printing

Introduction:

Textile printing is the process of applying color to fabric in definite patterns designs. Improperly printed fabrics the color is bonded with the fiber, so as to resist washing and friction. Textile printing is related to dyeing but, whereas in dyeing proper the whole fabric is uniformly covered with one color, in printing one or more colors are applied to it in certain parts only, and in sharply defined patterns. In printing, wooden blocks, stencils, plates, rollers, or silk screens are used to place colors on the fabric. Colorants used in printing contain dyes thickened to prevent the color from spreading by capillary beyond the limits of the pattern or design.

Printing:

Printing is one kind of dyeing. When different types of color used to make a particular design on the textile goods is called printing. Normally printing is performed on the textile goods in dry condition. 

Flow Chart of Printing 

Greycloth
↓
Brushing and Shearing
↓
Singeing 
↓
Desizing 
↓
Scouring 
↓
Bleaching
↓
Mercerizing 
↓
Stentering
↓
Washing 
↓
Drying
↓
Winding/Beaming 
↓
Printing 
↓
Finishing

Different types of printing method:

Block Printing:

The blocks are usually made of wood and the design is hand carved, so that it stands out in relief against the background surface. 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. 

Block printing 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.

Roller Printing:

Roller printing has traditionally been preferred for long production runs because of the very high speeds possible. It is also a versatile technique since up to a dozen different colours can be printed simultaneously. The basic roller printing equipment, shown in Fig. 7.1, consists of a number of copper faced rollers in which the design is etched. There is a separate printing roller for each colour being printed. Each of the rollers rotates over the fabric under pressure against an iron pressure roller. A blanket and backing cloth rotate over the pressure roller under the fabric and provide a flexible support for the fabric being printed. A colour doctor blade removes paste or fibres adhering to the roller after contact with the fabric. After the impression stage the fabric passes to the drying and steaming stages.

Screen Printing:

This type of printing has increased enormously in its use in recent years because of its versatility and the development of rotary screen printing machines which are capable of very high rates of production. An additional significant advantage is that heavy depths of shade can be produced by screen printing, a feature which has always been a limitation of roller printing because of the restriction to the amount of print paste which can be held in the shallow depth of the engraving on the print roller. Worldwide, some 61% of all printed textile fabric is produced by the rotary screen method and 23% by flat screen printing. 

There are two basic types of screen printing process, the flat screen and the rotary screen methods. 

Heat Transfer Printing:

Transfer printing techniques involve the transfer of a design from one medium to another. The most common form used is heat transfer printing in which the design is printed initially on to a special paper, using conventional printing machinery. The paper is then placed in close contact with the fabric and heated, when the dyes sublime and transfer to the fabric through the vapor phase.

Ink-Jet Printing:

There has been considerable interest in the technology surrounding non-impact printing, mainly for the graphic market, but the potential benefits of reductions in the time scale from original design to final production has led to much activity in developing this technology for textile and carpet printing processes. The types of machines developed fall into two classes, drop-on-demand (DOD) and continuous stream (CS).

Carpet Printing:

The printing of carpets only really achieved importance after the introduction of tufted carpets in the late 1950s. Until then the market was dominated by the woven Wilton carpets and Axminster designs were well established, but by the 1980s tufted carpet production accounted for some 80% (by area) of UK production. Much of this carpet production was printed because the range of patterns possible to produce using tufting machines was limited and there was a desire to produce a greater flexibility of design for these types of carpet

Warp Printing: 

The printing of a design on the sheet of warp yarns before weaving. The fillingis either white or a neutral color, and a grayed effect is produced in the areas of the design.

Resist Printing:

A printing method in which the design can be produced: (1) by applying a resistagent in the desired 

design, then dyeing the fabric, in which case, the design remains whitealthough the rest of the fabric is dyed; or (2) by including a resist agent and a dye in the pastewhich is applied for the design, in which case, the color of the design is not affected bysubsequent dyeing of the fabric background.

Photographic Printing:

A method of printing from photoengraved rollers. The resultant designlooks like a photograph. The designs may also be photographed on a silk screen which is used inscreen printing.

Pigment Printing:

Printing by the use of pigments instead of dyes. The pigments do notpenetrate the fiber but are affixed to the surface of the fabric by means of synthetic resins whichare cured after application to make them insoluble. The pigments are insoluble, and application isin the form of water-in-oil or oil-in-water emulsions of pigment pastes and resins. The colorsproduced are bright and generally fat except to crocking.

Burn-Out Printing: 

A method of printing to obtain a raised design on a sheer ground. Thedesign is applied with a special chemical onto a fabric woven of pairs of threads of differentfibers. One of the fibers is then destroyed locally by chemical action. Burn-out printing is oftenused on velvet. The product of this operation is known as a burnt-out print.

Direct Printing: 

A process wherein the colors for the desired designs are applied directly to thewhite or dyed cloth, as distinguished from discharge printing and resist printing.

