Faster than a speeding bullet to apply!" "More powerful than a UV varnish!" "Able to leap through thousands of feet of label stock with a single roll!" Although this may sound like the signature of one of America's oldest and most popular super heroes, it also describes the role of self-wound overlamination in the narrow web label industry today.
Humor aside, not every product can achieve super hero status, but self-wound overlamination has certainly been performing this role for many years. An industry staple since the mid 1980s, it's been a Superman for label printers disguised as a mild-mannered Clark Kent — the anonymous, invisible protector. It's a commodity product that when applied provides an invisible layer of strength and durability — a product that performs a crucial function but one that most label printers take for granted and often overlook when ordering plates, ink, dies, and stock.
Just as the flexo printing industry has grown and evolved, so too have the technology, materials and processes of overlamination changed over time. Who could have imagined that a printing method first achieved in 1890 on a press nicknamed "Bibby's Folly" would, over the next century, evolve into the most versatile, fastest growing conventional printing process in the world! Now, thanks to its ability to perform many different converting processes in-line on almost any substrate — from simple paper to exotic synthetics — narrow web flexographic printing is the predominant, most innovative and cost effective source of pressure sensitive labels in the world today.
Ever since the first Greek labeled an amphora of wine with the year it was produced or an ancient Mesopotamian merchant decorated a wooden barrel of grain to appeal to his client's aesthetic sense, labels and labeling have been an integral part of our culture, our lifestyle and our economy. For centuries, merchant-manufacturers put information related to their wares on earthenware pots, crocks, jugs, sacks, barrels, and crates. The first "modern" labels appeared during the Industrial Revolution in the late 1800s when the fundamental nature of production and distribution changed. It's hard to believe that probably the most significant event in label history didn't occur until the 20th Century. It wasn't until the early 1930s that R. Stanton Avery manufactured the first self-adhesive label and turned it into a thriving business that is still successful today. Now into the 21st Century, self-adhesive or pressure sensitive labels are the dominant technology for product information, decoration and identification. But pressure sensitive labels are being asked to meet many diverse demands — graphic, financial, environmental — as they continue to be an integral part of a multi-billion-dollar packaging industry that is driven by sophisticated consumer preferences and ever-increasing industrial needs in a globally competitive marketplace.
Two of the biggest demands that labels are asked to meet are durability and cost. Obviously, the most basic and yet most important function of any label is that it retain its legibility and durability for its specific application. In order to achieve this, the information and graphics contained on a label must be legible and last over time. Unless you're printing labels with disappearing ink for a top-secret organization, it really doesn't matter how expensive or exotic the base stock is — or how many process ink colors are used —or even how good the registration or print quality is if the label doesn't last or if it becomes illegible.
The primary role of overlamination, in general, is to provide protection from all of the chemical and environmental contaminants that work to destroy a label from the outside in. It protects the label from a host of potential intruders including moisture, chemicals, oil, acid, dirt, abrasion, and even the fading UV rays of the sun. Unlike a release liner, for instance, which is usually discarded long before the label reaches its final destination, overlamination becomes an integral part of the label itself. Because it performs this important function, it is a critical component of most label constructions. Choosing an overlamination (based on each label's function and specific end-use application) that provides protection, increases durability, is cost effective, and is easy to use, is a crucial part of the label construction process.
Unsupported (linerless) or self-wound (wound on itself) overlamination has been the flexo label printer's choice of an economical and trouble-free way of giving labels increased durability, tear-resistance and protection from hostile environments for nearly 30 years. Today, a majority of narrow web flexographic label printers prefer self-wounds and just about every narrow web converter, whether they use it or not, is familiar with this lamination method.
The first self-wound overlamination was, quite simply, carton sealing tape. Its coat weight, film thickness and even the roll length were identical to machine length rolls of clear polypropylene carton sealing tape. The cost savings that this early product afforded when compared to pressure sensitive adhesive films on release liners (commonly known as "supported" or "linered" overlams) was enormous. Not only was it significantly less expensive, but over time its composition and put-ups changed to meet the specific needs of flexo label printers. Thinner, more conformable film gauges were produced; different adhesives with better wet-out properties were developed; and lighter coat weights for easier unwind were achieved. Slit roll lengths went from 3,000 feet to 2,500 and 5,000 feet in order to "match" the roll lengths of label stock. Further improvements in manufacturing technology and coating/slitting capabilities have led to the development of an array of self-wound products. Today, self-wound overlamination is the most versatile lamination process available because it provides the label printer a variety of films, finishes and thicknesses to suit almost any application; it is available in roll widths and lengths to fill almost any production requirement; and its ease of use and convertibility on press are unsurpassed.
