Teacher Name: R.S. KunduSub:- Printing Tech.
PRINTING : ITS SIGNIFICANCE AND METHODS OF PRINTING
The Term ‘printing’ is used to signify the production, by various means, of coloured patterns or designs on textile materials other than woven, embroidered or painted designs.
Printing is a branch of dyeing and is generally defined as ‘localised dyeing’ i.e. dyeing which is confined to certain portions of the fabric that constitute the design. Thus it is really a form of dyeing – the essential reactions involved are the same as those in dyeing – but differs from it, in that, in printing, the colour is applied not in the form of a solution as in dyeing, but form.
METHODS OF PRINTING
Mehods of printing denote the means of appliances used for producing the printed effect: depending upon the means employed, different methods have been developed for printing. Thus in Block printing, wooden blocks are used and in roller printing, engraved rollers are used for producing the printed effect.
The methods employed are as under:
(i)Block Printing
(ii)Stencil Printing
(iii)Screen Printing by hand
(iv)Automatic Flat-bed Screen Printing
(v)Rotary Screen Printing
(vi)Roller Printing and
(vii)Transfer Printing (an indirect method of printing)
All the above methods represent a means of transferring a pattern to the fabric. Basically the difference is in the speed with which an original design is transferred on to cloth.
BLOCK PRINTING
Block Printing is the oldest and the simplest method of printing. Because of its artistic and decorative value and the purity and richness of colour produced by it, the method is still used in many countries of the world.
The blocks used in this method are made of several layers of common timber which are cemented together and the portions to be printed are carved or raised in relief on a thick block of wood. Metallic blocks such as ‘T’ japs’ are used for a special work a in Batik printing. Designs with fine lines which are too fine to be cut on a wooden block, are made by inserting short pieces of copper strips and pins.
Colour in the form of a thickened paste is applied to the raised parts of the block and the impression of the design is obtained by stamping the block by hand with a wooden mallet ( or hammer) on the cloth to be printed. Since the block has to be lifted and stamped on the cloth repeatedly, its size as well as weight should not be unduly excessive so that it can be manipulated easily.
EQUIPMENT REQUIRED:
Besides blocks, long tables and a number of sieves along with a trolley are required to carry out printing. The table is generally made of wood and is covered with a resilient pad of a wooden felt cloth or several layers of gunny cloth on which a backgrey is fixed. The cloth to be printed is then gummed or pinned on this: it is sometimes stiffened a little to prevent it from wrinkling.
PROCEDURE:
The colour paste is spread on the woolen cloth of the sieve ‘a’. The block is carefully placed on this sieve and pressed twice (or more) so that it picks up a uniform layer of paste: it is then stamped on to the cloth on the table by giving blows or striking it with a small but heavy mallet (or hammer) to ensure a clear impression.
DISADVATAGES OF BLOCK PRINTING:
As it is difficult to cut minute spots or fine lines without breaking the grain of wood, it is not possible to obtain fine sharp outlines.
Due to separate impressions of the block, it becomes somewhat difficult to join up each impression or repeat perfectly with the other impressions resulting in an imperfect joining.
STENCIL PRINTING
This is also one of the oldest methods of printing but it is not used to any great extent. Really speaking it is not a printing process at all as the colour is applied to the fabric not by impression as in block or roller printing but by brushing or spraying the interstices of a pattern cut out from a flat sheet of metal or waterproof paper or plastic sheet or laminated sheet.
A stencil is prepared by cutting out a design from a flat sheet of cartridge paper, metal or plastic, with a sharp pointed knife, Stencils made from thin sheet of metal are costly and less easy to cut with accuracy and more difficult to handle than those made from cartridge paper. Metal stencils are durable but are liable to damage during working because of their thinness.
Procedure:
In actual Practice, the stencil is laid perfectly flat on the fabric to be printed and the colour paste is then brushed through its perforations with a brush. The plate is then lifted when the pattern appears on cloth as a coloured silhouette corresponding with the cutout parts of stencil. In patterns with two or more colours, a separate stencil is required for each colour as in block printing. The cloth is stencil printed throughout in one colour first and then the other colours are printed.
The method is mainly confined to printing of wall hangings, decorative panels, curtains, bedspreads, table covers, cushion covers etc. Any kind of colour can be used for the pattern in this method. Oil Colours are often used for heavy woolen goods, velvets, jute cloth, buckram etc. Even Water Colours can be used for lighter goods if fast colours are not required.
SCREEN PRINTING
Screen printing is simply an improved method of stencil printing.
