UV printing on plastic: what sticks and what does not

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Plastic is not the same as plastic. On some types UV ink holds so well that you can hardly get it off again. On others it almost beads off. The reason is surface energy, and anyone who understands it can estimate before the first print whether a material will work.

Why some plastics refuse

For ink to adhere, it has to wet the surface. That only works if the surface has a higher surface energy than the ink. It is measured in mN/m, and in the industry often still in “dyn/cm”.

Untreated, PP and PE are at about 30 to 32 mN/m. For good adhesion of inks and adhesives, about 40 mN/m and more is the target. That is why ink beads off PP and PE, similar to water on a waxed surface. Silicone is lower still.

With test inks from specialist suppliers you can roughly check the surface energy yourself. For everyday use, though, the tape test after printing is usually enough.

The overview

Acrylic has its own article: Printing on acrylic and glass.

Primer for plastics

On ABS, PVC and PET you usually do not need a primer. On glossy or unknown surfaces, a comparison test is still worthwhile: one half with primer, one without.

For PP and PE there are special primers of their own. Normal plastic or universal primers are usually not enough there. Even with a special primer, the result remains less certain than with other plastics. Expect that some of the prints will fail the tape test. How to apply primer is covered in the article on primer.

Before every print, remember: plastic parts straight from production often have release agent on the surface. Clean with isopropyl alcohol, let dry, and do not touch again.

Flame and corona treatment

In industry, PP and PE are treated with a flame, corona discharge or plasma before printing. This briefly raises the surface energy. The effect fades over time, so the treatment is done shortly before printing.

For beginners and small workshops this is rarely an option. Flame treatment with a torch is hard to reproduce, can deform parts and is a safety risk with flammable plastics. If you regularly have to print on PP or PE, UV DTF as a detour is the better choice. More on this in UV DTF.

Phone cases: PC or TPU?

Phone cases are a popular product, and the material decides how long the print lasts.

Hard cases made of PC can be printed well. The surface is stiff, so the ink does not crack. You can also print transparent PC cases from the inside, so that the design sits protected.

Soft cases made of TPU bend when you put them on and take them off. Hard ink cracks in the process, especially at the edges. Flexible ink holds much better. Some cases combine a hard PC back with a soft TPU rim. In that case you only print the hard surface.

With silicone cases, you should avoid direct UV printing.

Printing on 3D prints

UV printing and 3D printing go well together: you print a shape and then give it a coloured design. PLA and PETG generally work well. A few points help:

• Light filaments in white or light grey save the white underbase.
• Print direction: the top of a flat plate is often less smooth than the underside on a smooth build plate. The underside is therefore often better suited.
• Layer lines: UV ink traces grooves. Sand side surfaces smooth if needed.
• Flat surfaces: ink does not reach deep recesses cleanly. Plan for flat areas.
• Heat: PLA softens at only about 55 to 60 °C. UV LED lamps get less hot than old mercury lamps, but thin parts should still not lie under the lamp for too long.

Which filaments suit which purpose is covered in the Filaments section.

For business use

When buying blanks, always ask for the exact plastic. “Plastic box” or “plastic bottle” is not enough. Many cheap blanks are made of PP or PE, and that is exactly where complaints about flaking prints come from later. A quick tape test on a sample saves you a whole batch of waste.

Continue with: Primer and the material overview.