Guhan S K - Ink testing

Maths and me @ Ink testing

What is yield in Ink testing?

Yield is the area that can be printed with given quantity of ink in grams (g/m²). In practice, however, it has become more usual to indicate the ink quantity in g/m² at a particular density value as yield indicator; in other words, the lower the ink quantity, the greater the yield. In ink manufacturing yield is determined by producing proofs with increasing ink thickness (0.7 to 1.3 g/m²), and determining the density value with densitometer. Plotting the density value against the ink quantity produces the yield curve.

From the image ink A reach the target density 1.75 D in 0.99 g/m² , but ink B quantity is 1.13 g/m² than its counterpart. This shows variation in particular ink composition to get perfect density at lower ink quantity (as in Ink A) is considered as good ink.

How do we find g/m² of ink ?

With the help of Analytical balance scale, first measure the substrate (paper of m² size) then apply a thin layer of ink, next measure the sample with ink we get ink quantity by difference.

Using Printability tester we can measure rubber blanket disc before and after printing on sample and with the difference amount of ink applied on paper is calculated.

What is colour strength in testing ;

Like yield testing we use reference value CIELAB to match against the sample of ink given at particular ink quantity, here ∆E in Y axis and < 2 value is considered as good ink. Reference value may be ISO standard for particular printing ink type, for example as IFRA26v5.ICC profile value as reference.

Colour testing is done mostly using absolute substrate that is recommended by ISO for testing. L*a*b* value of sample and reference value is tested using formula;

∆E = √ (L*r-L*s)²+(a*r-a*s)²+(b*r-b*s)² [were r - reference and s - sample]

With the help of high chromatic ink we can achieve larger gamut, like paper with brightness increase gamut volume and thus we can print more colours. So ink colour strength is an important factor for good reproduction.

Ink Yield Vs Mileage ;

Talking about yield, we said a gram of ink applied on sq.meter of paper (g/m² - substrate size is fixed) evenly and ensuring correct optical density for standard. Were mileage is vice-versa a gram of ink consumed for printing in sq.meters (m²/g - quantity of ink is fixed) eg. Cyan 18 m²/g ( 18*1000/0.19 = 94737 pages/kg - here 0.19 is product size).

So ink mileage is calculated to find performance of ink at production and yield is used while finding the quantity of ink to check optimum density in ink manufacturing.

Ink quantity is nothing, but the thickness of ink applied (density), this can be done in printing by increasing TIC and supply of ink. According to substrate property quality of ink can be increased or decreased without affecting print quality.

It is advice to have more than one type of ink for testing quality in printing. If a particular type of ink not able to achieve the target, then we can test with alternate brand. Each batch of ink and brands have different Yield and colour strength - that give different ∆E and this is an important point to note in case of quality checking.

What will be the ink pigment size ?

Well grinding of ink pigment deside the pigment size of an ink, for example 0.05 ɥm, 0.5 ɥm and 5 ɥm - Can you say correct range of offset ink pigment size ?

Before moving into this, we have to know something about pores and pores density of paper (substrate). Coated paper has 0.6 -0.8 ɥm pore radius and A4 bond paper had 1.2 ɥm pore radius. Pore density is the number of pores in sq.inch of paper, now we came to conclusion that Newspaper has more pore density than others because they have less pore radius. Capillary test is done to identify this pore size of paper in paper mill laboratory.

Now come to the point, we have the following queries on ink pigments ;

What will be the correct pigment size for web offset ink?
Is there the same size of pigments for all offset process inks? and
If any, then what is the range of sizes maintained for such process colour pigments?
How will you test the pigment size?

Flush ink:

A process used to produce a printing ink pigment that involves dispersing the pigment particles in oil, rather than water. Typically, pigments begin life as a water suspension of pigment particles, which are then filtered out of the suspension and dried into a press cake. In flushing, the particles are not dried after filtering, but are left with a water content of 30:80%; the resulting press cake is mixed with oil, which then disperses the water. The remaining bits of moisture are removed via suction and/or heat. Flushing is performed on pigment particles that, in the traditional method of drying, form hard-to-grind clumps. The paste generated by flushing produces finer-dispersed particles. In some cases, the manufacture of pigment is faster, easier, and less expensive when pigment is produced using the flushing process. Pigments can be flushed in a variety of different liquids, depending on the end-use characteristics of the ink. Newsink is flushed using mineral oils, and other types of inks use anything from litho oils to gloss ink varnishes. Pigments produced by flushing are called flushed colors.

