Flexography- Prepress and Photopolymer plates

 The author has already posted 2 Flexographic printing related articles on this blog. The first one was “Developments in flexo printing and label production technologies” https://www.labelsandpackagingworld.com/2020/10/developments-in-flexo-printing-and.html  and the second one was based on a successful entrepreneur Deepanshu Goel of Creative Graphic Solutions Private Limited who has established Prepress and Photopolymer plate making units around the country. https://www.labelsandpackagingworld.com/2020/11/creative-graphics-service-to.html . Deepanshu Goel has cooperated in putting together this article by providing necessary inputs on flexo prepress and Photopolymer plate making.

Flexographic printing has been around for many years, but the biggest advances came about in the 1990s with the development of photopolymer plates. In the last three decades, a lot of development has taken place bringing ease of use in this printing technology to print on a wide range of substrates and its subsequent adoption by diverse converters of labels and flexible packaging. Providing quick make ready, faster change overs and steadily improving processes to achieve high print quality has led to its fast proliferation. Since the prepress and photopolymer plate is the essence of achieving the right print we will dwell on the nuances and process involved from prepress to plate manufacture. Photo polymer printing plates for flexo are flexible polymer-based sheets on which raised images are created as described in process later in this article and they are mounted or wrapped around print cylinders. Ink is transferred to the raised images or characters to be transferred to the substrates. Aniloxes are the ink delivery system in flexo. The purpose of the anilox roller is to transfer ink from an ink tray after removal of excess with a doctor blade, metered and uniform amount ink to the printing plate, which in turn would transfer the image to the substrate.

 Pre-Press

Pre-press is a process or activity that needs to be undertaken before a plate is produced to achieve the graphic print result as close to the artwork of the intended design. To start with, the Prepress house gets a file that needs to be printed on different substrates from a brand owner, converter or printing company. Since the file so received may have been prepared for a different printing technology like offset or rotogravure, the prepress house has to convert it to be suitable for flexo printing specifically conforming to the particular printer’s equipment considering their limitations and capabilities. A good prepress house knows how the plates are going to be used to print, on which machines and which aniloxes, inks, substrates, etc., they must standardize the plates accordingly. The flexo prepress house will take care of all the variables. When the file is received colour separation is done using specialized software. Deepanshu Goel of Creative says, “we have the largest automation engine in the country”. Aniloxes are an important part of the prepress, if a customer comes before buying a machine then they are advised on the purchase of their anilox library in terms of brand names, number required and specifications. If a customer comes to them after having purchased the machine and aniloxes then the prepress house does the optimization as per the anilox library they already have. The resolution of the printed image is directly dependent on the aniloxes size and the dot size. There is direct correlation between the anilox cell size and the dot size. You cannot have a finer dot with a course anilox, so depending on the anilox library with the printer, the dot size is decided.

 

On receipt of an artwork the prepress house will, first do the finger printing of the printing press to assess how the press will print with different variables like aniloxes, inks, tapes, etc. that are available and intended to use. Fingerprinting is a process whereby the variables available are optimized so as to analyse the ink density they are getting on the different aniloxes, the dot gain, the trappings, the pressure settings they have, what are the number of colour limitations they have. Optimization is identifying the best set of variables to achieve your design requirements. In fact, the whole data of the machine is taken in fingerprinting process and then a profile of the machine is made by the prepress house in their system to study the limitations of the library and decide which LPI or Lines Per Inch of the plate will not go beyond a certain point and that the customer has capacity to print a particular LPI only. After analysing the fingerprinting, colour correction is done as per their capacity to manage colour and as per the substrate to finally move on to producing the photopolymer plate.

Plate making equipment is supplied by various companies that include Kodak, Dupont, Glunz & Jensen, Esko, Flint besides many more. The plate making equipment essentially include the following.

·      Processors

·      Exposure units

·       Dryers

The plates are supplied by Dupont, Flint, Kodak, McDermid, etc. In 1974, DuPont introduced the first elastomeric photopolymer printing plates under the Cyrel brand to the printing packaging industry along with the first solvent workflow equipment in 1974 and by the 1980s began developing automated inline platemaking and processing systems, recognizing the advantages of pairing plate and equipment to improve quality and productivity. Light finishing replaced chemical finishing and DuPont commercialized the first safer alternative plate processing solvents in 1990, which enhanced the sustainability of flexographic printing. Later DuPont introduced the DuPont Cyrel Digital Imager (CDI), the first digital Cyrel plate, and a fully digital prepress workflow at DRUPA in 1995. This enabled the industry to move away from film based prepress workflows and implement a distributed digital workflow, using laser ablatable masks (LAMs) to achieve compute to plate (CTP) output that allows design and production files to be easily shared by brand owners, trade shops, and printers prior to printing.  Prasenjit Das, Sales Manager - South Asia adds, “DuPont Advance Printing across four decades, has introduced a broad range of innovative Cyrel® flexographic platemaking equipment, plates and workflow solutions that enable printers and printer converters to optimize quality, productivity and sustainability. From solvent and thermal workflows to platemaking equipment, peripherals and more, we've raised the standard for flexo technology”.  According to Prasenjit, a full range of digital and analog Cyrel® flexo plates and sleeves are available from Dupont for both thermal and solvent workflows, the introduction of the thermal process Cyrel® FAST platemaking workflow has brought rapid access platemaking to a packaging industry where every minute counts. The elimination of solvents or the need for a drying step has cut down flexo plate processing time to as little as a half hour. Dupont’s Cyrel® FAST thermal workflow is a complete plate production system supported by complementary Cyrel® equipment, a workflow solution for a variety of printing applications, enabling fast processing, cost savings, sustainability and improved press performance

