UV-Curable TIJ on PE and PP Film: Why Surface Treatment Still Matters

A converter runs out of polypropylene film and substitutes a roll from another supplier. The batch code prints crisply, the UV lamp is lit, and the first samples feel dry. Two days later someone in the warehouse rubs several codes off while stacking finished bags.

The reflex is to blame the lamp, or the cartridge. But the resin name is not the variable that changed. Two films can share the name PP and still present entirely different surfaces to the same ink — different treatment, different coating, different slip additives, different storage history.

Think of two identical kitchen counters. One was wiped clean; the other was polished with a waxy spray. The same sticker behaves differently on each, even though the counters are the same material.

“PP” is not a printing condition

Ask the film supplier to name the intended print side and the surface-treatment specification, and to say in writing how a change will be communicated. Then walk the packaging: after the film has been formed into a bag, does that treated side actually face the printhead? Film can be correctly treated and still be printed on the wrong face.

Treatments are also not permanent. Corona and flame treatment decay during storage, so stock that was treated last quarter can behave differently from a fresh roll. The diagnostic order for adhesion failures covers how to measure that and why surface energy should be ruled out before anyone changes ink.

Take samples from the beginning, middle and end of a representative run, and print both on flat sections and at the position the real code will occupy. Use film that has passed through the same rollers, guides and forming shoulders as production stock. A clean sheet from a supplier’s sample book is an optimistic test that will not survive contact with your line.

Curing and bonding are two different things

A UV-curable ink film can be fully cured and still lift off the surface. Picture a pancake that cooked through and then slid off a nonstick pan: the cooking finished, the adhesion never developed.

Exposure and interface are separate variables, and on a printed label they fail in ways that look similar enough to be confused:

Under-exposure. The mark is tacky, smears before it is rubbed, or shifts under a thumb. Look at the lamp setup, the working distance and the line speed.

Interface failure. The mark looks clean and comes off in a coherent patch, or takes the film’s own surface layer with it. Look at the surface, the coating, or the ink chemistry.

Both at once. This is common on a new film lot, which is exactly why the order of the checks matters more than the elegance of any single one.

An exposure study has to run across the whole web, not just under the brightest part of the curing unit. A long strip of film can bow or flutter beneath the lamp, which changes the working distance and the dose. The centre of the web and the two edges rarely see the same exposure, and the gap widens as speed rises. Sample at the edges and in the middle, at production speed.

Record what the lamp does to the film as well as to the ink: does the film warm, distort, block when wound, or change appearance? A route that marks beautifully and blocks on the reel is not a route.

Keep the two failure families in separate columns of the record. Photograph the failure, retain the printed package, and label the ink, the film lot, the lamp setting and the time. A supplier can work with that. “UV does not stick” is not a test result.

Compare routes with everything else held constant

The right answer is sometimes a surface change, sometimes a coding change, and sometimes both. You cannot tell which until the comparison itself is fair.

Hold the message, the resolution, the print location, the speed and the post-print test constant, and vary only the route: the ink you run today, a UV-curable candidate, and any pretreatment under consideration. Comparing a treated sample of one film against an untreated sample of another tests the film rather than the ink.

Write the acceptance rule before the trial starts. Readability after folding, a named rub procedure, resistance to whatever the pack will actually meet in transit, and stable scanning after secondary packaging. A criterion invented after the results arrive is a criterion shaped to fit the results.

Give every combination of film lot, ink and treatment its own ID, and keep several printed specimens rather than one winner. If the product is a refill pouch, flex a realistic number of samples near the code and run them through the real secondary packaging. If the customer scans with an automated reader, test with that reader, in the orientation it will actually see.

Two published figures frame the whole trial. An untreated PE or PP surface sits in the 26 to 32 mN/m band, and a coding ink has to beat it by roughly 8 to 10 mN/m before the drop wets rather than beads; corona treatment lifts the same surface to about 38 to 42 mN/m, and that gain decays in storage. A trial run on stock treated months ago is measuring the warehouse, not the ink.

Cost comes last, not first

Only once you hold technically acceptable combinations does the commercial comparison mean anything — and then it should be cost per compliant package, not a cartridge price.

A UV route carries acquisition, electricity, cleaning, shielding, maintenance and operator training. Pretreatment carries its own equipment and process controls. A solvent system may already meet the durability target and be simpler to run. Comparisons that stop at the cartridge quote miss most of the difference and usually favour the wrong option.

A code that survives a flat coupon is not yet qualified

A household cleaning brand codes PP refill pouches. The code rubs off mainly at the edge of a gusset, after the finished packs have been compressed inside a shipping carton. The technician checks the printhead distance first and rules out missing nozzles.

The team then tests the current ink and a documented UV-curable candidate on the same film lots, at the gusset and on a flat panel. The UV candidate improves the flat panel. At the flexed seam, both inks perform badly.

The useful conclusion is not “the new cartridge failed”. It is that the code sits where the pack cannot protect it. Changing the code location, repeating the fold-and-rub sequence, and asking the converter which face is treated produces a purchasing specification that names the approved film face — plus a rule to requalify the code if the laminate or coating changes.

This example is a constructed troubleshooting scenario, not a customer case. If the code is a 2D symbol and your customer grades it, the parameter set is ISO/IEC 15415, the standard for two-dimensional symbol print quality — agree the grade before the trial, not after it.

What we will and will not claim

XinmoJet manufactures TIJ ink cartridges and formulates the inks we supply. For low-energy films that means a solvent system formulated for non-porous surfaces; for covert marking verified under a lamp it means a UV security ink — a different product from a UV-curable ink, and worth keeping separate in your head as well as in your specification. The distinction is physical rather than commercial: a UV-curable ink is cured by photo-initiation, in the 365 to 395 nm band that UV-LED curing heads emit, so the mark sets by chemistry. A covert UV ink is not cured by that light at all — it is read under a 365 nm lamp.

We do not supply UV-curable TIJ ink or curing systems. If your durability target can only be met by a UV-curable route, we would rather tell you that on the first enquiry than quote you a cartridge that fails on your line.

What we can do is run the comparison on your material instead of on a generic coupon. Send roll stock from the lot you actually run, the print face, your line speed and the distance from the printhead to the first thing that touches the code. Samples are printed on your own stock, free of charge, on the second working day after it reaches us, and we test rub and tape resistance against it rather than against a lab sheet.

Where to go next

If you have not yet ruled out surface and layout causes, start with the diagnostic order for adhesion failures — it is the cheaper sequence. If you are choosing a cartridge for a non-porous surface, begin at the cartridge range. And when you are ready to have the material tested rather than described, send us the film.

The same wetting question comes up on moulded parts, so coding on HDPE and PP covers the rigid side, and see how our TIJ ink cartridge factory is run explains how the solvent cartridges used here are filled and tested.