FLAT SCREEN PRINTING; Printing techniques

Screen printing is an extension of the technique of stencilling in which a coloured image arises from the transfer of colour through open areas in the sheet placed upon the fabric surface. In flat screen printing, the screen consists of a woven polyester material, tightly stretched across the underside of a light, rectangular frame. During printing, the screen usually lies across the fabric width. The polyester fabric only allows the printing paste to pass through the mesh in those areas corresponding to the design being printed. A suitable coating blocks the remaining areas. There is a screen for printing each colour and each successive screen sits on the fabric in the exact position for accurate registration of the multicolour design. In manual screen printing, the fabric lays on a long table on top of the printing blanket. This blanket is typically a cotton/polyester cloth, water-proofed by a coating of neoprene rubber. If the fabric slips or deforms during printing, there is a loss of print definition. The fabric is therefore usually pinned to the printing blanket, or stuck onto it with a water-based gum or thermoplastic adhesive. The latter type coats the blanket surface but only becomes sticky when heat softens the polymer. After printing, the fabric and printing blanket separate. Washing the blanket removes any gum and dye paste transferred through the cloth or beyond its edges. Thermoplastic adhesives are resistant to repeated washing and very durable when using water-based printing pastes. Beneath the printing blanket are several layers of cloth forming a uniform cushion called a ‘lapping’. In some cases, a ‘back-grey’ cloth is inserted between the fabric and blanket. This grey cotton fabric absorbs any paste that transfers through the fabric and thus avoids smudging of colour on the back of the printed fabric. If used, it too must be washed and dried for re-use.

The printing paste is poured into the screen that sits on the fabric surface. Drawing a flexible rubber blade called a squeegee across the inner surface of the screen spreads the viscous dye paste and forces it through the open areas to print the fabric beneath. Two or four strokes across the screen are usual, the number depending on the porosity of the screen and the paste viscosity. Each passage of the squeegee should transfer the same amount of paste to the fabric in any given print. The angle, speed and pressure of the squeegee must therefore be the same for each print. In semi-automated screen printing, a mechanically driven squeegee transfers the colour. This often has a pair of parallel rubber blades, with the paste held between them. When passing across the screen during printing, only the trailing blade is in contact with the screen surface. At the end of the stroke, the leading blade drops and the trailing blade rises ready for the back stroke. This eliminates the need to lift the squeegee over the residual paste at the end of each stroke. The Zimmer rolling rod applicator moves across the printing screen along the length of the fabric, driven by an electromagnet under the blanket. The roller is small enough that paste can flow over it so that lifting is not necessary.

This type of roller gives less screen wear. In manual and semi-automated screen printing, the end of each screen rests against a guide rail running along the table edge. The screen fits against a ‘stop’ that defines its position relative to the fabric. The screen for a particular colour gradually moves down the fabric length, fitting against the appropriate ‘stop’ as each repeat is printed. ‘Pitch marks’ printed on the fabric selvages will verify the alignment of the screen for the next colour. Printing the entire length of fabric on the table with one colour design allows some intermediate drying before application of the next colour. The next screen does not then crush or mark the paste layer already present so that the images are sharper.

In fully automated flat screen printing, productivity is higher. The screens for each of the colours have the correct positions on the guide rail for exact registration of their patterns. After printing all colours simultaneously, the screens are lifted, and the printing blanket moves the fabric stuck to it so that a printed pattern has the correct position for printing the next colour. Clamps securely hold the sides of the blanket and they move the fabric by the exact required distance after each print. The spacing between individual screens is usually one pattern repeat. Thus, if a screen has two pattern repeats, and screens are spaced one pattern repeat apart, the blanket must move the fabric two pattern repeats down the table (Figure 23.1). In this way, gaps are avoided. Correct fabric placement is vital for accurate registration of the different coloured patterns. In general, a slight pattern overlap prevents a white gap between two printed colours.

At the end of the printing table, the fabric separates from the blanket and passes into the dryer, while the blanket is washed, dried and recycled beneath the printing table. Various mechanical devices compensate for the intermittent movement of the fabric during printing but allow uniform movement of the fabric during drying, and of the blanket during washing.

Printing

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Printing involves localised coloration. This is usually achieved by applying thickened pastes containing dyes or pigments onto a fabric surface according to a given colour design. In particular, the viscosity of a print paste is critical. It determines the volume of paste transferred to the fabric and the degree to which it spreads on and into the surface yarns. The paste must colour all the visible fibres on the printed surface, so it must penetrate somewhat into the yarn structure. If the paste is too ‘thin’, it will spread, giving poor print definition, and penetrate too far into the yarns decreasing the colour yield.

