The logo, address or usage warning on a bag is added in the printing step that follows film extrusion. Not every bag is printed, but a large share of carrier, shop and courier bags go through this step. Printing turns the bag from a means of carrying into a communication surface.
Polyethylene is a flexible, thin material that does not readily hold ink on its own. So printing is not just a matter of mounting a roll on the press: surface preparation, the right choice of ink, colours landing on top of one another and drying are all handled together.
- Unwinder The unprinted film roll is unwound at a controlled tension and fed into the press.
- Plate cylinder The plate or engraved cylinder transfers the design to the film passing over the impression cylinder.
- Ink unit and doctor blade Ink is fed to the plate in a metered amount; the doctor blade wipes off the excess.
- Drying Hot air removes the ink solvent, so the colour reaches the next station dry.
- Rewinding The printed film is wound back onto a roll and prepared for the cutting step.
Before printing: surface preparation
Polyethylene has low surface energy; ink easily beads on this surface or may flake off later. To prevent this, the side to be printed undergoes corona treatment. Carried out with an electrical discharge, this treatment makes the film surface more receptive to ink.
Corona treatment is mostly applied during film extrusion, just before winding. Because its effect can fade over time, rolls that have been stored for a long time may need their surface checked again, or the treatment repeated, before printing. Some additives in the film can also migrate to the surface and affect ink adhesion.
Flexographic and rotogravure printing
The most frequently used method for bag films is flexographic printing. In flexo, the design is formed in relief on a flexible printing plate. Ink is transferred to the plate in a metered amount via the anilox roller, whose surface is covered with fine cells, and the plate prints the design onto the film. The relatively quick preparation of plates and their suitability for different run lengths make flexo widespread in bag production.
In rotogravure printing, by contrast, the design is engraved into the surface of a metal cylinder. The cylinder dips into the ink, a thin blade called the doctor blade wipes off the excess from the surface, and the ink left in the cells is transferred to the film. Rotogravure gives strong results in fine detail and tonal gradients; cylinder preparation, however, takes more work, and it is generally preferred for long runs.
Which method is chosen depends on the level of detail in the design, the number of colours, the order quantity and the structure of the film. A single-colour logo and a photo-heavy design do not call for the same approach.
Preparing the printing plates
Printing starts at the design desk, before the press. The approved design is separated into colours, and a separate plate is prepared for each colour. In flexo these are usually plates made from photosensitive polymer sheets; in rotogravure a separate cylinder is engraved for each colour.
The flexible nature of the film is taken into account when preparing plates. Slight spreading of the ink during printing can thicken fine lines and fill in small gaps. For this reason, line weights and colour gradients in print files prepared for film are chosen more cautiously than for printing on paper.
Once the plates are mounted on the press, each station is adjusted one by one: printing pressure, ink amount and alignment between stations. The print from the first metres is compared with the approved sample and corrected if necessary.
Colours, drying and registration
In multi-colour printing, each colour is printed at a separate station. The exact alignment of colours on top of one another is called registration in the industry. Because polyethylene film can stretch under heat and tension, colours can shift relative to one another; operators therefore constantly monitor film tension and alignment between stations.
On central impression flexo presses, the film receives all the colours as it travels around a large drum. Because the film runs supported against a single surface, stretching is limited, and this arrangement helps maintain colour registration on thin films.
The ink needs to dry after each station. Drying temperature is also kept balanced: because polyethylene is sensitive to heat, overly hot air can soften and stretch the film. If drying is insufficient, the next colour can smear, or the print can transfer to the opposite face in the wound roll. After the last station, the film is wound back onto a roll.
What to know when preparing a design
For a business ordering printed bags, the most useful thing to know is the limits of printing on film. Very fine lines, small text and large solid areas may not come out as sharp on flexible film as on paper.
- Number of colours: Each extra colour means an extra plate and setup; simple designs give more consistent results.
- Seal and handle areas: Important text should not be placed in areas that coincide with the seal line or the handle cut.
- Film colour: On coloured or opaque film, light tones may look different from what is expected; a white base print is planned if needed.
- Sample approval: Before the production run, colour and position should be checked on a trial print or sample.
Common printing problems
Most printing defects fall under a few familiar headings. Knowing the cause makes it easier for both the manufacturer and the business receiving the bags to describe the problem accurately.
- Misregistration: Colours do not sit exactly on top of one another; usually related to film tension or station alignment.
- Ink lift-off: The print comes off when rubbed or pulled with tape; surface preparation and ink compatibility are reviewed.
- Set-off: Ink that has not dried enough transfers to the back of the layer above it on the roll.
- Fill-in and voids: Small text fills in or speckles appear in large areas; ink amount and printing pressure are adjusted.
For packaging that comes into contact with food, the choice of ink and application is especially important. Such uses should be assessed according to the application and the relevant regulations.
Once printing is complete, the film carries the outer face of a bag, but it is still a roll. In the next step, this roll is turned into individual bags by cutting and sealing.