology.

Many methods of producing textile transfer prints have been described in the literature. Many of them

exist only in patent specifications but several have been developed to production potential. They may

be summarised most conveniently as below.

Vacuum Vapour Transfer

This method depends on the use of a volatile dye in the printed design. When the paper is heated the

dye is preferentially adsorbed from the vapour phase by the textile material with which the heated

paper is held in contact. This is commercially the most important of the transfer-printing methods. It

depends on controlled temperature, pressure and time.

Procedure

The transfer printing is accompanied by passing a transfer paper and fabric between hot oil heated

cylinders reaching a temperature of about 180°C. As the paper and fabric pass the cylinder, a partial

vacuum is created by sucking air through perforations. The heated dye sublimates or vaporises in

partial vacuum and diffuses into the fabric. This method is restricted to synthetic fibres and highly

volatile dyes which has limited its use.

Wet Transfer Printing Process:

In this method, water soluble dyes printed on paper migrate to fabric having fixing chemicals. The

principle behind wet transfer processes is the use of water as the medium through which dye diffuses

in passing from the paper to the fabric. Thus it resembles vapour transfer printing in that it is a

diffusion-controlled system, but since water is used instead of vapour it is not restricted to the use of

dyes that can sublime. There are two methods involved. The fastran process and the dew print transfer

process. A wet transfer printing process for fabrics is known which comprises the following three

stages.

1. Printing the required design on a selected grade of paper, using specially chosen dyes dispersed in a suitable paper printing medium.

2. Impregnating the fabric (nylon, wool, acrylic or the like) with an aqueous solution which may, for example, contain a dye fixation catalyst and a thickener, the latter to act as a dye migration controller and film-stabiliser.

3. Bringing the printed paper and the impregnated fabric into close conformity, by applying pressure, and maintaining this conformity under pressure for a period which may vary from a few seconds to several minutes, while maintaining the paper and fabric in a moist condition at a temperature of at least 100 on the printed paper are almost completely transferred to the fabric and, provided sufficient contact time is allowed, are fixed in the same operation.

Fastran method

This involves padding wool garments with an aqueous bath containing fastran powder, the powder

contains a low cast gum derivative that holds a moisture film over the material during the transfer as

well as a surface derivative that facilitates high dye absorption. The pre-wet garment is brought

into contact with a transfer paper and is placed over a silicon rubber sheet.

The composite is then heated in a press for several minutes during which time the dye migrates to

the fabric and becomes attached in the usual manner. This may be followed by an after wash.

The method is not highly productive but is quite suited for the printing of high-value articles such as

knitted woollen garments and cottons especially when novel design effects are obtained at the same

time. The dyes mostly used are acid and reactive dyes.

Dew print method

The DewPrint machine introduced in the late 1970s offered an ingenious solution to the problems

posed. It is well suited for nylon, spandex, and acrylics.

The major difficulty to be overcome in a system of this kind is how to maintain the contact pressure

holding the paper to the fabric continuously at the required level over a period of time. This cannot

be achieved simply by using a stretched blanket as in sublimation transfer. Consequently the DewPrint

machine was fitted with a series of pressure rollers around the heated transfer cylinder, which exert

a steadily increasing pressure up to but not exceeding the pressure of the mangle used initially to

impregnate the fabric.

The material is padded with a preparatory emulsion which aids dye transfer from paper to fabric. It is

passed along the printed transfer paper over a cylinder heated at lower temperatures. The fabric is

the washed to remove the emulsion assistant. The major drawback of this solution to the problems

was the capital cost involved, which seriously affected the adoption of the approach.

Continuous wet transfer continues to be of interest, and new equipment has been recently introduced

by Küsters in association with the Cotton Art process developed by Dansk Tranfertryk for transfer

printing of reactive dyes using their specially developed transfer paper

Melt Transfer

This method has been used since the 19th century to transfer embroidery designs to fabric. The design

is printed on paper using a waxy ink, and a hot iron applied to its reverse face, presses the paper

against the fabric. The ink melts on to the fabric in contact with it. This was the basis of the first

commercially successful transfer process, known as Star printing, developed in Italy in the late 1940s.