Printing was originally done by hand using wooden blocks with a raised printing surface, much as children do potato printing. The two main techniques used for transferring paste onto fabrics involve engraved rollers carrying paste in the recesses corresponding to the colour pattern, or screens with the open mesh in the pattern areas. There will be one roller or screen contacting the fabric surface for each colour to be printed.

The most important printing method today is pigment printing. This involves printing the coloured pattern onto the fabric surface and curing the printed areas by heating in air. The print paste contains coloured pigments and a binding agent. On curing in hot air, the binder forms a solid film of transparent polymer that holds the pigments in place on the yarn surfaces. The great advantage of pigment printing is that the fabric does not require washing after the fixation process. Soluble dyes used in printing a fabric are of the same types as those normally used to dye it a solid colour. Apart from in pigment printing, the usual sequence of operations is printing, drying, steaming and washing. Dyes for printing must have high solubility because there is only a limited amount of water in a thickened print paste and after drying the dyes must re-dissolve in a limited amount of condensed steam. The paste must dissolve the dyes to allow their diffusion into the fibres. It will also contain all the other required chemicals for fibre wetting and dye fixation.

The final washing removes the thickening agent, unfixed dyes and other auxiliary chemicals from the printed surface. During washing, it is critical that the dyes removed do not stain any white ground or other printed areas. For this reason, dyes for printing often have relatively low molecular weights so that their substantivity is not very high. When a manufacturer sells the same dye for both dyeing and printing, the two product formulations will invariably be different. Pigment and dye printing are both direct printing methods. Two other important indirect printing methods called discharge and resist printing also give coloured designs. In discharge printing, a uniformly dyed fabric is printed with a paste containing chemicals that destroy the colour leaving a white pattern. If the paste contains other dyes, stable to the chemical that discharges the ground colour, they can dye the treated area. In this way, multi-colour effects are also possible. In resist printing, the fabric is dyed after printing. The printed areas resist dye absorption so that these design areas are reserved. After removal of the resist agent, the design may be white or coloured if other dyes and appropriate chemicals were present in the original paste.

The rapid development of CAD computer systems for print design has had a significant impact on this activity. The development of digitised textile printing using, for example, ink jet printers is well underway. Such computer assisted manufacturing will considerably influence the textile printing industry in the near future. Its other preoccupation, as for the dyeing industry in general, is that of reducing the amounts of biodegradable and potentially harmful chemicals in the effluent leaving the works so that its environmental impact is limited.

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Adhesive systems of automatic flat screen printing machine

Adhesive system is very important part of an automatic flat screen printing machine. When fully automatic flat-screen printing machines were first introduced, it was quite common to combine the fabric to be printed with a back-grey, especially if the GSM of fabric was low. As adhesives and way of printing of their application were improved, this practice of printing became less important. The method of automated printing then became established, and which is still in use in many plants, is to apply a liquid-based adhesive to the blanket at the entry end, by means of a brush running in a trough containing the adhesive solution, and to spread the layer more evenly with a rubber squeegee; the textile material is then pressed against the tacky blanket with a pressure roller. A hot-air dryer is sometimes employed to dry the adhesive before the material is printed by automatic flat screen printing machine.

The available as another chose of the continuous application and removal of aqueous adhesive is the use of a tacky semi-permanent or ‘permanent’ adhesive on the blanket. Such adhesives, often based on acrylic co polymers, can withstand the washing necessary to remove excess print paste without becoming detached from the blanket. Their permanence is limited, however, replacement being needed after perhaps two weeks’ printing, and permanent thermoplastic adhesives have proved more satisfactory. These adhesives are coated on to the blanket and are only tacky when heated. Heat can be applied either directly to the adhesive layer or to the textile material, in order to achieve the required bond. Such thermoplastic adhesives often remain serviceable during the printing of several hundred thousand meters of textile material.
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FULLY AUTOMATIC FLAT-SCREEN PRINTING

FULLY AUTOMATIC FLAT-SCREEN PRINTING METHOD:

Figure 1. Fully automatic flat-screen printing machine (Buser)

Figure 2. Fully automatic flat-screen printing machine (simplified diagram); rollers 1 and 2 move as shown, to maintain the lower side of the blanket in constant motion, 3 is the pressure roller, 4 the temporary adhesive application unit and 5 the blanket washer (Buser)

Flat screen printing system is one of the most popular printing systems in all over the world. The flat screen printing is widely used in United States of America (USA), United Kingdom (UK), Korea, China, Japan, Bangladesh, India, Pakistan etc. Manual flat screen printing machine and printing process is too much slower and production in flat screen printing is also lower. The best solution for reducing time of printing and increase the productivity in printing is to use the automation in flat screen printing.   