Esko offers specialized software for flexo and CDI machines i.e., Imaging machines or imaging engines and software which empower a digital file to be replicated on a flexo plate in terms of dot structure and the pixel size. According to Karan Talwar Director Sales – South Asia, Esko’s business model comprises of Software, Hardware & Services integrated strongly with each other, driving optimized performance. Esko’s portfolio of software products performs a wide range of specific functions for packaging, labels, and sign and display applications:

• structural design of boxes, flexible packaging or other specific shapes. 
• integration of graphic and structural design. 
• prepress functions: image retouching, color management, plate preparation, content  management and control, printing or finishing specific preparations like screening or registration marks;
• process management functions: automation of prepress functions and the prepress workflow;

Esko’s portfolio encompasses digital flexo solutions from narrow-web label printing to wide format corrugated printing. The range of CDI computer-to-plate imagers offers high quality and productivity for digital flexo platemaking for all markets and applications. Karan further adds, “With complete portfolio of Packaging Management software, Packaging Design software, Packaging Prepress software & Automation software strongly integrated with Esko CDI Spark & Crystal range of Flexo plate imaging products, we can proudly state that over 80% of all digital Flexo plates worldwide are produced with Esko technology. Esko’s developed technologies like HD Flexo, Full HD Flexo, Crystal CDI, Crystal XPS UV LED Exposure are driving high level of quality and consistency in Flexo printing for various applications”. 

Miraclon is the new home of Kodak Flexcel Solutions. Miraclon is fully focused on the development, production and sale of Kodak Flexcel technology. This includes the Flexcel NX Imager and the Flexcel NX Laminator which form the equipment component, along with the consumables. Explaining their process Hersh Lulla Marketing Manager, Asia Pacific Region; The flexo plate-making process when utilizing Kodak Flexcel Solutions in pre-press includes:  

• The 1-bit TIFF file is prepared, at which stage the pre-press operator selects the right Digicap NX pattern (proprietary Miraclon technology) which allows for higher densities and optimum ink laydown.
• The file is then sent to the Flexcel NX System, which outputs the file on the Thermal Imaging Layer (TIL) utilizing Kodak SQUARESPOT imaging technology. The TIL is a uniquely designed film that provides rapid, high-resolution imaging for true 1:1 dot reproduction.
• This Thermal Imaging Layer (TIL) is then laminated onto the Flexcel NXH photopolymer plate using the Flexcel NX Laminator. This lamination process eliminates oxygen from plate exposure and creates higher-performing plates that hold stable and consistent dots. This is a patented technology process.
• The laminated TIL-NXH is then main-exposed with UVA light.
• The exposed, delaminated NXH plate is then put through processing, drying and light finishing (under UVC lights) to complete the plate-making process.

Kodak Flexcel NX Solutions simplifies the overall plate-making process while offering significant productivity and image quality benefits. Consistent plate-making and accurate image reproduction mean fewer plate re-makes in pre-press, while also enabling a much higher level of sustainability on press by reducing start-up wastage and ink consumption through optimized ink laydown to achieve higher densities.

Deepanshu Goel

According to Deepanshu Goel of Creative graphic Solutions explaining the process employed by them, similar plate making equipment is offered by suppliers like Flint, Kodak and Dupont. First with help of special flexo prepress software from Esko one has to do all as mentioned above up to color management. Then with the special imaging engine they rip the file into pixels, then these pixels are imaged on a Dupont, Flint or kodak plate depending on the machine. There are different processes in use now to produce a photopolymer flexo plate. These are as follows:

• Analog aqueous, or water wash: Plates using a negative to expose on the plate exposure and wash using a water system.
• Analog solvent wash: Same as above but using a solvent wash.
• Digital ablation aqueous: LAM (Laser Ablation mask) Plate is exposed with an electronic image sent through a laser ablation unit, which ablates a plate covered with an aluminum mask. Followed by UV exposure and water washing.
• Digital ablation solvent: Same as above that the plate is covered with Carbon and washed with solvent.
• Digital ablation thermal: Digitally ablating the masked plate, exposing to UV and finished in dry thermal processing unit.
• Digital laser engraving: Laser engraves complete image into exposed plate or sleeve.

He further explains that the LAM plate is imaged in the CDI engine and this image is exposed on the plate and the exposed part is polymerized by UVA light action. While the exposed portion gets polymerized, the unexposed portion is washed off in washing equipment with different chemicals, dried then given de-tacking with UVC machines. One plate takes minimum 2-3 hours to make.  Plate has drying time of 90-120 minutes depending on plate thickness and plate size. From receipt of artwork to plate delivery is about 12 hours.

Evolution and changes keep coming to flexo printing like flat top dots, changing polymers for different surface tension enabling better transfer of different inks and different shore hardness. The plate manufacturers keep altering the chemistries of the plate materials to achieve improved printing. Improving imaging  is also an ongoing process, especially with small pixel size by altering  the shape of dots. Changes are taking place with RIP engines to achieve different dot shapes and angles to get a better transfer and get a sharper image. So, depending on the targeted quality of image different dot size and shapes are made.

Flexo has prospered due to ease of use and cannot be really compared to other technologies as all have their own advantages. It makes converting much easier with in line converting. With the advent of UV inks, quality of flexo has come closer to offset with higher LPI, however with lower consumption of ink, higher densities, printing half tones, offset still has a better edge. The constant evolution of flexo has enabled the gap to be narrowed and the print quality is now becoming the preferred one universally.

 Written by Harveer Sahni Chairman, Weldon Celloplast Limited New Delhi India December 2020