UV-Curable Technology Profiling Considerations

UV-curable technology (not to be confused with "UV" water-based inks) is the next generation of outdoor durable printing for rigid and roll-to-roll printing. A UV-curing inkjet printing system brings additional advantages compared to other digital ink-jet systems. One of the most significant is the instant drying, which occurs in the ink curing step. This can occur in 0.2 seconds, is independent of the printing speed and lamp temperature, and can yield faster job turnaround.

UV inks are also applicable for more media than water or solvent inks, increasing the range of media that can be used without pretreatment. Untreated media is less expensive than coated media or substrates, which require a conditioning step. The durability of the cured ink also often eliminates the need for a lamination step to protect the surface, again eliminating a production step that can often be a bottleneck in production environments. It is generally accepted that cured prints can be placed outdoors for 2 to 3 years. Because the inks stay on the media surface and generally do not absorb into the media, print and color quality is more consistent from media to media and results in reduced set-up time.

This ink system and printer technology has special consideration when profiling. This information covers general UV tips. There are also some tips on how to profile white ink as a spot channel.

Lamp Temperature

• Determining the proper lamp temperature is vital for optimal cure of the ink. Some printers allow you to modify the lamp temperature by degree or percentage. Contact your printer manufacture for details.
• Just like in solvent printing, "Heat" effects how the ink and media combinations perform. Luckily with both of these systems it seems like you can profile just about any substrate, but it is still important to make sure you are maintaining a consistent light source.
• When using existing profiles from a 3rd party, verify the print speed and lamp temperature.

White Ink as a Spot Channel 

• If you are using spot channels you must use a CMYKW ink configuration. This allows for the additional spot channels to separate properly when images are processed.
• In Media Manager > Printer Capabilities you will want to enable this ink configuration so it will display in the wizard.
• The processing options remain CMYK even when you use a spot ink because the spot channel is not used in the ICC build process and does not effect the color output. 
• When setting up the media click > Spot Layer to add the spot channel before setting mode options.
  • See the white paper titled: "Specialty Ink Tools" for complete details.
  • See the Calibration and Ink Limit information below for additional tips on profiling white ink.

Ink Restrictions

The next thing to consider is ink restrictions. Because the inks are "instantly" cured, it is hard to determine where the optimal percentage of ink is achieved. Most users think "Well, if the ink isn't bleeding I can keep my ink restrictions at 100%". In theory this may be true, but if we take a moment to think about that is happening on the substrate, 100% ink may not be the answer. Think of the ink drop as a "ball" when it hits the media, when the lamp cures the ink the ball remains on the surface hardening while adhering to the media. At 100% the ink drop will seem to adhere when in actuality only about 80% of the ink drop has properly hardened.

Although "Ink Restricting" isn't usually needed because of bleeding or puddling, be aware that excessive ink can cause adhesion problems when it is cured (or not because more ink was applied than could be thoroughly cured during the print process.)  Even if there are no adhesion problems, some cured inks can give a rough appearance if too much ink is used.  This can result in poor linearization readings too.

• How can we test for this? Think about how durable this print needs to be in the real world and consider doing an "Abrasion or Scratch Test".
• Feel the texture, if you can remove the ink from the surface of the media by simply scratching the surface with the back of you nail, you know the ink is not cured properly at that percentage.
• Consider if you are printing on foam board, take a scrap piece and "rub" it on the surface. This simulates multiple sheets stacked together when shipped and handled by an installation crew.
• The main purpose of ink restrictions is to remove wasted ink while keeping each ink channel at an optimal primary saturation. For achieving maximum gamut, this means the highest percentage swatch that looks "good".
• If you decide to leave the ink restriction at 100% because you need the coverage, make sure you pay close attention to the Ink Limit step.

Note: When you add volume with UV ink it can cause Reflectance which makes the patch seem like it is getting darker then all of a sudden gets lighter.

Setting Light Inks

• If your UV printer is CMYKcm, it's best to determine your dark ink levels first by using Advanced Ink Restrictions and setting your darkest level to 100% and all other levels to 0%.
• Print the "Ink Resrtiction Swatch" and set your dark values based on the method above.
• Next, enter in your light ink values. Determine this percentage by taking 30-50% of your dark ink values.
• You can use the "Set Light Ink Levels" Macro button to do this automatically (unless your printer uses double-stirke or multiple levels).
• Load the "Ink Restriction Verification" swatch and reprint to verify your settings. Repeat the "Abrasion or Scratch Test" on your new 100% patch and analyze the test swatches in the upper left-hand corner for excessive bleed.