Due to increase the operating force and speed of flat-screen printing, it must be needed to develop a method of printing that could be delivered all the colors spontaneously. Perhaps flat screens for printing are not reasonable coloration units for a real continuous process of printing, and in all the successful printing machines for total automatic flat-screen printing the color is applied through the screens while the textile material is fixed.

The automatic flat screen printing contains many features and facilities. The main features of a typical automatic flat-screen machine are illustrated in figure1 and shown diagrammatically in Figure2. All those screens for the design (a single printing screen for each single dyed color) are placed perfectly along the top place of a long endless belt that named as blanket. A printing machine intended to print conventional furnishing styles and more place is kept for 15 more screens. Those automated printing screens are reserved for more printing color variation.  The width of the space in to the areas printed by any two adjacent screens must be a whole number of length ways design repeats. This need not necessarily be the same as the lengthways screen repeat as there may be several design repeats per screen repeat; for example, If there are four design repeats per screen repeat, the space between adjacent screens need only be one fourth of a screen repeat.

For automatic fully screen printing the textile material is gummed to the blanket at the entry end and moves along with the blanket in an intermittent style, one screen-repeat distance at a time. All the colors in the design are printed simultaneously while the fabric is stationary; then the screens are lifted and the fabric and blanket move on. When the fabric approaches the turning point of the blanket, it is pressed off and passes into a drying chamber. The soiled blanket is washed and dried during its return passage on the underside of the printing machine.

Current utilization of automatic flat-screen printing machine:
Scientist is trying to develop the automatic flat-screen printing machine. In the UK and USA fully automatic flat-screen printing is primarily used for the printing of good-quality furnishing textile material. The systems are perfectly suitable for the large repeats and large motifs often used on these fabrics. In addition, the printing speed is relatively low (300–600 m h–1) compared with rotary-screen or copper-roller printing, and this allows time for any printing faults to be mentioned and fixed before much expensive cloth is damaged.

The main part of the fully automatic flat screen printing machine:
1.    Adhesive systems of automatic flat screen printing machine
2.    Squeegee systems of automatic flat screen printing machine
3.    Intermittent movement of the printing screen and fabric

HISTORY OF SCREEN PRINTING

HISTORICAL DEVELOPMENT OF SCREEN PRINTING

Screen printing is very familiar terms in textile printing sector. Innumerable kids developed with simple stencils by cutting out shapes from card, and brushing or spraying paint or ink through the holes on to cloth beneath. Industrial stencil sets for lettering are made of waxed card or metal, and incorporate ties to hold the solid areas together and to prevent the centers of letters such as O or P from falling out. The ties produce unsightly lines across the stenciled letters.

History of screen printing in centuries ago, when the Japanese developed the stenciling technique for textile screen printing and brought it to a fine art, they overcame this problem by using human hair or silk threads as ties. These were so fine that the colour spread underneath them, disguising their presence. By the 19th century, the use of this method for printing fabric had spread far beyond history of Japans screen printing and was used worldwide.

History of screen printing in the mid-19th century, French printers introduced the use of a woven silk fabric to provide a continuous support for the paper stencil. For the absolute results the support cloth was stretched across a frame, and the combination became known as a screen. The development was important because in this way not only were ties automatically provided, but the amount of colour paste applied could also be controlled. Soon after, the paper stencil was replaced by a durable paint on the screen fabric.

From this time onwards the advantages of screen printing became increasingly appreciated, especially in fashion houses. Designs are relatively easy to transfer to screens and the frame size can be readily varied. The designer, freed from the restrictions of copper rollers, thus had far greater freedom to choose repeat sizes. In addition, the pressure applied in screen printing is much lower than in roller printing with the result that surface prints with an improved ‘bloom’ or colour strength are obtained, and textured surfaces are not crushed.

The development of screen printing to its modern, highly productive form ran parallel with improvements in the printing screens themselves. Accurate printing of multicolored designs requires stable screens. Screen fabrics made from hydrophilic yarns, such as silk, cotton, viscose rayon or cellulose diacetate, are apt to sag when in contact with water-based print pastes. The introduction of hydrophobic synthetic fibres such as nylon and polyester, especially the latter, made it possible to manufacture stable screens that maintained tension when wet. Their high tensile strength also allowed the fabric to be stretched more tightly over the screen frame, thus improving the accuracy that could be attained. Further improvement came with the introduction of metal screen frames to replace the wooden ones used hitherto, which tended to warp when subjected to a regime of continually alternating wetting and drying.

