A foundation in flexography

Who isn’t curious about flexography these days and its growth in many print markets? Is a move into flexo the magic solution for printers who are concerned about their market in today’s economy? It is widely acknowledged that offset litho offers outstanding quality and digital allows for profitable short runs and variable data, but there is an undeniable buzz about flexo in our industry today. Is flexo the solution to changing markets for conventional printers?

Clients have heard how the quality of flexography has improved dramatically over the past decade, and printers are curious about the flexo market. Flexo is a unique and, pardon the pun, tremendously flexible technology offering continuous print, variable repeat length, enormous variety of substrates and a wide range of in-line finishing options. And who can forget flexo’s undeniable access to the lucrative consumer packaging market? But is flexo right for you, and what does your company need to know about it? Together, we’ll explore some of the concepts surrounding flexography.

The flexo market

Flexo print technology finds its roots in converting: the manufacture of paper bags and cardboard boxes over a century ago. Converters sought a way of printing an image directly on their product, and flexography called aniline printing at the time, was born.

Today’s flexographic print market can be differentiated in a number of ways. For printers, one of the easiest ways is to consider press specifications and capabilities. Web presses, quite common in flexo, can be divided into narrow and wide-web. Additionally, there are the massive sheetfed corrugated presses and specialty presses such as flexo print units on envelope and bag converting machines.

Others describe the flexo marketplace according to the type of substrate: corrugated, paper, film and foil. Corrugated board and paper are fibrous substrates, meaning the stock is made from cellulose fibres and easily absorbs ink. Films – meaning plastic-type substrates – and foils are non-fibrous. Non-fibrous substrates create special challenges with ink laydown, drying and adhesion, but flexo is well-suited to these difficult substrates.

Narrow-web flexo presses are those with maximum web widths typically from six to 20 inches. Most commonly, individual printing units or “print decks” are arranged in-line, similar to most offset presses. Narrow-web presses are most commonly associated with the label market and represent a typical entry point into flexography.

Wide-web flexo presses, those ranging from 20 inches and up – some with web widths in excess of 100 inches – are often used to print paper or unsupported film. Think of bags for dog food, kitty litter, lawn care products or road salt. Wide-web high speed presses are commonly constructed with a massive temperature-controlled central impression cylinder surrounded by six to ten print decks. The CI press allows for a large press to have a relatively compact footprint and its CI design effectively supports substrates, minimizing stretch for better register.

A range of substrates and inks

One of the greatest strengths of flexography is its ability to print with good quality on a wide range of substrates, due to the flexible and resilient plate, a wide range of inks and ability to lay down a thick ink film when needed. Even the smaller entry-level narrow-web presses will effectively cope with a considerable range of paper stocks, adhesive label substrates, films and foils.

Paper substrates can be coated or uncoated, and include a wide variety of adhesive label stocks. Pressure-sensitive adhesive label stock consists of a three-layer sandwich: the face stock, which becomes the printed label, the adhesive layer itself and the peelable waxy-surfaced release liner. Face stocks are manufactured in a range of colours, finishes, weights and strengths. Adhesives come in a variety of applications, including permanent, extra-strong or peelable. Release liner is available in different weights and thicknesses to facilitate die cutting and handling requirements.

Films include white, coloured, translucent and clear plastics, vinyls and other synthetic substrates. Some films have considerable stretch – think of plastic grocery bags – while others offer incredible strength and dimensional stability. Many are heat-sealable. Bi-axially oriented polypropylene (BOPP or poly, for short) is a popular transparent substrate that provides strength, hardness and exceptional clarity. Poly is well-suited to consumer product packaging such as snack food bags and labels on food, cosmetics, personal care products and household products. Polyethylene (PE) is another important film, used for the manufacture of dry-cleaning bags and plastic grocery bags. Films can also be manufactured as pressure-sensitive label stock with an adhesive layer and release liner.

Foils are metal substrates, made from rolled paper-thin aluminum or other metals. Common foil applications include butter wrappers and peelable lids for plastic yogurt and cheese spread containers. Some premium labels are printed on metal-surfaced paper or film substrates for effect, such as beer and wine labels, and labels on personal care products. Some plastic films are “metalized” with a very thin inner foil layer between layers of polypropylene to improve barrier properties.

In most cases, flexo printers and print buyers begin by selecting the right substrate for the end-use application, and then choose inks which will meet the requirements of the specific substrate and end use. Thankfully, ink manufacturers are incredibly helpful and work closely with substrate suppliers and printers to understand the unique chemical and surface properties of the substrate, end use of the product and any special graphic print requirements. Ink manufacturers will recommend or formulate an ink to work with any substrate, and it is not unusual for flexo printers to run a wide range of inks across their inventory of substrates.

Flexo inks are liquid and pourable. Inks come in water-based, solvent-based and ultraviolet-curing (UV) formulations. Historically, water-based inks were used on fibrous substrates and solvent-based inks were used on films. However, with today’s ink technology, many printers are successfully using water-based ink in a wide range of paper and film applications, making for faster wash-up and easing venting and ink disposal requirements.

