Mind Your Banners
Banners are great devices for displaying graphics, but printing them can sometimes be a challenge. Find out how to match substrates and applications and learn how to prevent a banner job from turning into a hulk of a problem.
What comes to mind when you think of banners? If you're like most, banners are what the new car wash on the corner uses to announce its grand opening, or what the local superstore hangs to highlight the week-long sale on sweatsocks in aisle 14. Sure, banners will likely remain a popular choice for promotions such as those, but banners are gaining more and more ground in high-end, long-term promotional and display applications. Cities use pole banners to create a unique image for the area and highlight upcoming events. Museums drape them along their entryways to draw attention to new exhibits. These are venues in which banner materials can most affect the quality of printed graphics. But even the short-term outdoor graphics for grand openings and the indoor banners announcing special sales have requirements for printability and durability. That's why it's critical to select the right type of banner media for the job.
Plastic-based substrates are popular for banners, and vinyl-faced--sometimes called vinyl-reinforced--products and polyethylene materials are the favorites. But as common as these materials are, they still pose challenges to screen and digital printers. This article will focus specifically on vinyl and polyethylene banner substrates for UV and solvent-based screen and digital inks and examine the things to keep in mind when working with these materials. Let's start with vinyl.
The vinyl-manufacturing process
The manufacture of vinyl banner stock usually involves sandwiching a scrim layer--a reinforcement that is mesh-like in construction--between two sheets of vinyl. Laminating equipment applies heat and pressure when bonding these components. You might think that heat would distort the plastic material, but as Glenn Shull, business manager of Herculite's graphic-products division explains, "It helps the dimensional stability of vinyl. The number of rollers vinyl goes through and rollers it goes around really helps to flatten and stabilize the vinyl."
The laminators used to produce vinyl banner media bear little resemblance to those you might use in your own shop to encapsulate or mount printed graphics. These industrial-grade machines can be two stories tall and half a football field in length--and the wider the banner material, the larger the laminating equipment. Producing laminated banner materials in widths greater than about 98 in. requires what Shull describes as an "insanely big machine."
That's why those who want media wider than that typically must purchase coated banner material. But don't confuse the term with top coating; we'll address that later. In this case, the manufacturing process involves using a much tighter scrim than what is found in laminated vinyls and coating it with liquid vinyl.
Printing vinyl banner material
As with any vinyl substrate, vinyl banner materials have characteristics that, if left uncontrolled, can derail your best efforts to print them successfully. For example, the manufacturing method used for vinyl banner materials can leave the product with an apparent surface texture. The pressure used in creating vinyl banner substrates may produce a pattern that mimics the scrim, particularly when a stronger, heavier scrim, such as one with 9 threads/linear in. (also referred to as 9 x 9), is used. These divots in the banner substrate's surface can lead to visible pitting in printed graphics. While this may not be too big of a problem when printing vinyl banners for long-distance viewing, it can degrade prints designed to be viewed close up.
Shull has some suggestions for those who cannot risk having visible surface textures. "The best way around that is to request a product with an 18 x 9 scrim, rather than a 9 x 9," he says. "But if you absolutely must have 9 x 9 scrim, take a look around at different manufacturers, because we all have products that have better or worse surfaces--depending on what market we're selling into."
Shull notes that it's difficult to avoid the scrim-like texture when printing onto heavy duty banner, and he explains that if printers are still having that kind of problem when using very smooth banner materials, they may not be getting a good, clean squeegee hit. The ink may actually be hanging up in the screen.
Molly Waters, technical account executive at Avery Graphics, says that setting up inkjet printers for printing on vinyl banner material is pretty straightforward. But she urges screen printers to keep a close eye on their printing parameters when working with very wide substrates. "You need to make sure your squeegee pressure is even across the width," she says. "The wider you get, the more in spec your equipment needs to be."
Ink adhesion on vinyl
Just hearing the term plasticizer migration is enough to give printers a good scare. The manufacturing process used to produce vinyl entails the use of plasticizers--chemicals that keep the vinyl flexible. Over time, these plasticizers can actually move, or bloom, to the vinyl substrate's printing surface and cause potentially serious ink-adhesion problems. Plasticizer migration is a known evil when it comes to printing on vinyl--whether the banner is a two-sided product for indoor use or a one-sided, heavy-duty material for a long-term display outdoors--and it can affect the production workflow and hinder the quality of the printed graphic.
