Definition of Flat Screen Printing | Advantages of Flat Screen Textile Printing | Screen Printing Technology | Heat Transfer Printing | Advantages of Transfer Printing | Disadvantages of Transfer Printing

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

A variety of different machines can be used for printing fabrics. The most commonly used are described below.

Flat Screen Printing Machine
Flat-screen printing machines can be manual, semi-automatic or completely automatic. One type of machine, which is still commonly found in printing houses, can be described as follows. The fabric is first glued to a moving endless belt. A stationary screen at the front of the machine, is lowered onto the area that has to be printed and the printing paste is wiped with a squeegee. Afterwards the belt, with the fabric glued on it, is advanced to the pattern-repeat point and the screen is lowered again. The printed fabric moves forward step by step and passes through a dryer. The machine prints only one color at a time. When the first color is printed on the whole length of the fabric, the dried fabric is ready for the second cycle and so on until the pattern is completed.

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

In both machines the continuous rubber belt, after pulling away the fabric, is moved downward in continuous mode over a guide roller and washed with water and rotating brushes to remove the printing paste residues and the glue, if necessary. After this, the belt is sent back to the gluing device. In some cases the glue is applied in liquid form by a squeegee, while in other machines the belts are pre-coated with thermoplastic glues. In this case the textile is heated and then it is squeezed by a roller or simply pressed against the rubber-coated belt, causing the glue to soften and instantly adhere.

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

Heat Transfer Printing
Approximately 7% of printed goods are printed using Heat Transfer Printing

Advantages of Transfer Printing

• High quality prints
• Fewer seconds
• Economical for short runs
• Practically pollution free

Disadvantages of Transfer Printing

• Slow
• Primarily only for polyester

Discharge Printing | Resist Printing | What is Discharge Printing | Discharge Printing Process | After-Treatment of Discharge Printing

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

Resist Printing

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 pre-printing, 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.

Dye Fixation

After printing, the fabric is dried. Water evaporation leads to an increase in dye concentration and at the same time prevents the colors 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 fibers. 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 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 fiber 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 carbonate at the boiling point. 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 and where the dyestuff is (nearly) completely fixed (e.g. printing carpet tiles with digital jet printing techniques)

Textile Printing Operation

• At the end of each batch and at each colour change various cleaning operations are carried out
• The rubber belt, to which the fabric is glued during printing (see description below), is cleaned in continuous mode with water to remove excess adhesive and printing paste. Some machines are equipped with water re-circulation systems
• The printing gears (all systems responsible for feeding and applying the paste to the substrate) are cleaned by first removing as much as possible of the paste residues and then rinsing with water. In some companies the paste residues are directed back to the appropriate printing paste batch containers for re-use
• The remaining paste in the containers, in which the paste is prepared (paste vats), are in general previously cleaned up by means of sucking systems before being washed out with water. The residual printing paste collected in this manner is then disposed of. 

Discharge Style Printing Process | Discharging Agents

Discharge Style Printing:

Discharge means removal and discharging system means the process which can produce a white or colored effect on a previously dyed ground.

This discharging of color from previously dyed ground is carried out by a discharging agent which is actually a oxidizing and reducing agent capable of destroying color by oxidation and reduction.

Discharge Style of Printing

Discharge styles have been important since the earliest days of textile printing..With any industrial process there must be sound technical and commercial reasons for its conception and continuation. In the case of discharge printing, the following considerations determine the usefulness of the process compared with other printing techniques.

1. Printed materials with large areas of ground colour can be produced, the depth, levelness and penetration of which would be difficult, if not impossible, to obtain by a direct printing process.

2. Delicate colours and intricate patterns can be reproduced on grounds of any depth, with a clarity and sharpness that have become the hallmarks of this style. Intricate white patterns lose their crispness if left as unprinted areas in a direct, blotch print, because the print paste spreads unequally in different directions. In addition, a coloured motif fitted into a blotch print either leaves unprinted white margins or forms a third colour where fall-on occurs. In some cases such effects are acceptable, but they can be eliminated by using the discharge technique.

3. The extra processes required and the additional costs of discharge pastes mean that production costs are higher, but the aesthetically superior results give the product a higher value and enable profit margins to be maintained or even improved. The higher costs of discharge printing are often offset when applied to long-lasting designs used for scarves, ties, cravats and dressing gowns. As already indicated, in discharge styles the pattern is produced by the chemical destruction of the original dye in the printed areas. The discharging agents used can be oxidising or reducing agents, acids, alkalis and various salts. An early and, one might say, classical example is the discharge printing of cotton dyed with indigo, the characteristic colour of which can be destroyed either by oxidation or reduction.

Sequence of Discharge Printing Process:
Discharge printing is done as the following way.

Fabric preparation

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Table preparation

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Fabric plaited on the table

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Apply printing paste with the help of screen

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Curing at 190ºc (belt speed 3m/min)

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Delivery

Discharging Agents:
Clearly, the most important methods of discharging are based on reduction. This general method can be varied and adapted to give discharges with most classes of dye in use and on most types of fibre. Indeed, to many printers the terms ‘reducing agent’ and ‘discharging agent’ are synonymous. The most widely used reducing agents are the formaldehyde sulphoxylates. The stability of these compounds is such that only limited losses of sulphoxylate occur during printing and prior to steaming. The use of sodium formaldehyde sulphoxylate (CI Reducing Agent 2, sold as Formosul or Rongalite C) was established as long ago as 1905, when it was recognised that methods based on this reducing agent offered many advantages.

Advantages:

• Large areas of ground of ground color are possible.
• Delicate colors and intricate patterns possible on deep ground color, excellent depth and clarity possible.
• Higher production cost but long lasting unique styles.

Disadvantages:

• It is an expensive process.
• Two stage application involved in dyeing or padding and discharge printing.
• Limited choice of ground and motif colours. Requires rigid process care that any default will lead to damages