Cloning Ink Lab to Press Doctor-Bladed Hand Proofer Correlation
With Laser Engraved Anilox
By Bill Poulson, Harper Corporation of America

Ink technicians know that as helpful as the conventional hand proofer can be, it is not a perfect solution for ink-matching. The conventional hand proofer has a mechanically engraved anilox with a rubber transfer roller, and the metering of the rubber roll has the same inconsistencies that it did years ago when the flexographic industry used rubber metering rolls on press. Technique, pressure, speed of drawdown, durometer and diameter of the transfer roll and anilox roll are all variables that the ink technician must take into account when using the conventional proofer.

When doctor blades were introduced to the flexographic printing industry, they changed the future of flexography. The same modifications to the roll on a hand proofer will have the same effects. The doctor-bladed hand proofer in this article uses a 45 durometer transfer roller to lay down the ink. It is equipped with a laser-engraved anilox roll, thus the proofer simulates the same conditions as the pressroom environment. The remaining variable is the ink deposit rate between press anilox rolls and the doctor-bladed hand proofer.

As of the date of this writing, there is no data showing correlation between hand proofers and press anilox volumes. It is well known that ink deposit rates are different between a chrome anilox and a laser-engraved ceramic anilox. Why would you use a chrome anilox to match or correlate to your ceramic anilox in press?

The purpose of the testing was to demonstrate the correlation between ink film thickness in press anilox volumes and doctor-bladed hand proofer anilox volumes. This allows the user of a doctor bladed hand proofer to choose the proper volumes required to match the press anilox roll volumes used in the pressroom. Laser-engraved ceramic anilox rolls are more precise than chrome anilox rolls. The ink release characteristics match that of laser engraved ceramic anilox rolls in the press.

Standardized Inventory

It is critical that the converter has a standardized pressroom anilox inventory. This means, have a systematic and methodical approach to specifying anilox rolls for every job that goes on press.

Below is a suggested anilox inventory. Volumes can vary based on substrates being used. Every substrate should be standardized.

• Wide web: 2.4 bcm for Process Print, 3.5 bcm for Combo screen, 4.0 bcm for spot colors, 4.8 bcm for heavy solids.
• Narrow web: 1.7bcm for Process Print, 2.2 bcm for fine line, 2.8 bcm for combo screen, 3.5 bcm for spot colors.

The tests listed in this article were conducted at a wide-web printer using a fast dry solvent ink system. Since the inks and substrate were the same on the press and for the proofer, a densitometer was chosen to measure the draw downs and print instead of a spectrophotometer. It was determined that the ink film matched when the densitometer gave the same readings on both samples. All proofer samples were created on the same substrate as the press sample, and all press pulls were run on popular wide-web presses. The draw downs were created with the doctor bladed hand proofer using ceramic laser engraved rollers. Results may vary from converter to converter.

Certify the Anilox BCM

A roll audit was conducted on all of the press rolls for all the presses that were to be tested. This helped to determine the exact volumes of the press rolls. If you can, use new rolls to ensure an exact bcm and print quality.

Various line screens and volumes of choice can be used in the proofer. The proofer rolls were measured using the same roll audit equipment as the press rolls. A wide range of anilox proofer rolls with various volumes were available to choose from that correlated to the press bcm's.

Sampling Procedure

A variety of PMS and customer-approved colors, as well as the four process colors, were run on press, and all press samples were taken from supervisor-approved pressruns. Printed samples were taken from the press along with wet samples of ink. The wet samples were taken to the ink lab where draw downs were made with the doctor-bladed hand proofer. Density readings were taken to compare ink film thickness on the substrate. All data was documented.

With this set of components, it was found early on that compensations in volume will be needed to match volumes between press anilox rolls and proofer anilox rolls. This is due to the dry rate of the ink system being tested. This is not necessarily the case with all ink systems.

Results

As indicated in the first table, there was an average variation of 7 percent.

