How flexo and gravure differ from offset · the key design implications
Offset printing transfers ink from a flat plate to a rubber blanket and then to paper under controlled impression pressure. The process is extremely precise, dot reproduction at 175 LPI with ±0.05mm register is standard. Flexo and gravure operate differently and have different inherent characteristics that affect what artwork can and cannot be reproduced reliably.
Flexo prints from a raised relief plate (polymer or rubber) using a metered anilox roller to deliver controlled ink to the plate surface. The plate surface contacts the substrate directly. This direct contact means that plate pressure, plate hardness, and ink viscosity all affect dot reproduction, making flexo inherently more variable than offset at fine detail levels. Gravure prints from recessed cells in a chrome-plated copper cylinder, with ink wiped from the surface by a doctor blade and transferred to the substrate under impression pressure. Gravure is extremely consistent on long runs but has a characteristic highlight limitation, the smallest cells that can be reliably engraved and printed.
Flexo artwork specifications · the complete reference
| Parameter | Standard flexo | High-quality CI flexo | Notes |
|---|---|---|---|
| Minimum positive line weight | 0.4mm | 0.3mm | Lines below these weights print unevenly, varying from light to heavy as plate pressure varies. For keylines and rules, never go below the specified minimum. |
| Minimum reversed line width | 0.6mm | 0.4mm | Reversed lines (white lines on dark) need to be wider than positive lines because ink spread encroaches from both sides of the line. |
| Minimum positive text size | 8pt | 6pt | Fine serif fonts need 2pt larger than the minimums above. Use sans-serif fonts for any text approaching minimum sizes. |
| Minimum reversed text size | 10pt | 8pt | Reversed text in flexo is very sensitive to ink spread. Bold weight only at minimum sizes. |
| Trap value | 0.3–0.5mm | 0.2–0.3mm | Flexo register varies more than offset, trap values must be larger. Confirm with the converter's stated register tolerance and set trap to ≥50% of the register tolerance value. |
| Minimum isolated dot | 3mm diameter | 2mm diameter | Small isolated dots (registration marks, fine details) may print poorly in flexo. Avoid design elements smaller than these minimums. |
| Screen ruling | 100–120 LPI | 133–150 LPI | Confirm the press room's maximum screen ruling. Specifying 175 LPI for a flexo job that runs at 120 LPI produces suboptimal halftone reproduction. |
Flexo tonal range · highlights, shadows, and the bridging problem
The most significant artwork adjustment required for flexo is the tonal range. Flexo cannot print the very lightest highlight dots that offset can, the smallest dots that can be reliably produced on a flexo plate and transferred to the substrate define the minimum highlight. Below this minimum, individual dots may print intermittently, creating a speckled, uneven appearance in highlight areas.
Minimum highlight dot · the critical flexo artwork rule
Standard flexo on film with analogue plates: minimum highlight dot 5%. Modern CI flexo with HD digital plates: minimum highlight dot 3%. Any tonal value below the minimum will print as either paper white (completely absent) or as a speckled, uneven tone. In gradients that run from a solid to a highlight, the minimum dot value means the gradient cannot actually reach white, it fades to the minimum dot value and then abruptly drops to paper white. This creates a visible "jump" in the gradient at the minimum dot value.
To prevent this visual jump, gradients in flexo artwork must be designed to end at the minimum dot value rather than fading to zero. Specifically: adjust any gradient that ends at 0% to end at the minimum dot value (3% or 5% depending on the process). This eliminates the abrupt jump to paper white. This adjustment is made in the artwork, not in the press room. The pre-press step that adjusts gradients to the correct minimum highlight is called "highlight clipping" or "dot range limiting."
Shadow dot maximum
Flexo also has a shadow limitation, at very high dot percentages (95–100%), the dots begin to merge together (fill in) and shadow areas lose detail. Shadow clipping to approximately 95% maximum is appropriate for most flexo packaging applications. Confirm the specific shadow limit with the converter.
Flexo register · designing for wider colour-to-colour tolerance
Flexo register tolerance (±0.2–0.4mm depending on the press) is significantly wider than offset (±0.1mm). This wider tolerance has specific design implications that cannot be addressed after the artwork is finalised, they must be built into the design from the start.
Design rules for flexo register tolerance
- Trapping: all adjacent colours must be trapped to ≥0.3mm. Trapping in flexo is not a pre-press afterthought, it must be considered in the artwork design itself. Where two saturated brand colours meet at a hard edge, a visible misregister gap is unacceptable. Design the colour boundaries to accommodate trap: either build a small overlap zone, use a knockout with designed trapping built in, or use a colour transition (gradient) at the boundary that naturally accommodates misregister.
- Multi-colour fine text: text built from two or more process colours (e.g., dark blue text built from C90 M50) will show colour fringes when any colour is out of register. On flexo, this means any text built from more than one ink that is below approximately 14pt should be rebuilt as a single-ink colour. Knock out all other inks completely under such text.
- Tight registration design elements: very fine design elements that require tight register between two colours, a 0.2mm outline on a detailed logo, for example, should be reconsidered for flexo. If the element cannot be reproduced within the process register tolerance, it must be redesigned for the process, not force-fitted.
