Paper and ink testing

A variety of tests are used to determine ink and paper and paperboard quality, and to measure their interactions. They are necessary to balance print quality, cost, and wear on the press.

List of paper and ink tests

Some of the paper and ink tests are listed here:

I. Paper test A wide variety of paper tests are available, depending on the specific needs and on the governing contract or specification:[1]

II. Ink and Paper (Printability Tests)

  1. Print density
  2. Ink requirement
  3. Printed colour values
  4. Wet on wet printing (Two colours) - Trap
  5. Print Through
  6. Set Off
  7. Strike Through
  8. Abrasion resistance

III. Ink

  1. Fineness of the grind

Grammage Test

The mass per area of the paper is measured as gsm (grams per square meter). Paper sheets are cut to a size of 10 cm x 10 cm, then the sheets are massed individually. The mean of the masses is used to calculate the mass of the paper per square meter, and the standard deviation is determined from all samples. If the mass is higher than specified, it is a loss for the publisher.

Paper weighed in a precision balance

Colour of the paper substrate

Paper shade plays a significant role in the quality of colour reproduction. The shade of the paper affects the colour values of the primary and secondary colours and hence the colour gamut of a press. The standard for paper shade of newsprint as per ISO 12647-3 is mentioned in the table.

Backing L* a* b*
Black backing 82 0 3
White Backing 85.2 0.9 5.2

The maximum tolerance specified is ∆L 4, ∆a 2 and ∆b 2. When selecting a paper for production, it is very important to measure the paper shade. A spectrophotometer or a spectrodensitometer can be used. Publishers should also check for batch to batch variations. The ultimate aim is to use the paper which has a shade closer to ISO specification.

Colour values of primary and secondary colours

The colour produced by ink depends on the ink composition, pigments and also the paper shade. This test can be done in two ways. 1. Same ink printed over different paper substrates and 2. Different ink samples printed over a standard newsprint that matches ISO specification.

The most important thing that should be noted is the colour of the inks at the ISO specified density i.e. C 0.9, M 0.9, Y 0.9 and K 1.1. The ∆E value between ISO specified colours and the colours produced by the sample ink gives the ink's deviation from standard.

The colour values of the secondary colours (R, G and B) are also measured. From the colour values of C, M, Y, R, G and B, a 2-D curve of the colour gamut can be plotted and compared with the ISO colour values.


Ink requirement is defined as the amount of ink needed to print a unit area with a standard Solid Ink Density. For newsprint, the ISO 12647-3 specification is C 0.90, M 0.90, Y 0.90 and K 1.10, Status E, D50, 2o, density minus paper with Polarization filter. Ink requirement is measured as g/m2 (grams of ink required to print 1 m2 of paper)

Different paper substrates require different amount of ink to achieve the standard SID. A highly porous substrate generally requires higher amount of ink compared to a less porous substrate. Therefore, to study the ink mileage of different paper substrates, this test can be done on all the newsprint samples.

Ink requirement also depends upon the ink formulation and the colour strength of the pigments. From this test, the ink mileage of all the four process colours, when printed in the same paper, can be studied.

Trap

Trap is a measure of the ability of a wet ink film printed on the paper to accept the next ink printed on top of it. Preucil’s formula is used to measure trap. % Trap = {(Density of overprint – Density of 1st ink)/Density of 2nd ink} x 100

Trap affects the colour values of the secondary colours, and plays a major role in defining the color gamut of a press. Poor trap leads to colour shift in the secondary colours. Trap is also affected by the ink tack, viscosity, ink film thickness and the impression pressure.

Set-off

Set-off is defined as the tendency of ink to transfer from a freshly printed surface to another paper when pressed without any friction. Set-off is an unwanted behavior of paper and ink. It is influenced by the characteristics of the paper and ink: a porous paper surface absorbs the ink fast results in lesser set-off, as do inks that dry (or set) quickly. The higher the speed of the press, the higher the set-off will be.

Set-off is measured at two different time intervals, 0.35s and 3s. A freshly printed surface is pressed against a paper after 0.35s and after 3s. Then the set-off density is measured. The idea behind selecting these two timings is to know what will be the set-off when the materials are used in a high-speed press and a low-speed press. The table below gives an example of set-off measurement.

Ink 0.35 s 3 s
Cyan 0.09

0.07

Magenta 0.11 0.09
Yellow 0.09 0.07
Black 0.10 0.08

Pick test

The de-lamination of the paper surface while printing is called picking. Papers with lower surface strength lead to fluff accumulation and cause the blankets to need cleaning more often. The velocity at which the paper shows the first sign of picking is measured. A higher pick velocity indicates a higher quality paper substrate, and offers better runnability and productivity in the press.

Print through is measured as the print density of the reverse side of the printed-paper which is printed at a standard print density. The lower the print through, the better the paper.

Fineness of ink grind

Fineness of ink grind is an important parameter that describes the quality of dispersion of the pigments in the ink. A grindometer is used to test the fineness of the pigment particles. The lower the particle size the better is the dispersion of the ink.

References

  1. ↑ Caulfield, D. F.; Gunderson (1988), Paper Testing and Strength Characteristics (PDF), Proceedings of Symposium on Paper Preservation, TAPPI, pp. 31–40
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