Rotary screen printing
As a complementary process, rotary screen printing can enable converters to offer a look and feel of true distinction for labels and cartons, taking products to new levels of quality. For this reason, Rotary screen printing can help achieve this aim and be more competitive in high-value label and packaging markets.
by Hank Guitjens
To supply distinctive branding and communicate quality means considering a combination of processes, in order to create a multitude of effects, and creatively give the label or package a unique look and feel. It is important for label and packaging converters to think beyond the basic process.
Capable of applying opaque ink or heavy varnish deposits between 6 and 250 microns in one pass, rotary screen printing delivers opaque, coloured and tactile impressions and thus puts several high value features at the printer’s disposal that enhance aesthetic appeal or offer functional benefits.
A complete workflow is available so that printers can adopt the process and meet demanding quality expectations, whilst maintaining productivity and cost efficiencies. In addition to a range of rotary screens, this includes units that integrate smoothly with flexo, offset, gravure, letterpress and digital presses, and laser imaging systems.
Rotary screen printing is a stencil process. A cylindrical screen containing the image rotates in a fixed position. As the web moves between the screen and the impression roller, the screen rotates at a rate that directly matches the speed of the substrate through the press. The squeegee is fixed with its edge making contact with the inside surface of the screen precisely where the screen, substrate and impression roller come together. Ink, fed into the cylinder, collects in a wedge-shaped well formed by the leading side of the squeegee and the screen interior’s surface. The screen’s motion forces this bead of ink into the stencil openings. The squeegee shears the ink where the stencil and the substrate make contact with each other, allowing a clean transfer of ink.
Thus, it can be seen as a smooth, continuous process, suited for reel-to-reel printing configurations. Due to its productivity, versatility and ease of integration alongside flexo and other analogue processes rotary screen printing can complement the label converting workflow.
Specific benefits of rotary screen printing to the flexo printer are:
- Application-dependent speeds of up to 150 m/min (492 fpm) are possible
- The process is compatible with a range of ink sets and substrates such as plastic and paper commonly used for narrow-web applications
- It provides a fast return on investment: modules and cassettes can be added without major press refurbishment or disruption
- A range of tactile features from coarse and solid graphics to fine lines and vignettes are possible
- The process offers low total cost of ownership thanks to high-performance, reusable and re-imageable screens, and digital screen imaging systems
A flexo printer can enhance the aesthetic appeal or functionality of the printed label or package by adding UV-screen effects including opaque coatings, tactile varnishes, braille and metallic finishes.
In order to block out light and eliminate transparency, an ink covering with an opaque coating is usually the solution and can be achieved with a single screen printing pass. An advantage of using rotary screen for opaque applications is strengthened when the design requires a combination of fine and solid graphics. These can be printed with the same screen, whereas in flexo, different anilox roll cell-counts would be needed.
One of the most common opaque applications is the no-label look, which features a striking design on a clear film that in turn adheres to a clear container. This requires an opaque white in the first printing position, to prevent any transparency of the overprinted visible colours. These are normally printed afterwards by the main analogue process. The no-label look is commonly used to promote quality and purity in such diverse markets as cosmetics, beverages, household and industrial chemicals.
A package succeeds because it stimulates the consumer’s sense of touch and has visual appeal. Screen-printed varnish effects, typically from a subtle 26 microns, are proven to achieve both. They are added to a label design to create raised images, smooth glossy (“brilliant”) solid areas, or fine line patterns.
Raised tactile images over solid areas, created by printing a clear varnish over an existing image. This adds gloss and thus vividness to an icon or brand name, and thus enhances retail shelf-impact. Using rotary screens to apply varnish offers a waste-free alternative to the embossing technique.
A combination of varnishes
For instance, a combination of varnishes gloss over a matte surface, offers a subtler way of arousing curiosity. One sees a shape under the light, or notices a smoother feel against a more resistant matte solid area. Furthermore, 26 microns fine line varnishes can be created with a high-performance screen that has strength at high mesh-counts.
The “texture varnish” (typical print thicknesses are around 70 microns) features large particles in the ink to create a coarse feel and give a so-called ice-look to the pack. The larger the particle and the thicker the coverage, the coarser the substrate feels. It is a striking way of printing reversed-out images of icons or logos.
