Printing on corrugated polypropylene is routine for signage and for branded packaging, and it is also the thing people get wrong most often – because ink behaves on polypropylene the way adhesive does, which is to say badly without preparation.
Why it does not just work
Polypropylene has a low surface energy. Ink sits on it rather than bonding to it, and it scratches off or rubs away within weeks. Anyone who has printed on a sheet and watched the print come off on their hands has met this.
The same property is why labels lift off polypropylene boxes – covered in do barcode labels stay on PP picking boxes. It is one underlying issue with several symptoms.
Option one: treat the surface
Corona or flame treatment raises surface energy so that ordinary inks bond. It is done at manufacture, on the area to be printed, and it makes screen printing and most digital processes work properly.
The catch is that the effect decays over months. A sheet printed a year after treatment will not behave like one printed a week after. If your sheets sit in stock before printing, say so and ask for a treated grade with a longer window.
Option two: use the right ink
Inks formulated for low-surface-energy plastics will bond without treatment. They cost more and the range is narrower. For a simple one or two colour job this is often the simplest route, particularly at low volume where treatment minimums are awkward.
Option three: do not print
Applied vinyl, or a printed label with the right adhesive, or a card in a holder. For a message that changes – a batch code, a destination – printing is the wrong answer anyway, because the sheet outlasts the message.
And for a sign that needs to last outdoors for years, the print is usually the first thing to fail. Consider whether the message can be cut rather than printed – routed lettering or a stencil.
Two practical points
Print on the smooth face. Embossed or textured sheet will not take a clean print; the texture breaks the image. Specify a smooth face on the side to be printed.
Print before fabrication if you can. Printing a flat sheet is easier and cheaper than printing a made-up box, and the registration is better. If the part is simple, print the blank and then fold it.
Durability, and what to expect outdoors
Printed sheet outdoors fails in a predictable order: the print goes first, then the sheet chalks, then the sheet becomes brittle. If the sign needs to last years, the print method matters more than the sheet.
Screen printing with a properly bonded ink lasts longest of the cheap options. Digital prints are faster to set up and generally less durable. Applied vinyl sits between the two and has the advantage of being replaceable without replacing the sheet.
Worth matching the method to the life you need rather than the quantity – a short-run outdoor sign printed digitally will look tired well before the sheet does.
What we can do
We supply PP corrugated sheet with a corona-treated print face, and cut to size so the print area is where you want it. Tell us the print method and the volume, and we will tell you what the sheet needs – including when printing is the wrong approach.