It is used in the so-called ‘hot-split’ transfer papers extensively used today in garment decoration.

Dry heat transfer

This is a popular method for synthetics especially polyesters using disperse dyes. However the method

had some limitations of penetration and fastness on other synthetic fibres. It is divided into two

techniques

Conventional heat transfer printing: This utilises electrically heated cylinders that press against

transfer printed paper on a heat resistant blanket held under tension. The dry heat causes the dye to

sublime in pattern form directly from paper to fabric.

Infra-red heat vacuum transfer printing: This method operates at lower temperatures and pressure.

The transfer paper and fabric are passed between infrared heaters and a perforated cylinder which

are protected from excessive heat by a shield. It is best suited for certain fibres which are heat

sensitive such as acrylics and spandex. The system is also quite effective for dye penetration of pile

fabric e.g coduroys and velours.

Film Release

This method is similar to melt transfer with the difference that the design is held in an ink layer which

is transferred completely to the textile from a release paper using heat and pressure. Adhesion forces

are developed between the film and the textile which are stronger than those between the film and

the paper. The method has been developed for the printing of both continuous web and garment

panel units, but is used almost exclusively for the latter purpose. In commercial importance it is

comparable with vapour transfer printing.

DUPLEX PRINTING

Duplex printing simulates a woven pattern by printing the fabric on both sides. The fabric may be

passed through the roller printing machine in two separate operations or through a duplex printing

machine in a single operation. Duplex printing produces an equally clear outline on both sides of the

fabric. The difference can be detected only by raveling out the yarns out of the fabric.

BOLTCH PRINTING

This is a continuous technique for randomly dropping or sprinkling dye liquor on a fabric to produce

multi-coloured patterns. Streams f dye run from a trough into individual channels. The doctor blade

oscillates, breaking up the streams of dye into droplets and scattering them over the fabric. The fabric

is continuously moving in pen width by means of a conveyor, the fabric is then padded to force the

dye into it. It is later taken for drying. The process is not only versatile but can also be used in place of

some more expensive techniques at a minimal costs.

ELECTROSTATIC PRINTING

The dye resin mixture that has dielectric properties is spread on a screen bearing the design. The fabric

is passed through an electrostatic field which is held about 12mm above the cloth. The resin mixture

is pulled by the electrostatic field through the pattern by the electrostatic material. The dye is then

initially fixed by infrared heat then, fixation is subsequently reinforced with whatever process most

suitable with the kind of dye used.

PHOTO PRINTING

The fabric is coated with the chemical that is sensitive to light. Then any photo may be printed on a

fabric under dark room, conditions.

DIGITAL/INKJET PRINTING

Digital or inkjet printing is a non-contact printing technology. The key characteristics of digital printing

technologies include:

Variable data. The input data is not constrained by size, nor does it need to repeat. Because

the data is drawn from a computer file that is, in principle, unrestricted in size.

Non-contact with substrate. The ink is dropped onto the substrate, making it possible to print

on flat or curved, smooth or rough, delicate or hard substrates.

Versatile. In most cases, inks can be developed that are compatible with any chosen surface.

Multicolour, Based on the cyan-magenta-yellow-black colour gamut, thousands of colours can

be printed without the requirement for a colour kitchen.

High speed. The printing rates are of course, dependent on resolution, the type of printing

required, the head technologies, etc.

No moving parts. Printer motions are limited to oscillators within heads and a system to move

the heads in relation to the substrate. It is the ink that moves during printing, not a mechanical

device. Consequently, ink jet printers are inherently reliable.

The principle of digitising the printing process can be grasped by observing a dripping tap, as in Fig. (a)

below. Each drip allows a drop of water to fall onto the surface below. If the position and movement

of the faucet is controlled, the drops of water can be made to wet a substrate in a controlled way. If

further controls are introduced so that for each position of the faucet, the drop can either fall or not

fall, then the system mirrors the digital printing process reasonably well.

