Thermoforming Part II
What the inks need to do
Thermoforming uses thermoplastic substrates, so it should be no surprise that the ink must also be thermoplastic. This means it must soften with heat, resist melting and bubbling, and cool to a clean, durable finish--and it must do this after being forced around all matter of mold shapes and draw depths. There are high expectations for thermoformable inks, including some familiar features you would demand of inks used for flat parts: resistance to abrasion, chemicals, moisture, and blocking. But there are other requirements, as well.
Adhesion is even more critical for this application. As Steve Presutto, technical service manager of Coates Screen Inc. (St. Charles, IL), notes, "If you have only borderline adhesion , when you form a part, you'll get cracking and failure."
Just as important are the ink's flexibility and elongation characteristics, which are not an issue when producing flat parts. The ink must remain intact whether it is stretched, folded, or bent, and in some cases, it is expected to elongate 800% without so much as a stress crack. That can be a very tall order. This stretching and bending also places a burden on the ink in terms of color. If you pull a piece of taffy, its color weakens, and the same can occur on a thermoformed part if the ink is not sufficiently loaded with pigment. The deeper the draw, the more color density that is lost.
How do you quantify these performance characteristics? How much adhesion is enough? How much must the ink stretch? How much pigment do you need? The answer to all of these questions is that you test. For adhesion testing, everyone uses the crosshatch-tape test to match their own preset standards of what is acceptable. This test is particularly important for the deepest thermoformed draws.
As for the ink's pigment load, the depth of the draw will determine how much pigment is needed. The most suitable inks are heavily pigmented. And if you have an especially demanding application, you might even get the manufacturer to mix the ink for you at the desired opacity. Everything requires testing, readjustment, and then more testing.
Ink behavior in the real world
Historically, screen printing for thermoforming has been done with solvent-based inks, which are inherently the most friendly to the process for a number of reasons. First, solvent-based inks are thermoplastic, and as such, they bend, fold, flex, and elongate right along with the substrate when subjected to heat and pressure. (Water-based graphics inks are also thermoplastic, and although they are occasionally used for heat-bending applications, we found no evidence of their use in deep-draw vacuum forming.) The degree to which conventional inks respond to thermoforming is determined by the particular resin system used, which explains why screen printers report varying degrees of success with different ink series.
Second, the suitability of a solvent-based ink is also determined by its pigment load, and those products which offer the best combination of opacity and flexibility are typically used for forming. Conventional inks also "bite" into the substrate, creating what one manufacturer termed "a married unit." This is important for good ink adhesion after forming. For some applications, a specialty plastic ink or multipurpose product might be fine, but for extreme draws and/or detailed definition, a printer might select a an application-specific ink like Coates Screen's C37 Flexiform, designed just for thermoforming.
Finally, solvent-based inks are evaporative, which means what comes out of the dryer is very similar to what came off the press--less the solvent. The flexibility imparted to the ink by the resin system remains intact, because physically, the dried ink film has not gone through a molecular metamorphosis. This brings us to the subject of UV inks, whose suitability for thermoforming has long been questioned, due to their basic nature.
UV-curable inks are not evaporative. They are reactive, relying on chemical crosslinking to form a solid ink film. Crosslinking generally reduces an ink's ability to flex and elongate, as its molecules become more and more interconnected. So what comes out of the curing unit is very different than what entered it. And generally speaking, UV-curable inks aren't very thermoplastic. We say "generally," though, because like their solvent-based counterparts, UV-curable inks rely on various resin systems for varying degrees of flexibility. (In fact, one UV ink that we initially believed to be a specialty thermoforming product was described by its manufacturer as a highly flexible, highly opaque signage and fleetmarking ink.)
Opacity is another challenge presented by standard UV inks. Because curing is affected by the amount (as well as color) of the pigments, standard UV inks are not loaded with enough pigment to retain strong colors over deep draws and severe bends.
Nazdar's vice president of sales, Harold Johnston, comments, "With some UV inks, you can do a 1/2 in. or 2-3 in. draw, but you've got to draw the line somewhere, because the ink is not thermoplastic. Whereas with solvent- and water-based inks, which are primarily acrylic-based resin systems, you can load them up with pigment and put them through a coarse enough mesh so you have enough opacity to stretch out the image for 5-6 in." Johnston also points to the irony of the inflexible cured ink film. "That's one of its advantages. Once it's polymerized, there's not much you can do to it. So the whole process of thermoforming with UV inks is going to require much more research and development."
