Photo purchased from … www.depositphotos.com
Every so often I like to take the time to study in depth a commercial printing technology of which I have only a cursory knowledge: i.e., something new. Flexography fits nicely into this category.
For most of the last two decades I had known the flexographic process was good for food packaging. And during the decade in which my fiancee and I installed standees in movie theaters, I had learned that all of the flat black standee components (those not printed on gloss litho paper laminated to the cardboard) were flexo printed.
Back in the 1990’s when I was an art director, I had seen flexo-printed labels, and I could point out the slightly mottled or uneven solids on the matte litho label paper. At the time, these were less crisp than offset printed products. I also had known that flexography used rubber relief plates (i.e., unlike the planographic, or flat, printing plates used for offset lithography).
But when I was doing some reading recently, I learned the process was essentially modern-day letterpress, so I became intrigued. It touched my love of fine art, fine craftsmanship, and history as it relates to commercial printing.
This is what I learned.
The Technology
Flexography is similar to letterpress because it uses relief plates. The image areas are raised above the plate. When the plates are inked, only the raised areas print. Unlike most letterpress, however, the plates are rubber instead of metal, and the press is rotary (the rubber plates are wrapped around sleeves that are attached to the press rollers) rather than flatbed (straight up and down like most if not all letterpress equipment). In contrast, offset printing depends on the fact that oil-based ink and water stay separate from one another, so the image area and non-image area of an offset plate can be on the same flat surface. (Therefore, the process is described as “planographic”—or flat–rather than as “relief” like letterpress, or as “intaglio”–or recessed like engraving.)
Furthermore, flexography can print on almost any substrate, such as the plastic sheeting used to make bags containing bread at the grocery store. If you tried to print these on even a web offset press that would hold the plastic sheet under tension, you’d still have a mess. But you can print such plastic sheeting, the cardboard used for milk cartons, and other food packaging, or any number of other things including metallic foils, with flexography.
Back in the 1990’s, when I was an art director, flexography was not great (i.e., accurate or precise) for 4-color work and halftones, and even the labels my staff and I printed were a little blotchy. But this is no longer the case. Major improvements have been made to the inks, the press equipment, and the photopolymer plates since the ‘90s (Wikipedia). This now allows for control over halftones and gradients and incorporates ways to mitigate the substantially higher than usual (compared to offset) dot gain of flexography.
The Process
Ink rollers (also known as fountain rollers) distribute the water-based ink (unlike the oil-based ink of offset lithography) across anilox rollers, which are coated with fine, laser-etched wells (similar to gravure) that hold only a fixed amount of ink. Excess ink is removed with a doctor blade (again, like gravure). The anilox rollers apply a controlled amount of ink to the raised image areas of the photopolymer or rubber plates affixed to the sleeves on the plate rollers, and the plate rollers apply the ink to the substrate. (That is, this differs from offset printing, which prints on the press blanket first and then transfers the image from the blanket to the substrate.) These image or plate rollers are backed up by impression rollers that keep the substrate (which travels between the image rollers and impression rollers) flat and tight against the plates.
To back up just slightly, the rubber plates used in this process are imaged using either negatives (UV light hardens the image areas, and the remaining coating can be washed away) or direct-laser engraving technology (similar to the process used to burn offset custom printing plates). There is a third option that uses a negative of the image area and then actually makes two separate molds (yielding one raised printing plate). These printing plates are then taped to the sleeves wrapped around the plate rollers.
If conventional inks are used for the process, hot air (i.e, from dryers) is blown across the surface of the printed substrate to dry the ink. If UV inks are used, then UV light is used to instantly cure and solidify the ink on the surface of the custom printing stock (i.e., the ink doesn’t seep into the substrate).
The substrate usually travels from one web roll (i.e., so it can be under tension) through the press (through presumably four or more inking units), through the dryer, and on to a rewinding web roll so the printed product can always be under tension and therefore flat.
Finally, post-press operations can occur (cutting printed milk carton flats and then assembling and gluing them into completed boxes, for instance).
A wide variety of inks can be used. These include solvent inks, water-based inks, UV-curing inks, EB or electron-beam curing inks, and two-part chemically reactive inks that cure as the chemicals interact (Wikipedia).
What this means is that you can use a wide variety of substrates, including non-porous materials like plastic sheeting and metallic foils, both of which would not work on an offset lithographic press. Granted, even with the advances since the 1990’s, the are various levels of quality. The lower end is good for flat colors (like the black backgrounds for cardboard movie standees) and lettering (also appropriate for some corrugated board printing). The next level up would include more detailed corrugated board printing. And the highest level of quality would be appropriate for custom label printing (up to four-color process work matching the quality of offset commercial printing).
According to Wikipedia, here’s a list of potential substrates: “plastic, foil, acetate film, brown paper, and other materials used in packaging” (Wikipedia). This is good for corrugated board, shopping bags, “food and hygiene bags and sacks, milk and beverage cartons, flexible plastics, self-adhesive labels, disposable cups and containers, envelopes, and wallpaper” (Wikipedia). Interestingly enough, the process has improved so much that some newspapers prefer flexo to offset. Flexo inks are thinner than offset inks (i.e., “of lower viscosity”) (Wikipedia), so they dry faster, and this speeds up the overall manufacturing process and saves money.
Plus, the overall process yields web rolls of printed stock, which can then be unwound, slit, and processed, to create the bags, cartons, and other packaging products. (Therefore, the post-press work can be done very efficiently.)
The Takeaway
So this is my challenge for you, one that I undertake regularly as well. When you are in the grocery store, be mindful of the packaging. This is a huge and lucrative arena of commercial printing work. Notice the kind of design work (the creative) applied to the boxes of frozen food, the bags of bread, even the corrugated cartons from which the stocking clerks are removing products to put up on the shelves.
Consider the permeability of some of the packaging materials (like the bread bags). Notice that the artwork is detailed but not as detailed as offset commercial printing work. Look for the registration marks used to keep the colors aligned. (Registration can be challenging in flexography.) And keep in mind that all of the inks have to be food grade, acceptable to the FDA for being in contact with products that will be consumed.
A simple walk through the grocery store will open your mind to a whole new world of product packaging produced with technology derived from the raised lettering of the flatbed letterpresses Gutenberg might have used in the 1400’s.
This entry was posted
on Monday, December 13th, 2021 at 7:51 pm and is filed under Printing.
You can follow any responses to this entry through the RSS 2.0 feed.
Both comments and pings are currently closed.