Historical Developments

The first application of digital printing technologies was with carpets: a patented technology exclusive

to Milliken. This was launched in the 1970s and made use of a pulsed air jet to carry ink to the carpet

substrate, Using carpet squares, large scale patterns could be produced that were customised for

particular customers and specific rooms. Both hardware and software have been enhanced

continuously since then. Subsequently, in the period 1991-2000, developments with textile substrates

were limited to sampling (for photoshoots), the printing of flags and banners, and some other niche

products (such as silk ties). The past 5 years, however, have seen major breakthroughs in the use of

digital printing of textiles. There are many more commercial machines, many improvements in inks,

and significant developments affecting software systems. The range of applications is now very

extensive, and mass-customisation is a realisable goal.

Inkjet mechanisms

Dot on Demand (DOD) or Impulse Print Heads

This broad category of technologies provides mechanisms for delivering a drop of ink when there is a

demand. The demand is generated by the printing software: for each pixel, the instruction is either to

fire a drop or not to fire. The drop of ink then falls to the substrate under the influence of gravity and

appears as a dot on the surface. Drop sizes are measured in picolitres (the prefix pico means a

trillionth, so one picolitre is 10–12 of a litre). Typically, drop sizes are 20–30 pl, but there is a significant

trend for heads to produce drop sizes below 10 pl and one head has been released recently that can

achieve 1 pl.

Thermal Ink Jet Heads

A very localised volume of ink is heated in the head and this causes a bubble to form and burst. The

heating pulse has a timescale of 2–10 µs and during this time the fluid in the immediate vicinity of the

heater vaporises and expands explosively. The bubble expansion causes a pressure wave within the

fluid and, as a result, some of the ink is ejected from the nozzle. This ink forms a drop and falls onto

the fabric. As soon as the heating pulse finishes, the bubble collapses and ink is drawn in from the

supply channel. The ink jet heads do have a limited life, but as they are cheap to produce it is normal

practice to work with disposable cartridges.

Piezo Ink Jet Heads

A piezo material experiences distortion under the influence of an electric field (or conversely, it

generates an electric field when deformed). The word piezo is Greek for ‘push’, drawing attention to

its mechanical behaviour. The ink chamber is partly constructed from a piezo-electric material, so the

volume of the chamber is changed when an electric field is applied. Just as for thermal ink jet heads,

the effects operate Drop on demand concept. (A drop is produced when the signal to fire the nozzle

is given) When the chamber volume reduces, a drop of ink is forced out of the nozzle. When the

electric field is turned off, the chamber shape snaps back to normal and this draws ink in from the

supply channel.

Piezo heads give more control over drop production than thermal heads (for example, making it

possible to have drops of different size), and they can operate at higher production rates. Since the

ink is not heated to form a bubble, there are fewer constraints to affect ink formulation. Piezo heads

tend to last longer than thermal heads. For all these reasons, piezo heads are the most popular for

textile digital printing.

Electrostatic Ink Jet Heads

These heads use slight pressure to form a meniscus of an ink drop at each nozzle. There is no chamber

firing mechanism, however. Instead, an electrostatic attraction force from the substrate is used to

eject selected droplets to form the desired pattern. These printers are faster but have lower resolution

and are less versatile.

Continuous Ink Jet (CIJ) Heads

The continuous ink jet concept a jet of any fluid is unstable and tends to break up into small droplets.

This means that a continuous jet of ink emerging from a nozzle becomes a stream of droplets in a very

short time. CIJ technologies manage the movements of these drops electrostatically: the drops are

given an electrostatic charge and are steered, using an electric field, either to the substrate or to a

recirculation system. The earliest work on textile ink-jet printing made use of CIJ technologies, but

commercial exploitation has been slower than the DOD route.

Pulsed Ink Jet Heads

This concept is a hybrid between DOD and continuous jetting. The head produces a jet that quickly

becomes a stream of drops. However, instead of the jet being continuous, it is pulsed, depending on

the design of the print. There is therefore no need to put a charge on the drops, nor to steer them

electrostatically. With this delivery system, the option of using CMYK colour is not available, so each

head has its own solid or spot colour ink. As with the screen printing process, colour separation of

graphic images is needed and inks are prepared accordingly. This technology is used in the Chromojet

(for carpet printing) and the Chromotex Flatjet (for artificial furs, blankets and other high-pile fabrics)

Inks for printing textiles

Inks are comprised of a colourant, a carrier base and various additives. The carrier base (or vehicle)

may be water, solvent, oil, phase change fluid (hot melt), or UV curable fluid. However, to date, nearly

all the inks for textile printing are water based because they are designed for heads that require water-

based inks. The usefulness of the carrier base is over once the ink is deposited on the substrate. Water

is ideal in that it can evaporate relatively easily and is non-toxic. Other carrier fluids may need to be

evaporated in a controlled way and fumes exhausted or reprocessed to ensure a safe working

environment. The carrier base may constitute 80% of the ink.

