Developments in flexo printing and label production technologies

Flexographic printing in India has undergone a sea change in the last few decades. My own experience dates back to the 1980s when as siliconiser I was selling silicone coated release paper sheets to screen printers manufacturing stickers. I was under pressure to brand my silicone sheets to get higher brand positioning, value and increase in demand. Since the reverse of the silicon sheets was blank, few other suppliers of release paper located in Mumbai started printing the same image of a diamond to differentiate the uncoated side from the coated side so that sticker manufacturers would know which side to use as a protective layer for the pressure sensitive adhesive on the sticker. It was this need to print that lead me to learn about flexographic printing process and print my own registered trademark. It was a very rudimentary process whereby a butanol based ink was picked up by a rubber roller and transferred through an intermediate roller to a rubber plate mounted on another roller and the image was then in a moving web transferred to the paper. The rubber printing plate was natural rubber based and made more like a rubber stamp after vulcanizing the patterned sheet. In fact, Ram Nagar in Paharganj area of Delhi was home to manufacturers of rubber stamps. I located a vendor who would make the sheets for flexo printing for me to print the backside of coated release paper by flexographic printing in a secondary process after siliconising. Nowadays the same back printing of release liners is done in line by roto gravure process while producing the silicone coated release paper or filmic liners.

Developments in flexographic printing process or Flexo as it is now referred to has made it a preferred printing technology in recent time. It is now being considered almost at par with offset printing and the last two decades of the new millennium 2000, have seen the rapid adoption of this technology in PSA or Self Adhesive Label production or narrow web printing may it be labels, shrink sleeves, lamitubes or flexible packaging. Biggest advances in flexographic printing came about in the 1990s with the development of photopolymer plates. These are synthetic plates made with light sensitive polymers that harden and cross link at exposed areas in design on exposure to light. Further developments in platemaking have made it possible by computer aided etching the images and designs on the plates. The factors that aided proliferation and use of flexo in label production include the whole gamut of printing, embellishing and converting. These are as follows and each process is a complete technology and warrants attending individually;

1.       pre-press

2.       plate making

3.       plate mounting

4.       Web movement controls

5.       Press design

·       Inks

·       Ink delivery system/Aniloxes and their cleaning system/doctor blades

·       Print head construction

·       Hot air/UV/LED UV

·       Web path

·       Tension controls

·       Auto Registration

·       Online inspection

·       Turn bar

·       Delam/Relam

·       Lamination/cold foiling/Hot foiling

·       Embossing/ die-cutting/ slitting/rewinding

·       Waste matrix handling

·       Higher production speeds

·       Other specialized processes

·       Automatic butt splicers/reel changers to achieve 24X7 production

·       Hybrid/combination

Plates:

Developments in prepress and plate making has provided the biggest impetus to the improvements in flexo and its implementation in label production. Most of the important development in flexographic printing is attributed to photopolymer printing plates, including improvements to the plate material and the method of plate creation. Digital computer to plate systems has done well for the industry. Companies like Asahi Photoproducts, AV Flexologic, Dupont, MacDermid, Kodak and Esko have pioneered the latest technologies, with advances in fast washout and the latest screening technology. In India, the credit for upgrading and bringing flexo printing to a quality almost similar to that of offset, goes largely to Dupont and Esko. A good plate can ensure good printing if and only if the plate has been mounted on the print cylinders accurately. Flexographic plate mounting is a critical step in achieving optimum print quality. Using correct mounting tape, proper techniques in preparation, mounting and plate removal will allow the optimum performance of the mounting tape through the entire flexographic printing process. From the manual mounting of plates in the 1990s we now have reached a level where automatic plate mounting machines are available for achieving perfection in print quality and registration. Mounting tapes play a crucial role in flexo. High print quality at high press speeds can be directly associated with the quality of the foam used in the plate mounting tapes, a wrong plate mounting tape can impact print quality adversely. Specialized soft plate mounting tapes that act as a shock absorber as they compress and return to the same thickness are now available. The tape adhesive is also an important part, these adhesives have been developed such that they stick firm to avoid edge lifting and also be cleanly removable for the reusability of plates. Plate thickness and tape thickness are matters of importance as they impact the circumference and repeatability, but technical support is now easily available on these parameters from the suppliers.

