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What is Pad Printing?

If amateurs ask a pad printer about his work, he can quickly and easily explain the things you can achieve with pad printing. Almost everybody is often handling pieces which were decorated by pad printing, such as lighters, pens with logos, the mobile phone and other daily items. Taking a look around, you will find further pad prints like logos on computers and computer equipment, all the small signs inside your car (handles, controls, radio) or scales on thermostats, cameras and household appliances. The modern person is surrounded by the results of a printing process, most people have not heard of.


How does pad printing work?

The pad printing process is an indirect gravure process for printing of any type and form (almost) of shaped materials. The principle is as follows:



With the spatula the ink is moved over the whole cliché
The doctor blade removes the excess printing ink from the cliché. The ink film in the depressions remains.
The pad goes down to the cliché in a roll-off movement. Again in a roll-off movement the printing ink is picked up.
The pad moves to the substrate.

In a roll-off movement the pad is pressed on the substrate.

The ink is released to the substrate.


FEATURES OF PAD PRINTING

The following are the unique properties of the pad printing process:

  • Printing almost independent of the shape, e.g. concave, convex, curved parts
  • Printing of different surface structures, like uneven or structured surfaces
  • Possibility to print into hollows
  • Possibility to print mechanically sensitive products

COMPARED TO OTHER PRINTING PROCESSES THERE ARE MANY ADVANTAGES:

  • Variety of substrates
    Almost any material can be printed with suitable inks.
  • Ability to print fine subjects
    Resolution is far better than that of screen printing
  • High resistance of printing inks
    Depending on ink type used extremely high resistances against mechanical abrasion or chemicals can be achieved.
  • Easy handling and little maintenance
    Compared to other printing processes pad printing is easy to learn.
  • Multi-colour printing: wet in wet
    Possibility to apply multiple prints without intermediate drying
  • Short tooling-up times
    Plates and inks can be exchanged within a few minutes
  • Low set up cost
    Plates can be produced in-house
  • Relatively low space requirement
    Compared to other printing machines pad printing equipment is very small.
  • Low drying cost
    In the most simple case air drying at room temperature is sufficient.
  • Integration into complex systems, inline production and assembly lines
    For years now there has been a successful combination of pad printing systems with injection moulding equipment or assembly lines.

LIMITATIONS OF PAD PRINTING:

Size of motive
Motive sizes are limited by plate, pad and efficiency of the pad printing machine. The diameters of the largest efficiently printed motives are currently approx. 30 cm.

Layer thickness of ink film
The pad process uses plates up to a depth of approx. 20-25µm (at the most 35µm). Thus conventional inks will result in printed ink films of approx. 7µm. This layer thickness can be increased correspondingly by multi-layer printing. Rough particles (e.g. glitter pigments) are difficult to print in an efficient manner.

Printing speed
Even substrates can be printed a lot faster with other printing processes.

Whether pad printing is the only choice for a printing job, or if other printing processes would be better has to be evaluated individually. For printing of golf balls pad printing will continue to be the best method for some time, whereas for printing of T-shirts or beer crates screen printing will naturally be the more efficient choice.

INK SYSTEMS AND THEIR POSSIBLE APPLICATIONS

The principle of pad inks is quite similar to that of screen inks. Partially there are still screen inks sold and used as pad inks. Ink systems, especially developed for the pad process are required to obtain optimal results.

One can distinguish the following systems:

1-Component Systems
These inks dry physically by evaporation of solvents. If resistances required are not too high these inks are ideal for printing of mass articles such as lighters or pens (e.g.
TP 287, TPI)

2-Component Inks
2 component inks are mixed with a hardener prior to processing. The hardener chemically reacts with the ink resulting in a cross-linked ink film. These inks are best choice for high resistance requirements or difficult substrates. The disadvantage is the so-called "pot-life". As the reaction of ink with hardener is initiated when mixing the two components, processing time is limited, mostly approx. 8 h ( TP 218, TP 260).

1- and 2- Component Inks
Depending on requirements these systems can be either processed with or without hardener. They comprise so-called universal inks which show adhesion on various substrates. Such ink systems are ideal for printers often changing the substrates they print or having to meet changing requirements (TP 300, TP 313).

UV-Inks
These inks combine the quick drying of 1-component inks with the excellent resistance of 2-component inks, however they have no "pot life". The disadvantage, however, is the necessity of a quite expensive UV dryer and the limited applications of the individual ink ranges. Thus UV-inks will not be efficient for printers of advertising items and their changing jobs, whereas they are successfully used for industrial applications.

Which ink system to use often depends on the requirements. Solvent based 1-component inks are most efficient, however, often use of 2-component systems is indispensable. On the other hand UV-inks can be a very efficient solution for some printing jobs.

Epilogue
Pad printing is a simple and sophisticated process at the same time. The principle is always the same, be it printing on lighters with a simple machine or be it printing of thousands of closures with rotation printing equipment. The variety of possibilities of pad printing may imply that the system is difficult. The whole system, consisting of machine, plate, pad, ink and substrate has to be matched in order to achieve perfect results. It is the variety and uniqueness of pad printing which makes the process indispensable. This does not only apply for decoration of advertising articles but also to design or toys as well as for technical marking.


SUBSTRATES

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).

Metals
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.

Glass
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




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