Strong, stable screens enabled the hand screen-printing process to be mechanised. The first development was the introduction of a movable carriage, in which the screens are mounted one at a time. The squeegee (a flexible rubber blade used to spread the printing paste across the screen and force it through the open areas) was driven across the screen by a motor attached to the carriage. In this method, which is still in use, the fabric being printed is stuck down on long tables and one colour is printed at a time, just as in hand-screen printing.

The 1950s saw the advent of fully automatic, flat-screen printing, the Buser, Stork and Johannes Zimmer machines being prominent. These machines print all the colours in a design simultaneously along the top of an endless conveyor belt (blanket). The blanket and fabric are stationary while the printing operation takes place and then move on when the screens are raised; hence the fabric movement is intermittent.

Fully continuous printing is best achieved using cylindrical (rotary) screens and many attempts were made to form flat wire mesh screens into cylinders, despite the necessity of a soldered seam. When printing through a cylindrical screen with a seam, a line will show across the fabric once every cylinder circumference, unless the seam can be hidden within the design. This was the approach used in the screens manufactured by A J C de O Barros for the Aljaba machine, first introduced in 1954. Barros has written an interesting description of the Aljaba screens and machines.

Wire-mesh screens are too open for printing purposes, and on the early Aljaba screens electrodeposited copper partially filled the holes. This process was later discarded, to be replaced by the use of an outer seamless woven nylon sleeve. Later still, in fact after the closure of the Aljaba company, W Sword introduced a new version of the wire-mesh screen, the Durascreen, in which the holes in the mesh were partially filled with a flexible polymer by electrophoretic coating. The same process can also be used on electroformed nickel screens, extending their life considerably, since the flexible polymer coating reduces the risk of creasing.

An important innovation of the Aljaba company was the duplex machine used by some printers to print both faces of curtain fabrics. The fabric ran vertically upwards between pairs of screens, print paste being forced through the screens by metal roller squeegees.

The invention of seamless screens of electrodeposited nickel was the really significant step which heralded the rapid expansion of rotary-screen printing. Peter Zimmer (Austria) introduced the galvano screen in 1961, and Stork (Holland) the lacquer screen in 1963. These screens soon proved to be superior, in many respects, to Aljaba screens. When Stork introduced their machine, based on the lacquer screen, at the 1963 ITMA Textile Machinery Show at Hannover it was an immediate success, so much so that Stork decided to stop manufacturing fully automatic flat-screen machines. Between 1964 and the end of 1972 Stork sold 600 rotary machines throughout the world.

Machines using rod or roller squeegees, such as those manufactured by Peter Zimmer and Mitter, have been very successful in printing wider substrates, such as carpets. Rotary-screen machines have also been used to print paper for the transfer printing process. Currently rotary-screen printing is the predominant printing method worldwide, having substantially replaced copper-roller (intaglio) printing.

ENGRAVED-ROLLER PRINTING; COLOUR VARIABLE PRINTING

Two-colour engraved roller printing machine

Principles of operation of engraved roller printing:
A traditional two-dye engraved roller printing machine is shown diagrammatically in Figure. Each engraved cylinder (F) of roller printing machine, mounted on a steel mandrel (L), is forced during printing fabric as it travels around a pressure bowl (A) of printing machine with resilient covering (B). The engraved roller printing machine must be of robust construction because pressures of several tonnes are applied and each mandrel is driven by a single large crown wheel. As the rollers rotate, a furnishing roller (G) transfers print paste (colour) from a colour box (H) to the engraved cylinder, filling the engraving and smearing the whole surface. This surface colour is almost immediately removed by the steel blade known as the colour doctor (J). This doctor must be precisely ground, sharpened and set, at the optimum angle and tension, to leave the surface perfectly clean. Engraved areas retain the colour in parallel grooves and the doctor blade is ‘carried’ on the crests between the grooves.