For shorter run lengths, printers may be able to get away with wiping down the vinyl substrate with alcohol. Doing so can remove plasticizer that has bloomed to the surface, and it has the added benefit of reviving the substrate's surface energy--a factor we'll talk about in a moment. However, treating surfaces with alcohol really isn't feasible for longer runs. Would you like to wipe off 10,000 banner blanks?
The best way to avoid plasticizer migration is to store the vinyl media properly and pay attention to its shelf life. The manufacturer or distributor can give you these details. However, even if you think you've done everything right, you may still have to deal with plasticizer migration. Why? Well, when you buy vinyl banner stock, you may be purchasing an older material that was intended for sale into the sign market for use in pressure-sensitive applications. This situation is common with colored vinyls. So when you order your substrates, be sure to specify that they must be printable. This should score you a fresher product.
Plasticizer migration causes a loss of surface energy. A certain level of surface energy, measured in units called dynes, must be present for inks to wet out properly. As mentioned previously, the alcohol treatment can bring surface energy back to a printable level--but it's tedious work. A more efficient alternative is corona treatment, in which a discharge of high-voltage electricity restores the surface energy necessary for reliable ink adhesion.
"If you have a corona treater in your plant, you can run banner product through it and turn right around and print it," Shull says. "You can also save a lot of money by asking to buy product that's beyond its shelf life at a discounted rate."
However, Shull cautions that while corona treatment can negate the issues caused by plasticizer migration for a short time, it's not a cure-all. "A product might be printable for a year, but once plasticizer has bloomed to the surface sufficiently and you treat it, that doesn't mean you get another year. It means you get another couple of weeks."
The chemical composition of the vinyl itself also plays a role in shelf life. Vinyl is either monomeric or polymeric. Shull explains that monomeric vinyl is less expensive and tends to have a short shelf life. Polymeric vinyl, on the other hand, is the opposite in both cases. "[Polymeric vinyl] can sit on the shelf longer, it has better dimensional stability--it's overall a much better product. But it really makes no difference in the long-term quality of the display graphic," he says.
Handling is another factor to consider. If you use bare hands when working with vinyl, you may run the risk of transferring oils from your skin onto the printing surface. When this happens, you may notice visible hand marks in the printed graphics, whether screen or digitally printed. A simple solution is to wear gloves. Along with proper handling comes shop upkeep. Keep a close watch on contaminants that may be lurking in the shop, and make the equipment you use to cut banner vinyl is clean.
Selecting the right vinyl
Vinyl banner materials are available in a variety of forms to meet the needs of indoor and outdoor applications. To simplify matters, let's group these substrates into three categories: indoor, general-purpose outdoor, and heavy-duty outdoor. Shull explains that a typical vinyl banner product for indoor use, such as in-store graphics (Figure 1), is one that is constructed with blackout material, is two-sided, uses a very tight scrim, has a very smooth face and reverse, usually comes in weights between 10-13 oz., and--generally speaking--isn't too strong. This material also may be used successfully outdoors, in agreeable weather, for very short periods of time.
For general-purpose outdoor applications, such as the promotional graphics at the car wash, the banner material is most often a one-sided product that uses a much heavier scrim to enhance its tear strength. These substrates, which are commonly available in weights from 10-13 oz., are designed to be used outside for up to a few months. Shull notes that two popular constructions for general-purpose outdoor banners are 18 x 9 scrim at 400 denier (the strength of each thread in the scrim material) and 9 x 9 scrim at 750 denier.
Heavy-duty vinyls for long-term outdoor applications--the pole banners and museum displays--are constructed to withstand the elements: tears caused by wind whip, cracking from exposure to the cold, and more. Heavy-duty vinyl banner media is commonly available in weights from 15-21 oz.
Shull explains that with heavy-duty banner vinyls, the reinforcement is more critical than the weight. "A typical, heavy-duty thread count would be 9 x 9," he says. "Then you have denier, and that can range anywhere from 500-2000. A super-strong scrim would be a 9 x 9 and 1000 denier."
