Will the ink stick? Surface energy, primers and the tape test

“The ink is not sticking” is the most common complaint in UV printing and the least useful description of the problem. Adhesion is physics you can measure, and most failures come down to a surface that was never going to accept ink, a primer used wrongly, or a test run too early.

The rule of thumb is 10, not 7

For ink to wet a surface rather than bead on it, the surface’s energy must exceed the ink’s. Enercon, who make surface treatment equipment, put the working margin plainly: the substrate should have a dyne level “at least 10 dynes higher” than the surface energy of the ink. Inks at 35–38 dyne/cm therefore want a substrate treated to 45–48.

You will see “7 dynes” repeated widely. We could not source it anywhere. Every manufacturer publishes ten.

And the standard itself is careful to limit the claim. ASTM D2578 states: “Wetting tension in itself is not a completely acceptable measure of ink, coating, or adhesive adhesion.” Surface energy tells you whether the ink will wet the surface. It does not tell you whether it will stay.

Where common substrates sit

Figures from ACCU DYNE TEST, aggregated from several hundred sources, in dyne/cm:

Polycarbonate at 44 is easy. Polypropylene at 30.5 is hard. Silicone at 20.1 is essentially impossible — which is why silicone phone cases do not print, no matter whose machine you use.

A caution on glass: you will see figures near 300 dyne/cm quoted for it. Those are theoretical clean-surface values. Real float glass on a workshop floor carries a contamination layer and behaves far lower. Do not put glass and acrylic in the same table as if the numbers were comparable working figures.

The finding that breaks the rule of thumb

Here is the part almost nobody accounts for. Two UV inkjet inks from the same manufacturer can differ by 12 dyne/cm.

Nazdar publishes surface tension on its technical data sheets. The 270 Series sits at 32–36 dyne/cm. The 759 Series sits at 21–24 dyne/cm.

So “ink is about 30 dyne, treat to 40” collapses the moment you do not know your own ink. Apply the +10 rule to the number on your ink’s data sheet. If you have never seen that sheet, that is the first thing to ask your supplier for — it is a one-page document and it decides your entire pre-treatment strategy.

Dyne pens: how to use one properly

A dyne pen is the cheapest diagnostic in the shop and almost everyone uses it wrong.

• Start with a pen slightly below the level you expect.
• Draw two or three parallel passes, discard the first, and read only the last.
• Film holds for three seconds or more → go up a pen. Beads within one second → go down. Holds for one to three seconds → that is your level.
• Never retest the same spot. Test at least three points across the width.

Know the limits. Accuracy is about ±2 dyne/cm. Enercon: “What may look like 32 to one person may look like 35 to someone else.” Use it as a pass/fail gate, not a measurement.

Two situations where pens are invalid. Do not use pens on metal — the tip contaminates; use bottled fluid and a swab. And they are meaningless on porous substrates like paper or raw MDF, where the fluid permeates and falsely reports easy wetting.

One safety note carried on the manufacturer’s own documentation: pregnant women should not perform this test. Shelf life is around six months.

Corona treatment decays — sometimes within a day

If you buy pre-treated film, treatment is perishable. Vetaphone: the increase in surface energy “begins to decay immediately following treatment”, and film with high slip additives — over 1200 ppm — “can be totally resistant to printing just 24 hours after treatment.”

The cause is additives blooming to the surface. Paper, by contrast, holds treatment for at least six months.

Practical consequence: test the roll you are about to run, not the roll you tested last week.

Primers: two real data sheets, and a conflict worth knowing

Two manufacturers, two quite different windows:

Nazdar qualifies its 24 hours with “possibly less in very humid environments” — which in a coastal Indian summer is not a footnote.

Worth flagging: Marabu’s own document says five hours; at least one distributor repeats 24. Trust the manufacturer’s sheet, not the reseller’s page.

And one storage rule that saves jobs, from Nazdar: “If the primer has been exposed to excessive heat and has become cloudy in the bottle, discontinue use.” Cloudiness means the silane has already condensed in the bottle. It will not bond.