The fabric is then forced into the engraving and most of the printing paste is transferred. The cushion between the pressure bowl and the engraved roller clearly plays a critical role in the uniform transfer of colour across the fabric width and along its length. The cushion is obtained by rolling about ten layers of a lapping fabric with linen warp and woollen weft around the pressure bowl. Any irregularity, including any ridge at the end of the lapping that may develop if it is not perfectly matched with the first end, can cause a visible fault in a critical print. The cushion progressively hardens with use and requires changing from time to time. Because the lapping must be protected from colour printed beyond the fabric edges or forced through the fabric, an endless printing blanket (C) must be used and washed and dried continuously before returning to the point of printing. In addition, a back-grey (D) is used to absorb colour and give greater resilience, unless the blanket provides enough resilience and is able to hold the excess colour satisfactorily. In the printing of lightweight or knitted fabrics the woven back-grey is often ‘combined’ with the fabric to be printed, using small amounts of a suitable adhesive. This helps to maintain dimensional stability under the tension applied during printing.

After transferring its colour, the engraved roller is cleaned by a brass blade known as the lint doctor (K). This removes any loose fibres that may have stuck to the surface of the roller and could become trapped under the colour doctor, where they could cause a colour streak to be printed. The lint doctor on the second (and any subsequent) roller may be seen to remove a significant amount of the previously printed colour that the roller takes up from the fabric. Without the lint doctor, this colour would be carried into colour number two and cause rapid contamination, although some contamination is unavoidable because the lint doctor will push a little colour into the empty engraving.

The printed fabric is now separated from the back-grey and blanket and carried on to the drying section of the engraved roller printing machine, avoiding any contact of the print face until some drying has been achieved. In the UK drying cylinders were often used in the past, but hot-air dryers are more universally appropriate.

On a multi colour machine the printing rollers, with colour boxes and other auxiliary equipment, are arranged around a larger pressure bowl with minimum separation of the rollers. Setting and maintaining the correct registration (that is, fitting each colour of the design relative to the others) requires an arrangement for separately rotating each roller a small distance while the drive to all the rollers is engaged. The original box-wheel device was elegantly simple but introduced a serious hazard to the printer’s hands as it required the insertion of a rod, or tommy key, into a hole in a wheel rotating near the meshing drive gears. A safer device has been described by Gleadow, who has also given a more detailed account of engraved roller printing.

It is necessary to correct the pattern fit from time to time during printing, because each roller will have a different effective circumference if there are significant differences in the area engraved or the pressure applied. Thus the fabric will be stretched or allowed to relax to a minute degree between rollers and the fit of the design will gradually be lost. As already noted, combining with a back-grey can improve stability.

Good pattern fitting also requires a perfectly rigid fit of the engraved cylinder on its mandrel. To this end, the cylinders and mandrels are tapered and a forcing jack is required to mount and to remove the mandrel. In addition a key, or ‘tab’, on the inside of the cylinder engages in a slot along the mandrel.

MACHINE USED FOR BLOCK PRINTING

Mechanization of block printing:
In 1834 a machine that automatically performed all the actions of block printing was invented by Perrot (it became known as the Perrotine), and achieved some success in block printing process. Mechanization block printing was limited to three dyes and a maximum repeat of only 15 cm and operation of the printing could not be truly continuous, but the three colours were printed simultaneously in that printer. Storey gives an illustrated account of this printing machine, and of hand block printing.

Much earlier, attempts had been made to obtain continuous surface printing using wooden rollers, but the difficulty of uniform application of the printing paste to the roller was the common problem. In about 1805, however, a rotating woollen fabric ‘sieve’ was introduced in Accrington, and used successfully thereafter. The other essential step of block printing was the preparation of a raised pattern on the roller by inserting copper outline strips and felt, just as in hand block printing. This technique of surface block printing was used until recently for printing furnishing fabrics and for wallpaper, but was never as important as the copper roller method. It had the advantages of requiring low pressure and avoiding the dyes contamination that occurs on engraved roller printing machines, because no contact was made with previously printed colour, but the cost of roller making of roller printing was high. Wallpaper printers found that inexpensive rollers could be cast from epoxy polymers and the nonprinting areas were easily cut away, but they did not carry enough colour to print most fabrics.

Essentially the same method of block printing has been used for printing polymer film, other packaging materials and transfer paper, but is then usually known as flexographic printing. The design is built up on wooden rollers by the application of rubber mouldings and the colour is applied by uniformly engraved metering rollers.

Surface printing machines with woolen fabric sieve

A Pressure bowl
B Surface roller
C Furnishing blanket moving over rollers 1, 2 and 3
D Colorant trough
E Plain furnishing roller rotating in colorant
F Second furnishing roller spreading colorant evenly on the blanket
G Blanket for extra resilience round the pressure bowl
H Back-grey
I Fabric to be printed
J Printed fabric