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A brief on rotary screen preparation

The term textile printing is a combination of prints on fabric with 1 to 20 colors according to design and color. Printing is the process of painting a design drawn by an artist and fixation to the fabric or materials with a mixture of different chemicals and dyes with the help of applying different types of machinery and technology.

There are various methods for textile printing, first comes developing the design. Which is why design is called the heart of printing. To create a textile printing design, each design has to be converted into a 100% Vector image by using graphic software.

rotary-screen-printing- preparation
Figure 1: Rotary screen printing is a continuous printing process with a perforated cylinder or spherical screen are used which is automatically fed with paste from inside by a special pump. Courtesy: SPG Print

Depending on how many colors are used in a design, all the color needs a separate file for each color, a separate rotary screen is used for developing the design. Then each separated color file has to be transferred to the separate Rotary screen. For that, various machinery and chemicals are used.

Here a list of machinery and chemicals is given below: –

Chemicals Machine
Universal emulsion Exposing machine.

A-     Inkjet

B-     Laser

Sensilax sensitizer Cool dryer machine/Climatizer
Retouching Coating machine
Endring remover Polymerize machine
Endring adhesive (35-55 cc) Light stand and developing stand
Degreaser Water tank/developing tank
Acetone Endring fixer/endring gluing machine
Chromic acid Unpacking
Stripper- NTS
ETC
Carbolic acid
Oxalic acid

The steps of rotary screen preparation are given below: –

Screen rounding and degrease: The new screen is removed from the packet and the tension ring is carefully attached to both sides of the screen, which can be easily removed from the screen or reconnected.

After attaching tension ring the screen has to be kept in the polymer machine at a temperature of 160/160 degrees for 40-60 minutes. The amount of grease applied to the screen hardens the screen due to the grease hit and rounds the diameter of the screen perfectly. And this process is called screen rounding.

After rounding the screen, the screen needs to be degreaser. A degreaser is a type of soapy water that can clean the grease or oily substance on the screen. The screen is degreased with foam by mixing one part of water with four parts degreaser and inserting the screen inside the developing stand.

Alternatively, after soaking the screen in a degreaser tank for 15/20 minutes and washing it with water, it becomes a screen degreaser. After rounding the screen, the screen has to be inserted inside the lighting stand and needs checking by turning on the light.

Whether the mesh hole is closed or if there is any dirt on the screen, if any hole is closed or dust is on the screen, it should be cleaned with an air compressor or water gun machine. Then the screen is polymerized at the temperature of 40-120 C for 20/30 minutes or at normal humidity with water for 4/6 hours to achieve a suitable form for coating.

rotary-screen-printing
Figure 2: Rotary screen printing machine. Courtesy: Zimmer

The steps of rotary screen preparation is given below: –

1) Screen rounding and degrease: The new screen is removed from the packet and the tension ring is carefully attached to both sides of the screen, which can be easily removed from the screen or reconnected. After attaching tension ring the screen has to be kept in the polymer machine at a temperature of 160/160 degrees for 40-60 minutes. The amount of grease applied to the screen hardens the screen due to the grease hit and rounds the diameter of the screen perfectly. And this process is called screen rounding.

After rounding the screen, the screen need to be degreaser. A degreaser is a type of soapy water that is able to clean the grease or oily substance on the screen. The screen is degreased with foam by mixing one part of water with four parts degreaser and inserting the screen inside the developing stand.

Or after soaking the screen in a degreaser tank for 15/20 minutes and washing it with water, it becomes a screen degreaser. After rounding the screen, you have to insert the screen inside the lighting stand and check it well by turning on the light. Whether the mesh hole is closed or if there is any dirt on the screen, if any hole is closed or dust is on the screen, it should be cleaned with air compressor or water gun machine. Then the screen is polymerized at the temperature of 40-120 C for 20/30 minutes or at normal humidity with water for 4/6 hours to achieve a suitable form for coating.

2) Screen coating: The screen is to be coated in a light yellow light room. Excessive light dissipates the heat of the emulsion. To keep the temperature of the emulsion perfect, AC should be installed in the screen preparation room, if the emulsion contains any tiny sand particles or any other dirt or dust however, during screen coating, those sand particles will form a pinhole on the screen or appear in the form of a pinhole.

