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DIGITAL PRINTING MACHINE

Digital textile printing, often referred to as direct to garment printing, DTG printing, and digital garment printing is a process of printing on textiles and garments using specialized or modified inkjet technology. Inkjet printing on fabric is also possible with an inkjet printer by using fabric sheets with a removable paper backing. Today major inkjet technology manufacturers can offer specialized products designed for direct printing on textiles, not only for sampling but also for bulk production. Since the early 1990s, inkjet technology and specially developed water-based ink (known as dye-sublimation or disperse direct ink) has offered the possibility of printing directly onto polyester fabric. This is mainly related to visual communication in retail and brand promotion (flags, banners and other point of sales applications). Printing onto nylon and silk can be done by using an acid ink. Reactive ink is used for cellulose based fibers, such as cotton and linen. Using inkjet technology in digital textile printing allows for single pieces, mid-run production and even long-run alternatives to screen printed fabric.

PRINCIPAL

Digital printing is an invention that involves art work being process by a computer, and then printed directly on to the surface of fabric. Digital printing is not a heat transfer, as the ink is directly adhered to the fabric.

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Digital printing is an easier process than screen printing. A customer can get a multiple colour design without having to have a screen made for each colour. Digital printing does not use screen allows for a photographic print, with much more details than traditional screen printing. This provides significant saving in both time and money. Digital printing is often used for design with a lot of detail, colour and high resolution. As the ink isapply thinner, Digital printing is best used on lighter coloured ground, to allow the design to shine through The fact the design is processed and printed digitally allows for a quantity one, since there are no screens or physical set up.

MECHANISM

Digital Textile Printing delivers the ability to print designs on fabric, directly from a PC or Mac.Inkjet printing is done on fabric in the same manner as it’s done on paper, and just as easily.

Printed textiles can be used for a vast range of applications including flags and banners,

exhibition signage, Clothing etc.

Digital textile printing is a process of printing on garments and textiles using specialized or modified inkjet technology.

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The two keyrequirements of a digital textile printer are a transport mechanism for the garment andspecial inks that are applied to the textile directly to be absorbed by the fibres.

Direct to garment Visual communication

The predominant textile media used in visual communication is a polyester based fabric.

the type of ink chemistry needs to fit requirements for the media (such as polyester, nylon, cotton, silk). Based on the media and ink combination, the choice comes for infra-red fixation, heat-press sublimation or steaming. The structure of the fabric also needs attention, for example whether it is woven, non-woven or knitted.Polyester fabric is printed mostly with dye-sub or disperse direct ink, although UV and solvent inks (including HP’s latex formulation) can also be used. The great benefit of sublimation ink is the fact that the colorants will bond with the fibre during sublimation or fixation.

The biggest advantage of direct to media is drastically reduced waste. This method doesn’t need printing on transfer paper first before calendering (or heat-pressing) it onto the media. Waste is both an economical and an ecological factor in print production. Print speed doesn’t account for much if a large portion is being thrown

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away as waste due to incompatibility of media, ink, treatment or lack of know-how.

CONSTRUCTION AND WORKING

 The origin of DTP SystemDigital Textile Printing System was first started to develop from the Inkjet type of plotter for sample development and with day to day improvements in resolution & speeds, now it has become a standalone shop. Also applications of these printers are varied with fast developing printing skills, media, ink etc. it is used to produce big size of photos and hanging banners that hang on the buildings to a small multicoloured designer silk handkerchief & Ties.▶ Construction of DTP SystemPrinter Head : A 'Printer Head' is the most important part of the DTP system. Mostly these are PIEZO drop on demand print heads.Piezo Head - A 'Head' of 'Piezo type' was developed by 'Epson' for Inkjet printers and this type of head is used for DTP system. There are many other head manufacturers now trying to adopt to textile inks &industrial textile printing. Normally the resolution used for textile printing is 720 dpi.Body : The construction Digital Textile Printing system currently being sold, depends mainly on the kind of fabric