A UV ink just for thermoforming?
So what can you do when you want to eliminate conventional inks from all areas of a shop, print thermoform projects on UV inline graphics presses, or do contract thermoforming for fellow UV screen printers? Is it possible to overcome the flexibility and opacity issues presented by UV inks?
Many screen printers who do thermoforming are looking to switch to UV, not because they are dissatisfied with conventional products, but because they want the speed, reduced emissions, and all the other benefits that come with the change. To some degree, they have had successes with some of the more flexible standard UV inks.
But the most extreme demands of the thermoforming process require a specialty ink, and only recently have there been UV-curable products marketed for this purpose. Three manufacturers offer such a product: Polymeric Imaging (North Kansas City, MO) has developed Thermoform TF, TW Graphics (City of Commerce, CA) markets the 5000 Series VacuForm VF, and in Europe, Argon s.r.l. (Milan, Italy) offers a product that is simply called Thermoform.
Although they are relatively new ink lines, users in the field have reported that they do offer the flexibility, opacity, and other features required of a thermoformable ink. Some users have claimed to achieve draws up to eight and nine inches without cracking or delamination of the images. Overall, those printers who have integrated the new UV inks into production seem to be pleased with their appearance, durability, opacity, and formability.
The new UV thermoformable inks reportedly carry higher pigment loads than standard UV inks, and they offer substantial flexibility and elongation characteristics. Considering the inherent advantages of ultraviolet curing plus the ability to stretch the inks 600-800%, manufacturers, as well as the printers we interviewed, are optimistic about the future of thermoformable UV inks. As for what makes these inks so much more flexible than standard UV products, the secret is in the resin system--and because we are just beginning to see these products on the market, it remains a secret.
Working with UV inks for thermoforming
In many respects, thermoformable UV inks are printed and cured just like standard UV inks. But there are some important differences to consider. Ink laydown is one. "We have found that the greater the ink film, the more you can maximize the flexibility and elongation of the ink," says Caril Duncan, managing director of Polymeric Licensing, Inc., a division of Polymeric Imaging. "We recommend that printers put down as much ink as they possibly can and still cure it, and that will help them maintain the desired opacity and color consistency."
Getting the deposit just right requires some test prints and thermoform draws. Ultimately, it's not uncommon for a printer to select 300-thread/in. mesh to deposit enough ink to handle a deep draw. And when you consider the higher pigment load and thicker ink film, it stands to reason that these UV thermoformable inks also require more energy to cure.
Curing is always a critical function of UV printing, and because these inks are so new to the market, there are differing opinions on how critical curing is to the thermoforming process. Some say it is impossible to overcure the ink. Others claim that a complete cure, followed by thermoforming, effectively overcures the coating to the point where it cracks. Wolff, who has worked extensively with UV inks for thermoforming, names curing as the key variable in the process: "The issue with UV is the depth of cure," he says. "It has a narrow curing window, and overcuring can result in cracking [during thermoforming]. The ink should be just on the verge of being undercured."
Duncan reports similar feedback from customers. "The response I've heard is similar to what Bron is saying," she says. "Thermoforming [UV ink] requires more energy than a typical UV ink, but there is a point at which you're essentially curing the ink further than you need to, and that may cause it to embrittle slightly and lose some of its flexibility. But you won't see the broad cracking you'd have if you ran a typical UV ink through the same process. Still, you'd see some fractures, and that is not a desirable quality."
Both Polymeric and TW Graphics recommend a very slight undercure ("a minimum or 90% adhesion"), followed by post-curing or immediate thermoforming. Wolff relies on the heat of the thermoforming equipment to finish off polymerization. In fact, he has determined the curing window for every ink color he uses and, at times, changes the printing sequence to lay down the most forgiving color first. Of course, this also means changing the artwork to accommodate the new printing sequence. But these steps cut down on intercoat adhesion problems and help the image remain intact during forming.
All agree that UV inks are sensitive to some degree of heat, but does this mean that UV-thermoformable inks are more sensitive to it? "Yes, I think they are," says Polymeric's president, Don Sloan. "But I preface that by saying that thermoform inks are always subjected to heat anyway; that's part of the curing process. All of this happens at the same time. The ink is curing, stretching, elongating. If you were a molecule in that ink film, you'd be very busy." What printers and manufacturers all seem to agree upon is that 1) once a printed part has been through the heat and pressure of the curing process, the ink layer is set for life, and 2) you won't know exactly what will work for a particular job unless you run documented tests.