Newsink:

A type of printing ink used for printing on newsprint. Like newsprint, which is an inexpensive, low-quality paper made from groundwood pulp, newsink in turn is an inexpensive ink produced from low-cost raw materials, primarily mineral oils and Carbon Black. Newsprint has a high degree of ink absorbency, and the high speed of web offset presses makes rapid vehicle absorption necessary to prevent smudging and ink setoff. Newsink, then, has a highly fluid body and low viscosity which allows for quick absorption. The addition of resins to the ink also helps reduce the capacity for smudging. Ink that is too fluid or oily, however, can soak through the paper producing ink strike-through. Newsink is also specially formulated for letterpress printing, although very little newspaper printing is done on letterpress machines any longer.

Source from Printwiki

Offset ink pigment size lies in range between 3 ~ 10 ɥm. Mostly offset ink pigments are around 5 ɥm in size. In case of web offset inks they are made of flush inexpensive ink pigments size below 1 ɥm may be 0.5 ɥm in size.

Using Grindometers or Fineness of Grind gauge, LB-550 Dynamic Light Scattering Particle Size Analyzer are some example that can measure the ink pigment size. Hegman gauge has reading 0 to 25/50/100 ɥm, but more accuracy with fraction microns <1 ɥm unit measure is not in possible with this type of gauge.

Hegman - Mils - Microns

0 - 4.0 - 101.6

1 - 3.5 - 88.9

2 - 3.0 -76.2

3 - 2.5 - 63.5

4 - 2 - 50.8

5 - 1.5 - 38.1

6 - 1 - 25.4

7 - 0.5 - 12.7

8 - 0 - 0

Given table is from Hegman gauge Ref. Wikipedia, - Hegman unit compared with mils - a unit of length equal to 0.001 of an inch (0.0254 mm). Microns as we know it is 1/1000 th of mm or 10^-6 meter.

In addition there are gauge with PCU - North units scale with microns, which is used in paint and Ink industries. PCU is inverse of micron scale,were 0 micron is 100 PCU and 100 micron is 0 PCU.

Optical density Vs IFT (ink film thickness)

IFT has more relation with quantity/yield of ink (g/m²) , amount of ink as layer in specified area - when quantity increase then ink film thickness also increased, unit of IFT is measured in microns ɥm. We can say yield and IFT are same but measured differently as g/m² and ɥm respectively. Measuring yield is easy as discussed above but in case of IFT is not so easy. Microscopic view may get the detail but it is not a common method, only way is to relate them with quantity and pigment size we can predict the IFT value.

Here we plot IFT in x-axis and density in y-axis and at certain point density remain same (saturation level) as IFT increases. Understanding ink film thickness, density and their relationship are more important in Printing industry.

In this case of web offset inks, when we satisfy with less IFT as we said above of good density (high chromatic ink) then mileage will be higher [example Cyan > 18 m²/g of ink]. Else when IFT is more to get good density, then ink mileage get dropped [example < 13 m²/g of ink], from this we understood the relationship between IFT and Density in printing. [Note: here good density refer ∆E <2 with reference ISO standard] . High speed machines using Newsprint paper with more pore density required more ink consumption apart from IFT.

WAN-IFRA - Conformance of Indian Newsinks to ISO 2846-2 report on IFT gives standard specifies of the ink colours and transparency requirements for the newsinks.

Ink film thickness (IFT) for different printing process:

In Lithography offset printing process, ink IFT is more in wet printed copy than in dry condition, ink particle settle / bind with substrate and reduce to 10 - 30 % in thickness. So approximate IFT range will be 2-8 ɥm and it mainly depend on substrate quality.

Using nanography printing we can get same print quality with all type of substrates range from tissue paper to high end coated papers. This is because technology indirectly deposits dried ink particles to substrate, so there is no further penetration of ink. Image in this process stick ink in thin layer as stickers of IFT 0.5 ɥm as shown in picture above.

Why my Newspaper coldset (CS) ink quality is not good as Heatset (HS) ink Newspaper?

Here I correlate another sentence " When red apple, in print absorb all colors of wavelength and reflect its actual color we see apple with exact red color".

Web offset machines are high speed printing with > 500 m /min, has some limitations to dry or settle ink particles quickly by CS flush inks.

When both substrate and ink film thickness in printing helps to reflect image exact colors, then we get quality output. IFT is most important criteria here, when IFT is uneven due to substrate surface and ink drying nature (eg. coldset ink) produce less quality - say below 60% of PANTONE® colors reproduction. In case of Heat-set ink, IFT after impression (ink transfer) go through heat treatment and at flash point ink gets dried, ink film has gloss and even spread on substrate with more thickness than cold-set ink, so it can produce little bit more PANTONE color - that is color gamut bigger in volume compared to coldset newspaper. Basically ISO standard for newsprint can reproduce only up to 65% of PANTONE colors.

But in case of nanography printing technology can produce upto 90% PANTONE colors, with CMYK gamut larger than any other printing process. It is archived by latest technology that produce very thin dried polymer like IFT spread evenly on surface of the substrate (range from tissue paper to coated high grade paper).

Control in correct IFT and its spreading nature on substrate is the only factor to get good quality printing.

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