In flexography, inks are often delivered to printers in concentrated liquid form. Prior to going on press, the ink is measured for viscosity and pH to ensure correct flow, laydown and drying. A draw-down proof is taken to determine the colour strength of the ink. If necessary, the viscosity can be reduced by adding water as solvent, or the colour strength can be reduced by adding a colourless extender. Inks are monitored press-side, sometimes using automated systems, to ensure viscosity and pH remain within target ranges despite exposure to air and substrate.

Plates and design rolls

The name “flexography” originates from the characteristics of the printing plate – a flexible rubber or polymer sheet with a relief image of line art, type or halftones. The raised image area is inked directly by the anilox roll in a short, straightforward inking system. Flexo plates are available in a variety of materials and degrees of hardness (measured in “shore” using a durometer, similarly to offset printing blankets). The hardness or softness of the flexo plate affects its ability to hold fine lines and reverses and influences dot gain, also called tonal value increase. However, it is the same property that allows flexo to print on a range of challenging, irregular or fragile substrates including corrugated board and heavily textured stocks.

Flexo plates are made either from film (analog plates) or using a CTP exposure unit (digital plates). Analog plates are used in some plants, but CTP technology is becoming increasingly affordable. Many printers have found that, as in offset litho, CTP provides a simpler and shorter workflow, offers savings from eliminating photographic film and, most importantly, facilitates superior quality through better control of halftone dot values.

Many flexo printers rely on prepress providers for their plates. Prepress houses are quite common in flexo, and offer value-added expertise in colour correction, selection of plate material and quality control. By means of ganging multiple jobs for different printers on the same large polymer plate, the prepress house is able to offer efficiencies and control costs and waste to an extent that all but the largest of flexo printers could not achieve alone.

In flexography, plate cylinders (or “plate rolls”) are removable from the press and interchangeable. Any given press will accommodate a wide range of plate rolls of different diameters. By changing the plate roll for one with a larger or smaller circumference, the press can print a different repeat length. Of course, this necessitates having on-hand a range of rolls for different repeat lengths. The advantage of variable repeat length is that waste is minimized and versatility of the press is maximized. For example, a label press can print a label with a 3-inch length, with three labels around a cylinder with a 9-inch circumference. If the next job calls for labels with a 2-inch length, a change of plate cylinders will allow four labels on a cylinder with an 8-inch circumference. This practice allows for equal spacing of images on the printed web and minimizes waste.

A unique ability of flexo is that it is capable of printing a continuous image of various repeat length by means of replacing the plate with a design roll. A design roll is an engraved roll with a continuously repeating image around its circumference. Applications include wallcoverings, greeting paper or continuously “flood coating” a stock with a base colour.

Flexo plates are often gang-produced by sophisticated laser imaging equipment on large sheets of polymer plate material. While plates contain register marks and control strips, they are not punched prior to exposure as in offset lithography.

Plate mounting in flexography is accomplished by means of attaching the plate to the steel or aluminum plate roll using a specialized double-sided mounting tape, sometimes called stickyback. The plate is mounted to the cylinder with the cylinder removed from the press. A mechanical or camera-equipped mounting station hold the plate roll and allows the operator to squarely and consistently mount each plate in sequence, but a skilled operator remains essential to the mounting operation.

Mounting tapes can be made of cushioned foam or uncushioned plastic. Tapes come in a variety of hardness ratings, which influence print quality. Some tapes will yield better results on heavy solids, others on line art or halftones. The correct mounting tape is as important as the plate for maximum print quality.

Anilox rolls – uniquely flexo

The anilox roll is the heart of the flexo inking system, applying the ink directly to the plate and precisely controlling the ink film thickness. The anilox roll is a hard ceramic-surfaced roller that is precision engraved with microscopic cells to hold ink. The size and depth of cells control how much ink an anilox roll will deliver to the plate. Cell volume is described in a billion cubic microns (BCM) per square inch of anilox roll surface. The higher the BCM of an anilox roll, the more ink it will deliver to the plate.

Another property of the anilox roll is the cell frequency or line ruling. Anilox rolls can range from 200 to 2000 cells per linear inch. The higher the line ruling of the anilox roll, the smaller each cell is. Small, deep test-tube shaped cells will not empty of ink properly, so small cells cannot be engraved as deep as larger diameter cells. Generally, this means that a very high line ruling anilox roll will not carry as much ink or have as high a BCM volume as an anilox roll with a lower line ruling. While the concept of cell frequency of anilox rolls is similar to that of halftone screen ruling, the two should not be confused. Anilox rolls have a higher cell frequency than typical halftone screen counts.

The printer will use guidelines, rules of thumb and experience to select the correct anilox rolls for the job. Factors such as the type of substrate, amount of coverage, image properties (solid, line or screen) and halftone screen count will determine the correct anilox roll to use for a job. Desired ink density and the colour strength of an ink may require changing an anilox roll for one with a higher or lower BCM cell volume.

