Pre-Treatment of Substrates in Industrial Printing.

One of the greatest challenges in moving from traditional “contact” forms of product marking to inkjet is achieving a durable adhesive bond between the ink and the substrate. The challenge is compounded by the irregular topographies of many industrial parts and by the increased use of polyolefins — and by the harsh environments parts must survive, from under the hood of a car to a medical device inserted into a patient.

On top of adhesion, UV-LED inkjet inks are highly viscous — as they must be to lay down enough ink at the high throughput speeds many production lines require — and therefore inherently have low surface tension. Polyolefins such as PTFE have very low surface energy, so pre-treatment of the substrate is almost always called for to achieve adhesion and “wettability.” There are several methods of pre-treatment in the industrial printing world; this paper examines the pros and cons of each. But first, a few definitions.

What is adhesion?

Adhesion is the strength of attraction between a substrate and the ink, determined by the relative surface energy and surface tension of the materials. The higher the substrate’s surface energy relative to the ink’s surface tension, the greater the molecular attraction — drawing the ink closer for high bond strength. The lower the substrate’s surface energy relative to the ink’s surface tension, the weaker the attractive forces.

Adhesion is measured through scoring (the “cross-hatch” test) and the “tape test.” In the cross-hatch test, the printed substrate is scored with a special tool; in the tape test, tape is applied to the scored surface to test how well the ink holds. The relative surface tension of substrate and ink is measured with a dyne test, which gauges the angle formed by a drop of ink on the substrate. An angle of less than 90 degrees indicates good wetting and adhesion; greater than 90 degrees means pre-treatment may be in order. Typically, the surface energy of the substrate needs to exceed the surface tension of the ink by at least 10–15 dynes/cm² for good adhesion and wettability.

What is “wettability”?

Wetting is the ability of a liquid to form an interface with a solid surface. “Wetting out” means the ink flows and covers a substrate to maximize the contact area — and the attractive forces — between ink and substrate. A waxed car hood has a lower surface energy than water, so water beads up rather than wetting out, reducing its contact area with the surface. For ink to effectively wet out a surface, the surface energy of the ink must be as low as — or lower than — the surface energy of the substrate. Or the surface energy of the substrate must be raised through pre-treatment.

There are several methods of pre-treatment, among them corona, flame, plasma, Pyrosil®, and chemical pre-treatment such as primer wipes.

Pre-treatment options

Corona

Corona pre-treatment is a high-frequency electrical discharge that raises the surface energy and surface tension of the substrate. It comes in two forms. Anode-cathode corona flamers consist of a positively-charged and a negatively-charged electrode; the arc passes through the substrate, changing its surface energy. Blown-arc corona flamers combine the anode and cathode with blown air, under which the part is passed — providing surface treatment only.

Plasma

With plasma pre-treatment, the substrate surface is bombarded with positive ions, causing oxidation or reduction of the substrate and facilitating strong ionic bonds between the ink and the substrate.

Pyrosil®

Pyrosil® is a proprietary flame technology for pre-treatment and adhesion promotion across many substrates. By evaporating a proprietary chemistry mixed with propane or natural gas and then burning it, SiOx (silicon dioxide) is deposited onto the substrate. SiOx creates high surface tension for improved wettability and also changes the surface for improved adhesion. Pyrosil is used by major glass and plastic manufacturers and container decorators to increase the bonding and durability of UV inks on glass and other substrates.

Primer wipes

Simply wiping the surface of the substrate with primer can increase adhesion — but for high-speed product-marking solutions, this method is not feasible.

Issues to consider

When deciding on a pre-treatment method, several factors matter — chief among them safety, physical space, throughput, and cost.

Safety

High-speed industrial inkjet machines contain sensitive electronic controls, so any pre-treatment option must be grounded separately from — and on a different circuit from — the print machine. Care must be taken to avoid arcing, a concern with both plasma and corona pre-treatment due to the piezo-electric spark required to ignite the corona flamer. And wherever open flame is present, normal fire-safety precautions apply.

Footprint

Virtually any pre-treatment option requires physical separation from the print station. Depending on your production line, this requirement may determine the placement of the pre-treatment station and may dictate custom part-loading — from linear conveyors to rotary tables. (A rotary table can pass parts under the flamer twice, enabling pre-treatment at high throughput speeds without the cost of a second flamer.)

Throughput

Project parameters may dictate a throughput speed at which parts don’t spend enough time under the pre-treatment method — corona, flame, or plasma — to achieve the desired adhesion and wetting characteristics.

Cost

Finally, cost is always a consideration. At Engineered Printing Solutions, our Sales Engineers work with you to find the low-cost solution to your pre-treatment needs, whatever your production schedule demands. This may involve multiple pre-treatment stations or custom part-loading. One thing is certain: no two machines that EPS builds are alike.

Conclusion

With the proliferation of new polymers — including low-surface-tension polyolefins — pre-treatment of parts prior to inkjet marking and decorating is becoming more and more necessary. Pre-treatment provides two benefits: increased adhesion and increased wettability. Several methods are available, from simple primer wipes to corona, flame, plasma, and Pyrosil®. Based on production requirements such as throughput, parts-handling before and after printing, and unit cost, our Sales Engineers will work with you to find the pre-treatment method that works for you.