Gravure artwork specifications
| Parameter | Specification | Notes |
|---|---|---|
| Minimum positive line weight | 0.2mm | Better than flexo because gravure cells are engraved precisely. However, very fine lines are subject to "missing dot" skipping in highlights, lines at exactly the minimum may print intermittently. |
| Minimum reversed line width | 0.3mm | Reversed lines in gravure can be very fine if the press is well-maintained. But confirm with the converter for the specific substrate and press configuration. |
| Minimum text size (positive) | 6pt sans-serif | Gravure holds fine positive text well on film substrates. The cell structure means serif fonts at very small sizes may have gaps in thin strokes, use sans-serif below 8pt. |
| Minimum text size (reversed) | 8pt | Similar to flexo, reversed small text is limited by the minimum cell size that can be reliably engraved. |
| Trap value | 0.2–0.3mm | Gravure register is tighter than flexo. Standard trap of 0.2mm is adequate for modern gravure. Some Indian gravure converters specify 0.15mm for their best presses. |
| Screen ruling | 60–80 L/cm (150–200 LPI equivalent) | Gravure uses a cell-based screen with lines per centimetre. Confirm with the converter. Modern electronic engraving at 70 L/cm produces excellent photographic reproduction. |
| Minimum highlight dot | 4–8% depending on cell depth and substrate | The minimum printable highlight in gravure is determined by the minimum cell depth that can be reliably engraved and still transfers ink. This varies significantly by converter. |
Gravure tonal characteristics · skipping and the gravure highlight
Gravure's most characteristic quality issue in highlights is "missing dot" or "skipping", individual cells in the lightest tonal areas fail to transfer ink to the substrate, producing a speckled, uneven appearance. This is the equivalent of flexo's minimum highlight dot limitation and requires the same artwork remedy: tonal values below the converter's stated minimum highlight should be clipped to paper white (0%) rather than left at very low percentages that will print speckled.
Unlike flexo, gravure also has a specific limitation related to the cell structure at very fine line widths, when a positive line is narrower than one or two cell pitches, the line may print as a row of individual dots rather than a continuous line. At 70 L/cm engraving, a cell pitch is approximately 0.14mm, lines approaching this width will be rendered as discrete dots. The solution is the same as for flexo: maintain minimum line widths at or above the specified minimum.
Gravure colour gamut
Gravure achieves a wide colour gamut on film substrates because solvent inks can be formulated with very high pigment concentrations, and the cell-based ink delivery system applies consistent, dense ink films. Premium gravure on BOPP or PET film with high-density inks produces richer, more saturated colours than standard offset on coated paper. This is one reason gravure remains the preferred process for premium Indian FMCG flexible packaging where photographic image quality and colour saturation are primary brand requirements.
Colour limitations · what both processes cannot reproduce
- Very light neutral tints: tints below 3–5% in any process colour are at or below the minimum dot. Specify paper white (0% of all inks) rather than a 1–2% tint that will print unpredictably.
- Smooth gradients to zero: gradients from a solid colour to paper white must be clipped at the minimum dot value (3–5%) to avoid the visible jump described above. This is a standard artwork adjustment for both flexo and gravure jobs.
- Hairline registration between saturated opposing colours: a hard boundary between cyan and magenta, or between red and green, that requires ±0.1mm register to appear clean will show coloured fringing in flexo. Either trap the boundary, use a transition zone, or accept that a visible colour fringe is part of the printed result.
- Very fine reversed type in multiple inks: reversed type in a multi-ink build below 10pt will show fringing from register variation. Convert to a single-ink version or increase the type size.
File delivery for flexo and gravure · what to send and how
- File format: PDF/X-4 for most flexo jobs. Some gravure converters prefer high-resolution TIFF or EPS for each colour separation, confirm with the specific converter before preparing files.
- Colour mode: CMYK with all spot colours correctly identified by Pantone number and with correct overprint/knockout settings.
- Trap: trapping should be applied in the artwork and confirmed in the PDF. Do not rely on the converter's pre-press to apply trapping, confirm in writing whether the converter will apply trapping or expects it in the file.
- Repeat length: for flexo and gravure, the print cylinder has a specific repeat circumference (the print repeat). The artwork must be sized to fit within the print repeat length. Confirm the available repeat sizes with the converter before finalising the artwork dimensions.
- Colour profiles: flexible packaging does not use ISOcoated_v2, the substrate (BOPP, PET, PE film) has different optical properties from coated paper. Gravure and flexo converters use press-characterisation profiles specific to their press, ink, and substrate combination. Request the converter's ICC output profile and use it for soft proofing the artwork before delivery.
Flexo and gravure artwork checklist
| Check | Flexo | Gravure |
|---|---|---|
| Minimum positive line weight | □ ≥0.3mm (CI flexo) or ≥0.4mm | □ ≥0.2mm |
| Minimum reversed line width | □ ≥0.4mm | □ ≥0.3mm |
| Minimum positive text | □ ≥6pt (CI) or ≥8pt, sans-serif preferred | □ ≥6pt sans-serif |
| Minimum reversed text | □ ≥8pt bold | □ ≥8pt bold |
| Trap applied | □ ≥0.3mm | □ ≥0.2mm |
| Highlight clipped to minimum dot | □ ≥3% (CI) or ≥5% | □ ≥4–8% (confirm with converter) |
| Shadow clipped to maximum dot | □ ≤95% | □ ≤96% |
| Multi-ink text above register limit checked | □ Multi-ink text ≥14pt | □ Multi-ink text ≥10pt |
| Artwork fits within print repeat | □ Confirmed with converter | □ Confirmed with converter |
| Converter ICC profile for soft proof | □ Requested and applied | □ Requested and applied |