GMI Ink & Substrate Interaction 2 — Questions and Answers
Question 1: How does increased substrate moisture content typically affect ink drying time in heatset web offset printing?
- It accelerates drying by increasing ink solvent activity
- It extends drying time because moisture competes with solvent evaporation in the dryer (Correct answer)
- It has no effect because heatset drying relies solely on temperature
- It causes faster ink setting but slower final drying
Correct answer: It extends drying time because moisture competes with solvent evaporation in the dryer
Higher substrate moisture content increases the energy load on the dryer and interferes with solvent evaporation, extending the time required for complete ink drying.
Question 2: When printing UV-curable inks on non-porous substrates such as plastics, which property is most critical for ink adhesion?
- Ink viscosity matching substrate porosity
- Substrate surface energy being sufficient to promote wetting (Correct answer)
- Ink tack being lower than the substrate tensile strength
- Substrate caliper being within press specification tolerances
Correct answer: Substrate surface energy being sufficient to promote wetting
UV inks on non-porous substrates rely entirely on surface energy for wetting and adhesion since there is no absorption mechanism; insufficient surface energy leads to adhesion failure.
Question 3: Dot gain on an uncoated substrate compared to a coated substrate of the same nominal stock weight is generally:
- Lower, because uncoated paper has higher surface energy
- Higher, because ink spreads more in the porous uncoated surface (Correct answer)
- Equal, since dot gain depends solely on press pressure
- Lower, because uncoated paper absorbs ink faster, fixing it in place
Correct answer: Higher, because ink spreads more in the porous uncoated surface
Uncoated substrates have open porous surfaces that allow ink to spread laterally as it penetrates, significantly increasing physical and optical dot gain compared to coated stocks.
Question 4: In printing, 'show-through' and 'strike-through' differ in that:
- Show-through is caused by UV ink, while strike-through is caused by solvent ink
- Show-through is the visibility of printing from the reverse side, while strike-through is complete ink penetration through the substrate (Correct answer)
- Show-through occurs only on coated stocks, while strike-through occurs only on uncoated stocks
- Show-through is a quality standard, while strike-through is a defect classification
Correct answer: Show-through is the visibility of printing from the reverse side, while strike-through is complete ink penetration through the substrate
Show-through is the optical phenomenon of seeing print from the opposite side due to low substrate opacity, while strike-through is actual physical penetration of the ink through the substrate.
Question 5: Which ink sequence principle is applied in multicolor offset printing to optimize wet trapping?
- Print the highest tack ink last to maximize adhesion to dry ink layers
- Print inks in decreasing tack order so each subsequent ink has lower tack than the previous (Correct answer)
- Print inks in increasing tack order to progressively build adhesion
- Tack order does not affect wet trapping; only ink viscosity matters
Correct answer: Print inks in decreasing tack order so each subsequent ink has lower tack than the previous
Printing in decreasing tack order ensures each subsequent wet ink transfers properly to the previous ink layer, as lower-tack ink can adhere to higher-tack ink without splitting issues.
Question 6: Static electricity in substrates during printing is most problematic because it:
- Causes ink to repel from the substrate surface, reducing ink lay
- Causes sheets to stick together and misregister, disrupting sheet travel and feeding (Correct answer)
- Increases ink tack, making ink harder to distribute on rollers
- Prevents proper ink drying by insulating the substrate surface
Correct answer: Causes sheets to stick together and misregister, disrupting sheet travel and feeding
Static charge causes substrates to cling to each other or to press components, leading to feeding failures, double sheets, and registration problems during printing.
Question 7: Paper whiteness and paper brightness are different measurements because:
- Whiteness measures reflectance at 457nm only, while brightness includes UV fluorescence effects across the visible spectrum
- Brightness measures the spectral uniformity across the visible range, while whiteness only measures a single UV wavelength (Correct answer)
- They are equivalent measurements using different instrument calibration standards
- Whiteness is measured with a densitometer while brightness requires a spectrophotometer
Correct answer: Brightness measures the spectral uniformity across the visible range, while whiteness only measures a single UV wavelength
Brightness (ISO 2470) measures blue-light reflectance at 457nm and includes fluorescent whitening agent response, while whiteness (CIE whiteness) evaluates spectral uniformity across the full visible spectrum.
GMI Ink & Substrate Interaction 2
How does increased substrate moisture content typically affect ink drying time in heatset web offset printing?