ΔE and Density Variation on a Brand-Red Label
A label printer in Cuttack runs 10,000 copies of a coated-paper label in Pantone 185 C red. The customer's approval limit is ΔE ≤ 2.0 against the approved master. On the first press sheet, the left side reads magenta density 1.25 and ΔE 4.2; the right side reads magenta density 1.45 and ΔE 1.8. The plate step wedge shows a 50% dot printing as 58% on the left and 54% on the right. Fountain solution pH is 5.2, conductivity 1,200 µS/cm, and the ink is from a new batch. The pressman proposes to increase ink feed across the whole width to bring the left side up, and to pass the job because the average ΔE is 3.0. The customer will inspect a random sample of 20 labels and reject the lot if any label exceeds ΔE 2.0. The shop has 8,000 sheets of the same paper left and a two-day deadline.
1. Interpret the density, dot-gain, and ΔE readings. Is the sheet acceptable? Explain why the average ΔE is not a valid acceptance measure.
Read the worked answer
The right side is within tolerance: density 1.45, dot gain 54%, ΔE 1.8. The left side is outside tolerance: density 1.25, dot gain 58%, ΔE 4.2. The left side is lighter in ink film but has higher dot gain, which points to an ink-and-water imbalance, a worn or differently exposed plate, or uneven roller settings rather than a simple global ink shortage.
The sheet is not acceptable. The customer's rule is that any label above ΔE 2.0 rejects the lot. The average ΔE of 3.0 hides the fact that a large part of the sheet is at 4.2. Averaging is invalid because the customer samples individual labels, not the sheet average. The job must be corrected before acceptance.
2. Evaluate the pressman's proposal to increase ink feed across the whole width and to pass the job on average. What would you do instead?
Read the worked answer
The proposal is wrong on both counts. Increasing ink across the whole width will push the right side above its current density, increase dot gain, and likely take the right side outside ΔE 2.0. It may also cause set-off and poor drying. Passing on average is unacceptable because the customer rejects any single label above the limit.
Instead: stop the press. Adjust only the left-side ink keys and check the left dampening unit. Inspect the left plate for wear or exposure difference using the step wedge. Check roller pressures and ink batch viscosity. Mix fresh fountain solution to the maker's pH and conductivity range and confirm the new ink batch against the approved master. Run a 100-sheet test, measure at least five positions across the sheet, and accept only when every position is ≤ ΔE 2.0 with stable density.
3. Given only 8,000 sheets left, 10,000 labels required, and a two-day deadline, recommend an acceptance and recovery plan.
Read the worked answer
Do not pass the current sheet. First correct the press and qualify it on a 100-sheet test. Measure ΔE at left, centre, and right; if any position exceeds 2.0, continue adjusting rather than running production.
If the press is qualified, run the remaining 8,000 sheets and inspect against the limit. Because 8,000 sheets must yield 10,000 labels, confirm the layout and count before running; if the sheet layout cannot produce the required number, the shop is already short and must tell the customer immediately. Sort out any labels from the left side that exceed ΔE 2.0 rather than averaging them into the lot. If qualified production cannot reach 10,000 good labels, negotiate a reduced quantity, a deviation, or additional paper with the customer in writing. Do not ship a lot that can fail a random 20-label inspection.