LESSON 15 EXECUTING A SCREEN PRINT DESIGN

STRUCTURE

15.0 OBJECTIVES

15.1 INTRODUCTION

15.2 THE SCREEN PRINTING SET-UP AND THE PROCESS

15.3 HEAT SETTING

15.4 CLEANING THE SCREEN

15.5 PRECAUTIONS

15.6 ASSIGNMENTS

15.6.1 CLASS ASSIGNMENTS 15.6.2 HOME ASSIGNMENTS

15.7 SUMMING UP

15.8 POSSIBLE ANSWERS TO SELF-CHECK QUESTIONS

15.9 TERMINAL QUESTIONS

15.10 REFERENCES

15.11 SUGGESTED FURTHER READING

15.12 GLOSSARY

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15. EXECUTING A SCREEN PRINT DESIGN

Printing is the final phase after the design is selected, screen is prepared, and the colour paste with required combinations is ready. In this section the execution of the screen printing process will be covered along with precautions, care and after-treatments to be taken while carrying out the process.

15.0 Objectives

After going through this lesson you will be able to:

• Perform a screen printing operation efficiently,

• Know about pre-treatment, post-treatment of the fabric and

• Learn the precautions while doing the work.

15.1 Introduction

In Lessons 13 and 14, the following aspects of screen printing were discussed:

Equipment required, preparing the print paste, mixing of the colours and how to make them bright, dull or pastel, how to prepare the fabric, to do the markings on it for different types of designs, cleaning the screen before transferring of the design prior to printing, and then finally how to transfer the design with the help of different methods.

15.2 Printing

Printing is the final phase after screen making and the drying operations are over. The printing can start, as the set up is ready for the same. The total set up or arrangement consists of:

• The printing base with markings for registration.

• The correct printing media with respect to the type, colour, consistency, sequences etc, or those for blocking, as may be the case.

• The proper squeegee with respect to the size, type, etc.

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• The material to be printed upon properly stacked up at a convenient place to help in quick procurement.

• The correct type and size of drying rack or equipment, placed conveniently so that the stock can immediately be place one after another as printing progresses.

• The appropriate chemicals, thinning and thickening agents for the printing media, block out liquids for stencils

• A place with proper illumination and ventilation (Fig. 15.1).

• The printing base (15.2) should be neat and clean with an even, flat surface. A helper may aid placing, removal and stacking on drying racks of the stock for and during printing.

• Place the screen in its first position for printing and put the paste on the far side of the screen. For a 24 inch x 22 inch frame, 6 table spoons of paste should be enough. Do not put less paste as any excess can be returned back

to the jar.

• Hold the Squeegee firmly in both hands. If you are a beginner, it is better to have someone hold the screen down while you are printing as otherwise the screen may move and smudge the print. Place the Squeegee on the far side of the Paste and, holding it at an angle of 45°, pull the paste gently over the mesh towards you (Fig. 15.3).

• Return the Squeegee to the far side of the screen and repeat the process, this time pressing more firmly to remove all excess Paste and to ensure that the Paste is evenly distributed.

Fig. 15.1 Proper illumination

Fig. 15.3 Holding the squeegee

Fig. 15.2 Printing base

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In screen printing the fabric to be printed is laid on a long table. A screen with length slightly greater than the fabric width is fitted in a wooden frame. The design to be printed is transferred to the screen (for each colour, there is a separate screen) and the space in the screen not covered by the design is covered with an impermeable membrane so that the colouring material can only pass through that part of the design which is to be in one colour.

The colour formulation has to be carefully worked out. The well-known capillary effect between the warp and weft threads results in quick spreading of a coloured solution when a drop of such a solution is placed on an absorbent textile material. To ensure a sharp, fine print with clear edges, the colours must be held in position without spreading. A thick paste containing the colour (dye or pigment) prevents migration of the colour and the paste holds the colour till it is fixed after printing and then the unwanted paste ingredients can be removed. For this, appropriate thickening agents are used which increase the viscosity of the paste (Fig.15.4).

The printing paste is pressed through the screen with the help of a device having a rubber blade. The screen is moved one repeat and the process repeated. The different colour screens follow and thus the whole length of the fabric is printed.

When screen printing is carried out by hand, alternate repeats are normally printed along the full length of the table and then the gaps are filled in. This allows time for the print paste to penetrate the fabric and also undergo partial drying before the frame is placed on each printed area.

The printing process, especially if you are using a large screen, is much easier if someone else helps you. Your assistant stands on the other side of the table and holds the screen steady by its outer edges. Place the squeegee upright at the end of the screen furthest away from you. Leave a little gap between it and the screen wall. All along this gap pour a line of well mixed paste.

Now grasp the Squeegee firmly with one hand on each end of it and slope it slightly in your direction. Pull it firmly across the screen, maintaining even pressure all the time.

Fig. 15.4 Thick Paste

Fig. 15.5 Intermixing of colours

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Repeat the stroke; this time pushing the Squeegee away from you, back to the far end of the screen, at a slight slope in that direction.