The two major components of self-wound overlaminates are its film and adhesive. As mentioned, biaxially-oriented polypropylene (BOPP) was the original and only substrate available for many years. It is still the most commonly used overlaminate substrate. The same characteristics that make it a good carton sealing tape make it a great overlam — it's tough, durable, tear-resistant, and cost efficient. Because it is a film and not just a coating, it provides an effective protective barrier for labels. This is especially important for refrigerated food labels such as milk and dairy product labels where the label may go in and out of refrigeration. Polypropylene holds up exceptionally well to the moisture caused by repeated condensation and evaporation as well as a host of other grueling conditions. It also protects the label from the contents of the product it identifies and is commonly used as a laminate on corrosive, caustic and acidic products — everything from pickles to car batteries. Because it is scuff resistant, it is used to "tamper-proof" important data printed on labels such as lot numbers and expiration dates. Polypropylene's superior internal strength provides additional strength and stability to whatever label stock it covers, giving finished labels added "body" and enhanced gloss. In fact, many label converters have found that they can save money by substituting overlaminated paper stock for expensive vinyls or styrenes without sacrificing strength.
Today, clear polypropylene (BOPP) is joined by a variety of film substrates including matte polypropylene, clear polyester (PET), matte polyester, and thermal transfer printable polyester, to name a few. Each film has its specific application and use. For instance, the matte finish films have a non-reflective surface that make them the logical choice for bar code labels that need a non-glare surface in order to be picked up by sensitive scanners. They are also ideal for cosmetic and beauty product labels that require a "muted" or "softer" look and feel or when a write-on surface is needed, since the matte surface tends to be ink and pencil receptive. Polyester film provides even greater resistance to moisture, chemicals, abrasion, heat, and UV than polypropylene, making it the overlam of choice for more demanding applications and especially when a label will be outdoors.
The thermal transfer printable films were developed in response to the need by end users to print variable data and images on a printed, overlaminated label. Thermal transfer printable polyester provides all of the advantages of regular polyester film as well as printability with most brands of wax/resin and resin ribbons on the market today. Film thicknesses generally range from 1 mil to 2 mils, although advances in film formulations have allowed manufacturers to supply even thinner gauges.
Aqueous or waterborne emulsion acrylic is the most widely used adhesive for self-wound overlaminates because of its many attributes, outstanding clarity being only one of them. It is long aging and UV resistant. In addition, its non-yellowing, non-toxic and non-polluting properties make it vastly superior to both hot melt and solvent based adhesives. The strength of its bond to a substrate builds over time (ultimate adhesion) and holds up under extreme temperatures. Recent advances in adhesives technology have resulted in new generation, "super clear" formulations that require lower solids content (i.e., less clumping) and lower coat weights, resulting in self-wound overlaminates that "wet-out" almost instantly and virtually eliminate three big challenges: bruises, hazing and "fish eyes". Yes, fish eyes! Besides being a delicacy of Chinese cuisine, this is also the rather curious term label printers use to describe the irregularly shaped, randomly distributed marks that appear in self-wounds. Fish eyes are an inherent characteristic of self-wound films caused by air entrapment between the film and adhesive that occurs during the coating, slitting and rewinding processes. They are not defects, per se, and become virtually undetectable once the film is applied to label stock. As you might imagine, they are usually more apparent on labels with dark heavy ink coverage or on shiny metalized stocks. The pressure of finished labels wrapped on rolls literally "squeezes" them out within a relatively short period of time. It's not uncommon to hear amazed label printers report that the "fish eyes" that once covered the finished labels "magically" disappeared overnight. New generation "super clear" films practically eliminate "hazing". These films are being combined with "harder", "bruise-free", water clear adhesives to produce super clear, fish eye-free self-wounds to rival linered laminations. They can be used to create the perfect no-label look when overlaminated to clear label stocks and provide label printers with easier color matching and better internal quality control.