Discharge Printing: 

In “white” discharge printing, the fabric is piece dyed, then printed with apaste containing a chemical that reduces the dye and hence removes the color where the whitedesigns are desired. In “colored” discharge printing, a color is added to the discharge paste inorder to replace the discharged color with another shade.

Different types of textile printing processes

Whatever clothes we wear is full of attractive with prints. The printing is done is fabric form or in garments. When fabric comes out from mills, power loom or others means it is generally grey in color. To make fabric and cloths attractive and valuable dyeing and printing are done on fabric surface. But how the printing is done on cloths? Different printing process and advantages and disadvantages of each printing methods has been explained here.

1. Automatic Flat Screen Printing: In this method fabric is laid on flat table. A flat screen with perforated pattern is used for printing. Number of screen depends on no. of colors of the printed design. First the flat screen is placed on fabric from top and printing paste squeezed on screen by rod or scrapper from one side another.

    a. Advantages:

          I. Larger repeats

          Ii.Multiple strokes for pile fabric

    b.Disadvantages:

          i.Slow

          ii.Non continuous pattern

2. Rotary Screen Printing:

    a. Advantages

          i.Fast

          ii.Quick change over of pattern

          iii.Continuous pattern

   b.Disadvantages

          i.Design limitations

          ii.Small repeats

3. Engraved roller printing

     a. Advantages:

          i.High design capability (fine details and multiple tones)

     b.Disadvantages:

          i.Copper cylinders very expensive

          ii.Not economical for short runs

          iii.Required highly skilled worker

4. Heat transfer Print:

    a. Advantages

          i.High quality print

          ii.Fewer seconds

          iii.Economical for short runs

          iv.Pollution free

   b.Disadvantages:

          i.Slow process

          ii.Primarily for polyester

Style of Printing:

Style refers to the manner in which a particular action is performed; thus style of printing means the manner in which a printed effect is produced as distinct from the method which involves the means by which the pattern is produced.

 Style of printing involves certain mechanical operation and chemical reactions. Different styles of printing are described below:

Direct Style of Printing:

 A style of printing in one or several colours where the dyes are applied and then fixed by ageing or other appropriate means. The fabric is usually initially white but may sometimes have previously dyed.

Flock Style of Printing:

 A method of fabric ornamentation in which adhesive is printed on and then finely chopped fibers are applied all over by means of dusting-on, an air-blast, or electrostatic attraction. The fibers adhere only to the printed areas and are removed from the unprinted areas by mechanical action.

Discharge Style of Printing:

 This process is used to remove color from a dyed fabric in a desired pattern. A paste containing a chemical with a bleaching effect is applied through a printing process.

 The fabric is then placed in a steam unit for a specified time. The steam has the effect of removing the color from the fabric, leaving a lighter pattern in the printed areas.

 If desired, a non-dischargeable dye can be incorporated into the printing process to achieve a multicolored design.

Dyed Style of Printing: 

This style consists of two steps: printing with a mordant and dyeing. After dyeing color only fixes in places where mordant was applied as a result a design is produced on the fabric.

Resist/Reverse Style of Printing:

 In this case fabric is first printed with a white or colored resist salt and then dyed. A design is thus produced, as the dyestuff does not get fixed in places where the resist salt was applied. 

Rasid Style of Printing: 

In this style fabric is first printed with a dead salt and subsequent wet treatment produce color in printed places.

Azoic Style of Printing:

 Fabric is printed with coupling compound of azoic dye and then padded with a diazo compound. Color show up in printed areas as only there the reaction between two compounds occurs.

Metal Style of Printing:

 In the metal style of printing fabric is printed with silver or gold so;ution or non-removable resins.

Crimp/Crepon Style of Printing:

 Fabric is printed with thickened sodium hydroxide solution and then immersed in water. Thus only the printed areas shrink and an effect is produced.

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:

• 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.

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).

Yarn Printing (Space Dyeing):

Yarn printing is also known as “Space Dyeing”. Although the printing of yarns for true patterned effects proved very difficult tom control, the random space-dyed effects that can be more readily attained by a variety of yarn-printing methods have continued to be popular. The patent literature abounds with systems for producing colored flecked effects on yarns but the two most successful methods entail either warp printing or color application to a tubular knitted ‘sock’. The essential process sequence begins with dye liquor application, followed by steam fixation, washing-off and drying . 

space dyeing

Various warp-printing methods have been used over the years. In most present-day systems several ends of carpet yarn are taken from wound packages on a creel and colour is applied, either by lick rollers or by some form of spray or spinning disc applicator, to the yarns. The yarns are carried past the print heads in warp form or lying on a brattice on which they have been laid down in a continuous circular or elliptical coil. Warp printing tends to give the so-called ‘long spacing’ designs in the tufted carpet produced from them.