Glass, and the tin side

Glass offers ink no diffusion path and no mechanical key. A silane coupling agent is the only bond available: as Gelest puts it, adhesion comes from “formation of a -Si-O-X bond with the substrate.” That is why primer on glass is not optional.

Float glass has two different sides, and this catches people out. The face that lay on the molten tin bath during manufacture takes up tin, giving it a higher density of surface hydroxyl groups and “higher adsorption of organic substances.”

You can tell them apart: the tin side fluoresces under UV illumination — and you already own a UV source. Nazdar’s own primer sheet instructs: “Apply to the non-tin side of float glass.”

Cleaning method matters measurably too: in one study, adhesion roughly doubled between a basic rinse and a plasma-cleaned surface. And silanes have a known blind spot — they “do not form stable bonds” with calcium carbonate fillers or marble.

More on the material at printing on glass.

Acrylic: the two things your shop does that crack it

Crazing on acrylic is usually blamed on the ink. Peer-reviewed work points elsewhere: alcohol acts on PMMA by physical diffusion, and machining leaves residual stress. Together, the crazing threshold falls from 31.4 to 22.8 MPa for regular PMMA — a 27% drop — and from 51.4 to 31.4 MPa for oriented acrylic, nearly 39%. Alcohol concentration made no difference.

Router or laser cutting, then wiping with IPA, are precisely the two factors that paper shows combine to weaken the material. If your acrylic crazes after printing, look at how it was cut and cleaned before you blame the ink. See printing on acrylic.

Testing adhesion properly — and the 24-hour rule

The cross-hatch tape test is ASTM D3359. Points that matter: Method A for films over 125 µm, Method B under; the scale is 0 to 5 and “users shall not use intermediate values”; tape is removed “at as close to a 180° angle as possible”; and “no single tape is likely to be suitable for testing all coatings.” The method was developed for metal, and its own precision statement admits plastics data “lacks precision and bias.”

But here is the single most commercially useful fact in this entire article, published independently by two ink manufacturers:

Final adhesion and chemical resistance are achieved after 24 hours.

Tape-testing a print that came off the machine ten minutes ago will fail a job that would have passed the next morning. If you reject work — or reject a machine at a demo — on an immediate tape test, you are testing the wrong thing. Print it, leave it overnight, then test.

What we will not publish

Primer coverage rates in m²/litre and pot life: neither figure appears in any data sheet we obtained. The D3359 percentage table, cut spacing, tape brand and dwell time are behind the paywalled standard — buy it if you need to certify against it.

And do not extrapolate European durability ratings to Gujarat. Marabu’s “at least 2-year vertical outdoor exposure” is explicitly “referred to the middle European climate.” The UV load here is materially harsher.

The short version

• Get your ink’s surface tension from its data sheet.
• Target the substrate at least 10 dyne/cm above it.
• Test with a pen — as a gate, not a measurement.
• Primer on glass; the non-tin side; not if the bottle is cloudy.
• Treated film decays — test the roll you are running.
• Wait 24 hours before you judge adhesion.

If a specific material is giving you trouble, bring a sample to a demo and we will print it. That is a faster answer than any article.

Sources

• Enercon — dyne margin, pen procedure and accuracy limits
• ASTM D2578 — wetting tension caveat
• ACCU DYNE TEST — substrate surface energy table
• Nazdar technical data sheets — 270 and 759 Series surface tension; 7020–7035 primers; non-tin-side instruction; cloudy-bottle rule
• Marabu technical data — P4 primer window, 24-hour final adhesion, European climate qualifier
• Vetaphone — corona treatment decay and slip additives
• Gelest — silane coupling chemistry and known failure substrates
• Float glass surface preparation study — tin side identification by UV fluorescence, cleaning method and adhesion
• PMC10052915 — alcohol and machining effects on PMMA crazing stress
• ASTM D3359-23 — cross-hatch adhesion test scope and limits