Regardless of the printing technology used, dot gain of printing ink on the print is an inevitable part of the printing process. During printing, each dot of the halftone exceeds the area for which it was calculated. When it comes into contact with paper, it loses its correct round shape, and its deformation must always be taken into account both when manufacturing the printing form and when working on the printing machine.

Dot gain always affects colour reproduction, especially when the colour is formed by superimposing three or four basic triad colours, each of which has a small percentage content. When viewed with the naked eye, such a print appears slightly darker, and in shadows with a density of more than 80%, everything may merge into a solid block.

When selecting a used printing machine, it is important to determine which of the factors is responsible for the appearance of printing defects – incorrect adjustment of the compensation curves when outputting printing forms, excessive pressure, incorrect selection of the offset blanket or the poor condition of the equipment itself.

Dot gain has two components: natural, i.e. inherent in offset printing, and secondary, arising from violations of technology and adjustments of the printing machine. Natural dot gain can be roughly represented as the flattening of a drop of ink under pressure in the printing zone. This is where the name of the phenomenon comes from. But in reality, the process is much more complex.

 

The physics of dot gain

The crushing of ink on the paper surface depends on the properties of the offset rubber, the amount of pressure in the printing zone, the viscosity of the ink and, of course, the thickness of the ink layer. These are phenomena that, in general, are largely independent of the printing machine. It is clear that a drop with a thickness of 2 microns will be flattened under pressure more than a drop with a thickness of 1 micron. Ink diffusion in the paper (spreading due to absorption and distribution of ink along the fibres) also increases the spot area. It depends on the fluidity of the ink, the speed of its fixation and the porosity of the paper.

Reflection of incident light from paper occurs not only on its surface, but also due to the scattering of light in the thickness of the paper. A shadow forms under the ink stain, so the intensity of the reflected light immediately next to the stain is lower than on the clean area of the paper. As a result, the stain appears to be slightly larger than it actually is. This effect mainly depends on the transparency of the paper.

The main thing to keep in mind is that it is impossible to determine the amount of dot gain correction once and for all. It occurs when using different types of paper, as well as increased pressure and ink viscosity. Even the same ink can spread differently at different temperatures.

Substrate

Coated paper absorbs ink several times less than uncoated newsprint. On the latter, therefore, dot gain is much more noticeable.

Printing Ink

Ink is applied to paper under pressure. Because of this, the area of fresh ink on coated paper increases. The degree of this increase, i.e. dot gain due to pressure, is mainly determined by the viscosity of the ink. Thick, highly viscous inks compress much less than more liquid ones. In offset printing, each dot of the halftone is surrounded by a thin halo of ink. This is because each dot has a microscopic relief and the ink flows to its edges.

 

Smearing of printing ink

In addition to these natural causes, various machine defects and process disruptions can lead to increased dot gain. The most similar to dot gain in terms of its effective nature is smearing. It occurs when the offset blanket slips relative to the paper in the printing zone or relative to the printing form in the contact zone.

Here are just a few reasons:

• poor quality or incorrectly selected offset rubber,
• incorrectly tensioned rubber,
• incorrect blanket thickness,
• printing machine cylinder runout,
• cylinder drive gear defect,
• cylinder bearing defect,
• malfunctioning print cylinder valves,
• incorrect plate installation (insufficient clamping or skew),
• warped paper,
• sheet curling during feed into the printing machine,
• excessive ink stickiness, etc.

When the printing machine is at fault

Even if only dot gain and ink smearing are taken into account, it is necessary to be able to correctly identify the nature of the defect when inspecting a used printing machine.

Technological causes of dot gain are eliminated by adjusting the printing process in a particular printing house. During the adjustment process, a specialist experiments with different papers and inks to find a solution to minimise the defect. In addition, the size of the halftone dot can be adjusted at the stage of printing plate output. The result of the adjustment will be the maximum approximation to the specified original and, accordingly, to the correct colour reproduction.

Problems caused by the condition of the printing machine are much more difficult and expensive to fix. That is why, when inspecting printing equipment before purchase, we focus on determining its mechanical condition. The causes of misalignment can vary greatly, from an incorrectly prepared foundation for the machine to maintenance problems.

But if you print a test in a printing house and find smudging, do not rush to blame the printing machine. Perhaps the printing house technician is to blame. Or the lack of a technician.

 

Determining dot gain

To determine the actual dot gain, it is necessary to print a halftone measuring scale. When exposing the printing plate, any dot gain compensation should be excluded. Using a densitometer, determine the optical density at the control points of the scale. The standard provides for measuring dot gain at 40 and 60 percent grey. In practice, it is often limited to determining the actual density at only one point – for 50% grey, since dot gain in the midtones and shadows leads to darkening of the image.

In the light areas of the scale, loss of detail and reduced contrast will be noticeable. In the deepest shadows, there will also be a loss of detail due to the mutual crushing of the halftone dots on top of each other. Density measurements should be taken at a minimum of five points. At the light end of the scale, at 100 shades of grey, use the cell where a regular halftone pattern appears. This usually corresponds to 1-4%. If the screen is detected at a higher density, it means that errors were made when exposing the form, and there is no point in taking further measurements.

At the opposite end of the scale, find the darkest point where the screen is still visible. For uncoated paper, this is around 80%, for good coated paper, approximately 90-95%. For the remaining points, use the cells where the density according to the densitometer corresponds to 25%, 50% and 75% with an accuracy of +/-1. The numbers of these cells (say, 2, 15, 42, 58 and 96, including the points at the ends of the scale 0 and 100) will be used to obtain the compensation curve.

 

Dot gain compensation

It is impossible in principle to eliminate dot gain during printing. However, based on the results of our measurements, it is easy to construct a graph showing the degree of dot gain as a function of the density of the original screen (specified dot size).

This requires a non-linear correction, rather than a simple increase in brightness or gamma function, as is often done without considering the nature of the phenomenon. We have already taken the data for constructing the compensation curve from the densitometer. The values plotted on the vertical axis (the results axis) at the reference points – 0%, 25%, 50%, 75% and 100% on the horizontal axis – correspond to the numbers of the squares we mentioned above.

Dot gain should be measured not only for black, but also for the other three channels.

Next, you need to enter the compensation curve into the application you are working with. In Adobe Photoshop, for example, it can be saved to disk and then loaded at the final stage of image processing using the Image/Adjust/Curves command.

To obtain normal colour reproduction, the appropriate correction is made during the form preparation stage. For example, where there should be 50% halftone, 32% is set during exposure. During printing, 18% dot gain is added, and the printout has the desired 50%. It is clear that such a correction will only work if the dot gain is always constant.

 

If you don’t have a measuring device

Nowadays, it is difficult to find a printing house that does not have at least a primitive densitometer. But in practice, anything can happen. Of course, if you do not have a densitometer and you have difficulties with the exposure of the test scale or its printing in the printing house, you will not be able to construct an accurate curve. This will also be pointless if you frequently and randomly change paper and ink.

What can be recommended in this case? You can use the highly averaged compensation curves offered in professional graphics and layout software. This is not a panacea, but at least they will allow you to take into account the type of paper on which your work will be printed.

Whichever method you choose to combat dot gain – constructing accurate compensation curves or using the profiles supplied with the program – remember that compensation only needs to be done once. Either when preparing illustrations in a graphics editor or when outputting from the layout application.

A good technologist rarely changes the types of printing inks, and for the most commonly used materials, there are always pre-prepared sets of compensation curves.