The variation in density is due to the difference in transfer rate from press to proofer and some slight variations in volume matches (which will also be addressed in the second group of test results). Once the volumes are fine-tuned to compensate for transfer rate, the density numbers should fall into place.

There also was a curve apparent in volume compensation. A 10-percent volume increase was needed from 200-300 line screens, and a 15-percent volume increase from 400-600 line screens. Line counts of 660-1,000 need an increase of 30 percent in volume. These numbers are based on the components used in this study. Again the dry rate of the ink system being used was the main factor. These results vary with different ink systems. Although the doctor-bladed hand proofer gives a consistent, better quality drawdown compared to conventional proofers, there are still allowances that need to be made in order to get an exact correlation between press and proofer. Speed of the drawdown and impression is not as critical with the doctor-bladed hand proofer because the doctor blade feature and the laser-engraved ceramic anilox apply an even ink film to the rubber transfer roller.

This eliminates a major variable that occurs with a conventional hand proofer. The doctor-bladed hand proofer offers consistency when proofing no matter who does the drawdowns, or how much ink is applied to the proofer. Throughout the study multiple samples were taken to prove the doctor-bladed hand proofer's consistency. All readings were consistent within +/-.02 on the densitometer.

Four variables are significant in causing inconsistencies in ink proofing:

1. Diameter of the proofer anilox roll versus diameter of the press anilox roll.
2. Rubber transfer roller versus polymer plates.
3. Press speed.
4. Drying speed of the ink. (For example, participants in this study noted that UV inks can be directly correlated to press by anilox volume. Again, drying speed of the ink is critical. UV inks wet out well with the doctor-bladed hand proofer and drying rate isn't a factor with UV inks. Though other substrates can be a factor, this study was tested on film substrates.)

Now that a benchmark for the deposit rate of the doctor-bladed hand proofer roll is determined, the compensation needed in volume for the next set of anilox rolls can be made. The follow-up testing should demonstrate an exact correlation to the press.

Final Correlation

The initial testing focused on determining the deposit rate when comparing proofer to press. This approach offered the data needed to fine-tune the doctor-bladed hand proofer anilox rolls so that they would match press-transferred volumes exactly. This follow up demonstrates that volume compensations can be made in any volume range, allowing an exact color match when proofed in the lab. This allows an ink technician to have matched proofer rolls at his disposal for proofing different volume ranges and creating accurate color matches. This is how to set the anilox standardization as well as set the standardization for the proofer rolls.

This correlation process demonstrates that as ink film gets thinner, the proofer volumes require a higher percentage of additional volume. This is partly due to the drying speed of ink.

Inks dry much faster at lower volumes. The results charted in Table 2 demonstrates that there is a curve in volume compensation:

There is a 10-percent volume increase needed for 200-360 line screens, a 15-percent volume increase for 400-600 line screens, and a 30-percent volume increase for 660-1,000 line counts. Once these compensations were calculated from the results of the initial study, a new set of proofer rolls with the adjusted volumes were produced for correlated ink matches.

Once the compensated anilox volumes were applied to the doctor-bladed hand proofer, the proofer density readings correlated with the press readings. All density readings were +.02 - .06. This proves that the doctor-bladed hand proofer is capable of transferring the amount of ink needed to get a correct color match. Based on the results of this testing, it is clear that the doctor-bladed hand proofer (with specific pre-determined proofer roll volumes) is a very efficient tool for the ink lab: When an ink match is needed, simply find the proper line count and volume to match the approved colors.

The doctor-bladed hand proofer also saves time when hosting customers for color approvals at press-side. Most importantly, on a day-to-day basis, the ink technician has the proper tool for matching color and making inks "press ready." This minimizes make-ready times for ink-matching. Printers are encouraged to try this comparison themselves. They will get the same positive results.

I have tested a variety of solvent ink systems and they all differ from each other. Do your homework when standardizing your color matching process and the results will be positive. Use the same solvent blend that is used on press when making ink for press. Simulate the press environment as much as possible.