Reduce the percentage values as needed if the ink on the surface comes off easily. Ink Restrictions is the most important step in creating a profile because it create the baseline for everything to come. Spend the necessary time to set your Ink Restrictions correctly. Proper adhesion and coverage is needed for a good calibration and final ICC. Also keep in mind that lack of coverage can cause grainyness.

Calibration

• When setting up swatches for linearization, reduce the number of patches from the default 31 to 24 or 19.
• Click > Setup> Options to change the number of swatches. If you need more information, see the the profiling workflow step-by-step section titled: Calibration - Linearization.

This gives fewer readings, and can eliminate spikes that are not really the fault of the ink, but rather are caused by readings that are inaccurate because of the way the spherical ink dots cause more than usual scatter.  This can be compensated for in the linearization options, but most users find it easier to do fewer patches and not worry about the options sliders.

• Also set the measurement device to (2) readings per patch, when you lower the number of patches. This will help a lot and give much better readings.

Usually, a more linear media model gives better results than what you may get with the normal "default" media model. Again, this is a suggestion, not a cast-iron rule. 

• Use the Basic Density Curve media model and set an N-factor of 2.5 as a starting point, if you are trying to create a maximum gamut profile. This smooths out the mid-tone curve for linearity.
• The Advanced Grayscale media model is less than ideal and only recommended if you are using a non-ICC workflow.  If no ICC will be used then you will need adjust the media model to determine the most appropriate target density.
• White Ink Calibration -If you are profiling UV printer with a white ink spot channel, you will use the black ink data as a place holder for the white ink channel. You can hand enter the data, or if you are using an Eye-One you can set the number of patches to 17 (as described above) which gives you "one line" per ink channel. Then when you are prompted to enter in the spot data, re-read in the black line.
• Build the linearization and media model as suggested above.

Ink Limits

As with ink restricting, pay attention to ink limiting as well.  

• If no color change is observed beyond 3.0, then go ahead and set the lower ink limit. 
• Watch to make sure that single K (pure black) is achieving a suitable darkness without going too far and causing a "dulling" appearance.  Use Black Ink Compensation to correct for this if your gradients turn gray before transitioning into black. By the same token, don't reduce the gray/black ink limit too low.  For the gray/black ink limit, if you need to go below 3.0, you should start over with the ink restriction process.
• White Ink Ink Limits -  If you are having issues with white ink coverage, use Advanced Ink Limits and set row (7) to 5.0.

ICC

• You will get good results if you print the "default" number of swatches and build with the "default" build options. You may print more swatches or change the build options as desired.
• If you are using a 6 color printer you may wish to change the default ICC Build options to use the Predefined " CMYKcm" options. This will automatically set the Start Black values and GCR curves.
• Another option you may wish to edit is the Gamut Mapping. Change it from "Classic" to " More Lightness Percerving".
• It's recommended to use spectral readings rather than L*a*b* colorimetric readings if possible when creating ICC profiles.
  • See the white paper titled: "ONYX ICC Build Options" for complete details.

Printer Specific Observations

• Nur Expedio -it is possible in the printer control software (the software running on the pc inside the printer) to set the K channel (as all the other channels) to 120% maximum. This is (of course) overriding the ink restriction setting in Onyx. When the Expedio is set to 100% (the default setting) the black will not be dark enough to create a proper profile. Setting the K to 120% is solving this problem.

• Vutek UltraVU 8C - you can use a MaxK GCR setting. K is used then from 0 to 100%. The 8C mode also includes light K. To create even smoother results you might use a GCR3 setting, the problem however will be the inaccurate grey balance, this has to do with the balance of the Vutek inks which is quite green. To eliminate the improper gray balance you have to create neutral gray by using the ink restrictions.

• Oce Arizona 250 GT/Fuji Acuity - This printer has a smaller dot size which allows you to set the Start Black value sooner for both your Achromatic and Chromatic Black Generation. This will help maintain neutral colors. During the Ink Restrictions step pay careful attention to the multiple levels of ink for optimal saturation and coverage.
  • For some media there is no need to set ink restrictions to anything but 100% for all levels except 2 because the curing is so good on these medias.  The only reason to restrict ink is to make sure the ink cures sufficiently to get good adhesion / durability.
  • Profiling is like any other printer except that the levels are actually increasing dot sizes.  According to ODGS level 2 should always be left at zero.  The other levels are set in increasing values to get more ink, for example:
    • C1 = 100
    • C2 = 0
    • C3 = 100
    • C4 = 100
    • C5 = 40
    • C6 = 0
    • C7 = 0
      This would be increasing the dot size / amount of ink to use levels 1 through 4 at 100 % [except level 2 is 0,] and then level 5 at 40% because you had determined that this was enough ink.