What exactly is the “washboard effect”?

What does a washboard have to do with the flexographic printing process? 

Quite simply, if the printed corrugated packaging looks very streaky like a washboard, this is known as the “washboard effect”. This unattractive, streaky printout is the result of direct printing on corrugated board. The reason for this streaky look is easy to explain:

Corrugated board is made up of several layers of paper, printing is done on the outermost web, the top web, with the fluted corrugated web directly underneath. The inked printing plate is rolled directly against the corrugated board. If the screen dots hit the crests of the corrugations, there is greater counterpressure than in the troughs of the corrugations. The screen dots are squeezed and print out partly elongated. This causes an increase in tonal value, which becomes visible in the form of stripes. The effect is intensified with corrugated board profiles with larger, coarser flute pitches. The use of thin liner paper also intensifies the effect.

This appearance often appears on printed pizza boxes, outer packaging, shipping boxes or on secondary packaging in the retailer or supermarket.

The washboard effect is not pretty to say the least and as a consumer, poorly printed packaging has an impact on the content. Ergo, not very pretty packaging, rather not so good goods. 

The question now is how to avoid the washboard effect? 

There are two ways to optimize the contact of the flexographic printing plate to the board.

Optimization of the substrate. Sure, it works, but it also makes the packaging expensive. One is to select the cover material with a sufficient grammage and quality and to keep the underlying corrugation as small as possible. The narrower the spacing between the tips of the corrugations, the better the printability of the corrugated sheets. To ensure that the stability and thus the actual protective function of the outer packaging is not lost, double flute corrugated board grades are often used. 

Here, printing is done on the outer, finer flute, while stability is achieved by the underlying, coarser flute. This approach definitely achieves the desired results, but is associated with higher substrate costs. 

The second adjustment screw is the right choice of printing plate or foam substructure. Super-soft, thick flexographic printing photopolymer plates can be used, which on the one hand reduce the washboard effect and at the same time achieve good ink density. The disadvantage of these thick plates is the cost and plate weight, which makes them difficult to handle during installation or mounting.

A better solution here is the combination of a thin, harder printing plate in conjunction with a compressible foam substructure. The foam substructure absorbs the unevenness in the corrugated board, thus reducing the contact pressure on the printing plate in the printing nip. This results in less dot gain, finer halftone images and significantly higher print quality. 

The foam substructures are characterized by their high recovery capacity after leaving the printing nip, they are resistant to printing inks, and their low weight makes them much easier to handle in the press. The latest, innovative foam substructure can be positioned many times better in the press thanks to its non-woven surface. This makes work much easier for the printer.

– This article was written by Sandra Kaiser. Visit Flexo consulting to know more.

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