Rotary Screen Preparation Process

Rotary Screen Making Techniques

Ayesha Shaikh, Bushra Rehman,
Ummul Rifa Khan & Kanza Zaidi
NED University of Engineering and Technology, Karachi, Pakistan

 

1. What Is Rotary Screen Printing?
The rotary screen is a screen in a cylindrical form. The color/printing paste is applied from inside while the rotary screen is revolving.[1] The pressure of the screen and the central cylinder allows the paste to transfer onto the fabric. Because of the high quality it can achieve, rotary screen printing is the most appealing method for printing designers and fashion apparel fabric. [2]

Rotary screen printing has increased enormously in its use in recent years because of its versatility and the development of rotary screen printing machines which are capable of very high rates of production. An additional significant advantage is that heavy depths of shade can be produced by screen printing, a feature which has always been a limitation of roller printing because of the restriction to the amount of print paste which can be held in the shallow depth of the engraving on the print roller. Worldwide, about 61% of all printed textile fabric is produced by the rotary screen method and 23% by flat screen printing.

diagram of Rotary screen printing machine

Schematic diagram of Rotary screen printing machine
Fig: Schematic diagram of rotary screen printing machine

The most important and fundamental element of rotary screen printing is the cylindrical screen or the engraved roller. There are many companies that are making these rollers for printing.

2. Printing Screens:
Print screens are required in order to create the desired image on the textile substrate. Each color of the image/design requires a separate screen, for example, a three‐color design requires three screens, one per color. Print designs are becoming more complex, and it is not unusual for 20 colors to be used in the design. This can be quite costly to set up since a screen is required for each color and setting up the registration of the design (print registration) can be quite time consuming.

2.1 Rotary Screens
Lacquered rotary screens are produced in much the same way as flat screens, the only difference being that the screens are cylindrical, not flat. The mesh has hexagonal holes and is nickel coated so that it can endure the printing process. Again the design may be laser cut along the cylindrical roller for ease of screen manufacture.

Galvano rotary screens are thin inflatable nickel tubes, coated with a light‐sensitive polymer. A full‐length negative of the design is wrapped around the tube followed by a negative of the mesh pattern. The screen is exposed to light, washed and dried. Only the non‐pattern areas and areas corresponding to the supporting mesh in the pattern have no polymer. The polymer acts as an insulator upon subsequent nickel plating, nickel only building up in areas where there is no polymer. This results in a thin nickel sheet with holes only in areas of the pattern. Below figure shows the different layers in the manufacture of a galvano screen.

Galvano rotary screen construction
Fig: Galvano rotary screen construction

3. Companies That Manufacture Rotary Screens:
Stork who invented screen technology. Till 1960 there was flat bed printing running at 6 m/min. In 1960 Stork brought the Rotary screen concept which increased printing speed to 60 m/min. One of the company is ROTHTEC®. If offers all mesh sizes from 40 to 195 mesh and can be engraved to all lengths up to 3500mm available in circumference sizes 640, 688, 820, 914 and 1018 mm. ROTHTEC® engraves screens for acid, vat, pigment and heat transfer printing. [3]

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Another company is GURJAR. It is also one of the leading manufacturers of a complete range of NICKEL PERFORATED ROTARY SCREENS for conventional as well as digital imaging systems for a range of industries ranging from Rotary Textile Printing, Coating, Wall paper, Transfer printing and other Special effect printing. GURJAR offers complete range of Nickel Perforated Rotary Screens starting from 14, 25, 40 and goes up to 195 mesh in screen lengths 900 mm to 3500 mm and various repeat sizes of 640, 820, 914, 1018 and 1680 (Other screen lengths and repeats on request are also possible). Similarly there are many such companies that are manufacturing rotary screens. [4] In India not only Gurjar but also Stovec, Screen O Tex, Roopa, Encore, Harish manufacture screens. China is the largest manufacturer of screens. The common meshes are 40, 60, 80 ,100, 105, 110, 125, 135, 155, 165, 195. Stomac ( Now Stovec or SPG Prints ) or Stork produce screen printing machines. Others are Zimmer Regianni, Embee, Laxmi, Harris, Sunshine.[10]

4. Print Management Process:
Behind engraving the roller there is a whole management which decides all the process. So the things which need to be done before engraving are:

4.1. Scanning Using Drum Scanner:
The customer’s design art is received either as original design to be scanned or soft copy on disk. At least there should be one repeat.

4.2. Color Separation:
Scanning is processed to separate the colors making sure to reproduce the intricacies of the customer’s work.

4.3. Data Send To Engraving Department:
Within the days the data is deliver as quality fabric inkjet print that perfectly interpret the customer’s design to rotary screen engraving department. Changes and approvals can be made fast and economical when in the form of print rather than engraved screen.

4.4. Engraving:
After approval engraving is done and shipment is made to the printing department or to the customer. [3]

5. Responsibility of Engraving Department:
In order to get the best print there are some responsibility of engraving department that are:

5.1. The Quality of File or Painted Art Submitted:
To make sure that the data which is received is perfectly matched to the customer’s design.