The typical flexo inking system is very short – one or two rollers. After the cells of the anilox roll are filled with ink, the excess ink is metered, or scraped, from the surface of the roll by means of a doctor blade or occasionally a metering roll. On some presses, the anilox roll is the only roller in the inking system, rotating in the ink pan and delivering ink directly to the plate. On other presses, there may be a pan or metering roller that delivers ink from the ink tray to the anilox roll before the anilox roll unloads ink to the plate.

In-line finishing

From the origin of flexography, with printing units installed in-line on box and bag converting equipment, flexo has been closely related to finishing operations. Many flexo presses are equipped for post-print laminating using a self-wound adhesive laminate. The laminate provides a thin, clear layer over the printed ink film to improve durability, water resistance and gloss. Post-print laminating protects the printed ink film and allows water-based inks to be used on poly substrates, even in areas where the print will be exposed to water, as in the case of shampoo labels.

Even narrow-web label presses commonly offer several stations for rotary die cutting, perforating or sheeting. Commonly, these stations are used for die cutting of the face stock of labels while leaving the release liner intact in order to allow labels to be delivered in rolls. Some flexo presses are equipped with folding, gluing, scratch or self-seal scent application or digital print units to allow for in-line processes, which improve efficiency and reduce work time.

Evaluating quality

Like any other printer, flexographers use tools to ensure quality at all stages in the printing process. Many of these tools, such as loupes, densitometers and spectrophotometers, are common to other printing processes. But flexo also makes use of some unique tools for ensuring quality at all stages of the job.

Because flexo inks are subject to evaporation, resulting in changes in viscosity and pH, it is necessary to monitor, adjust and test the ink before printing and during the press run. Ink viscosity – resistance to flow – is measured using a viscosity measurement cup, or efflux cup. The most common is the Number 2 Zahn cup, a small metal cup attached to a long handle with a precisely-sized small hole drilled in the bottom. By dipping the cup in ink and measuring in seconds the amount of time it takes for the ink to empty through the hole, the printer can evaluate viscosity. The longer it takes for the Zahn cup to empty, the higher the viscosity of the ink. If the ink viscosity is too high, the ink needs to be thinned using water or solvent. Once viscosity is controlled, the printer will use an electronic pH meter to verify that the ink is within the specified target pH range, usually between 8.0 and 9.5, or slightly alkaline, in the case of water-based inks. Proper pH control is necessary to ensure proper laydown and drying of ink. Small anilox roll hand proofers can be used to proof an ink on the final substrate prior to going on press to verify colour and density.

Flexo prepress providers and printers will evaluate plate quality before printing. Plates are checked for highlight, midtone and shadow dot values and line screen using a special flexo plate analyzer, which can analyze the surface of the relief plate. A microscope or plate analyzer can be used to check dot shape or inspect for potential damage to the plate. A conventional Type A durometer is used to measure the hardness of the polymer plate in units called “shore.”

Anilox rolls are critical to print quality. The surface of the anilox roll can be carefully examined using a handheld microscope with 400x magnification. Anilox rolls are inspected to ensure that there is no damage to the surface of the roll or the ultra-thin walls between anilox cells. Cells themselves can be inspected to ensure that they are not plugged with dried ink, which would reduce their ability to carry ink to the plate.

Flexo printers are increasingly following industry specifications. Progressive, quality-conscious flexographers rely on guidelines published by the Flexographic Technical Association (FTA). The latest is FIRST 4.0, which stands for Flexographic Image Reproduction Specifications & Tolerances. The FIRST 4.0 manual offers a wealth of knowledge, best practices and specifications to help flexographers achieve standard predictable results. Many flexo printers are also successful in implementing G7/Near-Neutral Calibration.

The flexo industry is largely represented by the FTA, who have assembled a range of materials, manuals and online training opportunities. Through its online training portal called TEST Virtual Campus, with TEST being short for Technical Education Services Team, learners can explore over 350 different courses, including three levels of operator certification and courses in management and environmental issues. In Canada, the Canadian Flexographic Training Committee (CFTC) supports educational programs in flexography, including hands-on night school operator courses in the Toronto area offered to the public at Gordon Graydon Memorial Secondary School.

Conferences and shows provide great opportunities for printers to connect. Major events include the annual FTA Flexo Forum and Info-Flex trade show, the next which will be held in early May in Indianapolis and the FTA Fall Conference, which will be held in Louisville, Kentucky in early November. Expect as well to find many flexo printers at Label Expo in Chicago this month and at Print World in Toronto in November.

Flexo has a strong connection to packaging and label printing, but is experiencing inroads in some other print areas as well. While offset litho remains the leader in image reproduction quality and press speed and digital offers benefits in short run and variable data, there is room for each printing process to flourish in the areas of the broader print market where each excels. Whether your company is currently involved in flexo, considering a move into flexo, or working in conjunction with flexographers, we can all benefit from a better understanding of this important growing conventional printing process.

Leave a Reply