Pad printing is an indirect photogravure process. An image is etched into a flat printing plate and ink is flooded and doctored across the surface leaving ink only in the etch. A silicone rubber pad then presses down onto the etched plate and picks up the ink, which due to solvent evaporation has become tacky.
The image is now on the surface of the silicone pad. Silicone rubber does not allow ink to penetrate its surface and therefore when the pad presses down onto the product to be printed it releases the ink as a clean film. The range of inks and solvents used enable the inks to adhere to the surface after releasing the ink the pad is then clean and free to repeat the process.
Because such a wide range of inks are available almost any material can be printed and due to the flexibility of the silicone rubber pads uneven, curved and awkward shaped surfaces can be printed. As the ink is quick drying, pad printing is capable of 'wet on wet' printing and high quality multicolour designs can easily be achieved including four colour process.
The more recent development of closed cup machines has made the process 'cleaner' and enabled the process to become more user friendly and automated.
Wood, cardboard, paper
1-component inks are mostly used for these materials. As various coatings show a very different printability it may be difficult to print on coated wood materials.
Thermoplastics (e.g. PVC, PMMA, PS, ABS, PC etc.)
Also mainly 1-component inks. For higher resistances 2-component systems will be of advantage.
Polyolefines (PE, PP)
Due to their low surface tension these substrates have to be pre-treated (corona, flame treatment). There are special systems for PP materials which can be applied to non-treated material.
Duroplastics (PA "nylon", PU, phenolic resins etc.)
For printing of these materials mostly 2-component inks are used. This is mainly due to the high requirements such prints have to meet (e.g. automotive industry).
Printability of metals is so different that various divers systems are used. It is of utmost importance that the material is free of residues (e.g. oil films) before printing.
Special 2-component inks are offered for glass. These systems are often also suitable for precious metals. However in reference to dishwasher resistance one has to consider that these may not only destroy the glass surface but may also remove the pad print
Benefits of Pad Printing
Attractive mouldings – the use of pad printing as a post moulding operation can significantly improve the visual aesthetics of a moulding. Complex graphics are possible and a plain moulding can be transformed into a very visually stimulating part.
Added component value – by using pad printing to add graphics to a part, after it has been moulded, a significant amount of value can be added to a component. Visually stimulating components are more attractive to the customer than plain coloured ones and this gives the part additional value.
High level of quality – as pad printing machines are controlled by microprocessor, the level of repeatability is very high, particularly when using the CNC printer. This means that each part will be printed exactly the same and all to a high level of accuracy.
Ease of handling – even with CNC controlled machines pad printing is relatively easy to learn and maintain. There are very few parts to change so jobs can be changed between very quickly. CNC machines are very easy to set up once the programming has been written. The use of lasers helps line up jigs so that parts can be set up in the correct place very quickly. To change the pad and cliché is a fairly quick process for both CNC controlled and manual printing machines.
Variety of printable surfaces – assuming that the correct ink is chosen, pad printing can be carried out on almost any material. So long as the ink is designed to stick to the surface of the material pad printers can be set up easily to print onto the material. Aki take pride in being particularly good at printing on Chrome Plated mouldings.
Printing on uneven surfaces – the pads of pad printing machines can be designed to be almost any shape and this gives the ability to print on surfaces of various shapes. For example on curved surfaces a flatter pad can be used to ensure print goes right round the surface. This helps to give designers greater freedom when designing graphics.
Wet-on-wet multi-colour printing – pad printing has the ability to print many colours without the need for intermediate drying time between each colour. This helps to greatly decrease the time to print multi-coloured parts as there isn’t the need to allow each colour to dry separately.