GMI Troubleshooting & Problem Resolution 3 — Questions and Answers
Question 1: A gravure print of a fine-screen tint shows missing dots in highlight areas on every other copy. The most likely cause is:
- Worn or damaged doctor blade leaving residual ink in cells unevenly (Correct answer)
- Incorrect screen ruling for the substrate
- Ink temperature too high reducing viscosity
- Impression roller hardness too low
Correct answer: Worn or damaged doctor blade leaving residual ink in cells unevenly
A worn doctor blade can skip, leaving uneven ink in shallow highlight cells and causing every-other-copy dot skipping as the cylinder rotation pattern aligns.
Question 2: After replacing a printing plate, a pressman notices that register is off on the first unit only. The corrective action is to:
- Adjust the plate's lateral and circumferential register on that unit (Correct answer)
- Re-calibrate the registration marks on all plates
- Increase impression pressure on the first unit
- Remount the substrate roll tensioner
Correct answer: Adjust the plate's lateral and circumferential register on that unit
When only one unit is out of register after a plate change, the plate itself was mounted incorrectly and requires lateral/circumferential adjustment on that unit alone.
Question 3: A print shop receives customer artwork with text set in a light-weight sans-serif font at 6 pt. The most likely press-related problem is:
- Ink spread causing fine strokes to fill in and become unreadable (Correct answer)
- Insufficient ink density to render fine text
- Screen moiré appearing within the text strokes
- Color fringing from misregister between CMYK channels
Correct answer: Ink spread causing fine strokes to fill in and become unreadable
Very fine strokes in small text are highly susceptible to ink spread (dot gain), causing hairline strokes to fill in and become unreadable on press.
Question 4: When measuring a printed neutral gray patch, a spectrophotometer reads a* = +3 and b* = −4. This indicates the gray has a:
- Slight red-green cast toward green with a blue shift
- Slight magenta cast with a yellow shift
- Slight green cast with a blue shift (Correct answer)
- Neutral balance with minor noise
Correct answer: Slight green cast with a blue shift
In CIELAB, a positive a* value means red shift and negative means green; a negative b* means blue shift — so a*=+3 and b*=−4 represents a slight green-blue cast.
Question 5: A flexo press operator notices that solid ink density is acceptable but halftone midtones are printing too dark. The most targeted correction is to:
- Reduce plate impression pressure to lower mechanical dot gain (Correct answer)
- Decrease ink viscosity
- Increase press speed to reduce ink transfer time
- Replace the anilox roller with a higher cell volume
Correct answer: Reduce plate impression pressure to lower mechanical dot gain
Excessive mechanical dot gain in midtones without affecting solid density points to too much impression pressure spreading halftone dots.
Question 6: A batch of printed labels is returned because the bar codes fail to scan. The labels measure within density specification. What should be investigated?
- Bar width accuracy and print contrast signal (PCS) between bars and spaces (Correct answer)
- Ink absorption rate of the substrate
- Laminate gloss level over the bar code
- Total ink coverage in the bar code area
Correct answer: Bar width accuracy and print contrast signal (PCS) between bars and spaces
Scanners read bar codes based on the contrast and dimensional accuracy of bars and spaces; even spec-density bars fail if widths are wrong or PCS is below scanner threshold.
Question 7: A screen printing shop finds that a fine-mesh screen is breaking down prematurely during long runs. The most likely cause is:
- Abrasive pigment particles in the ink eroding the emulsion
- Squeegee angle set too acute
- Under-exposure of the photopolymer emulsion (Correct answer)
- Ink viscosity too low for the mesh count
Correct answer: Under-exposure of the photopolymer emulsion
Under-exposed emulsion has not fully cross-linked, making it mechanically weak and prone to breakdown under squeegee pressure during long production runs.
GMI Troubleshooting & Problem Resolution 3
A gravure print of a fine-screen tint shows missing dots in highlight areas on every other copy.
The most likely cause is: