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



Table preparation



Fabric plaited on the table



Apply printing paste with the help of screen



Curing at 190Âșc (belt speed 3m/min)



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 
 

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