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being used for printing and the purpose. There are various constructions depending on printing being used for Flags & banners, direct disperse, sublimation paper printing or printing on fashion textiles using Reactive / Acid inks. Also it depends if you want to print directly on fabric or transfer.Fabric feeding system : There are several types of the 'fabric feeding system', 'Cylinder' or 'Blanket type' etc. and again the choice is dependent on the kind of printing & fabric used.Ink supplying System: An 'Ink supply system' can be varied depending on which DTP System is used. This has developed over the years and is a very important area for the performance & run-ability of the inks. The “bulk ink” systems over a “cartridge type” system makes the inks to run better and also coupled with a degassing system helps to remove the unwanted bubbles in the ink, hence giving a better ink performance and lower maintenance and down time of the machine

Inks :Types of ink that can be used for DTP system are ' Reactive Inks ' , ' Acid Inks ' , ‘ Disperse inks ‘  and ' Pigmented Inks '.① Reactive Ink - This ink is the mostly used ink in general textile industry. It has almost the same color density as screen print colors and it is especially suitable for home textiles & fashion fabrics. It is used for cotton ,

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silk or even wool. Post printing, it has to be steamed at +/-102℃ for achieving high quality of colors & fastness.② Acid Ink - It is used for general sportswear such as swimming suits etc made out of NYLON LYCRA or fabrics from Animal fibre such as wool & silk. Though it offers bright and fresh colors like reactive inks, but due to higher flexibility of Reactive inks, they are preferred more over acid inks until unless the production is restricted to only exclusive fabric like silk or nylon. Also high 'ultraviolet light resistance' can be obtained by steaming process like in case of reactive inks.③ Disperse Dye Ink - It is used for polyester and good for Home textiles, fashion wear, sportswear & also flag printing. There are various ways for post printing process for Disperse inks depending on the purpose of use.④ Pigmented Ink - It is used mostly for textile design printing & T-shirt printing. Thin fabric such as polyester, georgette, polyester-georgette, chiffon etc. can be also suitable fabric for this ink. Needs high temperature for fixing the inks, but no steam is required.

Properties Required of Ink-Jet Dyes: The two major classes of colorants, namely dyes and pigments. The choice of dye depends upon the ink used, whether it is aqueous, solvent or hot-melt and on the type of printer (thermal or non thermal). However irrespective of the solvent system, all ink-jet dyes have to satisfy a number of stringent criteria.

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1. Color:   Though Black is the predominant color, for full color printing the three subtractive primary colors of yellow, magenta and cyan are also required. The key parameters are the absorption maxima (peak wavelength), and the shape of the absorption curve, in particular its broadness and the presence (or absence) of unwanted secondary absorption. Ideally, bright dyes (which have narrow absorption curves) are required in order to produce a comprehensive balanced color gamut because bright dyes can be made duller (e.g. by adding a shading color such as a black), but dull dyes cannot be made bright.

2. Color Strength:   The color strength of the dye should be as high as possible for several reasons. One reason is to enable prints having high optical densities to be produced, Other one is the increased flexibility possible in the ink formulation. The ink staprecipitation and also kogation on thermal ink-jet systems.

3. Solubility:   A dye for ink-jet should have as high solubility as possible to minimize any tendency for the dye to crystallize and cause problems such as nozzle blockage. Generally greater the number of sulfonic acid groups per molecule, the greater the water solubility. Carboxylic acid groups, especially in the form of salts, also confer water solubility on dyes.

4. Electrolytes / Metals:  

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Anion such as sulfate and especially chloride are undesirable in ink-jet inks due to the corrosion problem they cause to the metal print heads. Certain metal cations particularly divalent cations such as calcium, need to be removed, since these can precipitate with certain anions such as sulfate and with dye itself so the permissible amount is 100 ppm. Which is generally accomplished by dialysis or ukrafihration8

5. Light Fastness:   The final printed document must have reasonable light fastness (resistance to fading by light) if it is to serve any useful purpose. However it is well-known fact that in general dyes have much poorer light fastness than pigments.

