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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