Print processes: screen printing

The facility given by the self-adhesive labeling system to apply labels when required at the same time as the filling process, removed the need to ‘direct’ print empty bottles. The opportunity to eliminate the expensive logistics associated with the storage of pre-decorated containers was compelling.

Screen printing is able to deliver a very dense white which can both mask product show-through and also provide a base that can be subsequently overprinted with other printing processes.

This characteristic vastly increased the graphic options available for product decoration i.e. very dense color linked to fine tone work.

The development of the rotary screen process was the biggest factor in bringing screen printing into the label sector and this move was further accelerated with the introduction of steel screen mesh and its use in combination with other UV ink systems.

With the flatbed system the screen material, which can be polyester or nylon, is stretched over a flat frame.

The screen is now ready for printing and can be placed in the press. Ink is poured onto the flat screen and with the substrate laying a short distance beneath the screen (this is called the snap off distance).

A rubber squeegee blade then moves across the stationary screen and the ink is forced through the open mesh area (the image area) onto the substrate creating the printed image (See Figure 6.2).

FLATBED SCREEN PRINTING

Compared to rotary screen, flatbed screen is a much slower process, because the substrate and the flat screen have to be stationary at the printing stroke. The flatbed system is a stop-go operation and therefore much slower than the rotary system. (See Figure 6.3).

The flatbed screen was the first to be used by the label printer and though it has now been largely overtaken by rotary screen; it is still used today.

The three important areas that control the quality of the screen printed image are the size and depth of the mesh material, (See Figure 6.4) the viscosity of the ink and the 'snap-off' distance between the screen and the substrate.

In flatbed screen the ‘snap-off’ distance is most important and if it is not correct will create inferior print quality.

The ‘snap-off’ distance is the gap between the mesh and the substrate and this determines the pressure required on the mesh by the squeegee blade. It is the squeegee blade which forces ink through the screen mesh, bringing the ink, screen and substrate into contact to produce the image.

If the ‘snap-off’ distance is too great the image may become blurred, if the ‘snap-off’ gap is too little then smudging of the image will occur.

Synthetic screen materials are now widely used in the manufacture of flatbed screen printing, with polyester being one of the most popular. These synthetic materials offer the flatbed printer a number of different mesh size options.

IMAGING THE FLATBED SCREEN

An overall photosensitive polymer emulsion coating is applied to the screen material and then dried before a positive imaged film is placed in contact with the flat screen.

The screen is then exposed to a UV light source which hardens the emulsion in the ‘non-image’ areas thus making it insoluble in water.

The emulsion in the ‘image area ‘ remains soft and the screen is then pressure washed to remove the emulsion from the ‘image’ areas before the screen is then dried.  (See Figure 6.5).

THE SQUEEGEE

The function of the squeegee blade in screen printing is to force the ink film through the parts of the screen mesh that forms the printed image. In flatbed screen the squeegee moves across the screen spreading the ink evenly across the back of the screen and creating the image.

On the return stroke the squeegee then lifts slightly and brings the ink back across the screen ready for the next print sequence. In rotary screen the squeegee is in a fixed position and remains in contact with the screen surface as the cylindrical screen rotates.

Print quality will be affected by the type of blade used and the printer will need to select the most suitable blade type. The edge of the blade which is in contact with the screen can be varied in hardness and profile shape and will vary dependent on the type of job being printed. Any damage to the squeegee will affect the print quality.

ROTARY SCREEN PRINT

The introduction of steel mesh for screen printing led to an important change within the label industry. It allowed the steel screen material to be formed into a cylindrical shape (See Figure 6.6) which meant that screens could be fitted into full rotary presses which are able to run at much higher speeds that the flatbed screen presses.

Steel mesh screens can be produced with a wide range of screen value options allowing the printer to choose the most suitable mesh for each job.

This choice provides more control over the volume of ink being printed and allows for very high coating weights of ink to be printed, way in excess of the other printing processes.

The rotary screen process adopts exactly the same Squeegee principles as the flatbed system, but with some key differences.

The imaged screen is formed into a cylindrical shape whilst the squeegee blade is placed into the screen cylinder in a fixed position.

The screen cylinder which then holds the liquid ink, rotates at the same speed as the web being printed and the ink is then forced through the imaged area of the rotary screen and onto the substrate (See Figure 6.6 and 6.7).

Unlike the flatbed system, which has no impression cylinder, the rotary screen prints against an impression cylinder, with the substrate running between the screen cylinder and the impression cylinder. This means that the two cylinders have a small point of contact which gives an excellent printed result. Ink levels are maintained within the rotary screen either manually or by the use of an ink pump. 
                                     
ROTARY SCREEN IMAGING WITH FILM

The imaging of a rotary screen cylinder when using a positive film is very similar to the imaging of a flatbed screen. The rotary screens can be supplied to the printer already made up into the cylindrical shape (Stork system) or can be supplied as flat sheets that are then formed into the rotary cylinder by the printer. (Gallus ‘Screeny’ system).

With all rotary screen systems an end ring has to be fitted into each end of the screen cylinder.

This gives the rotary screen the necessary stability and ensures that the screen rotates evenly during the printing operation.