The main difference between the two methods is that in screen printing the ordinary stencil palte is replaced by a tightly stretched screen of thin silk gauze on which the pattern is painted in such a way that the unpainted parts correspond to the perforations in a stencil plate and perform exactly the same function i.e. they allow the colour paste to pass through them freely on to the cloth placed beneath. In stencil printing, colour is usually applied by brush but in screen printing it is applied by means of a rubber or wooden squeegee which is almost as wide as the screen.
EQUIPMENT REQUIRED:
The main items of equipment are screens, squeegees, and wooden or concrete table. The talbes are similar to those used to block printing.
The table is covered with a woolen felt cloth about 6 mm thick. On this cotton backgrey is used if the cloth to be printed is to be fixed with pins: if it is to be gummed with an adhesive, then a waterproof cover is used.
PROCEDURE:
The colour paste is first poured at the bottom end of the shallow trough of the screen and drawn over with two or three strokes of the squeegee with uniform pressure whereby the colour is transferred to the cloth underneath. The screen is then lifted from this position (1) to position (3) repeating the operation and then the position (5) and so on (missing one position each time) to avid marking off of colour until the whole table length is covered. The cloth is them dried.
Each screen is washed with a jet of water immediately after use because if it left to dry with the colour paste on it, it becomes difficult to clean it later and can even block the mesh of the screen rendering it useless.
TRANSFER PRINTING
It is an indirect method of printing in which a sublimable dye is transferred from paper to a thermoplastic fabric under controlled conditions of temperature, time and pressure. In fact, it is not a printing process at all in the real sense of the term and is actually a calendaring operation. The process was discovered by a French engineer, De Plassey, in 1960 who, while hot pressing polyester printed cloth on a while piece of polyester cloth, accidentally found that the design had shifted from the printed cloth on to the white piece of cloth. He patented the process cloth on to the white piece of cloth. He patented the process of transfer printing.
Tranfer printing is a simple process requiring only heat treatment for a very short time. It involves two stages : first, the desired pattern is printed on paper with ink made from disperse dyes which are volatile – this is a vital step in the printing process. In the second stage, The pre-printed release paper is placed on the fabric and heat and pressure are applied to the back of the paper whereby the dye on the paper sublimes and diffuses into the fabric which is in a plastic condition at that temperature. The transfer of the dye is effected at 1800 to 2200 C within 20 to 30 seconds. After the dye is transferred, the paper is removed from the fabric which does not need any further treatment. Usually about 80% of the dyestuff is fixed on the fabric and about 20% remains on the paper.
The process is applicable mainly to polyester fabrics and garments made from it as well as polyester knitted goods. More than 90% of transfer printing is carried out on cent percent polyester material because the disperse dyes ideally suited for it are also suitable for transfer printing.
The most essential material required for transfer printing is paper printed with the desired design. A good quality non-fibrous and non-absorbent paper which would yield maximum dyestuff during heat transfer is used.
For printing paper, dyestuff alone cannot be used: it has to be in the form of ink. The ink is made up of a volatile disperse dye, a solvent (water or alcohol) and a thickener (polyvinyl acetate). Only a little ink is allowed on the paper in order to exclude the possibility of a halo effect.
MACHINES USED FOR TRANSFER PRINTING:
Two types of machine are mainly used for transfer printing viz. Flat-bed Press Transfer Printing Machine and Continuous Calendar or Drum Transfer Printing machine. In Flat-bed Press machine, the process is batch-wise or discontinuous and it is mainly employed for garments like shirts as also for knitwear, hose and ties. The continuous Calendar machine is used mainly for fabrics whereas the Flat-Bad Press Machine is used for garments as well as fabrics.
ADVANTAGES OF TRANSFER PRINTING:
I The operation of transfer printing is very simple and does not require expensive machinery.
Ii There is no after-treatment of the printed material-no drying, washing, steaming, curing or Developing.
Iii As the operation is simple, no skilled labour or a colour chemist is required; the cost of labour is Iow.
Iv The operation is quite clean ensuring freedom from soiling of hands and clothes.
V Faults do not occur in printing because the defects are rectified at the stage of paper printing itself, consequently the rejects are few.
Vi There is no need to store printed goods; the goods can be printed when required in the required Quantity.
Vii It is possible to pint knitted goods and garments as also grey fabric (provided it is free from Stains.)
DISADVANTAGES OF TRANSFER PRINTING :
I The process is mainly applicable to polyester material; the process is not applicable to natural
Fibres like cotton or silk as they are not thermoplastic and do not possess any affinity for Disperse dyes.
Ii The colour range i.e. he number of dyes used for printing is limited.
iii The cost of printed paper is high and its width is limited.
iv The process is not economical for short runs of the fabric and the designs on the pre-printedrelease paper cannot be modified.