Colorants for textile substrates have to be fast for washing, light and rubbing. Functionality issues are

just as important as putting colour on the textile. There are four main categories of colorant: (i)

Reactive dyes (for cellulosic fibres) (ii) Disperse dyes (for polyester) (iii) Acid dyes (for protein fibres

and nylon) (iv) Pigments (for all substrates) These materials have been developed for more traditional

dyeing and printing routes, and adapted for use in digital printing.

Pigments inks (Water, Oil or ‘Solvent’ Carrier)

Pigment inks have fine particles of insoluble colourant in suspension. They are used to obtain print

qualities of durability, lightfastness and UV-resistance. The particles, which are typically less than 0.1

micron in diameter, are bound to the substrate by the cross-linking (polymerisation) of a resin

component in the ink during a post-printing heat treatment. In the past, pigments have had a

reputation for a limited colour gamut, because the inks have carried a large proportion of monomer

resin and it has not been possible to get enough colourant onto the substrate. However,

improvements in ink technology have reduced this problem substantially. Recent developments have

focused on solvents, which reduce the tendency for pigments to settle and provide a good carrier for

monomers, but are less versatile regarding substrates.

UV-curable Inks

UV-curable inks are made up of liquids (monomers, oligomers and photoinitiators) that are converted

100% to solid under the influence of UV light, plus colourants (normally pigments) and additives

(surfactants to promote wetting and inhibitors to reduce low light cure). The technology has been

developed around acrylate monomers. In the presence of free radicals, these monomers undergo

rapid polymerisation. The mechanism involves a photoinitiator absorbing UV energy and decomposing

to produce free radicals. These, in turn, react with monomers to trigger a free radical chain reaction.

The result is a 3D polymer structure, binding with itself, with the pigments and with the substrate. The

oligomers can be acrylated epoxies, acrylated polyesters, acrylated urethanes, etc. These provide

desirable characteristics to the polymerised material.

Selected advantages are summarised below:

Instant drying by curing

No drying of ink in the jets (greatly reduced clogging)

Reduced problems of the inks ageing in storage

Minimal level of VOC (volatile organic compounds–solvents) and pollution

Good quality gloss, satin and matt finishes

Offer many options for adhesion chemistry with different substrates

Produces durable prints that are resistant to many chemicals

Compact curing equipment

On-going development of inks Drawbacks include:

Higher cost than water-based inks

Higher machinery costs

Requires controlled handling procedures

Ink film is thicker than produced by aqueous inks

Shrinkage occurs that can adversely affect adhesion

With fabrics, the polymer film will affect handle

Health & Safety issues relate to the materials causing skin irritation

General requirements for inks used in textile digital printing:

Purity (to avoid blockages of nozzles, so they are filtered to less than 0.2 microns)

Particle size (for disperse and pigment inks)

Viscosity

Surface tension

Conductivity (for continuous flow application)

Stability

pH value

foaming properties

In addition, the inks must be appropriate for the substrate and must satisfy the end use requirements

as; Substrate orientation, Production and end user fastness properties, Washing properties, Handle

Compatibility with conventional textile printing Ink developers must consider the textile substrate, the

pre-treatments given to the substrate, the selected print head technology, the post-treatment and

performance in use. The analysis necessarily varies with each case.

Applications for inkjet printing

Although the technique is already well established, it is still used mainly for special and short run

printing purposes). It is also being used for printing a variety of smart sensing materials on textile

fabrics for a range of biological and electronic applications, mass-customisation and interior textiles,

customised Products such as ties, scarves, etc, carpets, flags and banners.