What flexo printing on corrugated boxes is, the designs it suits, and how to prepare artwork
Flexo printing on corrugated boxes uses a printing plate with a raised image, much like a rubber stamp. A roller inks the raised surface, and the plate transfers that ink directly onto the corrugated board. Logos and text print sharply, solid colors print evenly, and the process suits large production volumes.
Photos show the limit of the process. A photo prints with coarser halftone dots than the original file shows. Because the board surface is uneven and absorbs ink, regular corrugated boxes typically use a screen ruling of only about 45–65 lines per inch (lpi).
How a flexo press inks the board
The two parts worth knowing are the printing plate and the anilox roll.
Each color needs its own printing plate
A three-color design needs three printing plates, one for each color. Each plate is made of photopolymer, with the image raised above the plate floor. Only the raised areas take ink and touch the board.
A raised point prints at full ink strength or not at all. Light tints and gradients are made by breaking the area into halftone dots. Smaller dots make the area look lighter to the eye.
The anilox roll controls ink volume
The anilox roll controls how much ink reaches the plate. Rolls with larger cells carry more ink and suit dark solid areas. Rolls with smaller cells carry less ink and suit detailed work.
The roll is a metal cylinder whose surface carries a very large number of tiny cells. Ink fills the cells, a doctor blade scrapes off the excess, and the roll passes an even, metered amount of ink to the plate.
Logos, text and flat colors suit flexo best
Photos and full-panel solid colors can be printed with flexo, provided the design allows for the corrugated surface from the start. The designs that print best use flat colors with clean edges: logos, product names, text, handling symbols on shipping boxes and barcodes. The table shows how each type of design tends to print.
| Design type | Typical result on corrugated board | What to consider in the design |
|---|---|---|
| Logos, text, small-to-medium solid areas | Sharp edges and even color | Specify each color as a Pantone number |
| Photos and gradients | Coarse halftone; small dots grow as the paper absorbs ink (dot gain), so a 3% dot may print as 10% | Reduce detail, increase contrast between dark and light areas, or use line art |
| Full-panel solid color | Flute lines may show as alternating dark and light stripes | See the section on flute lines below |
| Small text, or reverse (knockout) text on a solid | Lines can break, or ink can spread and fill the counters (enclosed spaces) of letters | Keep to the minimum sizes in the artwork section |
| Light or pastel colors on brown liner | Colors look dull and shift, because the brown of the paper mixes with the ink | Use darker colors or choose a white liner |
The board shapes the printed result
Liner and paper change the color
On a white liner, printed color comes close to what the screen shows. On a brown kraft liner, every color looks duller. Buyers often notice this only when the first production boxes arrive.
The same ink and plates can also give different color and sharpness on different papers. Corrugated box printing uses water-based ink, which dries by soaking into the paper, helped by a dryer unit in the press. Each paper type absorbs ink differently.
Paper grades also differ in how well they take print, as covered in the guide to corrugated paper grades and grammage.
Full-panel solids can show flute lines
For a smooth full-panel solid, plan the board spec with the print in mind. Options include a shorter, denser flute or a thicker liner. The corrugated flute guide explains how E, B, C, BE and BC flutes differ.
Solids can show darker lines along the flutes. These stripes show more on tall, widely spaced flutes than on short, dense ones, and on thin liners. This effect, called fluting or striping, comes from uneven print pressure. Where a flute tip supports the liner, the liner gets more pressure than above a valley, so ink lays down heavier.
Sometimes the pattern is already in the board before printing. The adhesive that bonds the liner to the flutes shrinks as it dries and leaves small waves along the flutes. This is known as washboarding.
Printing can lower box compression strength
Packaging research states that some flute crush during printing and coating cannot be avoided, and that the amount of crush can be controlled. During printing, the plate presses onto the board and crushes some of the flutes. The finished box then has lower compression strength.
Pressure during printing damages the flutes slightly, and they do not recover their shape. A 2021 study in the journal Przegląd Papierniczy linked this to a lower edge crush test (ECT) value and lower bending stiffness in board printed with a pressure-based process.
A thesis from Rochester Institute of Technology reached a similar result with printed regular slotted containers (RSC). Boxes printed with thick plates had clearly lower compression strength than boxes printed with thin plates. Printing more colors with thick plates lowered compression strength further.
The manufacturer can factor the print effect into the box spec when it knows the print details. For jobs with full-panel solids in several colors around the box, share those details during the spec discussion, together with the product details and how the boxes will be shipped and stacked.
The guide to boxes breaking or crushing in transit explains how to read box compression strength (BCT).
What a print-ready flexo file includes
The figures in this section come from guidelines published by international corrugated box manufacturers, and actual values depend on the press. A print-ready flexo file has:
- vector artwork
- colors separated as spot colors
- the design placed on the die-line for the box
- an allowance for slight misregistration between colors
Vector artwork and Pantone colors
Supply artwork as AI or PDF with fonts embedded. A low-resolution raster logo breaks up at the edges when it is enlarged to box size.
Specify each color as a Pantone number. Each color removed from the design removes one printing plate.
Trap where colors meet
Where two colors meet, let one overlap the other slightly, so no gap shows if the colors shift. Designs where two colors butt exactly edge to edge carry the highest risk of a visible gap at the joint.
One manufacturer guideline recommends an overlap of about 1/8 inch (about 3 mm), because color position can shift by about 1/16 inch (about 1.6 mm).
Clearance from score lines and cut edges
On regular boxes, keep key graphics and text about 1/2 inch (about 13 mm) from score lines and cut edges.
Minimum type and line sizes
- Positive (inked) text: no smaller than about 8 pt.
- Reverse (knockout) text, left unprinted on a solid: larger than positive text, because the surrounding ink can spread and close the letters.
- Lines: no thinner than about 2 pt.
Barcodes and QR codes
Print codes dark on a light background. A study on printing QR codes on corrugated board found that the code and background need a color contrast of about 60% or more to scan on the first attempt.
Printed die-cut boxes need two tools
A printed box die-cut to a special shape needs both a printing plate and a cutting die. The guide to die-cut boxes and how they differ from an RSC explains what each tool does.
Start artwork once the die-line is final
If the box size may still change, wait for the final die-line before starting artwork. Every clearance from score lines and cut edges is measured from that layout, so graphics can be placed accurately only once the size is fixed. The box styles and print options available for custom production are listed on the custom corrugated boxes product page.
Frequently asked questions
What ink is used to print corrugated boxes?
Most flexo printing on corrugated board uses water-based ink, and its color can drift slightly lighter or darker within the same lot. During a run, the paper keeps drawing water out of the ink, so the ink concentration gradually changes. The ink dries by soaking into the paper, helped by the dryer unit in the press.
Can photos be printed on corrugated boxes with flexo?
Yes, although the halftone looks coarser than the image on screen. Images with fine, light gradients suffer most, because small dots gain until areas meant to be light turn dark. Photos usually need file adjustments first: more contrast between dark and light areas, fewer small details, or conversion to a single-color image.
How can printed box color come close to the color on screen?
Specify the color as a Pantone number or send a physical color sample. Then check it on the same paper type used for production. A screen shows color with light, while box color comes from ink absorbed into paper that has its own color. Paper from different mills or lots can give different color with the same ink.
Why should graphics not run across score lines?
Graphics that cross a score line are likely to break or look misaligned, especially small text and thin lines. At the score, the board is pressed into a groove, so ink does not lay down fully. When the box is folded, the liner surface there stretches and may crack.
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Auto Boxes Technical Service Team