Noise is a challenging characteristic of self-wounds. All self-wound tapes and films "chatter" when being pulled off the roll. Most press operators use some kind of hearing protection, the most popular being good old fashioned ear plugs, when running self-wounds. Inexpensive, easy-to-install tension control devices that greatly reduce noise levels and "slapping", as well as stabilize unwind pull and provide more consistent and controllable roll tension, are also available. Finally, manufacturers have been able to combine new adhesive formulations requiring lower coating weights with special silicone release coatings that lower surface energy and reduce unwind force to produce self-wound overlams that are now both "super clear" and "super quiet".
"Curl", "wrinkles" and "tension" are popular topics of conversation at any local beauty salon, and they are, undoubtedly, the most common web handling problems associated with self-wounds. The principles at work are really quite simple. Curling (either up or down) occurs if the overlam is stretched when it meets the printed substrate. The resistance caused by the unwind force needed to pull the pliable overlam film from the roll "stretches" the film. The longer, stretched film over the shorter, stiffer label stock will eventually return to its original "relaxed" state (like an elastic band). The result is curl. A driven, clutched "pull" roll between the unwinding roll and the nip stations allows the operator to "feed" the overlam with a minimum of tension, allowing the overlam to relax before meeting the substrate. A dancer roll can be used on older presses that can't accommodate a driven "pull" roll. Press manufacturers are familiar with the specific modifications that are necessary to achieve this and can offer technical assistance. On most presses it's usually just a matter of changing the way the overlam is threaded through the press. If a particular press should require actual design changes, they are usually not costly, are easy to implement and well worth it.
"Wrinkles" or creasing and "bruise lines" are usually related to unwind problems on press that are caused by incorrect speed or tension control and will benefit from some of the same press modifications and web handling techniques used to eliminate curl.
Tension (and tension control) is an issue common to self-wound products. It affects converters and coaters alike. There is a fine line between too much and not enough. "Telescoping" is a common, tension-related problem that results when rolls are wound too tightly during the slitting process. This can occur due to uneven adhesive coating or film thicknesses in the jumbo roll, referred to as "gauge bands", or improper tension control during the rewind-slitting process. It's not immediately apparent, but manifests itself over time as the built-up pressure literally forces the film layers or "wraps" to bulge up, giving the rolls a "coned" appearance.
It might be worth mentioning that — as with all film products — careful material handling is critical. Carton packing and palletized shipping of finished rolls by the supplier as well as sensible storage, retrieval and web handling practices by printers will ensure that small tears or edge damage (the leading cause of costly web breaks on press) don't occur. The long aging, UV resistant acrylic adhesives and clearer films being used by most manufacturers today have virtually eliminated the problem of "yellowing", still a common complaint of self-wounds that are coated with hot melt adhesives.
Never before have narrow web label printers demanded so much from their materials: tremendous versatility, easy setup, quick changeover, low waste, environmental compatibility, outstanding quality, and superior durability, to name a few. Self-wound overlams meet these requirements head on. They have the clarity of linered lams at a fraction of the cost and without the added financial and environmental cost of disposing of costly liner paper. They are more versatile and easier to use than raw film and UV curable "cold glue" adhesive. Not only does this system lack versatility because the raw film is only available in thin gauges of BOPP, but it requires special equipment and additional energy to cure the adhesive. Its setup and clean-up are longer, its run times slower, and its adhesive requires the use of an ink station. A third type of lamination consisting of a UV varnish that is cured on press gives a laminated "look" to the label, but provides no extra "body" and only minimal protection, durability and moisture resistance.
Traditionally thought of as an inexpensive commodity product, self-wound overlamination has gained the respect of label printers worldwide and is now the overlam of choice for most converters. The wide variety of film substrates, film gauges and adhesive thicknesses means there is a self-wound lamination system to fit virtually every application. It doesn't require any special equipment; it sets up quickly; it requires no clean-up and runs trouble-free on press. Its durability, improved clarity and cost effectiveness offer narrow web converters many opportunities in the pressure sensitive label market.