Difference 1: Plates
As the name suggests, flexo printing utilises flexible plates for a rotary printing process. The plates are made from a photopolymer compound and are flexible enough to be wrapped around a printing cylinder. The relief image on the plate is fixed via a laser image-setter and the polymer in the ‘non-print’ areas is washed away in a processing unit, where it is dissolved into a solvent or water solution. Ink is transferred from the ink well via a rotating ‘anilox’ roller onto the flexo plate. A separate printing station & flexo plate is required for each colour to be printed. The image is then printed directly onto the substrate. The printing plates are quite durable and if stored correctly, can be re-used several times, before they eventually need to be replaced.
For offset printing, again as the name suggests, ink is transferred (offset) via a series of rollers onto the printing plate. This can be either a flat-bed or rotary process – depending on the type of offset press. The plate is usually made of aluminium. The complete wet image (either single colour or multicolour) is then transferred onto a ‘blanket’ and in turn onto the substrate, before drying.
Difference 2: Ink
Offset printing usually consists of four ‘process’ colours; cyan, magenta, yellow and key (which is black). Each colour has a dedicated printing station. Any spot colours are made from a combination of the process colours. Ink can be water based or UV curable.
Although process colours are also used in the flexo process, additional print stations tend to be used for spot colours. Spot colours can be supplied pre-mixed, or can be mixed in-house, as required. Flexo inks can also be water-based or UV curable. Solvent based inks can also be used. UV inks enable higher running speeds and can be left in the press at the end of the day, without the need to empty & clean each print station. Water based inks need to be removed from the press, when it is not in use – to prevent ink from drying on the rollers & in the ink trays.
Difference 3: Types Of Materials To Print On
Offset printing machines can print on materials such as; paper, metal, cardboard, cellophane and vinyl. The printing surface must be flat and smooth. It is excellent for printing newspapers, books, magazines, stationery, posters, brochures and so on. Generally, to print onto both sides of the substrate will require a second pass. Similarly, any die-cutting, slitting, folding, creasing, laminating, etc is done as a secondary, off-line process.
Flexography is used on both absorbent and non-absorbent materials, for example, cellophane, foil, cardboard, fabric, plastic, metal etc. It is mostly used for packaging; envelopes, retail bags, wallpaper, paper, newspapers, sweet wrappers, labelstock and so on. The main difference with both processes is that offset printing is only done on flat surface while Flexographic printing can be used on almost any substrate with a flexible surface. Flexo printing can achieve high speed production and many presses have multiple converting options integrated into the press, enabling a single pass operation.