The procedure for imaging rotary screens used in the label industry is as follows :-  

1. Metal screen formed into cylinder and end rings fitted.

2. An overall photosensitive polymer emulsion coating is applied to the screen material and then dried.

3. The positive imaged film is accurately positioned in direct contact with the screen and then secured to allow the screen to spin in the exposure unit

4. The screen plus the secured film is placed into the exposure unit and exposed to a timed UV light source whilst the screen is rotating.

5. The emulsion in the ‘non-image’ area is hardened and becomes water resistant

6. The rotary screen is removed from the exposure unit, the film is removed and the screen placed in the washout unit, in which the screen is pressure washed to remove the emulsion in the image areas.

7. The screen is removed from the washout unit and dried before making ready for the press.

ROTARY AND FLATBED SCREEN CTP IMAGING

CtP (Computer to Screen) imaging of both flatbed and rotary screens is now widely used. This method of imaging removes the need for film originals and eliminates the exposure process, power washing and drying of the screens.

The digital file which contains the image to be printed is transferred to the imaging unit. A high powered laser then ‘burns’ the emulsion away creating the image directly onto the screen. Laser engraving is a digital method of imaging both flatbed and rotary ‘nickel’ screens.

It involves the removal of the emulsion coating in the image areas (i.e. the open areas of the screen). After this the screen requires no further processing and is ready for fitting into the press. In the case of rotary screens, the screen is imaged using a rotary CtP unit, whereas in flatbed imaging the screen remains flat.

CtP imaging reduces the costs associated with the multiple process operation needed when imaging by the traditional method of film contacting. A direct engraved screen produces excellent quality and consistency, with screen resolution of 2540 dpi being produced to allow fine line work with high contrast to be delivered.

Screens can typically be imaged in 15 - 20 minutes and because the lengthy drying process is eliminated productivity can be improved, giving a much faster ‘turn round’ compared to the conventional screen imaging method. Screen material is expensive but the ability to re-use and re-image screens, especially the rotary screens, has allowed some printers to include a facility which involves stripping off the unwanted image and recoating the screen.

SCREEN MESH MATERIALS

Screen printing materials fall into two categories fabric and metal.

Nylon and polyester are the standard mesh materials used today for flatbed screen printing, but screens can also be made from stainless steel wire mesh.

In both instances a single thread of material is woven together to form an accurate gauze structure.

Nylon is generally used when screen stability and tight print register are not required and because it is durable it is generally used for high volume single color work and for printing onto an uneven surface.

Polyester mesh has the same qualities as nylon, but it is more stable and is the material of choice when printing multiple-colors where fine registration is necessary.

Stainless steel wire mesh is more costly than nylon and polyester, but there are some advantages to be gained from using steel mesh screens.

Screens made of steel mesh offer greater screen stability and are even more durable than nylon and polyester. If there is a requirement for heavy ink deposits, steel screens would be the preferred mesh, but if required it will also produce fine print detail. Steel screens have a longer life particularly when printing with very abrasive inks.

MESH GRADE AND MESH/SCREEN COUNT

Screen mesh materials are identified by their mesh count and grade.

The term ‘mesh count’ is the number of threads woven into the mesh per centimeter. Screen materials are available in various mesh counts from 12–200 threads/cm.

The greater the number of mesh holes the better control the printer has of the ink film, particularly when the graphics require fine text and line work.

The lower the number of mesh holes the heavier the ink deposit, but this also means that the printer will struggle to print fine detail.

Mesh grading is the term used to indicate the thickness of the thread used. The mesh grade will affect the overall stability of the screen and also the depth of the screen mesh which controls the thickness of the ink being deposited.

The selection by the printer of the most suitable material and mesh count is most important. It will be influenced by the graphic requirement and will take into consideration the print detail, the stability of the screen and the effect on print registration and the thickness of the ink film required.

 Figure 6.8 shows three grades of mesh assembled with the same thread count. Mesh thread thickness controls the size of the screen mesh open area. As the mesh count increases the print area of each cell is reduced giving greater control of the ink film.

ROTARY SCREEN MATERIAL

There are two main types of rotary screen materials used in the label industry. One is manufactured by SPG Stork and is called the ‘RotoMesh’ system and the other from Gallus is called the ‘Screeny’ system. There is a considerable difference in the structure of these two rotary screen systems.

The ‘RotoMesh’ material is a nickel-based flexible sheet onto which a honeycomb shaped cell structure is engraved using Mesh Count - an electroforming procedure.

This method of producing the mesh cell gives the printer an accurate and wide range of screen mesh options and print possibilities (See Figure 6.9) for producing both fine tone work and heavy coatings of ink. ‘Rotomesh’ screens are very stable and can be de-imaged and re-imaged. 

The Gallus ‘Screeny’ material is a microstructure formed from a stabilised fabric which has a photosensitive coating. The material has excellent structural properties which allow it to be formed into a cylindrical shape onto which end rings are fitted and glued. The forming of the rotary shape is carried out ‘after’ the screen has been imaged.

