When Should You Use Pad Printing Ink Hardener?

Pad printing ink hardener simply increases the adhesion resistance of the ink mixture once it cures. Depending on the type of printing ink, the hardener, technically known as a crosslinker, can act as a curing agent. Additionally, it can be a reactant or a catalyst in the chemical reaction that occurs during the ink mixing process. Overall, different ink series utilize catalysts (crosslinkers) to improve ink adherence to surfaces. But when should you use printing ink hardeners?

What is a pad printing ink hardener?

A pad printing ink hardener is a reactant that gets added to ink mixtures. This reactant acts as a curing agent or a reactant (catalyst) in the case of silicone inks. Note that a hardener hardens, meaning that it increases ink resistance. A catalyst, on the other hand, is reactive and acts as a curing agent. For regular solvent-based pad printing inks, a hardener makes the ink film durable. Additionally, it makes the ink withstand abrasion, chemicals, and, in some cases, weather elements. For these and many other reasons, it is important to use the correct mixing ratio of the ink to the hardener. Also, it is important to choose the right hardener for the ink series you are using.

Ink hardener helps a lot with ink adhesion. Irrespective of the density (soft or hard), ink hardeners are important to increase ink adhesion. Pad printing inks for flexible items require less or no hardener. EVA or nitrile inks, for example, require a hardener-to-ink weight ratio of 3%–5%. On the other hand, pad printing ink for plastic might require 10%. Generally, LSE plastics such as PP have a high density. As a result, increase the hardener ratio when printing plastics.  Check out our Natron Polypropylene Primer for polypropylene plastics.

On the other hand, ink for printing silicone rubber requires a minimum of 10%, regardless of silicone density. The reason is because the catalyst is a reactant, and it aids in vulcanizing the ink and silicone substrate. This is different compared to solvent-based pad printing inks.

Both hardeners and catalysts help with ink adhesion and increase image durability. When a catalyst or hardener gets added to the ink, a chemical reaction happens that reduces the mixture’s useful life (ink pot life).

Effects of humidity and temperature

All hardeners for solvent-based printing inks are sensitive to humidity. Store these types of hardeners in tightly closed containers. When exposed to humidity and open air, these hardeners will harden.

Print in a humidity controlled with controlled environment. Because the hardener draws water, items printed in high humidity may have poor ink adherence.

Another important factor that impacts the ink-hardener mixture and adhesion to the substrate is temperature. Dry the prints at 150 °C (300 °F) for 20–30 minutes after printing to get the best ink adhesion for 2-component inks like pad printing ink for glass or metals. This will have the best possible cross-linking, resulting in the highest chemical and abrasion resistance possible.

Ink systems can also be dried and cured at room temperature. Room-temperature curing takes up to 7 days. Generally, room-temperature curing will have reduced resistance compared to heat-cured substrates. The reason for this is humidity and the hardener-ink chemical reaction. Furthermore, heat promotes chemical reactions that aid the substrate-ink bond.

So, it’s important to use heat on hard substrates like glass, silicone, metals, and thermosetting plastics that need to meet high standards (like being dishwasher- and chemical-resistant).

When should you use pad printing ink hardener?

The first step is to check the ink manufacturer’s recommendation. Match the ink to the appropriate hardener. Second, make sure you use the correct ink-to-hardener ratio for your specific application.

There are also one- and two-component pad printing ink series. One-component pad printing inks do not require a hardener. Two-component pad printing inks require a hardener. It is important to note that all one-component ink series are considered one- or two-component inks. This means you can choose when to add a hardener. For example, when printing on substrates that require chemical or abrasion resistance with one-component pad printing ink, the use of a pad printing hardener is highly advisable.

When printing flexible items with solvent-based inks, you can print without the hardener or reduce the ink-hardener ratio. Reducing the hardener ratio increases the flexibility of the ink film. While pad printing ink series, e.g., the Natron ST Series Ink, are flexible, adding a hardener would make the printed ink rigid.

Learn More About Pad Printing

To learn how to mix pad printing ink, go to our YouTube channel: https://www.youtube.com/watch?v=JfATtFXi0rc