Every printing process has a resolution limit. On flexo, the plate is rubber-soft and the ink is delivered through an anilox roller. Both have a minimum cell size below which ink simply doesn't transfer. On screen printing, the mesh has gaps and a 0.1pt line falls between threads. On digital toner, a 0.1pt line is one or two pixels at imaging resolution, and a single missed pixel breaks the line.
The result on press is one of three outcomes: the line disappears entirely, the line prints intermittently (broken into dashes), or the line prints inconsistently (visible on some sheets, missing on others). All three are unacceptable for production.
| Press | Positive line | Reversed line |
|---|---|---|
| Digital (HP Indigo, Xeikon) | 0.25pt | 0.5pt |
| Offset lithography | 0.15pt | 0.25pt |
| Flexography | 0.5pt | 0.75pt |
| Screen printing | 1pt | 2pt |
| Foil stamp | 0.5pt | 1pt |
A "reversed" or "knockout" line is one where ink surrounds a thin gap. The gap itself is what has to hold. These are harder than positive lines because they fight ink spread on every side.
0pt strokes from Illustrator. A path with stroke weight set to 0pt renders at the device's minimum line width, which on a 2400 DPI plate is essentially nothing. Always set an explicit weight.
Imported logos. Vector logos pulled from the web or a low-res source often have hairline accents that looked fine at screen scale but vanish at print scale.
Table and chart borders. Spreadsheet exports default to 0.25pt rules. Going to a flexo press, every one of those will break.
Dieline guides. Some designers draw the dieline as a 0.1pt line so it doesn't visually compete with the artwork. This is fine if the dieline is on a non-printing layer, but if it's flagged as a print spot color, it'll be sent to plate and then disappear.
In Illustrator: select the path → Window → Stroke → set weight to a press-safe value. For very thin decorative elements, expand the stroke to a filled shape (Object → Path → Outline Stroke) so it can't be reduced further by RIP scaling.
For type, use a heavier weight or larger size. Reversed type below 7pt at a thin weight will break on flexo regardless of the technical stroke threshold. The counters of letters fill in with ink.
For dieline guides, set the stroke to a non-printing layer or a clearly named structural spot color (Dieline, CutContour) and confirm the press treats it as a guide, not a printed line.
Preflight scans every stroke in your PDF and flags any that fall below the press-safe threshold for your substrate.
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