6. Water Fastness:   The final print should be resistance to water so that if the paper gets wet or is rubbed by moist fingers, smudging does not occur. On the one hand high solubility is required for the ink, but on the other hand, water insolubility is required once the dye is on the paper. Hence a compromise position has to be reached to adequate solubility and adequate warer fastness.One of the promising approaches to achieving high water fastness is by using the concept of differential solubility.

7. Smear Fastness:   This is effectively resistance to smear when using a highlighter pen. Such pens usually contain aqueous inks and can cause a dye to smear by facilitating the formation

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of water soluble species. Again dyes from solvent or wax / resin inks are less prone to smear than those from aqueous inks.9 :

8. Shade   The shade or hue appears the same irrespective of the substrate on which it is printed. :The shade or hue appears the same irrespective of the substrate on which it is printed.This can be achieving science the substrate varies enormously in physical properties. For instance, paper varies in texture, adsorption, additives, and pH, and ideally the dye should be insensitive to this differences.10

9. Toxicology:   Because of the increased awareness of Environmental health and safely, in recent years. It has been made imperative that screening test for mutagenicity (in bacteria) is the Amens test, and it is normally required that dyes should be Amens negative, i.e. not cause mutations in bacteria. Therefore, ink-jet dyes should ideally be Amens negative

10. Thermal stabilityIf the dye is required for thermal ink-jet system such as canon's bubble jet or Hewlett Packard's Think Jet, then the extra parameter of thermal stability or kogation fastness is required. Since the temperatures involved are as high as 300°C, then dyes stable to these conditions are required. Good kogation is when these deposits are absent or very low, whereas poor koagtion is when the deposits substantial. Poor kogation produce by two factors The first

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is the presence of inorganic impurities such as iron, copper and silicon. The second is the degradation of the dye itself Solving the second cause of kogation, namely dye degradation, has proved more difficult

Aqueous Ink-Jet Dyes: Colorants comprising the first generation of inkjet dyes were selected for their vividness (high chroma), good aqueous solubility, and the stable inks derived therefrom which gave a reliable printing performance InkJet printers have become considerably more sophisticated over the years, and demand for dyes and inks capable of producing very high print quality has grown.Ink formulation research has complimented dye development and has been aimed towards achieving the following print properties:

  Good optical densities   No feathering   Minimal black to color bleed   Uniform and controlled drop spreading   Good water fastness   Rapid dry time   Smear resistance   Media compatibility

Solvent Inks: Solvent based inks find application in continuous printers in the industrial segment where they are used to print bar codes, batch numbers, sell-by dates and other such information on packaging materials. The used of solvent

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based inks rather than aqueous inks affords a faster dry time and makes the inks more suitable for printing hydrophobic or non-porous substrates. Traditionally, ketonic solvents like methyl ethyl ketone are used but these are being replaced by less flammable alcohol. Solvent dyes are usually used as colorants and the predominant color is black. A typical example of a black dye used in this area is the trisazo dye. Another dye used in solvent inks is CI Solvent Black 35. This dye exhibits high light fastness. 

Hot-Melt These inks are solid at room temperature but become fluid when heated at 60-125°C. They are fired using a piezo printer. The ink vehicle includes Cig-24 fatty carboxylic acids and alcohol. Dyes that are soluble in the vehicle tend to be hydrophobic, an example of which is the modified xanthenes magenta.Ink Jet Printing Technology for Textile Definition of type:There are two types of ink-jet printer. The coarse resolution type & Stork true cote within these two types are further sub groups. It is in this area of fine resolution that there has been the most recent research activity2.

Coarse Ink Jet Printer These are normally based on valve technology and have essentially found use only in the carpet industry. There are two main commercial available systems. The Millitron system use an array of jets with continuous streams of dye liquid which can be deflected by a controlled air jet. The chromo jet uses computer-activated on/off valve systems

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to control the flow of the dye liquid. The use of electrochemical valves which are computer controlled to open and close rapidly so that liquid is fired in a succession of short pulses. The contribution of BTTG in Manchester on the type of technology but with increased resolution.