The screen preparation room should be reserved at all times so that the pinhole in the screen is not created by the sand particles entering the screen preparation room.

For this, the screen preparation room should be maintained at all times so that no sand particles from outside can enter the room.

There are generally three types of chemicals used in screen coating

  1. Universal emulsion 2. Sensilax sensitizer 3. Water

200 grams of chemicals are used to make a screen coating. In 100 g of emulsion, 10% water, 5% sensitizer and the rest are photo emulsions.

Many times the percentage of chemicals has to be changed depending on the thickness & sharpness of the design… In this case 20% water has to be used and At the same time an exposer uses his practical experience to do this.Many new chemical companies are currently marketing readmit emulsions. It is usually used to make emulsions according to design variations.

For best results, mixing emulsion has to be mixed very well by stirring with a stick.

Using high mesh polyester or bolting cloth, filter 2/3 times. It is more convenient to do screen coating after filtering emulsion. Because then there is foam in the emulsion, you have to wait for 4/5 hours for this foam to dissolve. Because coating the screen with foam mixed emulsion does not penetrate the emulsion perfectly in the screen.

Screen coating is done in two ways

  1. Coating machine
  2. Manual process
  • Coating machine: 200 gm of the prepared emulsion is added into a clean scraper ring and coated on the clean screen with a speed at 10-12 min. then dried at 350c – 400c and 60% relative humidity in an oven with circle wind and on more than ± 20c temperature differences. Coat the binder evenly on the both surfaces beaten the enduring and the nicked screen with a scraper coup ting and removing the excessive binder.

Advantage:

  1. Uniform and smooth coating.
  2. Variable speed for coating.
  3. Coating pot run up and down up on solid beam.
  4. Easy to operate.
  • Manual system: If there is any problem in the coating machine or there is no coating machine then the exposers do screen coating on the basis of their skill hand. Manually coating the screen does not make the emulation penetration 100% perfect on the screen, resulting in various problems, but not so much the problem with the local design. Moreover, in the case of export design, this manual system is not acceptable, as well as manually screen coating is not perfect and time is wasted, for this reason, screen coating is no longer done manually. Exposers often solve many screen problems by reducing or increasing the viscosity of the emulsion, for example, making emulsions depending on the exposure or sharpness of the design. After coating the screen, the screen has to be kept upright for 15/20 minutes so that the emulsion applied on the screen comes down by rolling. Then the screen has to be kept in the emulsion machine at the temperature of 30/35 degrees Celsius for 20/25 minutes. This dries the emulsion applied to the screen. If there is no clematises machine, it can be dried in a light-dark room with normal fan air, in this case, it takes more time, about 2/3 hours. If the emulsion screen is not exposed within 4/5 hours, the emulsion temperature is lost. However, in a dark room with AC, the coated screen can be done for up to 18 hours.

3) Screen expose: After the screen coating, the screen has to be exposed. The device by which the screen is exposed is called the rotary screen exposing machine. There are also three types of exposing machines,

  1. Manual exposing machine
  2. Inkjet exposing machine
  3. Leaser exposing machine

When designers did manual design, they used to expose it in manual exposing machine.
There were various problems in manual expose but even after that manual expose was given, the main material for giving manual expose is design film. With this film, part by part exposure was given and the cost would be less.

With the change of technology, CAD exposing process started instead of manual. First Inkjet then Leaser exposing system is started. Any critical design can be perfectly exposed in inkjet and laser exposing machine.

In this case there are some conditions, the coated screen must be inserted inside the rubber tube of the exposing machine. Then with the help of automatic air system you have to tighten the screen with air and tap on both sides of the screen to prev air from escaping.

Comparison between Manual expose, Inkjet expose & Leaser expos:

Manual expose Inkjet expose Leaser expos
1. The first instrument of manual exposing is the film exposing the design by wrapping the film on the screen. 1. Film process is not required. Ink is used instead of a film. It has to be exposed through computer operating.

 

1. Film process is not required. Leaser is used instead of a film. It also has to be exposed through computer operating.
2. Manual system has to be exposed with part by part film.

 

2. Inkjet has to expose the whole screen at once.

 

 

2. The whole screen has to be exposed by computer operating.
3. In a manual system, sometimes a film joining sign or a repeating joining sign may appear on the screen.

