How to Prepare Artwork for Custom Packaging: Dielines, Bleed and Colour

1. Three files, not one

The most common delay in a packaging order is not production. It is a single flattened JPEG that contains the artwork, the dieline and a photo of a previous bag, sent as "the design". A factory cannot separate a JPEG into print plates, cannot move it onto a different dieline, and cannot tell which line is a cut and which is a crease.

What prepress actually needs:

• The dieline — we supply it once the size and construction are agreed. It carries every cut, crease and glue area, at 1:1.
• The artwork — built on top of that dieline, with bleed, in CMYK or spot colours.
• A reference — a photo of a bag you like, Pantone numbers for brand colours, or a physical sample. This is what settles arguments about "the green looked different on screen".

2. Dielines: cut, crease and glue

A dieline is the flat map of the bag: the outline of the blank, the fold lines, the glue flap and the handle position. On a bag it also defines the gusset folds and the base, which is where most structural mistakes happen.

Two rules that save a week: never scale a dieline (build the artwork at 1:1 or the die will not match), and keep the dieline on a separate layer with print switched off. If you are reusing a dieline from another supplier, send it before you design — bag templates differ by a few millimetres at the base, and those millimetres are the difference between a bag that stands up and one that bulges.

3. Bleed and safe margin

Bleed is 3 mm beyond every cut line. Paper is guillotined and die-cut to a tolerance of roughly ±1 mm, so any background colour or image that stops exactly at the cut line will show a white sliver on some pieces out of every thousand. Extend the background 3 mm past the line and the tolerance disappears into the trim.

Safe margin is 5 mm inside every cut and crease. Text, logos, barcodes and legal lines that sit closer than that risk being clipped, or folding over the crease. On a bag the danger zone is the top edge — where the paper is folded over the handle patch — and the base fold.

• Backgrounds and images: extend 3 mm past the cut.
• Text and logos: keep 5 mm inside the cut and the crease.
• Glue flap: keep completely free of print, varnish and lamination.

4. Colour: CMYK, Pantone and ink coverage

CMYK for images, spot for brand colour

Photographs and gradients belong in CMYK. A brand colour that must match across every reprint belongs in a Pantone spot colour, because a spot is mixed once and printed as a solid, while a four-colour build of the same green drifts with press conditions and substrate. On kraft paper the drift is large: the same CMYK green prints noticeably duller on brown kraft than on white art paper.

This is also a cost decision. Flexo uses one cylinder per colour, so a job printed in four spot colours needs four cylinders; a job printed CMYK needs four as well, but a fifth spot means a fifth cylinder. The relationship between colour count, cylinder cost and run length is covered in Flexo vs Offset vs Digital.

Total ink coverage

Total area coverage (TAC) is the sum of the four ink percentages in the darkest area of an image. Above roughly 300% on coated paper and 280% on uncoated or kraft, ink stops drying properly: it sets off onto the next sheet, scuffs in the bag machine and blocks in the stack. Convert rich blacks down before sending, and use 100% K only for small black text — a four-colour black on 8 pt type will look blurry.

White ink on kraft or dark stock

Printing white on brown kraft or black paper needs an opaque white base pass under the colour. That is an extra cylinder and an extra pass, so it is priced as an extra colour. It is also why a design that looks vivid on a white art paper proof can look muddy on kraft if the white base was not specified.

5. Resolution and file format

A 1000 × 667 px image placed on a 400 mm bag is about 60 dpi. It will look acceptable on a monitor at 30% zoom and visibly soft in print. Judge photographs at 100% zoom on screen at final size, not in a fit-to-window view.

6. Type, barcodes and legal marks

• Minimum type size. 6 pt on coated white paper, 8 pt on kraft. Kraft absorbs more ink and the counters in letters like e, a and 8 fill in — this is dot gain, and it is worse on uncoated stock.
• Reverse type. White text knocked out of a solid colour needs a heavier weight and at least one size larger, because the ink spreads into the letters.
• Barcodes. EAN-13 at no less than 80% magnification, printed in a single flat colour with at least 3.5 mm of clear space either side. A barcode built from four colours, or placed over a photograph, will fail a retail scanner test.
• Certification marks. FSC, recycled-content and composting logos are licensed marks. They are only applied when the material and the supply chain actually qualify, and the FSC mark carries a licence number that must be in the artwork.
• Origin marking. Many destinations require a country-of-origin statement on the item or its packaging. Settle the wording before the file is approved, not after.

7. Our eight-point preflight

This is the list our prepress team runs before a file goes to plate or cylinder. It takes about 20 minutes and prevents almost every reprint.

• Dieline present, 1:1, on its own non-printing spot layer.
• Bleed of 3 mm on every cut edge.
• Text and logos 5 mm inside cuts and creases.
• Glue flap free of print, varnish and lamination.
• All fonts outlined or embedded; no substitution warnings.
• Images at 300 dpi at final size; no RGB or web-sourced artwork left in place.
• Total ink coverage within 300% (coated) or 280% (kraft and uncoated).
• Spot colours named with a Pantone reference, and white base specified if printing on kraft or dark stock.

8. Five mistakes that cause a reprint

• Artwork built at 50% scale. The die is 1:1, so everything lands at half size. Fixing it means redoing the file and, if it was already plated, new cylinders.
• No bleed on a full-colour background. Shows up as white hairline edges on a small share of every run — enough to reject a shipment, not enough to notice on the proof.
• Rich black on small text. Registration drift turns the text into a coloured fringe. One black plate avoids the whole problem.
• Lamination over the glue flap. The most expensive mistake on the list, because it is not visible until the bags are filled and the bases open under load.
• Approving a screen proof as a colour match. A monitor is backlit and uncalibrated; kraft is neither. Where colour is critical, ask for a physical proof or a printed sample on the production paper.

The cost of each of these lands in the same place: plates, cylinders and a second make-ready. Those are the fixed costs that a quotation cannot absorb, which is why they are worth 20 minutes of checking. The rest of the quotation structure is in Custom Packaging Cost Breakdown.

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