Always discuss specialty applications with manufacturers. They can help guide you to the banner materials that best suit your needs. This is especially true if you plan to use a UV inkjet system to print banners. Waters suggests that printers test these inks on a job-specific vinyl substrate before starting production. "The inks are constantly changing," she says. "UV does work quite well, but ink manufacturers continue to work to make the inks more flexible.... What may be compatible today may not be tomorrow. On the screen-printing side of things, it's more consistent. Most people are using UV today."
The polyethylene-manufacturing process
Producing polyethylene banner substrates comprises several manufacturing steps. The first is extrusion, where pellets of raw polyethylene are converted to a molten state by way of pressure. The pellets are extruded in a corkscrew-like fashion, and the pressure from the squeezing action prepares the polyethylene to be forced out through a circular die into a ring or a tube, also known as a bubble. Next, the bubble is flattened or collapsed, which produces a sheet that Rich Witmer, marketing communications manager at Valéron Strength Films, describes as "sort of like a plastic bag that's been folded in half. It's really a tube if you open it up, but it can lay flat."
The extrusion process can create a grain in the polyethylene that is sometimes visible on the surface of the banner material. Witmer explains that tearing in the substrate will most often follow the direction of this grain.
Topcoatings applied to some polyethylene banner materials may hide this grain pattern, but they are more common with materials for inkjet printing, particularly those designed for use on aqueous inkjet printers. "[Top coating] is compatible with screen printing, but it's overkill," notes Ed McCarron, marketing manager for InteliCoat. "Screen printers would find the cost prohibitive."
But he adds that the coatings can offer benefits to those who use UV and solvent inkjet devices. "The prints I've seen direct on polyethylene just don't have the pop," he says. "A lot of times, without a top coat, you're limited to how much ink you can apply to polyethylene, so the image might not be bright--it might be dull or have a faded look to it."
Corona treatment is a standard part of the finishing process that most polyethylene-banner manufacturers apply to the banner stock. As addressed earlier, corona-discharge treatment raises the substrate's surface energy, in turn making the material more receptive to inks. Polyethylene must be corona treated before screen or inkjet printing. Corona treatment also prepares polyethylene for top coating.
To optimize printability, be sure to wear gloves when handling the media to avoid damaging the surface or contaminating it with oils from your skin. Also prevent the material from coming in contact with moisture, because moisture accumulation can adversely affect ink adhesion.
Polyethylene's place in banner applications
Polyethylene is best suited to indoor applications (Figure 2). Its surface is smooth--there's no scrim--so the material can be used for banners that involve close-up viewing. It's usually available in gauges between 4-10 mils, but keep in mind that the presence of a top coating increases the material's thickness.
"The smooth look of polyethylene and lower cost is popular for short-term promotions," McCarron says. "Scrim banners are typically more for longer-term outdoor. Polyethylene might be chosen for a campaign for three weeks to a month. Some can be outdoors but with much less tear resistance."
Polyethylene may be used more successfully in outdoor graphics, according to Witmer, when it's attached to another application surface (Figure 3), either in rigid displays or mounted to other types of hardware. Additionally, certain manufacturing processes, such as cross-laminating several layers of polyethylene into a single sheet, are used to produce banner materials that have greater tear strength than standard polyethylene media.
Polyethylene can be sewn and grommeted, and it can be produced and coated specifically for two-sided printing. It can also be recycled without the hassles associated with processing vinyl.
Polyethylene is hydroscopic, which means it is prone to absorbing moisture from the air. This action can potentially drive a microscopic wedge between the ink and substrate and adversely affect ink adhesion. When this happens, exposure to the elements, as well as airborne debris, can damage printed graphics. That's why polyethylene isn't a first choice for long-term outdoor graphics.
A banner in the works
The use of banners is increasing in a wide variety of display-graphics applications, whether they're high-end outdoor graphics for long-term use or simple, short-term aisle markers for trade shows. To capitalize on the trend, it's important to include manufacturers and distributors in the material-selection process and then thoroughly test the substrates for compatibility with your printing processes and inks before you start production. Once you get the hang of banner printing, you'll consistently produce high-quality graphics and create satisfied customers.