Knit–deknit applications, on the other hand, tend to give characteristically speckled ‘micro spaced’ designs, because of the limited degree of penetration of dye liquor achieved by the duplex printing rollers into the yarn sock. 

yarn printing

Although the end-effects produced by the two methods are basically different, the processes can be modified so that their results are more closely comparable. Thus the long spacing effects of warp printing can be imitated by overall application of a ground color followed by color spotting with segmented lick rollers or oscillating jets of dye liquor. Similarly oscillating jets of liquor can be applied to knitted sock and excess liquor squeezed out before steaming. This leaves large colored areas with good liquor penetration and, when tufted, the long spacing effect is achieved

Printing in Color - How Colors Combine:

•           Red + Green = Yellow

•           Red + Blue = Magenta

•           Green + Blue = Cyan

•           Cyan + Magenta + Yellow + Black = Four Color Print

For color printing pre-press technicians use digital color page makeup systems to produce an electronic image that mirrors what the printed pages will look like. They then use "off-press" color proofing systems and soft proofing techniques to print a copy or "proof" of the pages as they will appear. Once customer approval has been received technicians then often use laser image setters to expose the digital images directly onto thin aluminum printing plates.

Because of improved computer chips, an increase in technology use has had a major effect on the print sector. Over the last 3-5 years many printing business have altered their internal structures. In turn this has affected skills demands, with larger printing businesses now requiring workers with specialized skills. In these larger businesses the pre-press technicians are now responsible for the first stage of the print process only, as other processes have been automated.

Smaller print businesses often employ one printer who is responsible for all three stages of the printing process. These employees build content and structure page layouts with the materials supplied by the customer. They proofread draft documents for errors and clearness of print; they correct mistakes and then work with the print machinery to complete the print job. After the print run they may also need to bind the printed pages together (the job of a bookbinder in larger businesses). Working in these smaller businesses can be very fast paced and vacancies are generally filled by those that have a full set of skills and several years of print sector experience, due to the high level of responsibility. A trainee position within one of these smaller businesses would provide an excellent opportunity to learn a wide range of skills and that would provide you with many career development options.

Printing Competitiveness:

Printing companies are fully aware that an innovative product range is the best ally to ensuring the long-term competitiveness and adaptability of a whole sector to new market trends.

Full-colour management systems, together with the digitalisation of production processes themselves, enable substantial new quality standards to be introduced throughout the printing process. New market segments appear, making the printing industry a part of the wider "communications sector", complementing print with other "added-value services".

The most obvious examples are:

·         the introduction of customisation features in print;

·         the management of databases;

·         the production of short runs and electronic storage for later runs;

·         the production of electronic versions;

·         cross-media publications;

·         comprehensive communication solutions;

·         web-based communication solutions;

·         e-business solutions;

·         routing and dispatch, storage facilities, etc.

As well as creating additional value for their customers, the incorporation of these new services adds a new dimension to print buyers' communication strategies.

Printing Defects:

The defects which occurred in printing process are given below:

Printed defects fabric

Printed defects fabric

1.Flushing/Wicking 
2.Bleeding 
3.Misfits 
4.Stick-ins 
5.Scrimps 
6.Banding 
7.Unwanted pigment marking on Fabric 
8. Mottled 
9. Crack or Miss Alignment in Transfer Printed Fabric 
10. Printing Machine Stop 
11. Color out 

The causes of these defects are enlisted below: 

1. Flushing/Wicking: 
Caused due to Low viscosity of print paste. 

It occurs when the printed area bleeds out into the unprinted area. The result is a haloing or shadowing effect around the outline of the pattern design. 

2. Bleeding: 
Caused due to Low viscosity of print paste 

It is major defect as it happens throughout the fabric unless the viscosity is corrected. 

3. Misfits: 
A misfit is a print defect caused by improper alignment of the screens. Also known as out of registration, misfits leave unprinted areas in the design. 

For example, a green leaf may overlap its black outline or print over another color. 

4. Stick-ins: 
A stick-in occurs when a small fiber or piece of lint gets stuck in the screen opening. 

The result is a small unprinted circle in the design. A stick-in is very difficult to see and often goes unnoticed during a long run. 

5. Scrimps :
A scrimp defect occurs when the fabric creases underneath one of the screens during the printing process. 

The pattern is then printed on top of the crease, leaving a large unprinted area when the fabric returns to its relaxed state. 

6. Banding: 
Defect created by the print head’s movement over the substrate. 

Use of scanning print head, or a print head that moves back and forth across the substrate in straight line placing drops of ink at precise locations along the line. 

If the head is not properly aligned, or if the substrate advances unevenly, the result is a slight horizontal band or line of unprinted area. 

7. Unwanted pigment marking on Fabric: 
Caused due to screen has holes in it that should have been covered. This could be because of ageing of the screen and eventual damage or just improper exposure to light. 

8. Mottled:
Color applied unevenly during printing. 

9. Crack or Miss Alignment in Transfer Printed Fabric:
Incomplete transfer of design from paper to fabric on transfer printing due to removal of transfer of paper while the fabric was still hot. 

10. Printing Machine Stop: 
As a result of printing machine stop the dye sometimes is smudged along the width of the fabric. 

11. Color out: 
The result of color running low in reservoir on printing machine.