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HOW LAMINATION ACTS ON THE COLOUR RENDERING OF A PRINTED SHEET – SOME CONSIDERATIONS –

Colorno, 14 November, 2013

After lamination, the colours of the printed sheet may feature a different shade of colour compared to the non-laminated sample.

There are various factors that may determine the situation; we will try to clarify some of them below.

First of all, please note that the plastic lamina that is coupled with the printed sheet causes a variation in the path of the light hitting the sheet: first directly on the printed sheet and after lamination, through the plastic film. Since the colour perception is affected by the light’s path, this phenomenon must be taken into account.

The chromatic visual effect that is achieved is basically determined by the behaviour and by the reaction of the following elements:

– Type of plastic film used (opacity and transparency), surface structure, thickness.

– Technology with which the plastic film is applied.

– Type and quantity of adhesive.

– Type of paper.

– Type of ink (colorimetric characteristics, ink tack, viscosity, transparency, pigmentation, etc…).

– Printing machine, printing sequence or printing conditions (rubber blanket, plate, damping solution, etc…).

– Printing elements: relationship between graphism (printing part) and non-image areas (non-printing part).

Also, to achieve an optimum and repeatable result, it is crucial to know the densitometric and spectrophotometric values that were used for printing. Comparing a non-laminated printed sheet with the same laminated sheet, it is obvious how lamination significantly affects both its general appearance as well as the colour rendering of the printed sheet. In fact, the images with the plastic film may take on an unexpected and undesired dominant colour.

The colour change of the ink in the light is amplified by the plastic film.

By conducting several laboratory tests (spectrophotometry and densitometry) the first observation in relation to the values measured is that lamination significantly affects the colours of the printed sheet. This variation is not the same for all colours. Below are some details.

DEFINITION L*

The values measured show a different behaviour in relation to the colour examined. For example, with regard to white paper, the definition decreases with lamination, although minimally, while with black it increases significantly. In general: with dark colours (Black, Blue and C+M+G) the tendency with lamination is that it increases Definition, while it tends to decrease it with lighter colours. Finally, it tends to be slightly greater in solid backgrounds and decreases in screened areas (30%; 50%; 80%).

The results achieved suggest that the effect of lamination on the chromaticity of an image is not similar in all the percentages and particularly in solid backgrounds and screened areas; consequently an image may feature colour variations that, overall, may be different from each other.

SATURATION C* and SHADE h°

Even with saturation we have a different trend of the values in less saturated solid backgrounds and more saturated screened areas (30%; 50%; 80%). In relation to SHADE h°, this remains quite stable except in the case of 100% red and in Magenta surface coatings (30%; 50%; 80%).

Print contrast K/Dot gain ratio

A factor that helps us understand the differences between a laminated printed sheet and a non-laminated one is the dot gain value. This value increases significantly with lamination.

It is obvious that the dot gain is not physical but instead optical, i.e. due to the film layer that amplifies the visual perception of the dot dimensions. It is as if the film acts as a magnifying glass. Due to this effect, if the dot gain increases, as a result the print contrast K decreases.

The print contrast K/dot gain ratio determines the qualitative trend of the printed sheet during printing. By indicating the reference tolerances via the objective values, which can be repeated in the event the same printing conditions reoccur, it is possible to achieve a compliant printed sheet both during start-up and during printing, ensuring chromaticity.

The printing conditions vary gradually as the printed copies increase

During the print run the inking variation generates a chromatic variation. This condition entails that the greater the ink density range, the greater the possibility of seeing this difference when the sheet is laminated.

The use of the densitometer or spectrophotometer during the printing steps allows a lower tolerance range to be achieved.

Any further processing of the laminated material must not take place before the necessary and sufficient time has elapsed for the maximum adhesion of the film to the coupled support. This period may vary according to the various inks and finishing products used together with the type of support, whether it is paper or another material. It is good practice, therefore, to carry out preliminary checks to verify that applications after coupling take place properly.

TO SUM UP

– It is crucial that the inks are completely dry both on the surface and internally; otherwise the lamination process may determine an undesired reaction, resulting in anomalies that affect the plastic film, making the film detach and making the printed sheet non-compliant.

– The ink used for printing must be suitable for coupling with the plastic film. Great care must be taken, for example, when using metallic and REFLEX inks. This type of ink has in fact proven to be prone to delamination and colour change phenomena after lamination.

– Read the ink technical sheet carefully.  Colour change phenomena may start to occur half an hour after lamination and up to 48 hours afterwards.

– The ink most always be adapted to the type of paper.

– If you are not aware of the plastic film’s behaviour beforehand, when using certain types of ink, it is good practice to carry out a small print run to verify the final result.

TO CONCLUDE

With this document we wanted to share the laboratory analysis that allow us to objectify different phenomena that occur during the printing/coupling processes, with those involved in printing and laminating paper. The indications it contains aim to contribute to the knowledge already acquired by the operators. Finally, it was also concluded that non-compliances in colour rendering are in most cases linked to an incorrect selection/use of materials and/or improper execution of the work, rather than anything defective.

Mag Data Spa

Stefano Galderisi