Inks for Pad Printing

Inks & Additives

To achieve an optimal print quality, special pad printing inks have to be used. These inks have been specifically developed to show high pigmentation as such a small quantity is transferred during the printing process. The corresponding additives such as thinner and  hardener ensure a good processing of the ink. The ink should preferably possess the following characteristics: 
                • Long stability (potlife) in the ink tray/cup
                • Safe
                • Not harmful to the environment
                • Easy to clean
                • Adhesion on all parts and materials without pre- or after-treatment if possible
                • One ink type preferably for all materials to be printed

Depending on where they are being used ink requirements vary greatly. For marking only, the requirements are low, but for a decorative print the requirements are very sophisticated. The finished print on the substrate should show the following characteristics:
                • High opacity of the ink
                • Extreme adhesion and scratch resistance
                • High chemical resistance
                • Safe (eg; for toys)
All these demands cannot be met by one single ink type even today. Therefore, a variety of special inks have been developed which can meet different demands and applications.

In the printing ink range we differentiate between single and two component, baking, and UV-curable inks. Inks consist of different binders, pigments, extenders and additives. This leads to the special characteristics of each individual type and is the reason that different ink types cannot be mixed with each other without losing the original characteristics.
The mixture of a pad printing ink is as follows:

The binders of pad printing inks are based on one or more types of resins. The choice and the combination of the resins decide on the ink’s application eg. adhesion on various materials, level of gloss and chemical resistance. As the resins are usually available in the
form of granulates or powders, they have to be dissolved with adequate solvent or solvent combination, to achieve a printable binder.

Solvents differ mostly in their evaporation time and solubility. The combination of the solvents in the printing ink will be responsible for its drying behaviour, the printability, as well as the ink adhesion onto certain substrates.

The pigments in the pad printing ink will determine the colour and the opacity. There are organic and inorganic pigments. From the group of inorganic pigments, only those which are free of heavy metal substances in their chemical structure are still being used.

Additives are additional substances which are normally used in small amounts. Their effect helps to fine tune the ink characteristics, for example the flow, the viscosity or the opacity. They are the flow agents, thickening agents, and filling agents.

Pad printing inks can be divided into various groups because of their drying process. These groups are:
• Physically drying inks: one-component inks
• Chemically hardening inks: two-component inks
• Baking inks

One component inks
This ink system dries by the process of physical evaporation of the solvents. At the same time, the surface of thermoplastic materials (for example polystyrene (PS), polycarbonate (PC), PVC etc.) is attacked by the solvents. Through this attack of the substrate’s surface, a direct fusing of ink and material is created. In this case, a high scratch resistance and a good ink adhesion is guaranteed. Single-component inks are very fast drying.

Two component inks
These ink systems show a very high chemical resistance with good adhesion and scratch resistance, especially on difficult substrates. A hardener must be added to the ink, which will cause a chemical reaction with the binder. It is most important that the correct ratio is used. The addition should be done shortly before printing, as the ink with added hardener can only be used for a limited time span (pot life). Depending on the ink type, the pot life lasts for approx. 6-12 hours. The complete hardening and adhesion of these ink systems is sometimes reached only after a few days. Very often, a common mistake is to try adhesion and chemical resistance tests too early. It is important to follow the information contained in  the corresponding technical date sheets available from Miolford Astor.

Baking inks
The baking inks show similar characteristics to the two component inks. The chemical reaction of this ink system is a cross linkage of the binder, which will only take place under the influence of very high temperatures. More information about this can be found in the
corresponding technical leaflets.

Pad printing inks cannot be taken straight from the can into the ink tray of the machine. Depending on printing speed and print design the ink must have various viscosities, and for two-component inks, the hardener must be added. Here, already two handling mistakes can be made which would have negative results for printing. The ratio of ink/thinner/hardener must be very accurate, as the amount of ink needed to fill an ink cup or ink tray is so small (between 50 and 150 g), just a few grams error would cause a big deviation.
For example: Two component ink: ink/ hardener 10:1 + 10 % thinner amounts to 100 gram ink + 10 gram hardener + 11 gram thinner.
A deviation of 2 grams in this example when adding the hardener will change the mixing ratio by 20%!! This can cause terrible results because this wrong ratio will completely change the ink characteristics like adhesion as well as the potlife. A complete day’s production could be useless as the scratch resistance and adhesion of the ink can only be tested after 48 hours.

Rule of thumb for two component inks:
    too little hardener: longer pot life but bad adhesion
    too much hardener: shorter pot life and ink becomes brittle

For these reasons, an exact weighing of the components is a must for trouble free production. The age of the ink and the hardener can also be important. Hardeners are mostly hygroscopic (water attracting), and therefore lose their characteristics when an opened can is kept for a long time unsealed. Please refer to the shelf-life data of the ink manufacturer.

The choice of thinner is important mostly for extremely fast printing, double printing or multicolour printing. Fast thinner is used for fast printing or multicolour wet on wet printing. A slow thinner is used for slow printing processes or when printing extremely fine designs, to avoid drying of the ink in the plate.

PMS colour matching of pad printing inks is possible, but there can be slight variations in the end product, as the base range of pad printing inks is different to those used to formulate PMS colours. A great deal of experience and patience is required for optimal results. Milford Astor offers a PMS matching service if you require.

The use of inks and solvents calls for certain safety measures to be followed. Some of the contents are dangerous during processing (contact, inhalation, swallowing, flammability) and others during disposal (groundwater contamination). Specific information can be obtained from the Safety Data Sheets (SDS) available on request.

When handling inks and solvents, the following precautions should be taken:
Protective gloves should always be worn when mixing and cleaning; the printing area should be well ventilated.
Remnants of inks, thinners and other solutions should be collected and disposed of as special waste.
Read and follow all the handling and storage guidelines listed in the SDS available on request.