Optimizing Relief Height: First-Place 2018 FFTA Rossini Scholarship Recipient’s Research on FIRST Relief Guidance

Press Conditions

This study was conducted on an OMET Varyflex press using Siegwerk UV black ink, a low-volume, banded anilox roll and MacTac BOPP (LYTE) label stock. The banded anilox roll contained six bands with volumes of 3.58 bcm, 2.85 bcm, 2.22 bcm, 1.75 bcm, 1.53 bcm and 1.03 bcm. The substrate was a top-coated BOPP label stock with a surface energy readout of 40 dyne. The press was run at an ambient condition of 70 degrees with 80 percent relative humidity.

After plate-to-anilox impression was set, plate-to-substrate impression was set and a sample was taken to ensure both ink density and dot area were consistent on both sides of the print. Once the necessary measurements were taken, the speed was increased to 250 fpm, and another sample was taken to check slur and overall print quality. After the second evaluation, six samples were taken from each relief tested, and three were measured multiple times to determine sample variance.

After kiss impression was complete, the plate-to-substrate impression was slowly increased to induce over-impression on the plate. Each plate’s impression was increased to the maximum amount the press would allow, and the floor still did not print. An unintended effect of using a template created for a banded anilox roll was the additional support provided by the bearer bars, which prevented the floor from printing.

A secondary run was conducted to test the stability of the plate with the absence of bearer bars. Although more print defects appeared during the second run, the floor of the plate did not print regardless of impression or relief.

Measurements & Calculations

Each of the three samples taken from the kiss and over-impressions was measured with a calibrated Techkon spectrodensitometer. First, the density of the solid patch was measured. Next, the dot area of each sample was measured, and the average of each was measured and listed for comparison. As expected, there was above-average dot gain measured due to the absence of a compensation curve.

An optical analysis showed compression in highlight dots and vignettes in the over-impressed samples across all reliefs, as well as a lack of formation in partial percentage dots across all reliefs. After the print quality was analyzed, a small portion of plate material was taken to the Clemson Electron Microscopy Lab for viewing under an S3400 Scanning Electron Microscope (SEM), which contained a tungsten filament source. Each sample was cleaned and sputter coated with approximately 20-μm. of platinum to provide a better surface for the electrons to reflect off of, which in turn provides a better image through the detection methods of the electron microscope.

Once prepared, each sample was placed in the SEM for viewing. The tools within the viewing program allowed for the measurement of the dot diameter and the shoulder angle of both sides of the dot. The measurement of both dot angles allows for the final resulting angle calculation to take dot variance into effect. There is no standard calculation for determining shoulder angle, and many other calculations do not take variation in the formation of dot geometry into account. The charts list the shoulder angles (a & b) as well as the resulting calculation (angle c), and the dot diameter, as displayed in Figure 2. The following formulas were used:

180-a=xa

180-b=xb

180-(xa+xb)=c

(180-c)/2= average shoulder angle

Tables 1-4 display the measured angles of sides a and b of two dots randomly selected from each of the tint patches, and the calculated resulting angle across the tested reliefs. As demonstrated in Figures 3a and 3b, the 0.013-in. plate had the widest resulting angle per relief, and the 50 percent dot had the widest average angle contributing to the formation of the dot. The associated photos show the dot formation at each relief at a microscopic level.

The change in anilox-to-plate pressure on press showed a slight variation in solid ink density (SID) across reliefs, as shown in Figure 4. This increase from 12.1 to 13.2 on both 0.009-in. and 0.012-in. relief plates did show a slight decrease in SID compared to the greater relief plates. However, this does not explain the change between the 0.018-in. relief plate and the 0.020-in. relief plate.

As hypothesized, the plates with decreased relief had less tonal value increase (TVI) in the highlight areas compared to those with greater relief. Figure 5 shows the difference in average TVI across all tested reliefs, taking into account both the kiss and over-impression samples. The 0.009-in. and 0.012-in. relief plates provided a more linear gain compared to the 0.018-in., 0.020-in. and 0.023-in. relief plates, which experienced a bit of a spike in TVI at the 1 percent dot. There is roughly a 4 percent decrease in highlight TVI with lower plate relief.