Correlation Procedure

1. Have a standardized anilox inventory on all presses.
2. Start with a new anilox or an anilox that has a certified bcm.
3. Go to press and run the color needed.
4. Sign off on the color for final production.
5. Take a press pull or sample from the sign off.
6. Save a wet sample of ink from the color needed.
7. Go to lab and start with the same bcm in the doctor bladed proofer.
8. Do drawdown and qualify with your spectrodensitometer until you get the match you need.
9. Add or subtract bcm on the proofer anilox rolls if the matched bcm anilox does not qualify on the spectro reading.
10. Do this on the process volumes, line print volumes & the spot color volumes:
• Wide web: 2.4 bcm for Process Print, 3.5 bcm for Combo screen, 4.0 bcm for spot colors, 4.8 bcm for heavy solids.
• Narrow web: 1.7 bcm for Process Print, 2.2 bcm for fi ne line, 2.8 bcm for combo screen, 3.5 bcm for spot colors.

These are just two examples of the volume range that may be needed. The volume range may vary due to substrate that is used. Once this is done you will also be starting the standardization process for your anilox rolls in press as well.

Water-based Notes

Although it is not documented in this report, water-based inks transfer at a different deposit rate. It is important to keep in mind that matching line count to the press sample is not important.

The main concern is to lay down the same ink fi lm thickness or volume (bcm) as the press samples. The manufacturer's representative for your proofer should demonstrate and assist with choosing the proper ink volume. Results will vary depending on the substrate and ink formulation.

UV Notes

UV inks correlate exactly with this proofing system. This is due to the fact that UV inks are 100-percent solids and there is no dry rate. The ink wets outs exactly as it would on press. It is not critical that the proofer spec for line screen is a perfect match. BCM is the critical factor when using the UV inks systems.

Coating Application Notes

The proofer anilox rolls have a volume capability up to 22 bcm. The proofer can be an excellent tool for benchmarking coating applications. If the printer has an array of anilox rolls with a variety of bcm's it can match any pounds per ream requirement and make anilox roll recommendations accurately. I use this tool all the time for coating applications. Having an array of volumes in your proofer inventory will give the capability to test for any coat weight requirement needed up to a 22 bcm. This type of proofer allows you to benchmark any application that arises.

Summary

The conventional hand proofer has a mechanically engraved anilox which is limited to the volumes available as compared to a laser engraved ceramic proofer roll. The release characteristics are much different than a ceramic roller. This proofer is obsolete to today's standards. This makes it less flexible and volumes cannot be customized. Ink technicians have worked with the conventional proofer for years with much success, but technique is the key and this makes it a moving target. The transfer rate on the mechanically engraved chrome anilox roll is different than the laser-engraved ceramic anilox roll.

Correlation is impossible. Since most all press rolls used today are laser engraved ceramic rolls, it makes more sense to use the same rolls in the press and proofer.

As shown by the documented results, the doctor-bladed hand proofer matched the press samples, with the density variation numbers within an acceptable quality control range after we studied and understood the dynamics that was taking place as related to drying rate and solvent inks.

The combination of using a proofer that meters with a doctor blade and being equipped with laser engraved rolls that are made of the same material and engraved using the same method as the press rolls, gave us a tool that produced a repeatable and minimally variable result. The ability to dial in the proofer rolls, using the results from the initial test has allowed us to create a predictable environment for color matching to press.

Care & Maintenance

It is always critical to have control of the anilox investment. Have the tools needed to properly inspect anilox rolls coming off of press to re-qualify them as press ready for the next time they are needed on press. Proper handling and care of the anilox rolls on a daily basis is a major part of the success that I see in flexo environments that are successful when matching color on press.

Coordination and a systematic approach to have anilox rolls staged and press ready is paramount to the success of color matching to press as well. If the rolls are plugged with ink, then you are giving your ink lab a moving target. This makes it impossible for ink room personnel to be successful. The time invested in the anilox area will pay itself off at the end of the day.

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