Technical information
 

Intensity or coloring strength

By the intensity of an ink, we understand its colour strength, which is the result of the concentration, nature and development of its pigments.

Although they are closely related, it should not be confused with the colour intensity of the printed film, since this is influenced by other aspects: transfer, speed, stock type, etc.
It is measured comparatively with a known concentration standard. For this, each of the inks is degrated with a white ink and the difference in the intensities of each is evaluated.

Gloss

The gloss of an ink is measured by the amount of light reflected from the printed ink, and it is a function related to various factors: ink type and quality printed, paper used, amount of water used for the printing, etc. The gloss is measured with a glossmeter with a measurement inclination of 60º. To evaluate the gloss level of an ink in comparison with another the samples must be taken using the same paper and printing conditions.

Rub Resistance

The rub resistance of an ink corresponds to the greater or lesser tendency of the printed work to suffer from deterioration by rubbing with other unprinted stock of the same material and, besides the ink , it also depends on other factors, such as: the stock used and the printing conditions. Its measured with an apparatus called Rub Tester, which has an arm with a weight (normally of 1 or 2 kgs). the printed sample is placed below the weight and on another sample of the same stock, which is immovably fixed in place. The highest resistance corresponds to the highest number of cycles one sheet of stock can be passed over the other before any deterioration of the printed work occurs.

Transfer

This is the capacity of the ink for transfering from the plate to the blanket and from there to the stock. To obtain correct printing, the highest possible transfer of ink must take place and for this, the following factors must be taken into account:
The hardness of the rollers and the rubber blanket, together with gigh speeds, reduce transfer levels.
The increase in the smoothness and compactibility of the stock improve ink transfer.
Appropiate ink viscosity, yield value and tack are determining factors in transfer.

Inks drying

We can say that the drying of an offset ink is its capacity for passing from a liquid to a solid state and it is usually carried out in two phases:
- Drying by absorption (setting), where the most fluid components of the ink separate and penetre into the paper.
- Drying by oxidation (polymerisation) where the rest of the components are oxidized and polymerised, solidifying on the paper.
The external factors that can affect the drying of the ink include the following: the temperature, the stock (coated type, acidity, porosity, humidity, etc.), the pH of the fountain solution, etc.

Viscosity

The viscosity of an ink is the resistance it shows to following when a certain force is applied to it and it is the main factor characterising its rheological behaviour. Furthermore, there are other physical concepts related to viscosity, such as yield value and thixotropy.
Yield value is the minimum force required for an ink to begin to flow and corresponds to friction between its largest particles (usually, the pigments). Thixotropy is an apparent viscosity which gives the ink its body and a paste-like appearance, where when shaken, it tends to flow easily, returning to its initial status when left to stand.

An appropiate combination of these three physical phenomena will improve the dot sharpness, it will favour the distribution of the ink on the rollers and provide an appropiate ink-water balance.

Tack

The tack of an ink is the resistance of a film of printed ink between the two surfaces in which it stands.In other words, the tack of an ink expresses its level of stickiness. With offset printing, this property of the ink is very important and has the following influence:
- The distribution of the ink on the press rollers.
- Its tendency to emulsify with the water.
- The dot gain in the section.
- In particular, the transfer of the ink to the stock, since the effort the stock must make to separate the ink from the blanket can cause dificulties.

Resistance to light and chemical agents

The resistance to light of an ink is its capacity to maintain the shade and intensity of its colour under natural or artificial light and depends almost exclusively on the nature of the pigment. The resistance to light of an ink is measured on a scale of 1 (very low) to 8 (very high).
The resistance of an ink to chemical agents is a fundamental property of the pigment and results from its capacity for maintaining the shade and intesity of its colour under the direct action of chemical agents such as acids, alkalis and solvents. In the case, the scale runs from 1 (very low) to 5 (very high)

Colour

In Graphic arts, colour may be defined as the type of light reflected on the printed work and it is an optical property of the ink that is intrinsically relatedto the nature of the pigment with which it is made..
In order to measure the light reflection values in different inks, a spectrophotometer is used to read the printed samples converted into values of the CIE L*a*b system, which gives a numeric expression of the colour differences. Byusing this method, it is possible to determine the number of shades and tones that are visibly distinguishable on the printed work.
Whatever the case, the exact shade can be changed in accord with the spectral composition of the light and the characteristics of the stock.

Colour obtaining techniques

To make an ink of a certain color or according to sample, means giving in the qualities required for the specific work to be printed. Once the viscosity, drying, and setting properties, etc, have been adjusted, the exact shade requires a high level of care and precision. For this, current and appropiate technolofy must be used for the type of ink we wish to achieve.First of all a spectrophotometer measures the exact color to be obtained,which, connected to a formulation computer programme enables us to determine the proportions that intervene in each colour. With precision scales, we weigh the exact amounts of these proportions and once they have been well mixed and homogenised, we make a printed sample of the product. We then individually measure the colour, checking that the values obtained agree with the defined standard.

Opaqueness and transparency

The opaqueness of an ink is its covering power and determines the property of the printed film for hiding what it has underneath, whether it be the stock or another ink. It depends basically on the pigment used and consequentlyon its capacity for absorbing the light and deviating it in an endless number of directions.

Additives

The formulation of an offset ink is always aimed at giving it the qualities required for it to have immediate setting, fast drying, colour intensity, rub-resistance, etc. In such a way that the image is transferred exactly in a controlled way and with maximum quality onto the stockl.
Often, the printer, either to attend the urgencies and demands of his clients or as a result of the different factors that may occur during printing: temperature, humidity, stock type, water, etc., needs to highlight or vary certain properties of the inks. For this reason, certain additives which, although their excessive use is not advised, in certain circumstances help to obtain better results in print jobs.