Primer for UV printing: when you need it and which one
are explained briefly when you hover over or tap them.
The print looks perfect, and after a few days you scratch it off with a fingernail. Almost always the cause is a lack of adhesion. Primer is a thin, clear layer that acts as a bridge between material and ink. On some materials it is mandatory, on others superfluous.
What primer is for
UV ink adheres best to surfaces that it can wet well. Smooth, dense materials such as glass, bare metal or certain plastics have a low surface energy or give the ink nothing to grip. The droplets then merely sit on top instead of bonding.
Primer, also called an adhesion promoter, improves this bond. It raises the surface energy and itself forms a firm bond with the substrate and with the ink. A well-chosen primer often also improves colour appearance, because the ink spreads more evenly.
Primer does not replace clean preparation, though. Grease, fingerprints or release agents on the material prevent adhesion even with primer.
Which materials need primer?
| Material | Primer needed? | Note |
|---|---|---|
| Bare metal (stainless steel, raw aluminium) | yes | degrease thoroughly, metal or glass primer |
| Anodised aluminium | usually yes | depends on the anodising, test |
| Coated metal (photo metal) | usually no | coating is intended for printing |
| Glass, glazed ceramic | yes, always | glass primer |
| Acrylic | often yes | depends on the sheet, usually needed on smooth surfaces |
| Untreated wood | no | wood accepts ink well |
| Varnished or oiled wood | test | depends on the varnish |
| ABS | usually no | test on glossy surfaces |
| PP, PE | yes, special primer | very low surface energy |
| Silicone | hardly possible | unreliable even with primer |
| Leather | usually no | flexible ink matters more than primer |
This table is a guide. Plates, mugs and blanks differ depending on the supplier, even when they have the same name. You will find more detail for each material in the materials overview.
Which types of primer are there?
Primer for glass and ceramic. For very smooth, mineral surfaces. Many glass primers also work on metal.
Primer for metal. For stainless steel, aluminium, copper and powder-coated surfaces. Some manufacturers combine metal and glass in one product.
Primer for plastics. For acrylic, glossy ABS, polycarbonate. For PP and PE there are dedicated special primers, because normal plastic primers are often not enough there.
Universal primer. Covers many materials reasonably well. Handy for getting started, but on difficult surfaces usually weaker than a suitable special primer.
The safest choice is the primer that the manufacturer of your machine recommends for the material in question. Primer and ink are matched to each other.
Applying primer
There are four common ways:
- Wipe or wipes: Pre-soaked disposable wipes or a lint-free cloth with liquid primer. Good for individual objects.
- Primer pen: For small areas and curves. Clean, but laborious for large areas.
- Spraying: For larger areas and series. Even, but you need ventilation and masking.
- Printing it along: Some machines have their own primer channel and print it only where ink will land later.
This is how to proceed when applying by hand:
- Degrease the surface with isopropyl alcohol and a lint-free cloth, wiping in one direction. Wear gloves.
- Apply primer thinly and evenly, wiping in one direction. Go slightly beyond the later print area so that the edges definitely lie on primer.
- Let it flash off until the surface is no longer tacky. The time is stated on the product, often a few minutes.
- Print promptly. Many primers lose effectiveness after a certain time.
Too much primer is a common mistake. A thick layer stays soft, forms streaks or shines visibly next to the design. Thin is better.
Primer is a chemical. Ventilate, wear gloves and read the safety data sheet. More on this in UV safety.
Testing adhesion
Whether the primer works only shows after printing. After printing, wait at least half an hour, preferably longer, because adhesion still increases.
Tape test. Rub a piece of strong adhesive tape firmly onto the print and pull it off with a jerk. If ink stays on the tape, something is wrong.
Cross-cut test. The more precise test according to DIN EN ISO 2409: with a cutter, make six parallel cuts through the ink at a spacing of 1 to 3 mm, then six cuts across them. Press on adhesive tape, pull it off quickly and look at the grid. It is rated from 0 (no flaking) to 5 (large pieces detached).
Everyday test. Fingernail, coin, damp cloth. Not standardised, but close to what your customers will do later.
Always test on an offcut from the same batch as your material.
For business use
Primer costs little per piece but saves complaints. For every material you offer, keep a short note: which primer, how applied, result of the cross-cut test. If a supplier changes the material, you notice early. And for products under heavy use, such as key rings, varnish is also worthwhile as protection.
Continue with: Ink does not adhere and Varnish in UV printing.