Strock Trucolor Jet Printer The Strock Trucolor pronter is a development based on the continuous stream technology (using the binary method). Essentially a dye formulation is pumped at constant pressure through nozzle, 14.4in diameter. The continuous stream is brolen up into droplets by modulation 625kMz, meaning that 625000 drooplets of colorant are formed each second . Because of the number of droplets per colour in any one pixel area can vary between zero and 15,16 color level possible. On each pixel of pattern which allows the production of smooth continuous tones. This contrast with drop on demand printing, which has to relay on dither patterns to naked eye, instead of the snooth shade afforded by continuous stream method.

Pre and post Treatments Printing with reactive and acid dye inks generally involves pre and post treatment in order for the dyestuff to fix onto the fabric. This is a multi- step process with a substantial degree of complexity.

Reasons for Pre Treatments The main reasons for separating the dyes from thickeners

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and other chemicals and applying them separately to the fabric are as follows

  ‘All In’ inks are less stable and have lower storage stability, e.g. reactive dyes are more likely to hydrolyse when alkali is present in the ink.

  Chemical in the ink cause corrosion of jet nozzle; the detrimental effect of the sodium chloride on steel surfaces is well known, for instance; inks for use in ‘charged drop’ continuous printers should have low electrical conductivity.

  Thickeners in the ink often do not have the desired rheological properties.

  Some chemicals can be utilized in pre treated fabric but would cause stability problems in the ink e.g. sodium carbonate as alkali for reactive de fixation is acceptable on the fabric but not in the ink.

  The presence of large amounts of salts in aqueous inks reduces the solubility of the dyes; concentrated inks are required in jet printing due to the small droplets size.

When pre treated fabric has been dried and then jet printed there is usually little need to provide a drying station to dry the print.Fixation Steaming is the process normally used to fix printed textile. Reactive and acid dyes are steamed under atmospheric pressure at just over 100ºC. During the process steam condensed on the fabric and is absorbed by the thickners and hygroscopic agents in the pronted

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areas. Dyes and chemicals dissolves and form extremely concentrated dyenath within the thickener film. As the result of extremely low liquor ratio fixation is much more rapid than in exhaustion dyeing. High temperature steam is necessary for the fixation of disperse dyes on polyester. The Tg of polyester in steam is lower than it is in dry air, and fixation is more efficient. Usually steam is heated to 170-180ºC at atmospheric pressure, but sometimes pressure steaming at 130-150ºC is used. Pigment prints are cured hot air in a stenter or a roller baker.

- RIP softwareThe RIP software plays an equal important role in the Digital Textile Printing as the machine itself. It allows you to choose or change colors and also previews the print result on your screen before you actually print out. You can modify intensities of inks, saturation etc and a good RIP acts as a bridge between the design system and the printer as it allows you to print several meters of continuous fabric without resting. 

 - COLOR PROFILER

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This is system is indeed very important to match colors as shown on the monitor to colors appearing on the print, or colors over various fabrics. Though it seemingly involves a very complicated procedures, but if mastered gives brilliance to the print job.▶ Progress of Digital Textile Printing :For better print process and achieving the right color, pre-setting, steaming & washing processes are essential processes used during Digital Textile Printing.- Pre-setting process for fabric.To print via DTP System, pre-setting process is essential. The purpose of the pre-setting process is to get better absorption & stability of the ink on the fabric during & after the print. Subsequently, the post treatment can be made depending on what kind of ink & fabric you are using.- Finishing process for fabricPrinted result via DTP system (specially for Reactive / Acid & in some cases Disperse inks) should go through steaming or washing process to finish. Without it, you would not get desired colors & fastness as the inks are water-soluble.WORKING :

1. Pre-setting Process : Using 'pre-setting' system / Coating machine / Padding  mangle, appropriate treatment should be done for the fabric before printing.

2. DTP System : Print it via Digital Textile Printer after modifying colors & Designs with help of Design & RIP Software.

