Using UV and conventional inks in tandem
You can reap significant, tangible benefits from what might appear to be an unconventional approach to graphics screen printing: combining conventional and multipurpose UV-curable inks in the same printing job. By using these inks together, it's possible to successfully print some difficult substrates that would otherwise require the use of specialty inks.
In simple terms, the greatest benefit of hybrid ink usage is the ability to do more with less. In some printing applications, hybrid ink combinations can result in finished-print performance that equals or exceeds the best that can be obtained with a specialty ink. This reduces your reliance on multiple specialty ink series and cuts costs associated with ink storage and waste.
Any procedure that offers potential benefits in product performance and cost savings is worth further investigation. This article will profile jobs that were made possible or enhanced by using both UV and conventional solvent-based inks.
If the concept of hybrid ink usage appeals to you, take note of this important cautionary advice: Thorough testing is absolutely essential. The use of UV inks alongside conventional systems is certainly not new and has become regular practice for many printers--but not without considerable testing and documentation.
Not all hybrid ink and substrate combinations will print successfully due to a range of variables your testing will uncover. These variables include the following: environmental factors such as ambient temperature, humidity, and barometric pressure the properties of substrates from different sources the characteristics of inks from different manufacturers the performance of different types of printing, curing, and drying equipment
Because of these variables, it's virtually impossible to describe a single, feasible process that will work reliably for everyone. Therefore, the procedures offered here are more general and can be used as a starting point for testing under your own unique production conditions. It is always wise to test until you are satisfied about the compatibility of the materials, the suitability of the production process, and the performance of the finished product.
Using more than one series or type of ink to complete a single printed piece is not a new idea. In the early days of the modern screen-printing era, thickened house paints were widely used as inks. Printing jobs executed on durable substrates were often finished with an overprint of a clear, protective marine enamel to improve weathering resistance. And for many years, paper substrates printed with poster screen inks were overprinted with clear enamels or lacquers. In addition to giving the posters a high-gloss finish, these lacquers and enamels imparted resistance to scuffing and abrasion and improved flexibility retention in the printed poster inks.
More recently, when UV-curable screen-printing inks were introduced, they became popular for use as high-gloss protective clearcoats printed over graphics produced with conventional inks. UV clearcoats provided higher gloss than conventional alternatives, and they cured readily at high production speeds. Today, applying UV clearcoats over conventional printed graphics is so routine that it has become a mainstream production process.
The best candidates for hybrid ink usage printing plant with both UV-curing and jet-drying capabilities can sometimes benefit in more efficient use of its production equipment by taking a hybrid approach to large jobs. This is especially true when a single, conventional ink color is printed before or after the four colors in a process-color UV job on an inline graphics press. You can print the conventional ink using a single-color press with a conventional dryer either preceding or following the printing of the process colors. And if your shop is devoted exclusively to either UV or conventional inks, you can team with a company that specializes in the other ink system and partner on any hybrid printing projects that come your way. In general, printers who focus on P-O-P graphics are most likely to benefit from using UV and conventional inks in tandem.
UV inks, then and now
In this discussion, UV ink refers to the current generation of ink formulations that cure by exposure to radiant UV energy, especially the "multipurpose" types intended for use on a wide range of substrates. Conventional ink refers to solvent-solubilized formulations, which dry by evaporation. Although water-reducible UV inks and water-based inks containing co-solvents might also be candidates for hybrid applications, they are not being addressed here because presently they are much less widely used than UV and solvent-based inks.
Current multipurpose UV inks differ greatly from those that were common some years ago. Early UV formulations had little tolerance for variations in curing energy dosage. When fully cured, most of the early ink series developed very good solvent- and abrasion-resistance. Unfortunately, they were also prone to intercoat adhesion difficulties arising from overcuring of the first colors in the printing sequence.
Most UV inks currently available cure much more rapidly at lower energy dosages than they predecessors, and they possess wider tolerance for curing variations. The newer ink series are generally more vulnerable to attack by solvents, but this can constitute an important advantage. The low solvent resistance means these inks are less likely to have intercoat adhesion problems and easier to overprint with conventional inks for plastics. However, they also require that you use a hardening additive or a protective clear coat when job requirements include high solvent resistance.
Relative ink costs The price of a gallon of UV ink is greater than a comparable color of conventional ink. How much more varies considerably based on your location, but in use, the price differences might not be as great as they initially seem. Because UV inks are printed through finer meshes than those typically used for solvent-based systems, they cover more area per gallon. UV inks also require less energy to cure, and the curing units take up much less space than conventional dryers.
