After lamination, the colours of the printed sheet may feature a different shade of colour compared to the non-laminated sample.
Several factors can cause this. We will try to clarify some of them below.
First of all, please note that the plastic foil coupled to the printed sheet causes a variation in the path of light that hits 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, it is necessary to prevent this.
The achieved chromatic visual effect is basically determined by the behaviour and by the reaction of the following factors:
Furthermore, to achieve an optimal and repeatable result, it is crucial to know the densitometric and spectrophotometric values that were used for printing.
By 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.
However, it could happen that 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 carrying out various tests in the laboratory (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. Here are some details.
DEFINITION L*
The measured values highlight a diversified 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. Generally speaking, with dark colors (Black, Blue and C+M+G) the tendency with lamination is that it increases definition, while it tends to decrease with lighter colors. Finally, it tends to be slightly greater in solid backgrounds and decrease 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, particularly, in solid backgrounds and screened areas. Consequently, an image may feature colour variations that, overall may differ from each other.
SATURATION C* 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 difference 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 optical, i.e. due to the film layer that amplifies the visual perception of the dots dimension. It is as if the film behaves as a magnifying glass. Due to this effect, if the dot gain increases, 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 by means of objective values, which can be repeated in the event that the same printability conditions are repeated, it is possible to create a compliant print both in the start-up phase and in the printing one, ensuring its chromaticity.
Printing conditions vary as the number of printed copies increase
During the print run, the inking variation generates a chromatic variation. This condition entails that the greater the ink density range is, the greater the possibility of seeing this difference when the sheet is laminated.
The use of densitometer or spectrophotometer during the printing phases allows us to obtain a lower tolerance range.
Any further processing of the laminated material must not take place before the necessary and sufficient time has elapsed to achieve 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, be it paper or another material. Therefore, it is recommended carrying out preliminary tests to verify that applications after coupling take place properly.
TO SUM UP
- It is essential that the inks used in the printing phase are completely dry both on the surface and internally. Otherwise, the lamination process may cause an undesirable reaction, resulting in anomalies that affect the plastic film, causing its detachement and making the printed sheet non-compliant.
- The ink used in the printing phase must be suitable for coupling with the plastic film. Greater attention must be paid, for example, when using metallic and REFLEX inks. In fact, this type of inks has proven to be prone to delamination and colour change phenomena respectively after lamination.
- Read the ink technical data sheet carefully. Colour change phenomena may start to occur from half an hour after lamination up to 48 hours later.
- The ink must always be adapted to the type of paper.
- If you are not aware of the plastic film behaviour beforehand, when using certain types of ink, it is recommended to carry out a small printing run to check the final result.
TO CONCLUDE
With this document we wanted to share, with those who are involved in printing and lamination, the laboratory analysis that allows us to objectify different phenomena that occur during the printing/laminating processes. The indications given aim at supporting the knowledge already acquired by the operators. Finally, we concluded that non-compliances due to colour change are, in most cases, linked to an incorrect selection/use of materials and/or improper execution of the work, rather than anything defective.