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3. Finishing Process : Printed result has to undergo the finishing process, such as steaming, washing etc.

 ▶ Advantages of Digital printing :The most notable advantage of digital printing is that there is no limitation in colors during printing process. Theoretically 16 million colors & shades can be produced via DTP. The DTP system is ideal for sample printing as you reduce your cost and the result is immediate in comparison to the conventional ways where long wait time is required. Moreover process of making screens or etching the cylinder to find the right sample is too expensive.In addition, with Digital Textile Printing system, you can obtain good results for special fabrics and create a niche designer market for your range of prints.

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DIGITAL PRINTING OF COTTON FEBRICS

Textile printing involves the production of a predetermined coloured pattern on a fabric, usually with a definite repeat. It can be described as a localised form of dyeing, applying colorant to selected areas of the fabric to build up the design.Textile Printing, like Textile dyeing, is a process for applying color to a substrate. However, instead of coloring the whole substrate (cloth, carpet or yarn) as in dyeing, print color is applied only to defined areas to obtain the desired pattern. This involves different techniques and different machinery with respect to dyeing, but the physical and chemical processes that take place between the dye and the fiber are analogous to dyeing.

A Typical Printing Process Involves the Following Steps:

Color paste preparationwhen printing textiles, the dye or pigment is not in an aqueous liquor, instead, it is usually finely dispersed in a printing paste, in high concentration

Textile Printing

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The dye or pigment paste is applied to the substrate using different techniques, which are discussed below

FixationImmediately after printing, the fabric is dried and then the prints are fixed mainly with steam or hot air (for pigments). Note that intermediate drying is not carried out when printing carpets (too much energy would be needed for removing the highly viscous liquor)

After-TreatmentThis final operation consists in washing and drying the fabric (it is not necessary when printing with pigments or with other particular techniques such as transfer printing).

Pigment Printing

Pigment printing has gained much importance today and for some fibers (e.g. cellulose fibers) is by far the most commonly applied technique. Pigments can be used on almost all types of textile substrates and, thanks to increased performance of modern auxiliaries, it is now possible to obtain high-quality printing using this technique.

Pigment printing pastes contain a thickening agent, a

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binder and, if necessary, other auxiliaries such as fixing agents, plasticizers, defoamers, etc.

White spirit-based emulsions, used in the past as thickening systems, are used only occasionally today (mainly half-emulsion thickeners).

After applying the printing paste, the fabric is dried and then the pigment is normally fixed with hot air (depending on the type of binder in the formulation, fixation can also be achieved by storage at 20°C for a few days). The advantage of pigment printing is that the process can be done without subsequent washing (which, in turn, is needed for most of the other printing techniques).

Printing Paste Preparation

Dye Printing process traditionally starts with the preparation of the paste. Compared to pigment printing, the composition of the pastes is more complex and variable, being determined not by the dye used, but by the printing technique, the substrate, the application and the fixation methods applied.

Apart from the dye, printing pastes contain a thickening agent and various other auxiliaries, which can be classified according to their function as follows:

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Oxidizing agents (e.g. m-nitrobenzenesulphonate, sodium chlorate, hydrogen peroxide)

Reducing agents  (e.g. sodium dithionite, formaldehyde sulphoxylates, thiourea dioxide, tin(II) chloride)

Discharging agents for discharge printing (e.g. anthraquinone)

Substances with a hydrotropic effect, like urea Dye solubilisers, which are polar organic solvents like

glycerine, ethylen glycol, butyl glycol, thiodiglycol, etc. Resists for reactive resist printing (e.g. sulphonated

alkanes) Defoamers, (e.g. silicon compounds, organic and

inorganic esters, aliphatic esters, etc.).All the necessary ingredients are metered (dosed) and mixed together in a mixing station. Since between 5 and 10 different printing pastes are usually necessary to print a single pattern (in some cases up to 20 different pastes are applied), in order to reduce losses, due to incorrect measurement, the preparation of the pastes is done in automatic stations. In modern plants, with the help of special devices, the exact amount of printing paste required is determined and prepared in continuous mode for each printing position, thus reducing leftovers at the end of the run.

