A container’s curved surface can make printing seem difficult, but screen printing can decorate it. For food cans and two-piece cans, however, screen printing can be slow and require several stages. This archived technical article explores other container-printing processes and the developments discussed at the time.
Printing on plastic containers
Plastic containers can be decorated with labels and shrink sleeves, or printed using screen printing and letterpress offset, also called dry offset. Dry offset is particularly associated with thin-walled containers, often used for dairy packaging.
Inks for container printing
The article describes three ink systems for thin-walled containers: heatset inks for polyethylene, plasticiser-based inks for polystyrene, and UV-curable inks.
Plasticiser-based inks set through absorption of the plasticiser by the polystyrene. The source discusses their use in food packaging and the energy needed for drying, and describes UV inks as an alternative to both heatset and plasticiser-based systems.
These containers are used for dairy products and similar foods. They are printed at high speeds on machines that leave little time for curing under small tubular UV lamps. The inks therefore need high radiation sensitivity and pigment strength, because the deposited film is very thin: the article cites less than 1.5 g/m². Full curing must still be achieved at the maximum operating speed.
The article highlights Germany’s Zeller & Gmelin and its UK subsidiary InterColor as specialist manufacturers associated with research and development in UV inks, particularly in the European market.
It describes the period before 2000 as focused on fast drying, UV resistance and performance in cold storage. Later developments discussed in the article shifted towards higher print quality, stronger colour and more faithful colour reproduction.
Developments in curing and substrate performance
The source describes a development called cold UV curing, using specialised UV lamps with a wavelength of around 308 nanometres. It discusses the challenges of activating photoinitiators at this wavelength, the limited contribution of infrared heat to that UV process, and the difficulty of curing reflective white inks and strongly absorbing black inks. Highly UV-sensitive inks were developed to address these issues.
Another application discussed is UV printing on expanded polystyrene, used for cups and tubular containers. Its irregular surface and low ink receptivity create particular challenges. The ink needs good flow, strong surface wetting and low tack to produce a sharp, accurate image.
These substrates cannot be preheated in the same way as more heat-resistant materials because polystyrene can melt or distort quickly. The inks described offer good coverage, adhesion, colour strength and gloss despite these difficult surfaces.
Two-piece cans
Two-piece cans, widely used for drinks, follow a different decoration process. The source outlines the following manufacturing sequence:
1. Start with a coil of aluminium strip.
2. Lubricate the surfaces.
3. Form the cup and draw and iron its walls.
4. Trim and shape the edges.
5. Wash off lubricant residues.
6. Apply an exterior coating to support ink adhesion.
7. Pass through an oven to dry the exterior coating.
8. Decorate the exterior by printing.
9. Pass through an oven again to cure the decoration.
10. Apply an interior lacquer to protect the contents.
11. Pass through an oven a third time to dry the interior coating.
12. Form the neck and circular flange.
13. Palletise the cans for filling.
Thermal curing is used to obtain adhesion and resistance on aluminium and other metals. The article describes a specialised acid-catalysed polyester–melamine curing cycle of 6–7 seconds at 300°C. It also notes the development of UV-curable inks for two-piece cans, while reporting that their operating speed at the time was lower than that of the other processes discussed.