It is very worthwhile to calculate your actual ink usage costs for particular jobs. You may discover that by using both UV and conventional inks, your total ink costs will decrease. Additionally, you may find that the hybrid approach actually contributes to the functionality or durability of the printed part.
Controlled ink inventories Using a specialty ink series for each difficult substrate you print can force you to stock fairly complete color ranges for many different ink lines. This is especially likely to happen where scheduling is tight and jobs go to production almost immediately after the order is received. Few screen-printing ink suppliers provide rush delivery service, and not all of them find it practical to carry complete color ranges of all the ink lines they offer. Even when they stock a full color range, the volume of ink available can prove insufficient for prompt fulfillment of a large-volume order.
The challenge of controlling ink stocks grows steadily worse when numerous specialty ink lines are used. With the passage of time, leftover containers of single-purpose inks in custom colors begin to proliferate, and the printer is left with stockpiles of ink that will probably never be used. Working these leftovers into new matches is usually difficult because inks that are formulated for different specialty substrates are not reliably intermiscible.
The limited shelf life of UV inks compounds the problem. Despite careful stock rotation, color-matched UV ink can expire in storage before the next occasion arises for its use. The limited shelf life also naturally discourages ink suppliers from stocking large inventories of UV inks for specialty substrates.
From the printer's viewpoint, the fewer ink lines in the shop, the better. Stocking fewer ink lines not only simplifies the task of keeping ink inventory well controlled, it also reduces waste. Additionally, it merits notice that fewer ink lines means fewer ink modifiers, many of which are series-specific and can invite misuse. The objective is to rationalize the number of ink lines to a manageable number, without compromising your ability to satisfy a broad range of job requirements.
As stated earlier, making hybrid ink usage work involves thorough testing, as well as an understanding of the costs of using each ink series. Let's look at some jobs that were successfully produced using both UV and conventional ink systems in tandem.
Window display on clear acetate
Clear acetate is an economical substrate with excellent transparency, making it highly suitable for use in window advertising. Unfortunately, acetate is also very vulnerable to attack from the solvents and/or monomers contained in conventional and UV-curable inks.
Solvent attack manifests itself in several ways, including halo formation at the perimeter of the print. The solvent softens the acetate, which results in blocking of the sheets after stacking. The image can also "haze through" the stock to the next sheet in the stack, usually followed by crazing and embrittlement of the printed sheet. The window advertising pieces in Figure 1 were produced in the following way to avoid such difficulties:
1. The first color was an opaque white background, printed through 280 thread/in. mesh (or finer) with a multipurpose, solvent-based plastics ink that had been mixed with a minimal amount of low-attack thinner.
2. The print was jet-dried at a temperature not exceeding 50°C (122°F). Since the solid white print covers a relatively large area, the ink is delivered through the screen rapidly enough that drying in the screen is extremely unlikely.
3. Subsequent process colors (including black) were printed through 380 thread/in. mesh (or finer) using UV-curable multipurpose plastic inks, which had also been reduced minimally.
4. The print was cured at the low end of the recommended energy dosage range. (For one of the recently formulated, rapid-curing UV inks, this could be as low as 90-110 mJ for the colors and 135-155 mJ for the black.)
Some synergy seems to have taken place as a result of using the conventional ink with low-attack thinner as an island on which to apply the UV ink. First, it protected the substrate from attack by the UV monomer in the overprinted colors, which could have led to embrittlement of the substrate. Second, the slight residual presence of solvent in the conventional ink layer prevented or relieved internal stresses induced by shrinkage of the UV inks during the curing process.
This hybrid example takes advantage of the strengths of both types of ink: the inexpensive delivery of high opacity by the conventional multipurpose ink and the brilliant transparency, prolonged screen stability, and ability to reproduce fine detail provided by the UV ink.
Fiber reinforced vinyl banner
Fiber reinforced PVC banner stock has remarkable flexibility and is very tear-resistant. The better grades of such material retain these properties at low temperatures, making them durable even in cold-weather conditions.
To maintain these performance characteristics, PVC banner stock contains one or more plasticizing ingredients. This can pose problems for the printer because drying or curing with excessive heat can trigger migration of the plasticizer to the surface of the stock, impeding drying/curing and ink adhesion. Even when plasticizer migration is controlled in the drying or curing stage, the plasticizer can still affect the print's gloss level and result in an undesired contrast between the finish of the printed areas and that of adjacent, unprinted areas.
