The tin can.
Is it the can you tie to the wedding car with a length of string before the newlyweds drive off on honeymoon? Or is it the can on the supermarket shelf, where you skilfully fish out the perfect one and leave the others behind? Or is it the dented, scratched tin on your desk, in which anything that will fit collects?
It is all of the above, and it appears in many other forms as well. As an aerosol can for your daily shave, a stable container for wood varnish, a biscuit tin for the table on Sundays, a tobacco tin for smokers, a child’s toy, a lid on a jam jar, a crown cork on a beer bottle and a heart-shaped chocolate tin for your dearest.
- High quality decorated cans
The tin can starts its life in a steelworks. There, a steel strip is rolled out to the desired thickness, between 0.12 and 0.49 mm depending on the application. For weight reasons, and to save materials, product development is currently moving towards 0.1 mm. The strip is cut to length and cut into rectangular sheets. In the paper field you would talk about size 4 or large format, but metal printers use the bare figures: no more than 1,200 mm wide by 1,000 mm long. The weight of a sheet can quickly reach one and a half kilograms or more.
In the metal printing plant these sheets are coated first. A gold coating may be used for the inside of the can to act as a protective barrier between the metal and its contents. Tins for tomatoes are always coated in white for aesthetic reasons. Biscuit tins are not coated inside, because biscuits are often sealed in plastic. The coating is applied to the tin using coating machines specifically designed for this purpose. The accuracy of the coating is the key criterion here. On the one hand it should be possible to adjust the thickness of the coating very precisely, so as to use as little of the coating material as possible and thus save costs. On the other hand, the coating should be evenly distributed to prevent subsequent quality problems. After coating, the sheets go straight into an oven, where they are dried with air at 200°C. Drying generally takes 12 minutes.
- Coating line with thermal drying oven
It is not until now that the sheets are printed. Print quality requirements are high, because the can is a form of packaging that consumers in supermarkets decide whether to buy in a fraction of a second. This is why tin cans are only printed using the offset process. The exception is beverage cans for beer and soft drinks, where for reasons of cost the round can is printed. However, the graphical requirements for these cans are also considerably lower.
Metal printing was invented towards the end of the 19th century. The Mailänder Printing Press Factory was founded in Bad Canstatt in 1867, and still exists today under the umbrella of KBA-MetalPrint. It develops offset printing presses specifically for metal printing. This differs from paper printing not just because of the hard material used, which places high demands on the robustness of the press. Metal printing also stands out because the ink cannot penetrate the substrate. The ink lies wet on the surfaces of the sheet, and this calls for particular skill on the part of the printer when it comes to adjusting the fount solution and the printing parameters if the ink is to adhere to the substrate. On the other hand, the substrate’s impermeability is an important feature of the finished can. Food is packaged in it, and after heat treatment can be stored for long periods without preservatives. The tin also prevents the penetration of foreign matter from outside.
- Drying oven infeed
Like the coated sheet, the wet, printed sheet is dried in a conveyor oven, but only at 160°C. It is also possible to use UV-sensitive inks that can be hardened quickly using UV lamps. As a final operation the printed sheet is coated again. It is given a transparent protective coating that not only protects the printing against scratches, it also gives it a glossy finish. In the same process, sheets with the can bottoms and lids are also coated and, where necessary, printed
Then we come to the next stage in production. The finished sheets are cut into shape or punched out. The side portions are welded to make cylinders (this can be seen afterwards from the welded seam on the side of the can), and the bottoms and lids are firmly attached by crimping. Obviously, it’s important not to forget to fill the can first!
The environment is an important aspect of metal packaging. The tin can is already environmentally-friendly as it stands. Metal can be recycled completely and without loss of quality, which certainly can’t be said of plastic or composite packaging!
The production process has also been improved considerably in recent years. Coatings contain up to 60% solvents. Instead of releasing these into the environment, they are used as a source of energy for heating the ovens, with one kilogram of solvent replacing a cubic metre of natural gas. By using heat exchangers that adjust their efficiency according to the energy requirements of the oven, and by means of intelligent control systems, the gas consumption of modern lines has been drastically reduced.
Compared with the paper market, metal printing is clearly in a niche. However, the focus shifts if we consider the packaging market alone. A modern metal printing line turns out a million sheets per month on average. These are used to make 28 million cans, produced at a rate of 850 cans per minute, in other words 14 per second.
You can see the result in the supermarket. The high-quality printing on a can showing green Spanish olives in vivid colours. Or the fine chocolate leaves in the jet black-coated rectangular tin. Or the peppermints in the cool, flat tin with the alpine landscape printed in photographic quality on the inside of the lid. Or the top-quality assortment of chocolates in the red, heart-shaped tin with the embossed flowers.