Inks and ink delivery:

UV Curing image: courtesy Labelsandlabeling

Flexo liquid inks have also undergone a lot of developments in the intervening period. From solvent based to water based hot air drying to UV cured and then LED UV cured inks. The evolving of the process into UV ink has brought in an era of high-speed printing and reproducibility with accuracy.  With water based flexo the color matching of each job (even when it was a repeat) used to be a challenge due to ever changing viscosity of the ink lying in store shelf or in the pan while running on the press or due to changing temperatures it affected the print quality. Availability of UV inks in the late 1990s and full UV flexo presses changed the flexo printing scenario. Subsequently operators could easily operate machines, handle the inks without any difficulty eliminating the need for measuring ink viscosity, ink pH, color variation, setting time to get the exact color with reduced wastages. Anilox rolls play an important role of measured ink delivery system. Anilox rollers are cylinders highly engineered metal and/or ceramic roll that are laser engraved with minute cells of a given angle, line screen, and volume to deliver the required amount of ink to achieve fineness of print. Higher line count aniloxes enable printers to go to higher plate screen line count, fine print and reduced ink consumption with higher speeds. Laser-etched ceramic anilox rolls also play an important part in the improvement of print quality due to their ability to control Ink volume transfer making full-color picture printing possible Further developments in introduction of LED UV is resulting in reduced energy consumption. The subject of print quality, flexo dot size, dot gain, distortion etc. and their impact on colours or tones is a full subject that we shall discuss separately in another writeup.

Printing station:

The printhead has had an interesting journey from the simple set of rollers driven by gears as described in my opening paragraph to a highly technical workstation delivering print quality that matches the best produced. The quality of gears improved over the years, the line shaft was accurately designed, the number of gears reduced to reduce any visible gear marks, the whole cluster was re-engineered to achieve finesse and facilitate quick change overs, automatic registration controls with cameras or sensors and finally introduction of servo drives. The servo drive eliminated the gears, helped in reduced wastages, achieve better speeds and excellent registration control in both machine and cross direction in automation. The advanced print impression pre-settings, a computer control pad, register pre-settings and job storage data improve the flexo printing experience. This drastically reduced dependence on operators. The anilox placement was redesigned for easy lift off for replacement. The simple ink tray, doctor blade which is an important part of flexo printing process is now replaced in seconds after introduction of Chamber doctor blade which is a recent development in the flexographic inking process. It is an enclosed chamber doctor blade system in which two doctor blades are used along with an enclosed chamber to dispense the ink to the anilox roll. it enables printing with smaller quantity of ink without contamination from atmosphere and faster changeover. The whole ink cartridge system and anilox of printing head can be changed in seconds enabling fast changeover of jobs.

Drying System:

Hot air drying required a long web path and by the time an operator could reach the level of acceptable registration he would end up with over a 100 running meters of wastage impacting the cost. Ever since 2007 when short web path was introduced along with installation of UV lamps to dry and cross link backed with chill drums aided by fully automatic registration control systems with cameras at each station was introduced, the amount of wastages has come down substantially. On the other hand, the development of UV LED flexo inks give faster production speeds compared to traditionally employed UV curing. It also offers longer life of lamps, instant on or off capability, reduced maintenance, and faster changeovers.

Finishing:

Aiding better quality e.g. to eliminate pinholes when a solid ground is to be printed web cleaners are installed to have a cleaner web eliminating micronic dust particles or fibers that may have flown of  the blades during slitting and settled on the web. Corona treaters are now a standard equipment that aid the anchorage of the ink and coatings to the substrates. Inline embellishing and finishing have brought innovation into printing, decorating and converting of labels. These include cold foiling, lamination, hot foiling, turn bar for front/back printing all on movable rails to position at any station. An important part of the finishing in self-adhesive labels is die cutting and waste matrix removal. These could be any printer’s nightmare, especially when producing special shapes. A change of die with a change of magnetic cylinder meant anything between half an hour to one hour using crane to lift and replace the heavy magnetic cylinder. In today’s time when time is money when we consider it as an input in our costing, one cannot imagine such loss of time. Machine design have been modified by press manufacturers such that now one can just slide out and slide in the new cylinder in a matter of seconds. Complex die cut labels result in waste matrix that cannot withstand the rigors of highspeed converting. Most machine manufacturers have redesigned their waste matrix removal innovatively to keep performing efficiently at full machine speeds. Other processes that are added as required include inspection, embossing, debossing, slitting and rewinding all inline in a single process.

Machine design and engineering is an extremely important part of machine building. The precision in developing and creating each moving part such that there is no unwanted vibration or shiver when the machine speed is increased or decreased. Some machines may have all the features that are there in a high-end press, but it runs well until say 70-80 meters per minute while internationally acclaimed label presses run at 200 meters per minute. For price considerations some printers are tempted to opt for the former, not realizing that in the long run the more expensive, high speed and stable machine returns a better Return on Investment (ROI). It is thus advisable to dwell on each aspect of the machine, its setup, consistent quality output, speed and maintenance. Capital Investment is not like buying raw material repeatedly and where you can change your supplier whenever desired but capital equipment is a one-time investment which has a definite impact on your company’s future.

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

Printing magazine are permitted to reproduce the above article by giving due credit to author including the blog address http://harveersahni.blogspot.com