5.2. The Size of The Actual Repeat:
It will decide the repeats on the roller.

5.3. Screen Mesh:
Mesh size is the most important thing in the screen. More the number of mesh size, finer and pretty the print will be.

5.4. Length and Engraved Width:
Repeat size will decide the size of the screen. [5]

6. Screen Making:
Engraving of screen includes following six steps

  • Unpacking
  • Rounding
  • Degreasing
  • Washing
  • Drying
  • Coating

6.1. Unpacking:
The first step in the preparation of screen for engraving is unpacking of screens. Screens are usually received in a bean shaped package from which they are carefully taken out so that no dent is generated in the screen. It’s advised not to keep screens in the boxes for more than a year otherwise they can cause problems in rounding process and also not more than four screens should be kept on one another because weight of the screens can cause serious problems. While taking out screen from the packaging care of its dimensions are taken and we make sure that the screens are more towards rounder shape.

6.2. Rounding:
In order to bring screen in round form, rounding rings are used. On both sides of screen, rounding rings are inserted and then screens are put in polymerization chamber. Polymerization process for rounding of screen is done at 180 to 200°C for 10 to 15 minutes. After polymerization, rounding rings are removed from screen. Screens packed in boxes have a little larger size then the standard one. After polymerization, screens are put over a stand then they are cut into original size through a scissor. After it, screen both ends are taped with the help of a paper tape so that no handling mark appear on screen during coating.

6.3. Degreasing:
To get even coating throughout the screen degreasing of screen is done with emulsion SCR 35. Ratio is kept 1:4 which means 4 times water and 1 time emulsion. The time allowed is 5 to 10 minutes. Soft water is always used while making an emulsion.

6.4. Washing:
For the process of washing showering of screen is done, 4 times from top to bottom than bottom to top with soft water. If soft water is not used than carbonates will stick to screen and that portion of screen will not polymerize.

6.5. Drying:
The process of drying is carried out in dirt free room. It’s advised to conduct drying at low temperature around 20°C. Clamps are always used to hold the screen.

6.6. Coating:
The process of coating is usually done to close the meshes of screen. The screens are placed in cones and plate containing coating material which moves down slowly only ones. The only two chemicals used for screen coating are SCR 102 and SCR 101. SCR 102 is the Lacquer and it is a universal emulsion. SCR 101 is a sensitizer and it is also an emulsion.

Recipe for coating solution is as follows:

SCR 1021000gm
SCR 10150gm
Water100gm

After coating, screens are put in clamitizer, around 20 screens could be placed in one clamitizer. Coated screens are dried in clamitizer through air. 20 minutes are required for drying one coated screen. In case of using max wax jet engraving machine, screen should brought for engraving immediately after clamitizing otherwise screen will get hard. But in case of using laser jet engraving machine, after clamitizing screens are polymerized at 200°C for 1 – 1.5 hours. The polymerized screens should bring for engraving within 1 hour. The top ends of coating chamber are covered with tape otherwise emulsion may flow inside walls of chamber. Two squeegees are always used, one for coating and other for scrapping.

7. Techniques of Rotary Screen Making:
There are three types of engraving methods

  • Photoengraving machine
  • Max wax engraving
  • Laser jet engraving

7.1. Photoengraving Machine:
It’s the oldest and the most conventional way of photoengraving. Screen one end is held in cone of machine. Negative is put over screen and then according to repeat adjustment, this negative is kept over another area and so on. In this way, over 1 screen 1 negative is exposed across the entire screen length in steps. Through ultraviolet light, black area of negative is exposed over screen. After exposing, all of the max wax jet engraving processes will be followed.

photoengraving machine
Figure 2: Photoengraving machine

7.2. Wax Engraving:
In wax engraving, solid wax is melted to make deigns on rollers then screen is exposed to light and after that printing screens are developed.

7.2.1. Procedure:
The process of wax engraving consists of four steps:

  • Engraving
  • Exposing of screen
  • Developing of screen
  • Polymerization

7.2.1.1. Engraving:
After coating and clamitizing, engraving is first step in wax engraving. Here the wax used is in the solid form which starts melting due to temperature and flow to nozzles then the design is formed.

Figure 3: Wax engraved rollers
Figure 3: Wax engraved rollers

7.2.1.2. Exposing of Screen:
For exposing commonly halogen light is used. Time of exposing varies from 2 to 7 minutes. The exposing time is depending on mesh of screen, thickness of coating, light source and image pattern.

7.2.1.3. Developing of Screen:
Immediately after exposure screen is immersed in little hot water for 5 to 10 minutes .when all wax is removed clean the screen with sponge both from inside and outside till image comes out clearly. In end wash the screen with little pressurized water by pipe. Examine the screen under light if any open areas are there close it by means of brush

7.2.1.4. Polymerization- Curing by Heat:
After screen is developed at room temperature it is kept in polymerizer (heater) for curing. The temperature is 70-180°C for at least one and a half hour. The temperature should be constant throughout the chamber. It is desirable to turn the screen after 45 minutes.

7.2.2. Advantages of Wax Engraving:

  • The best quality screens are made in all aspects.
  • Effects like sharpness of design is best achieved.