This involves the welding or gluing of the two leading edges of the flat screen to form the cylindrical shape. Imaging is done with the screen laid flat onto an exposure unit fitted with a vacuum bed which can be contact imaged using film or direct imaged using CtP. Imaging time is typically 30 minutes.

The ‘Screeny’ system gives excellent print quality and is suitable for printing very fine line work and excellent solids.

COATING WEIGHT COMPARISONS

Figure 6.10 shows the ‘coating weights’ i.e. the thickness of the ink layer which can be achieved using each of the major printing processes.

It is clear that the screen process is able to deliver significantly more ink film than any other process.

These figures are an approximation and it should be noted that developments in anilox roller and ink technology are likely to impact on these coating levels going forward.  

SCREEN INKS

The majority of screen printing flatbed and rotary presses operating in the label industry use a UV ink system. Some dedicated label printing screen presses however do use solvented systems.

Screen inks used are of a relatively high viscosity.

The ink must be viscous enough to avoid passing through the screen cells until the exact moment of the printing cycle when the ink is forced through the screen cell by the pressure of the squeegee blade.

Inks with thixotropic properties (this is an ink that thickens up and affects the flow properties of the ink) can offer some advantage to the screen printer as it becomes fluid when agitation takes place or pressure is applied i.e. via the squeegee blade.

Drying or curing when printing UV screen is done using the same web UV curing system as the litho, flexo and letterpress printing systems. (See Figure 6.11).

Screen printing with solvented inks requires a closed system of infra red and or hot air drying and may also involve solvent reclamation. (See Figure 6.12). 

IDENTIFYING THE SCREEN PROCESS

The improvements which have been made in screen printing have made it more difficult to easily identify a screen printed image. Early screen printing could be identified by the ‘saw edge’ which appeared on the edges of the print.

Developments in screen materials now mean that finer woven meshes are available which allow much finer cells. Avoiding mounting the mesh at 90 degrees to the direction of the print has also helped improve the printing quality. Modern screen print can still be identified by the slightly uneven printed edge, but the viewer will now need a magnifying glass to correctly identify the screen process.

ADVANTAGES AND DISADVANTAGES OF THE SCREEN PROCESS

Advantages

• The main strengths of the screen process are:

• Prints very intense colors with excellent covering power

• Prints onto almost any substrate filmic, paper, metallic

• Prints onto curved, uneven and fragile surfaces

• Prints very thick, opaque inks suitable for tactile labels

• Prints unusual ink and coatings, micro encapsulation, metallic inks

• Prints brilliant colors, fluorescents, large particle size inks

• Controlled coating weights

• Solvented or UV drying

• Re-usable screens

• Fast makereadies

• Suitable for combination printing

Disadvantages

• The main process limitations are:

• Press running speeds tend to be lower, more so with flatbed

• Can be difficult to reproduce fine detail and small type

• Halftone reproduction requires coarser dpi rulings

• High costs of rotary screens

• Screen imaging can be a slow process

SCREEN PRINTING – SUMMARY

Screen printing in the narrow web segment has traditionally been viewed as a high-end add-on for those label converters pursuing the high added-value markets and who were willing to make significant investment in the technology.

Rotary screen printing is a very effective added-value printing process, being capable of printing thick ink layers with high accuracy at relatively fast speed.

Personal care packaging in particular has long used this process to achieve high opacity graphics, particularly on clear filmic materials (commonly referred to as ‘no-label' look).

The key feature of rotary screen printing is the total flexibilty of the integrated units which allows for screen print to be added at any point during the print process and is perfect for integration into a combination printing press.

NEW APPLICATIONS

The screen process is increasingly being used for tactile and raised images, glossy and high lustre varnishes, metallic features and many other security features. Screens are also being used to apply pattern adhesives to a variety of converted products.

Two of the more interesting areas where screen has emerged are in the creation of braille information and in the printing of flexible electronics.

Braille images are typically printed with a UV varnish, at printing speeds of higher than 40 meters per minute (130 fpm), with thicknesses that can reach 300µ.

Rotary screen printing too has become the reliable and accepted printing solution for the application of electronic inks, facilitating high volume, low cost production. The printing of electronics such as RFID antennae on common substrates such as paper, film and textile using standard printing processes is rapidly growing.

TECHNOLOGY DEVELOPMENTS

Significant technology developments are making the screen process more accessible within the industry, and are reducing the cost of ownership.

The development of dedicated screen units for leading press manufacturers, for example, continues to facilitate the smooth integration into existing workflows.

Evolution of the direct laser engraving process – a faster, fully digital, much simpler alternative to the conventional exposure method has also encouraged the greater use of screen.

Direct laser engraving saves time because there is no need for the exposure, washing and drying processes, necessary for imaging the screens in the conventional way. Direct laser engraving systems achieve excellent quality, with maximum resolution of 2540 dpi, with  very high contrast and precision in reproduction of positive and negative structures.

The availability of the pure non-woven nickel screen format to a wider market, and the introduction of a galvanic screen produced via an electro-forming process that requires no wire mesh base are interesting developments.

It is clear that the role of screen printing in the label sector will continue to develop and it has already carved out significant niches.

Polyester screen is stretched tightly over frame of wood or metal