Monsoon Curl and Set-Off on Maplitho Booklets
In mid-July, a small offset shop in Puri prints 2,000 copies of a 16-page booklet on 70 gsm cream maplitho. The pressroom relative humidity is 85% and the paper has been stored in the same room for a week. The job runs with a standard process black ink, fountain solution at pH 4.8 and conductivity 1,800 µS/cm, and a delivery stack of 1,000 sheets. Twelve hours after printing, the top sheets curl upward at the edges, and set-off is visible on the reverse of the top 100 sheets. To cut cost, the pressman had reduced spray powder by half and added 5% cobalt drier to the ink. The customer rejects the booklets because the text shows through and the covers do not lie flat. The shop has one week to recover the order and enough paper for only one more run.
1. Diagnose the likely causes of the curl, set-off, and show-through. Distinguish between paper, ink, and fountain-solution causes.
Read the worked answer
Curl: the paper has equilibrated at 85% RH for a week. Maplitho absorbs moisture, the edges swell, and the sheet no longer lies flat. Uneven drying after printing makes the top sheets curl upward at the edges. The 1,000-sheet stack traps moisture and solvent vapour.
Set-off: reducing spray powder by half removed the air gap between sheets, so wet ink transferred to the back of the sheet above. Adding 5% cobalt drier can skin the ink surface while the film below remains wet, which worsens blocking and set-off rather than curing the stack.
Show-through: 70 gsm maplitho is a thin, absorbent sheet. High conductivity at 1,800 µS/cm suggests contaminated fountain solution and excessive emulsification; the ink becomes waterlogged, penetrates the sheet, and reduces opacity. The acidic pH 4.8 is not extreme by itself, but combined with high conductivity it points to poor fountain control.
Conclusion: this is not one fault. The paper is too moist, the stack is too high, the spray powder is insufficient, the drier dose is excessive, and the fountain solution is contaminated.
2. Recommend corrections to paper handling, ink, fountain solution, and delivery. Be specific about what you would change before the reprint.
Read the worked answer
Paper: move the paper to a conditioned room at 45–55% RH for at least 24 hours before printing. Do not open the ream wrapper until the paper has reached room temperature. Print from a stack that has been fanned and aerated, and keep unused paper wrapped.
Fountain solution: mix fresh solution to the ink maker's pH and conductivity range. Discard the contaminated batch. Aim for the recommended pH, usually mildly acidic to neutral depending on the system, and check conductivity against a freshly mixed sample. Reduce water feed to the minimum that keeps the plate clean.
Ink: do not add 5% cobalt drier. Use a faster-setting ink suitable for uncoated stock, or add drier only at the maker's recommended dose. Reduce ink film slightly and check tack.
Delivery: reduce stack height to about 500 sheets, apply spray powder at the correct rate, and check the reverse of the top sheets after 30 minutes and again after 12 hours. Run a 50-sheet test and let it stand overnight before committing the full run.
3. The customer wants the order in one week and only enough paper remains for one more run. Decide whether to salvage or reprint, and explain the risk control you would use.
Read the worked answer
Decision: reprint, do not salvage. Set-off on the top 100 sheets might be trimmed or sorted, but the curl and show-through are not local defects. Curl affects the whole stack and the customer has already rejected flatness; show-through is a bulk paper-and-ink problem that cannot be corrected by trimming. With one week left, sorting 2,000 booklets is slower and less reliable than a controlled reprint.
Risk control: because there is paper for only one run, treat the first 50 sheets as a qualification test. Print, stack, and leave overnight under the proposed conditions. Measure curl, check set-off on the reverse, and check show-through against the approved sample. Only if the test passes should the remaining paper be run. If the test fails, stop and inform the customer rather than consuming the last paper. Record the fountain conductivity, pH, spray rate, and stack height for the reprint.