It is common practice in many printing houses to filter the printing pastes before application, using for example a filter cloth. This operation is especially important for thickeners to prevent free particles from blocking the openings of the screens.

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Printing (Paste Application) After preparation, the paste is applied to specific areas of the textile using one of the following techniques:

Direct printing (which also includes digital and transfer printing)

Discharge printing Resist printing.

In the case of direct printing the dye is applied to specific areas of a pretreated textile substrate, which can be white or pre-dyed (in light colours).

It is possible to speak of discharge printing, if in the fixation process that follows the application of the printing paste there is local destruction of a dye applied previously.

If the etched (discharge), previously dyed area becomes white, then the process is called white discharge. If, on the contrary, a coloured pattern has to be obtained in the etched area after the destruction of the previously applied dye, then the process is called coloured discharge. In this case the printing paste must contain a reduction-resistant dye along with the chemicals needed to destroy the previous one.

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As a result the pre-dyed background is destroyed according to a pattern and the dye, which is resistant to reduction, takes its place.

In the case of resist printing, a special printing paste (called «resist») is printed onto certain areas of the fabric to prevent dye fixation. In the case of physical resist the material is printed with a difficult-to-wet resin that inhibits the penetration of a dye applied in a second stage.

On the other hand, with a chemical resist, dye fixation is prevented by a

chemical reaction. Depending on the way the process is carried out, one can speak of preprinting, intermediate or over-printing resists. One common procedure is the wet-on-wet process in which the resist paste is initially printed, then the material is overprinted with full cover screen and finally fixed and washed. Over-printing resists can be applied only if the

dye, already present in the previously dyed and dried fabric, is still in its unfixed form, as in the case of developing dyes.

The difference between transfer printing and the techniques described earlier is that with this technique the surface of the fabric is not directly printed. Instead, the pattern is first created on an intermediate carrier (e.g. paper) using selected disperse dyes and then it is

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transferred from there to the fabric. The dye is usually fixed by placing the printed paper in contact with the fabric into a thermal pressure system. Under the influence of the heat the dye sublimates and diffuses from the carrier into the fibre of the textile substrate.

There isno need for further treatment such as steaming, washing, etc. This technique is applied forpolyester, polyamide and some acrylic fibres, using selected disperse dyestuffs according to the specific type of fibre.

Fixation :

After printing, the fabric is dried. Water evaporation leads to an increase in dye concentration and

at the same time prevents the colours from smearing when the fabric is transported over the guide rollers. At this stage the dye is not yet fixed.

The aim of the subsequent fixation step is to transport as much as possible of the dye, which is retained by the thickener, into the fibres. This is especially important with dyes, such as vat dyes,

for example, that are printed in the insoluble form and are converted into the corresponding soluble state only after

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reaction with the reducing agents during the fixation process.

Fixation is usually carried out with steam. Water vapour condenses on the printed material, swells up the thickener, heats the print and provides the necessary transport medium for the diffusion of the dye. The distribution of the dye between fibre and thickener is an important factor in determining the fixation degree of the dye, which is called the "retaining power" of the thickener.

The thickener, in fact, is often composed of polysaccharides and therefore competes with cellulose in retaining the dye. This is the main reason why the fixation rate of a given dye is 10 % lower in printing than in dyeing.

After-treatment :

The last step of the printing process consists in washing and drying the fabric. When printing with insoluble dyes such as vat dyes this operation also serves as a means to re-convert the dye to the original oxidised state. In this case, after an initial rinsing with cold water, the printed material is treated with hydrogen peroxide. The process is completed with a soap treatment with sodium.

As already explained, washing is not necessary with pigment printing and transfer printing. This holds for any dyeing/ printing system where thickeners are not needed

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and where the dyestuff is (nearly) completely fixed (e.g. printing carpet tiles with digital jet printing techniques).