The selection of a highly-flexible specialty ink might appear to be the natural choice for printing on a substrate like this. But if you need to produce a consistent finish on both printed and unprinted areas, you'll need to print a final clear coat over the entire material. A clear overprint may also be necessary if the finished piece will be subjected to stress from rolling, unrolling, and abrasion. Additionally, a specialty ink for flexible PVC can carry a premium price, and local suppliers do not always carry full stocks, so availability can be a problem.
The hybrid approach to the project shown in Figure 2 uses a standard, multipurpose UV ink, followed by an overprint of a clear conventional plastic ink that is suitable for use on the banner stock. If necessary, the conventional ink can be modified by adding a small amount (typically only 2 to 3% by volume) of an aggressive, "biting" solvent such as Gamma-Butyrolactone (sometimes also referred to as BLO). The modifier promotes adhesion to the previously applied UV colors. It also relieves residual stress from shrinkage of the cured UV ink. The result is a degree of flexibility from a multipurpose UV ink that might otherwise only be possible from a specialty UV ink. The job was produced as follows:
1. Process colors (including black) were printed with multipurpose UV ink, especially well stirred but unmodified, through 330-380 thread/in. mesh.
2. The ink was cured at approximately 135 mJ for the colors and at approximately 145 mJ for the black.
3. A conventional clear gloss ink, recommended for flexible PVC, was printed through 280-330 thread/in. mesh over the banner surface.
4. The ink was jet dried at moderate to low temperature. Stock surface temperature was kept between 48-51°C (118-124°F), but this range can change depending on the temperature tolerances of the specific stock you use.
In addition to giving the printed piece abrasion resistance and a uniform finish, the conventional clear coat offers a secondary benefit of improved flexibility and impact resistance in the underlying UV image. In comparative tests for recovery from cold folding (after eight hours in a freezer) and impact distortion, these prints equaled or outperformed prints which had been made on the same stock using inks specially formulated for use on highly flexible PVC.
Untreated metallized polyester mirror material
When a customer specifies a specific print-treated substrate, and it is not immediately available, you may have to substitute an untreated stock. This creates a challenging ink problem, and a hybrid ink combination can offer a practical solution. This is often the case with metallized mirror polyester, which seems to be most widely stocked by suppliers in an untreated form.
An untreated stock can be underbased with a conventional polyester ink and overprinted with a multipurpose UV ink in process colors. The polyester-resin-based conventional ink has a fierce odor and is not easy to jet dry, but careful testing before production and good process control can make it practical, if not fun, to use. The following list outlines the production procedure for the mirror polyester shown in Figure 3:
1. An underbase was printed using an opaque white conventional polyester ink mixed with 2% flattening powder to facilitate jet drying and to improve bonding of the subsequent UV overprints. The white was printed through 280-330 thread/in. mesh and jet dried.
2. Multipurpose UV process colors (including black) were printed through 380 thread/in. or finer mesh, cured in the same manner as the flexible PVC banner described previously.
In this example, the polyester ink formed a strong bond with the substrate and a suitable foundation to which the subsequent UV colors could adhere.
Hybrid applications that don't work
Printing a UV ink over a conventional specialty ink on treated polyethylene banner stock approaches being the "mother of all bad ideas." Although there may be some set of circumstances that allows this procedure, the results will usually be disastrous. The problem is that shrinkage of the UV overprint breaks the bond between the conventional ink and the substrate. The result is selective adhesion failure of the conventional ink wherever it is overprinted with UV.
Solvent-based fluorescent inks can also pose problems in hybrid ink combinations. Their brightness and outdoor durability are diminished by each pass through the UV curing unit. Each exposure to intense UV radiation is a mini-simulation of outdoor exposure to the damaging effects of sunlight--to which fluorescent colors are notoriously vulnerable. Applying more than one overprint of a UV ink over a conventional fluorescent print is not advised.
Attitudes toward hybrid ink usage vary widely among screen printers. On one end are the cautious printers who use one ink series per substrate and use the inks only as directed. On the other end are the bolder printers who dabble in alternate application protocols and experiment with various combinations of inks, substrates, equipment controls, and other process variables. If you fall into this second group, don't mistake this article as a recommendation always to use multipurpose UV inks. This is only one option for hybrid ink usage, and your own experiments may uncover other complementary ink combinations and production procedures.
About the author
Frank Greenway has been involved in the screen-printing industry for 28 years. His background includes serigraphy, commercial graphics printing, textile printing, equipment sales, ink development, and technical support in UV-printing applications. For the past nine years, he has worked as technical support specialist for Artisan Screen Print Ltd., Markham, ON, Canada.
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