When Is Dot Gain a Defect Rather Than a Curve?
If the dots across the whole plate and all colors are one step larger than target, and this runs smoothly from highlights to shadows, that is a curve or ink amount, not a defect. Only when, on the same sheet, just one color or one patch, or 50% suddenly measures more than 4 percentage points larger than the signed-off proof while other tones do not shift along with it, should you investigate it as a defect; do not change the plate curve.
Dots across the entire plate and in every color are one step larger than target, and from highlights to shadows they are smooth. This is a curve or ink-volume issue, not a defect. Only on the same sheet, if it is only one color or one patch, or if the 50% is suddenly more than 4 percentage points larger than the signed-off proof while the other tones do not move with it, should you investigate it as a defect; do not change the plate curve.
The boundary between curve and defect
Plate curves are used to compensate for stable dot gain. If the 25%, 50%, and 75% of each color—cyan, magenta, yellow, and black—are all higher than target and by similar amounts, this is normal gain for this machine and this pressure setup. It should go into the curve, or first bring the solid density back to the signed-off proof. At this point, washing the plate or looking for dirty spots will find nothing.
A defect is the segment that is not smooth. The 50% is larger, while the 25% and 75% are still close to the signed-off proof; or only magenta is larger while the other colors are close to the signed-off proof; or only the right side of the layout is larger. If this kind of gain still appears in the same position after changing to another plate, it is not a machine-wide condition that a curve can represent. 4 percentage points is the shop-floor warning threshold used to decide “whether to stop and investigate,” not a substitute for the dot tolerance in the contract.
Should It Go Into the Curve or Should You Stop the Press?
| Pattern of gain | Classification | Action |
|---|---|---|
| All colors and all tones high together | Curve or overall ink volume | Match the signed-off proof; if necessary, adjust the curve during idle time |
| Only the 50% jumps out | Defect | Check that color’s pressure, slur, and blanket |
| Only one side of the layout is high | Defect | Check the pressure and packing on that side |
First Actions for This Round
- Read at least three points: 25%, 50%, and 75%; do not read only one dot.
- If all three are high together, match the density to the signed-off proof; do not change the file.
- Only if one point jumps out by more than about 4 percentage points, stop the press and check the pressure at that point.
