Printing on plastic bags is mostly flexography on corona-treated polyethylene, and most of its limits come from the substrate: a low-energy, stretchy, often translucent film. That is why a logo that looks sharp on a paper bag can come out soft, shifted or flaking on polythene when the artwork and press set-up ignore what film does on press. Below: how bags are printed, how ink adhesion is achieved and checked, how many colours are sensible, and how to set up artwork that survives conversion into a bag. For formats and ordering, start at the custom printed plastic bags hub.
How are plastic bags printed?
Most printed plastic bags are produced by flexography: flexible photopolymer plates carry fast-drying liquid ink onto the film web, one plate per colour. The film is printed as a flat or tubular web, either in line with extrusion or as a separate step, and then converted into bags by sealing, gusseting and punching.
The typical sequence:
- Blown film extrusion produces a tube of PE film to the specified width, thickness and colour.
- Corona treatment raises the surface energy on the side or sides to be printed.
- Flexo printing, commonly on a central impression (CI) press, where all colour stations print around one large drum for stable registration, or on a stack press, where stations are stacked vertically and both sides of the web can be printed in one pass.
- Drying between and after stations.
- Conversion: side or bottom seals, gussets, handle punching or welding, patch or loop application, perforations and packing.
Other processes exist but are less common for bags. Rotogravure is used for very long runs of high-quality film, more for laminated pouches than carrier bags. Digital printing on film is growing for short runs and prototypes but is less established for bag converting. Screen printing is used for small quantities of finished bags. How these compare across packaging types is covered in flexo vs offset vs gravure vs digital printing.
Why does polyethylene need corona treatment?
Untreated polyethylene has low surface energy, typically around 30-31 dyn/cm, so ink beads up and rubs off. Corona treatment oxidises the film surface with a high-voltage discharge, raising surface energy so ink wets and bonds. Printers typically aim for roughly 38-42 dyn/cm or higher, with water-based inks usually needing the upper end.
Points that matter for buyers:
- Treatment decays. Additives in the film (such as slip agents) migrate to the surface over time, and surface energy drops. Film treated at extrusion and stored for weeks may need re-treatment before printing.
- Both sides, if both are printed. Specify one-side or two-side print so the film is treated accordingly.
- Recycled film varies. PCR film can contain additives and contaminants that affect treatment and adhesion, so levels should be checked per batch.
- Over-treatment is possible and can cause blocking (film layers sticking together) or affect sealing.
How ink adhesion is checked
- Dyne test pens or test inks (for example per ISO 8296 or ASTM D2578) indicate the film's surface energy before printing.
- Tape test: pressure-sensitive tape is applied to the cured print and pulled off; ink removal indicates poor adhesion. ASTM F2252 describes a method commonly used for flexible packaging.
- Crinkle and rub tests show whether ink cracks when the film is flexed or scuffs in handling.
- Block test: stacks of printed film or bags are pressed together under heat to check that ink does not transfer or stick.
Ask your supplier which tests are used for release, and agree any special requirement, such as resistance to cold or condensation for chilled-goods bags, at technical review.
Solvent-based or water-based inks for plastic bags?
Both work on corona-treated PE, and the choice is usually the converter's, driven by its press and drying equipment. Solvent-based inks dry fast, wet low-energy film easily and give strong adhesion, but release volatile organic compounds that must be controlled. Water-based inks cut solvent emissions and odour but dry more slowly and usually need higher surface energy and careful press set-up on film. For a buyer, the practical question is whether the chosen system holds up on your film and end use, which the adhesion tests above answer.
| Factor | Solvent-based inks | Water-based inks |
|---|---|---|
| Adhesion on PE | Very good; tolerant of lower dyne levels | Good with higher treatment and right formulation |
| Drying speed | Fast | Slower; needs more drying capacity |
| Emissions and odour | VOCs; requires extraction, often incineration or recovery | Lower VOCs; low odour |
| Print quality | Excellent density and gloss | Very good; can be slightly less glossy |
| Typical use | Wide use in film printing | Growing use, especially where emissions or odour are a concern |
UV-curable flexo inks are a third option on some presses. For bags that may contact food, inks are printed on the outside and must not transfer to the food side through set-off in the reel or stack; this falls under good manufacturing practice in Regulation (EC) 2023/2006, and EuPIA guidance and the Swiss ink ordinance are common references. Germany's Printing Ink Ordinance (Druckfarbenverordnung) applies from 1 January 2027 to inks on food-contact materials. See food contact packaging compliance for the documents involved.