7.2.3. Disadvantages of Wax Engraving:

  • When the wax is finished error will occur on computer and both the screen and head will stop.
  • Non-uniform temperature in polymerizer causes the screen defects.
  • Wax engraving is the long process especially washing. [6]

7.3. Laser Engraving:
In laser engraving, mask is applied and portions of mask are removed by laser then after the metal plating screen is formed.

7.3.1. Procedure:
The process of laser engraving consists of three steps:

  • Application of mask
  • Removing portions of mask
  • Deposition of metallic material

7.3.1.1. Application of Mask:
Polymeric electrochemical resist mask or black polymeric mask is applied to cylinder. Auto-coating machine is used for coating as this machine applied the mask evenly and smoothly.

Auto-coating machine
Figure 4: Auto-coating machine

7.3.1.2. Removing Portions of Mask:
The portions of mask are removed by the laser to make the design on screen and cylinder is exposed from where mask is removed.

Laser engraving
Figure 5: Laser engraving

7.3.1.3. Deposition of Metallic Material:
Immerse the cylinder having unremoved mask in nickel plating solution so areas where mandrel is exposed are filled with plating solution.

7.3.2. Laser Engraving Machine:
The main parts of laser engraving machines are:

  • Shaft which is rotatable, it rotates the screen.
  • Two end ring assemblies used to fix the opposite ends of rotary screen.
  • Laser engraving device including head moving back and forward along the shaft.
  • Supporter is also movable and it supports that part of screen which is being engraved.
schematic view of laser engraving machine
Figure: Schematic view of laser engraving machine

7.3.3. Advantages of Laser Engraving:

  • Laser engraving is non-contact method i.e. less wear and tear.
  • This process creates permanent design.
  • Today’s equipment is user friendly.

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7.3.4. Disadvantages of Laser Engraving:

  • Its initial cost is high.
  • Sharpness of design is not as best as in wax engraving. [7]

8. Modern Engraving Machines:
Latest rotary engraving machines include:

  • Rotary inkjet technology
  • Laser exposing technology

8.1. Rotary Inkjet Technology:
Rotary inkjet engraver is a digital equipment of computer-to-screen engraving. In this printing technique, design is directly coated on nickel screen with special ink and cures the resin with UV light. Comparing to traditional engraver, it saves the film cost. (1)

Engraved rottary roller
Engraved roller
rotary inkjet engraver
Fig: Rotary inkjet engraver

8.1.1. Advantages of Rotary Inkjet Engraving Technology:
Following are the advantages of rotary inkjet engraving technology:

  • It is a fast engraving process.
  • Rotary inkjet machines provide noiseless operation.
  • Rotary inkjet machines consume less power.

8.2. Laser Exposing Technology:
Laser exposing technology is same as laser engraving technology but the machines used in this are new and improved. It has the ability to produce high quality laser engraved rotary screens by simple and efficient means. Elimination of washing and dev eloping of exposed screens reduces chance of error, dent and crease. Laser exposing is a single step dry process, it simply engrave the coated screen with your desired design and print. It eliminates costly consumables like film or ink, and time-consuming processes like washing.

Rotary Screen Laser Exposing Machine
Figure: Rotary Screen Laser Exposing Machine

9. Comparison Between Different Engraving Methods:

S.NOPhoto EngravingMax Wax EngravingLaser Engraving
1.Engraving done using this technique is 35%Engraving done using this technique is 60%Engraving done using this technique is 5%
2.Time required for engraving is 3 hrs.Time required for engraving is 2-2.5 hrs.Time required for engraving is 45 mins.
3.Used for printing sharp images.Used for geometrical designs.Used for solid figures and floral designs.
4.Local work is done on photo/conventional engraving machines.Mostly export work is done on max wax engraving machines.Mostly local work is done on laser engraving machines.
5.Machines are manually controlled.Machines are automatically controlled.Machines are semi-automated.
6.Less checking or touching required.More checking or touching required.Less checking or touching required.
7.Design out problems occurred.No design out problems occurred.Design out problems occurred.

10. Conclusion:
Rotary screen making technique is a very important process in rotary screen printing. Mostly penta, hexa and nova screens are used. Screens are engraved according to design by different techniques. Before engraving screen is prepared by unpacking, rounding, degreasing, washing, drying and coating. After engraving of screens end rings are inserted on both ends of screen. Among all engraving techniques, photoengraving is the oldest one. Laser engraving and max wax engraving are now used. Max wax is mostly used engraving technique. When ends rings are attached to the screen and screens are fully prepared then these screens are mounted on rotary screen printing machines to start printing operation.

References:

Text references:

  1. Handbook of Value Addition Processes for Fabrics By B. Purushothama
  2. An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
  3. www.oecotextiles.wordpress.com
  4. www3.epa.gov
  5. www.rothtec.com
  6. www.gurjargroup.com
  7. TP notes
  8. www.mahavirtextilechemical.com
  9. patent.google.com

Images references:

  1. https://homesecurity.press
  2. https://www.diytrade.com
  3. highlightedimagewax.jpg
  4. indiamart.com
  5. mrprint.com
  6. electrotech.com

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