 

3. This problem does not occur during exposing the inkjet. 3. This problem does not occur during the leaser exposing.
4. To remove the oily surface, need to apply powder on the coated screen Before film wrapping. 4. Inkjet does not need to be powdered.

 

4. Laser also does not require powder.

 

5. The screen is opened in the emulsion part or black part of the film and the rest is hardened. That means the negative part will be hard and the positive part will be open. 5. In the case of inkjet, the design will be printed on the screen with black ink and if after washing the screen, the black part will open. 5. In the case of leaser, the negative part of the design will be hard and after washing the screen, the design area will be open.
6. The area of ​​the design is not bellied during exposing.

 

6. The area of ​​the design is bellied during exposing. 6. The area of ​​the design remains unchanged.
7. It takes 80-90 minutes to expose the screen. 7. It takes 20-25 minutes to expose the screen. 7. It takes 20-25 minutes to expose the screen.
8. A separate film has to be made for each color. 8. You have to create a separate file for each color. 8. You have to create a separate file for each color.
9. The cost of a film is high. 9. Ink’s costing is low.

 

9. Film and Ink are not required.
10. Lighting is required to harden the coated chemical of expose. 10. After printing with black ink on the screen, lighting is given to hardening the rest of the coated area. 10. The design is exposed by hardening the screen with direct light

 

11. Setting mark is given on each screen in the manual system. 11. setting mark is given through computer operating. 11. setting mark is given through computer operating.
12. Before exposing, it is necessary to attach the screen with the exposing machine and the coated screen is hardened by the automatic air system. 12. Inkjet machine attaches automatic air system coated screen is hard to print through Ink.

 

12. By attaching the screen to the leaser machine, the screen is tightened through the automatic air system and exposed with a laser.

 

4) Screen developing wash: After exposing the screen, the screen has to be developed within 10/15 minutes. If wait for a long time, the emulsion of the screen becomes hard, so there is a problem in screen wash.

The exposing screen has to be carefully immersed in water in the developing tank. Make sure that the screen does not rub against the body of the developing tank. Because if the tank is rubbed hard, the screen emulsion is likely to go up. Under of the develop tank has two semicircular stands, above which, carefully soak screen on it in water for 10/15 minutes. The screen should be removed from the develop tank and the outside and inside of the screen should be cleaned with water under high pressure through a shower. And sometimes, rub lightly with foam. In this way the design area is opened from the screen at water pressure.

Shawkat-Hossain-Unifill-Composite-Dyeing-Mills-Ltd
Figure 4: Md Shawkat Hossain (Sohel), Manager CAD, Unifill Composite Dyeing Mills Ltd

Then insert the screen inside the lighting stand and check each motif of the design. If a motif is not fully open, it must be opened with the help of water gun machine. At this time, it is very carefully noted that, the extra area of ​​the design is not opened. Thus the exposing screen has to be developed by the water very carefully.

After that it is necessary to remove the water from the screen, for this removing the screen from the light stand and keep it upright for 10/15 minutes. Then the emulsion on the screen surface should be kept inside the polymerize at the temperature of 120 degree Celsius for 20 minutes to harden it.

5) End ring gluing: To remove the screen from polymerize, end ring gluing is done. And before doing the end ring gluing should be cleaned well.

After that the end ring adhesive should be applied manually on both sides of the end ring (front side and back side) of the screen. After applying the adhesive, the adhesive should be dried for 10/15 minutes.

 

endring-gluing-machine
Figure 5: End ring gluing machine. Courtesy: Collected

Then with the help of end ring gluing machine, put the end ring inside the screen and hit for 10 minutes. After hitting the end ring will be firmly attached to the screen. If the end ring does not attach perfectly, then the screen will be destroyed by opening the end ring during fabric production. For this, the end ring should be applied very carefully.

After inserting the end ring, the screen has to be checked by inserting it in the check stand. If a pinhole or extra area of ​​the design area is opened on a screen, it should be closed with a brush with photo emulsion.

In this way, the screen has required to check more times, then it will be suitable for production.

The process of Rotary screen preparation may be described as the following: – 

Rotary-screen-preparation-process

Note of thanks: I will give special thanks to my dedicated student who always tries to learn something about textiles, Md. Mubashwir Moshwan, student of Textile Engineering, MBSTU, Santosh, Tangail-1902
for his co-operation to write this article in various ways, I wish him, he will be successful in his career.

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