How many colours can be printed on a plastic bag?
Plastic bags are commonly printed in one to six spot colours per side, and more on some presses. Most bag designs use one to three colours, because each colour adds a plate, a press station and set-up waste, and fewer colours register more reliably on thin film.
| Design approach | What works well on film | Watch-outs |
|---|---|---|
| 1-2 spot colours | Bold logos, text, simple patterns | Very fine lines in thin film |
| 3-4 spot colours | Brand palettes, flat illustrations | Registration between adjacent colours; allow trapping |
| Process colour (CMYK) | Photographs, gradients on good film and presses | Dot gain, moiré, colour shift on tinted or recycled film; usually needs white underlay |
| Full flood colour | Strong brand impact | High ink usage, blocking risk, affects recyclate colour; can show pinholes |
Spot colours referenced to Pantone are the standard approach. Colour on film will not match a coated-paper swatch exactly, because the film is translucent and its base colour shows through. Approve colour on a printed sample of the actual film. The trade-offs between spot and process colour are covered in Pantone vs CMYK.
Film colour and white underlay
On natural (translucent) film, colours look darker and less saturated than on white. Options:
- White-pigmented film: the whole bag is opaque white, giving a clean base.
- White ink underlay: a white ink printed first under specific colours or areas, so they stay bright on clear, coloured or recycled film. It uses a print station.
- Coloured film: the bag itself is coloured with masterbatch, and print in white or a contrasting colour sits on top. Good for bold, cost-effective designs.
Recycled film usually has a grey or off-white tint, which shifts pale colours. More on this in LDPE vs HDPE vs recycled PE carrier bags.
Why is registration harder on plastic film?
Thin PE film stretches under web tension and with heat during drying, so the position of each colour can drift slightly relative to the others and to the bag's handle, seals and gussets. Designs that depend on tight fits between colours, or exact print positions relative to the handle, need allowance built in.
Design rules that help:
- Trapping: overlap adjacent colours slightly so small shifts do not show gaps.
- Avoid hairline keylines around shapes and text; a thin outline is the first thing to show misregistration.
- Keep minimum line widths and text sizes generous; reversed (knock-out) text needs to be larger and bolder than on paper. Your printer will confirm minimums for the film and press.
- Allow a margin around handle cut-outs, seals and gusset folds. Do not place critical text across them.
- For two-sided printing, front-to-back alignment is less exact than colour-to-colour alignment on one side.
- One print repeat corresponds to one bag (its width on side-weld bags, its length on bottom-seal bags), so the design should still work if its position on the bag varies slightly.
The dieline should show the bag outline, seals, gussets, handle positions and safe zones. General artwork preparation, file formats and proofing are in the packaging artwork guide.
Does print affect recyclability or compliance?
Moderate ink coverage on a mono-PE bag is generally compatible with film recycling, but heavy solid coverage, metallic inks and non-PE labels can lower the quality of the recyclate. Print copy must also respect environmental claims rules.
- Design for recycling: under the PPWR, packaging placed on the EU market must reach recyclability grade A, B or C from 1 January 2030; the detailed criteria are still being developed, and ink coverage may be one of the factors considered by design guidelines.
- Claims and markings: environmental claims printed on the bag must be specific and evidence-backed under Directive (EU) 2024/825 in the EU (applying from 27 September 2026) and the CMA Green Claims Code in the UK. Some jurisdictions require specific wording on carryout or reusable bags.
- Market rules: the legal background for printed bags by market is in plastic carrier bag regulations.
Artwork and print checklist for plastic bags
- Vector artwork (PDF, AI or EPS) with fonts outlined and colours named as Pantone references.
- Bag dieline with seals, gussets, handle positions and safe zones marked.
- Number of colours per side, and whether both sides are printed.
- Film colour (natural, white, coloured, recycled) and whether a white underlay is needed.
- Any colour-critical brand elements and how they will be approved (printed sample on the actual film).
- End-use conditions: chilled, wet, outdoor, food contact or heavy handling.
- Required claims or legal markings for the market of use.
- Quantity per design; each additional design or colour change usually means new plates.
Send the checklist with your files through the custom printed plastic bags hub or our contact page. Handle positions affect layout, so it helps to decide the handle first; see plastic bag handle types.