The reticulating or “wrinkle” varnish is a coarse feature that stands out from smooth surroundings. Larger ink particles are used for this effect to create the coarseness. The reticulating effect occurs when it is cured by ultra-high frequency UV-A light.
A screen-printed varnish coating may be applied to protect the graphics for wet, durable or chemical applications, where resistance to corrosion, squeezing, microbes or water is needed. Screen-printed braille dots and – for hazardous goods such as chemicals – warning triangles, requiring laydowns of approximately 250 microns offer durability and can be printed in a single pass. A clear varnish is used to apply the dots over the image areas without compromising the impact or integrity of the branding. Braille is mandatory for pharmaceutical labelling in Europe. In some countries such as Spain, Braille is also required on a range of other retail goods. Information in Braille can include product name, product strength, ingredients and expiry date.
The latest rotary screen development includes a mesh that combines a screen size large enough to allow particles of up to 70 microns to pass through, but form a relatively low thickness, of around 20 microns, on the substrate. This new rotary screen, with a screen count of 40 cells per inch, enables the printing of an increasing range of metallic and reflective glitter effects, and optically variable security inks, at speeds exceeding 60 m/min (196 fpm). This application offers potential for differentiating gift wrapping and other products.
Fine meshes for oriental and small text
There are also new screens with fine meshes that facilitate the clear reproduction of Chinese and other oriental characters including three point text. This means that more information can be provided on a label, without compromising legibility including white or no-label look labels. With a high mesh-count of 164,000 cells per square inch and a relatively high open area, the new fine mesh screens achieve sharp edges while avoiding the risk of clogging. At the same time they offer the same strength and resilience as screens for standard or coarser effects.
Gold and silver inks are often used for communicating heritage and luxury. Rotary screens offer the ability to apply a range of metallic effects, from fine linework to solid areas e.g. for a crest or a medallion. Therefore, it can be seen as an alternative to hot foil and bypasses the need for costlier metallized labelstock.
Depending on the type of prepress system, the printing unit and the types of screens used many different options are available. The wealth of choice enables the printer to configure the workflow according to budget, level of use, the type of printed effects required and the end-user markets supplied. There are re-imageable screens, engraving systems and modules available that enable the process to be added on printing widths beyond 900 mm (35.43”), making it suitable for folding carton and flexible packaging as well as labels.
Recently, direct laser engraving has greatly simplified the way screens are imaged. A rotary screen engraver features a sealed CO2 laser that ablates the emulsion from the image areas on the screen without the need for film, chemicals or drying. The precision of the laser, together with the software optimizes the control when creating fine positive and negative images, and resolutions up to 5080 dpi. Print-width dependent repeat sizes range from 10” to 36” (254 mm – 914 mm). Adding rotary screen technology does not require investment in a new press. The existing press can be retrofitted with modules or lighter, easily changeable cassette units.
A printing module can be installed on a sliding rail system above the press. The rail system allows easy repositioning of the module, manually, to any position in the printing sequence: primary for opaque white; secondary for varnishes, for example. Module widths can range from 10” (254 mm) to 40” (1000 mm) and offer a range of repeat sizes. Lighter 40 kg modules with restricted repeat ranges may offer better investment return for narrow web printing situations.
Nickel screens for high performance
The quality of the screens has a significant impact on total ownership costs. Screen cylinders featuring nickel, electroformed non-woven meshes offer the highest levels of strength and stability and optimum ink flow. Thus they can run smoothly without risk of breaking or clogging at speeds of up to 450 fpm (150 m/min) – depending on the application. They also have longer printing lifespans, can be reused several times, and can be re-imaged for new jobs.
Alternatively, a non-woven nickel screen can also be supplied onto a cylinder. This mesh is supplied as a pre-coated, light-sensitive plate. These rotary screens can be used several times for printing a single image, making them a suitable solution for repeat jobs. The higher the mesh-count, the more detailed the feature: a range of screens is available catering for everything from coarse effects to fine linework and Chinese or Arabic text.
Finally, innovations from ink suppliers are strengthening the competitiveness of the rotary screen process. New LED UV-curable inks allow faster, safer and energy-efficient curing, while low-migration inks assure safety for food and pharmaceutical packaging applications.
In total, rotary screen printing as a complementary process enhances the chances of brand success with numerous appealing effects. With guidance and ongoing support from the technology provider, the rotary screen printer is best-placed to offer bespoke and more exciting branding solutions with a workflow configured to make a fast return on investment.