This article covers the main properties of tinplate inks, commonly used for offset printing on metal.
Printing on metal and tinplate
Metal is used to make cans and packaging for food, paint, oils, drinks, confectionery and other products. In some applications, the flat metal sheet is printed before the container is formed. In others, the container is made first and then decorated.
The ink needs lasting durability; otherwise the print may not withstand different environmental conditions.
The basic offset-printing principle does not change between paper, metal and other substrates. The differences lie in the ink and the requirements of the surface. The main considerations for metal printing are substrate properties, flexibility and adhesion, and heat resistance.
Metal is dense and non-absorbent, so the ink cannot penetrate it. Strong adhesion is needed to keep the ink on the surface. The printed film must also be resistant and fully dry before further processing.
Drawing and bending during container manufacture require the print to withstand deformation without scratching, flaking or other damage.
Food packaging may also undergo sterilisation with water or steam for an hour or longer. The print must resist fading, blistering and heat damage under the relevant process conditions.
Types of offset tinplate ink
Tinplate ink formulations use a suitable resin and solvent system that adheres to metal, with additives to control abrasion, slip and resistance. The source describes high-boiling solvents in the 200–300°C range and resin systems based on reactive polyesters and alkyd resins, themselves a type of polyester.
Conventional alkyd-resin offset ink
Conventional tinplate ink is similar in consistency to ink for paper, but the non-absorbent metal surface requires a more robust formulation. Its resins and vehicle need greater heat resistance and sufficient adhesion to keep the printed film stable before it enters the oven.
Alkyd-resin inks dry with heat. The article describes a drying tunnel after the press operating for 10–20 minutes at 130–200°C.
UV-curable offset ink for metal
Large thermal tunnels, which can be around 20 metres long, and their fuel costs encouraged interest in UV printing. UV inks have a physically and chemically different formulation from alkyd inks. High-quality pigments are used to maintain colour stability and resistance to external damage.
UV formulations cure when exposed to ultraviolet light. The light initiates radical reactions in the monomers and oligomers, allowing curing to occur rapidly, in a fraction of a second.
UV inks require sufficient energy to start the reaction. Unlike infrared or gas drying on web presses, heat is not the primary curing mechanism.
A photoinitiator starts the chemical reaction that polymerises the ink and rapidly creates bonds throughout the film. The relevant UV light energy must reach the printed surface uniformly and consistently during the run.
Ink selection and mixing are important because baking can change the printed colour substantially. Understanding the ink type and its mixing instructions helps achieve the intended final colour.