UV print not sticking: ink scratches off
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The print looks perfect, yet it can be scraped off with a fingernail, or it flakes off after a few days. UV ink does not soak into the material like ink on paper. It cures on the surface and holds only as well as it bonds with that surface.
Cause 1: The surface is not clean
A single fingerprint is enough for the ink to hold worse at that spot. Typical contamination:
- Grease and skin oil from handling
- Release agents from manufacturing, especially on injection-moulded plastic parts
- Silicone, for example on phone cases or coated surfaces
- Dust and remnants of protective film
What helps: wipe the surface with isopropyl alcohol (IPA) before printing and let it dry, then only handle it with gloves or by the edges. Ink manufacturers such as Fujifilm also recommend IPA for plastics that contain slip agents or plasticisers. With silicone, cleaning alone usually does not help, as silicone is fundamentally very hard to print on.
Cause 2: Low surface energy
Some plastics repel liquids the way a waxed surface repels water. The measure for this is surface energy, given in mN/m or “dyn”. For ink to wet well, the surface energy of the material should be clearly higher than the surface tension of the ink. In the printing industry, a gap of about 10 dyn is the guide value.
Untreated polyethylene (PE) is only at about 30 to 32 dyn, and polypropylene (PP) is also difficult. These materials need pretreatment. With test inks (dyne pens) you can roughly check the surface energy.
What helps: primer, lightly sanding smooth surfaces, and for PE and PP a flame or corona treatment. Which plastics are how difficult is covered under Printing on plastic.
Cause 3: No primer or the wrong one
Glass, metal, ceramics and many plastics need an adhesion promoter. Primers come for wiping on, spraying on, or as a separate ink channel in the printer. Important: primers are matched to specific groups of materials. A glass primer often does not help on PP, and vice versa.
- Apply primer according to the manufacturer’s instructions, not too thick
- Observe the flash-off time before you print
- Test different primers on a sample piece when a material is new
Details under Primer in UV printing.
Cause 4: Undercuring
UV ink only becomes solid through enough UV light. If it gets too little, it stays soft inside or slightly tacky on the surface and adheres poorly.
- Lamp: dirty lamp glass, aged LEDs or a power level that is too low
- Speed: fast print modes often mean less exposure per layer
- Layer thickness: many layers of white, colour and varnish on top of each other cure less thoroughly, especially at the bottom
- Contaminated ink: cleaning fluid in the ink can interfere with curing
What helps: clean the lamp glass, check the curing level in the software, choose a slower mode, reduce the layer thickness. A subsequent exposure can help with slight undercuring.
Cause 5: Hard ink on flexible material
UV inks come in hard and flexible variants. Hard ink is scratch-resistant and good for glass, metal, acrylic and wood. On bendable materials such as leather, silicone cases, films or thin plastic, it cracks when bent and flakes off. Flexible ink stretches with the material, but is usually slightly less scratch-resistant.
Which ink your machine uses is in the datasheet. More on this under UV ink.
Cause 6: Tested too early
Straight after printing, the ink is solid but often does not yet have its full final adhesion. With UV inks, a post-cure sets in. For its Uvijet inks, for example, Fujifilm states that maximum adhesion, scratch resistance and chemical resistance are only reached about 24 hours after curing. Other manufacturers give similar periods, so look at the datasheet of your ink.
A scratch test straight after printing therefore only tells you so much. Better to test after a day.
Testing adhesion properly
Tape test: press a piece of strong tape (for example packing tape) firmly onto the print and pull it off with a jerk. If ink stays on the tape, adhesion is too weak.
Cross-hatch cut: with a sharp cutter, cut a grid through to the substrate in the print, about six cuts lengthways and six across. Then press tape on and pull it off. This is based on standardised test methods such as DIN EN ISO 2409 or ASTM D3359.
| Result | Assessment |
|---|---|
| Cut edges smooth, nothing detached | very good adhesion |
| Small flakes at the intersections | sufficient for most decorative items |
| Whole squares missing | adhesion too weak, look for the cause |
| Ink comes off over a large area, also outside the cuts | surface or primer does not suit |
For business use
Adhesion problems often only show up at the customer’s: in the dishwasher, on the key ring or after shipping. Test every new material and every new batch of blanks before you go into series production, and keep the test pieces. Include clear care instructions, such as “not dishwasher safe”. How long a print lasts under different conditions is described in the article Durability of UV prints.
Quick checklist
- Surface cleaned with isopropyl alcohol and not touched afterwards?
- Material known to be difficult (PE, PP, silicone, glass, metal)? Then use a suitable primer.
- Primer applied correctly and flashed off?
- Lamp glass clean, curing sufficient, mode not too fast?
- Layer build-up not unnecessarily thick?
- Flexible material? Then use flexible ink.
- Adhesion only tested after about 24 hours with tape or cross-hatch cut?
Continue with: Primer in UV printing and Printing on plastic.