The colours can normally be intermixed to produce a variety of shades and colours. Since the media is of considerable cost, care should be taken in handling this to avoid loss due to unused paste drying up (Fig. 15.5).

Self-check Questions

1. State whether the following statements are True or False

i) Thickening agents are used to increase the viscosity of the printing paste.

ii) Clogged screen can be cleaned with the sponge dipped in urea water.

iii) The printing paste will keep the screen moist for about 10 minutes.

iv) Always print the darkest colour of the design first.

v) A screen can be used only for one printing.

15.3 Heat Setting

To make printed fabric colourfast, it is necessary to subject it to a high temperature. This process is called heat setting. The curing time in an oven/autoclave is three minutes at 150°-170°C (300°-340 °F). For smaller pieces, you can do the heat setting at home.

Occasionally you can use tumble dryer, putting in about two meters at a time and leaving it on the hottest setting for half an hour. (If you use this method, first make sure that the paste is completely dry, otherwise it will smudge.) It is also possible to use a domestic pressing machine to heat-set fabric, but care must be taken to ensure that every single square centimeter receives an equal amount of heat. The same applies to any heat setting done with an iron. (The iron must be turned to the setting for cotton, i.e. hot.)

15.4 Cleaning the Screen

A screen should last for many years, so take special care of it. Never let textile-printing paste dry on the mesh, as it is very difficult to remove. If you have to leave the screen for a few minutes during printing, either pull a fresh layer of paste over the surface or leave the screen on top of the last print made.

Fig. 15.6 Cleaning the screen

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The moisture present in the paste on the fabric will keep the screen damp for about 10 minutes. Flooding the screen with Paste will keep it moist for about 30 minutes, depending on the temperature. As soon as the printing has been completed, wash the screen using maximum water pressure in order to remove the last trace of paste from the mesh (Fig.15.6). If the screen does get a little clogged, clean it with sponge dipped in urea water.

15.5 Precautions

• Before starting the printing of the fabric, the printing paste may be used on a waste fabric to ensure that the print is satisfactory and that there are no holes in the screen.

• For good results, the cloth should be tightly stretched over the frame and cleaned thoroughly to remove all dust, grease etc (Fig. 15.7).

• Colour should be in the form of a thick paste; otherwise it will spread over the fabric (Fig.15.8).

• Coat the screen well with Enamel, so that no holes remain on the screen.

• Remove extra colour from the screen after each printing.

• Printing base should not wobble.

15.6 Assignments

15.6.1 Class assignment

i) List and sketch the equipment used in screen printing.

Fig. 15.7 Frame with tightly stretched and

cleaned bolting Fig. 15.8 Colour

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15.6.2 Home assignment

ii) Print a bed sheet with the floral theme.

15.7 Summing Up

Printing is the final phase; the print paste has to be carefully worked out. To increase the viscosity of the paste, appropriate thickening agents are used. The printing paste is pressed through the screen with the help of a squeegee. To print the whole fabric the screen is moved after each printing. A number of screens are used according to the colours used in the design. After the cloth has been printed, some after-treatment is essential to fix the colour onto the fabric.

15.8 Possible Answers to the Self-check Questions

1. State whether the following statements are True / False.

i) True ii) True iii) False iv) False v) True

15.9 Terminal questions

1. Write about the precautions that should be taken during screen printing.

2. How will you clean the screen after printing?

15.10 References

1. Allen, J. 1980. Colour Craft. The Hamlin Publishing Group Ltd., England.

2. Anonymous. 1977. Dyeing &Printing. Excalibur Books.

3. Bhatnagar, P. 2000. Screen Printing Techniques for Unemployed Educated Women. Govt. of India, Ministry of SSI & ARI, New Delhi.

4. Clarke, W. 1974. An Introduction to Textile Printing. 4th Ed. Newness Butterworth, U.K.

5. Pellew C.E. 1998. Dyes and Dyeing. Abhishek Publications, Chandigarh.

15.11 Suggested Further Reading

1. Clarke, W. 1974. An Introduction to Textile Printing. 4th Ed. Newness Butterworth, U.K.

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2. Storey, J. 1979. Manual of Textile Printing, Thames and Hudson Ltd., London.

15.12 Glossary

1. Capillary Tube

2. Clog Block

3. Considerable Much significant, relatively large in number or amount

4. Equipment Tools

5. Flooding Overflow of water

6. Impermeable Preventing liquids to pass through

7. Intermixed Blended

8. Moisture Water vapour

9. Opportunities Chance

10. Penetrate Go through

11. Precaution Safety measure

12. Procurement Something acquired

13. Sequence In succession

14. Smudge Disfigure

15. Stacking Piling up

16. Stock Store

17. Stretch Extend

18. Temperature Degree of Heat

19. Thickening Agent Material used to thicken another mixture

20. Transferring To move to another place