Typically, a customer buys a digital printer and ink to use with multiple products. Gregory Harwood advises on substrate requirements and selecting specialised UV inks for greatest versatility and performance.
Why specialised inks? It’s a frequently asked question: ‘Why do certain substrates need specialised UV ink?’ The performance of digital printing inks can vary depending on a substrate’s surface tension. In some cases, the substrate may have a certain surface tension, while the ink carries another. If the two surface tensions don’t align, the result is poor adhesion – or the ink will actually repel off the surface. Given this, the surface tension of the ink being applied must be less than the surface tension of the object. If not, the ink will bead up against the substrate, creating a kind of water effect, with no adhesive bond taking place with the product. It may also just not stick at all. UV-cured inks, for example, can somewhat ‘pin’ the graphic to the substrate, but then it may readily flake off. To maximise the service life of the printed product, inks must anchor, or bond, into the substrate in order for them to stick. Understanding surface tension or drying levels, therefore, is critical to picking the correct specialised ink and getting the results needed. It is essential that ink formulators understand the principles and relationships of surface tension for inks and the target substrates.
While there are different types of digital printing inks, for purposes of this article, we’ll focus on the popular and versatile category of UV inks. Within UV, there are different characteristics of
inks, such as ‘Tritan™ ink’, which is specifically for Tritan™ products (a BPA-free durable plastic). This specialised ink can also work for other items, but it’s specifically formulated for that material and its unique and popular substrate. There are also other inks designed for the sport bottle market. The impressions produced by these inks are squeezable and offer crease resistance; i.e. the objects can be bent and won’t crack. That same type of ink, however, might be too flexible to work well with more rigid substrates. When making selections, it’s critical to find the proper balance based on understanding a customer’s product line and determining their production volumes. If 80% of the printing is on Tritan™, then it makes sense to make ‘Tritan™ ink’ the main ink – and then find ways to utilise it for other products. The goal is to minimise the need for frequent ink changeover. Sometimes that comes as a result of an honest discussion and having the expertise to say, “We looked at your products and here are your top 10. This ink can handle eight of them. Does that justify the machine?” In most cases, it does, which allows users to consider a second machine for other items at another time – perhaps even using a different ink series.
When choosing an ink, it needs to be versatile. Typically, a customer buys a printer to use with multiple products, so a multipurpose type of ink is essential. However,different substrates may still require different pre-treatments in order to run with a single ink. This is because printers aren’t usually designed to pour in different inks on a rotating basis whenever the substrate changes. So, having one type of ink and then incorporating different pre-treatment methods to ensure proper ink bonding is the preferred process. Companies that print on Tritan™ might have to follow this pre-treatment-then-print process, making it 80% of their workflow. As it happens, some industry suppliers have developed a ‘Tritan™ ink’, such as Inkcups’ T2 UV ink, that can print on Tritan™ without any pre-treatment. In that case, users only have to make adjustments for the other 20% of their business, which could include glass, metal and other materials.
Ink pre-treatments comprise several processes that can be used to ensure proper adhesion. One method of pre-treatment is a flame process, where the plasma of a flame is used to change the surface tension. Another technique is to use a wipe-on chemical primer, where an operator wipes it on the object with a lint-free cloth before printing. For some applications it can be a combination of the two. Additional pre- treatments include:
There are a number of ways to pre-treat a substrate, but the rule is to have as few touches as possible in order to reduce the amount of labour involved.
As for post-treatment, it’s typically not needed. With proper ink selection and pretreatment if applicable, excellent results are generally attainable as part of the print workflow. That way, finished products coming off the machine may be immediately packed and shipped, optimising productivity and efficiency
Depending on the type of ink selected and the physical properties of the substrate, inks adhere in a few different ways. Some bond through absorption – typically with porous materials, such as wood or paper. Others adhere chemically by solvating or dissolving the surface and chemically uniting with the object. Finally, an ink can adhere by mechanically interlocking with the surface of the substrate, which is greatly dependent on how the coating wets the surface. Regardless of how ink forms its bond, its adhesion strength must be sufficient for the application or the impression will not remain fixed. Typical adhesion failures include peeling, flaking and blistering. To ensure that the ink-substrate bond is strong and long-lasting, the American Society for Testing and Materials (ASTM) has devised several standardised adhesion tests that can be performed. These include the scratch test, the tape test, and two crosshair tests. Additional tests can also be done, depending on the item, process, and customer requirements.
When a buyer considers a printing system, it’s typically based on the need to print on three or four popular substrates. Most ink suppliers will do an initial ink validation and recommend a formulation for the customer’s applications. Leading suppliers go one step further and in effect serve as consultants, offering the customer a range of options to consider based on more in-depth evaluations and testing. Such an evaluation process should include a variety of sample primers, in order to build a knowledge base of their effects on different substrates. Additional steps that could be taken include providing a primer wipe and testing all available primers. If one of these proves suitable, the customer can procure it in bulk quantities. In certain cases, if a customer is unsure, does not have the time, or simply prefers not to undertake testing themselves, they can send the product to be tested by the ink supplier. Lastly, a detailed report is provided on what was effective and what was not. These considerations taken up front, in collaboration with a knowledgeable and experienced ink supplier, will help ensure that ink and product are well matched for a long term bond.
This article was written for Specialist Printing Worldwide: Issue Two: 2020. To access this publication, visit: https://www.specialistprinting.com/digital-issue-2
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