If the logo on a printed bag does not rub off or lift when pulled with tape, there is usually an invisible process behind it: corona treatment. In this post we explain why polyethylene film needs this treatment, how it is done and the ways in which it affects print quality.
Why doesn’t polyethylene hold ink?
Chemically, polyethylene is a rather “indifferent” material. Its surface has few groups inclined to bond with other substances, so its surface energy is low. This property is exactly what protects polyethylene from moisture and many chemicals, but it is an obstacle for printing.
A liquid applied to a low-energy surface gathers up instead of spreading. Like a drop of water on a greasy pan, ink forms small beads on untreated polyethylene film. Even when the ink dries, it adheres weakly to the surface and comes off easily with rubbing or when pulled with tape.
For the print to last, the ink must wet the film surface well, that is, spread over the surface and adhere to it. Corona treatment is done precisely to make this possible.
How is corona treatment done?
In corona treatment, the film is passed between an electrode carrying high voltage and an earthed roller. An electrical discharge forms in the narrow air gap between the electrode and the film. In the dark, this discharge can be seen as a violet-blue glow, and it is this glow that gives the process its name.
Energetic particles in the discharge zone strike the film surface and change the molecular structure of the surface in a very thin layer. Groups more inclined to bond with ink form on the surface. The rest of the film, that is, its thickness, strength and appearance, is not affected by the treatment; the change is only on the outermost surface.
The treatment is applied only to the side of the film that will be printed. If both sides are to be printed, both are treated. Because excessive treatment can cause unwanted results such as film layers sticking to each other, the treatment level is adjusted to the need.
At which stage of production is it applied?
Corona treatment is mostly carried out during film extrusion, after the film has cooled and been flattened and just before it is wound onto a roll. At this point the surface is fresh and responds best to treatment. On many blown film lines, the corona unit is placed just before the winder.
Some printing machines also have a corona unit. This second unit is used to refresh the surface just before printing on rolls that have been stored for a long time or have come from elsewhere. Which arrangement is chosen depends on the production flow and on how long the film takes to reach printing.
Why does the effect fade over time?
The effect of corona treatment is not permanent. The treated surface gradually returns towards its original state; in the industry this is called the effect “dropping”. The rate of the drop varies with the film’s raw material, the additives it contains, the storage temperature and the length of time.
Additives are especially important here. Some additives that make the film slippery or prevent layers from sticking together migrate over time from inside the film towards the surface. This thin layer building up on the surface can get between the ink and the film and weaken adhesion. That is why, for films to be printed, the choice of additives is considered together with corona treatment.
In practice, this means corona-treated rolls should be taken to printing without long delays and should not be stored in hot environments. A roll that has been waiting for a long time should be checked again before printing.
How is the treatment checked?
The effect of corona treatment cannot be seen with the eye, so simple but regular checks are needed. Methods commonly used in workshops include:
- Test fluids and pens: Test fluids with different surface tensions are applied to the film. Whether the fluid spreads over the surface or beads up immediately gives an idea of how wettable the surface is.
- Tape test: After the print has dried, adhesive tape is stuck onto it and pulled off quickly. If ink comes away with the tape, adhesion is insufficient.
- Rubbing and crumpling: The printed surface is rubbed with a finger or another surface, and the film is crumpled. If the ink cracks and flakes off, the problem may lie in surface preparation or in the choice of ink.
When the results of these checks are recorded, it becomes possible to trace which roll was printed when and with what result. Finding the cause also becomes easier when a problem arises.
Common printing problems
Not every printing problem comes from corona treatment, but some symptoms point to surface preparation. Recognising them helps find the source of the problem faster.
- Beading and voids: If the ink does not spread over the surface and small holes or specks remain in solid colour areas, the surface may not be wetting sufficiently.
- Ink lifting off: If the print looks fine at first glance but comes off with tape or rubbing, adhesion is weak. The treatment level may be insufficient, or the effect may have dropped over time.
- Different results within a roll: If one part of the roll prints well while another causes problems, check whether the treatment is applied evenly across the width of the film.
- Layers sticking: If film layers cling to each other as the roll is unwound, the treatment may have been more intense than necessary.
These symptoms can have other causes too: unsuitable ink, a wrong drying setting or additives in the film. That is why corona treatment, ink and film are addressed together when solving a problem.
Differences by printing method
One of the most common methods in bag printing is flexographic printing. In this printing, done with flexible plates, ink is transferred to the film as a thin layer and dries quickly; a well-prepared surface is important for fine lines and solid colour areas to come out cleanly. In rotogravure printing, ink is transferred to the film from the cells of an engraved cylinder; here too the surface needs to accept the ink evenly.
Whatever the method, corona treatment should not be considered independently of the choice of ink. Inks developed for polyethylene are formulated to work with a treated surface. The ink supplier’s recommendations and trial prints are the most reliable guides.
What to look out for when buying bags
A business ordering printed bags does not need to deal with corona settings directly. Still, a few simple steps help ensure print quality:
- Request a printed sample before series production and carry out a simple tape and rubbing test on it.
- Share with the manufacturer the conditions in which the bags will be stored for a long time; hot environments can affect the print and the film surface.
- If the contents of the bag will come into contact with the print, assess with the manufacturer, within the framework of the relevant legislation, whether the ink used is suitable for this application.
Corona treatment is an invisible but indispensable preparation step for a printed bag. When done right, it goes unnoticed; when it is skipped or its effect has worn off, the print itself shows it clearly.