Nelson, whose company has also used both solvent-based and UV-curable thermoforming inks, reports some differences in working with these formulations. While solvent-based prints can lose some gloss in the thermoforming process, says Nelson, the UV inks produce an excellent gloss, which, if needed, can be brought down with flattening powder. He says that enhancing the gloss of a dulled solvent-based ink requires the application of a glossy clear coat after forming. The other difference is the rate at which the substrate can be brought up to thermoforming temperature. He adds that with thermoformable UV inks, his company gets the best results when the substrate is heated more slower than when solvent-based inks are used.
While you will always select a material to suit the application, in thermoforming, the substrate introduces a whole new world of variables. The slightest variations in material thickness can cause a part to form improperly. And since dark and light colors absorb heat at different rates, substrate color must be taken into account when setting the forming temperature.
You must also be familiar with the "thermoforming window" of your substrate because it's forming temperature affects the behavior of the ink film. The key is to avoid excessively high temperature, which present unwanted risks in terms of ink performance. Typically, polycarbonate will soften at about 350°F (177°C), acrylic at 325°F (163°C), and PETG at temperatures as low as 275°F (135°C). Polystyrene, an extremely popular substrate, has a very wide thermoforming window. And for printers who apply vinyl graphics prior to thermoforming, low temperatures are necessary to prevent melting the vinyl.
Other factors such as substrate grade, orientation, and impact resistance, as well as ink laydown, are equally important to ensure good forming and product performance characteristics.
Material grade A "registered grade" substrate is top-shelf product that should offer the most quality and consistency. If you are ordering material for thermoforming, make sure your plastics vendor knows what you'll be using it for. Nelson also recommends testing each and every substrate batch, even if you are rerunning a job. He notes that material with the same specs from the same vendor, may nevertheless contain different resins or differ in orientation.
Orientation The orientation of an extruded material refers its elastic "memory" relative to the axes of the web and forming machine axes. In the forming machine, the plastic will want to shrink in one direction or another, and the amount of shrinkage depends on the quality of the material. To be safe, Nelson advises that you print the sheets "all in the same direction so they will form consistently throughout the run."
Impact resistance Tests for impact resistance are performed on products such as bicycle helmets and vending machine panels. The substrate must pass cold impact tests to ensure proper product performance. Some plastics are tougher than others. Polycarbonate is at the high end of the scale. PETG, an amorphous co-polyester, is about 65-70% as tough as polycarbonate, and, according to Eastman Chemical's business market manager for structured products, Randy Beavers, "15-20 times tougher than a general purpose acrylic sheet." Equally important, if the plastic must pass impact tests, so should the ink. If a forklift bangs into a vending machine panel, and the substrate remains intact, cracking and delamination of print are not acceptable.
Image and ink coverage Wolff cites orientation as a critical factor as well, but credits the ink with the most effect on the quality of the form. "Heavy ink coverage affects the draw and the amount of distortion because it affects the amount of heat absorbed by the part," he says. On top of that, he notes, the color of the image can affect how the substrate responds to thermoforming. A black and white striped image, for example, would require careful setting of the heat zones on the thermoforming machine because light and dark colors absorb heat at different rates. Again, this is an area that requires testing and documentation.
Then there is the question of what gauge of material to order, because what you start out with isn't what comes out of the thermoformer. For example, the Coke vending machine panel shown here took several trials to determine the correct material gauge. The flat sheet began at 0.118 in. thick, but at its thinnest point, the formed part was 0.09 in., which was still sufficient for it to pass all the necessary impact tests.
Black art or science?
When you listen to printers describe how they produce thermoformed parts with screen printed UV inks, you get the feeling that this is a seat-of-the-pants exercise. It is extremely rare to successfully produce a job on the first attempt with no trial runs. Every new job, and every job rerun, for that matter, requires testing before production can begin. When you are thermoforming a full-flooded, gold metallic UV ink, such as Joliet Pattern did recently, you must, as Wolff says, "Have all your ducks in a row" before you begin. Nelson agrees. "It's not a black art," he says. "You have to be able to define the process, keep track of records, and profile the [thermoforming] heaters. It's a science that you have to keep working with. We look at this as a controllable process."