Control print elements (as well as printing standards at all) began to be developed in the 1960s. They became relatively widespread in the 1980s (only the most advanced printers used control scales as early as the 1970s). Before that, many printing houses did not even have a standardised triad of colours. The processes of colour separation and ink mixing were done manually, and not surprisingly, colour coverage suffered as a result.
How printers worked at that time is a rather interesting and vast topic for a separate story. Some production connoisseurs could recognise by eye which printing press their work was printed on.
It should be reiterated that when inspecting a printing press, the inspector’s task is to determine the nature of the defect. Most of them can be caused by problems in technology, and they are not related to defects in the printing press itself, but to incorrectly selected materials, climatic conditions.
Test printing we fulfill when the customer has chosen the extended inspection package. Or when mechanical defects of the equipment are suspected during a standard inspection.
Nowadays, everything from the colour of the inks to the amount of dot gain has been standardised. With the advent of the CIP 3 standard, it is now possible to quickly change over and even achieve repeatability in the printing of an order after a period of time. A lot of work had to be done before it made sense to objectively evaluate the results. Checking scales exist to facilitate this.
Let’s turn to the textbook for the theory.
A control stripe is a set of control elements (fields, test objects) on a print that allows to evaluate its quality. In doing so, both individual defects arising from certain defects in the printing process and their overall effect are controlled.
Since the control scale serves as an objective indicator of the print quality, the evaluation of the results (obviously detected by means of the scale) is carried out in an extremely objective way: by measurements on densitometers, spectrophotometers; but also visually, for example, with the help of a magnifying glass.
Control elements are distinguished:
(a) by purpose:
- for platemaking processes;
- for printing processes;
- for post-printing processes;
(b) by type of carrier:
- analogue (the carrier is photographic film; the scale is glued on the printing plate together with the printed photographic plate);
- digital (added to the layout at the stage of layout);
(c) by the method of control:
- operational control;
- for equipment debugging.
In the work to check the condition of the printing press, control elements are used to determine the existing defects of the sheet conveying system, clamshells and other elements. Most of the tests are not freely available, difficultly recognizable and that is why are used only by specialists.
Control elements of operational control of printing
The most familiar to us are the stripes of operational control of printing processes (even those who have never dealt with printing have seen them – they are sometimes not cut and remain on the finished product).
There are a lot of different parameters of the printing process that can and should be controlled, hence a lot of corresponding control elements. But not all of them are necessary for operational control. In most cases, a complete scale contains the following elements:
1) 100% ink fields (so-called dies).
Their number corresponds to the number of colours and the number of ink zones of the printing press. Plaques are distributed across the entire width of the sheet. They are used to control the total ink supply. The plates are measured with a densitometer. The resulting optical density data is compared with the standards for the ink in question. As a result of the measurements, the printer has an understanding of whether all inks are being fed evenly and can adjust the ink supply in time.
2) Trapping fields (so-called binary overlaps or binary dies).
These elements allow to evaluate the overlapping of two colours. It is important for raw printing, when one ink is immediately followed by another, i.e. for multi-colour, multi-section machines, especially offset machines. Trapping fields are the overlapping of two colours: for triad printing they are blue (C+M), green (C+Y), red (M+Y). Trapping fields can be signed “trapping”.
A common problem: CMY ink fields are reproduced well and match the reference. However, the overlay fields (i.e. trapping) are very different from the colour proof, indicating problems with the overlay colours. In this case, it is usually recommended to change the order of overlaying colours, change their set (because even a small contamination of colours has a significant impact on the result of their overlaying) or the substrate.
3) Grey Balance.
This is an overlay of inks in a certain (usually 75 C, 62 M, 60 Y) ratio which, when printed perfectly, ensures the same spread of the inks, and for ease of determining the presence of a tint this field is usually printed next to the “real” grey 80% field formed by the black ink. These fields must be identical, then the image is not tinted. The apparent tinting of the composite field, and therefore of the entire print, may be due to incorrect ink supply ratios or unequal dot gain. Often there is a signature “bal” (from balance) under the grey balance fields.
4) Slip and crush control elements (also called smudge and double fields).
These are usually two fields formed by parallel strokes, horizontal for one field and vertical for the other field. The word “slur” (French for “marriage”) is often written underneath these elements. It is especially important to control sliding and crushing in roll printing, where sliding of the plate on the paper is a common problem. The principle of the control element is as follows: if the sliding occurs in the vertical direction, the horizontal strokes will become thicker, but the vertical strokes will not (they will only become slightly longer, but this is almost imperceptible).
But the thickening of horizontal strokes, even a slight one, will immediately lead to darkening of the field formed by them, compared to the “vertical” one. This visual effect allows you to instantly detect the presence of sliding or crushing.
In addition to the system of perpendicular lines, control elements in the form of concentric circles are sometimes used for this purpose. The principle is the same.
5) Elements for control of raster point spreading.
These can be radial worlds, various elements combining raster or dashed elements of different frequencies, but the most common are fields with 40% and 80% raster points. They are measured with a densitometer behind the corresponding light filter, after which the tonal data gains are calculated (i.e. the measurement results are compared with the nominal values of 40% and 80%). It should be borne in mind that the increase in the size of raster elements may be caused not only by dot gain, but also by problems of the printing press – sliding, crushing. It is recommended to identify their nature – technological (due to incorrect selection of materials, pressure or climatic conditions) or mechanical (wear and tear of printing machine parts or incorrect mounting) – and eliminate them in time before measuring the increase in tonal data.
6) Fields for controlling relative print contrast.
There are usually two fields for each ink: with relative raster dot areas of 100% and 80%. The densities of these fields are measured and compared. If there is no difference, then the ink has bled into the gaps of the 80% field, which means a complete loss of detail in the shadows. It is “shadow blockage” that is the most common gradation problem, and this element allows you to identify it.
7) Elements to control small raster elements.
These are fields with raster points of relative area 1%, 3%, 5%, 95%, 97%, 99%. The reproduction of these dots is controlled with a magnifying glass. They cannot always be reproduced – due to unsmooth paper, poorly made printing plate, etc.
8) Elements for controlling alignment – crosses, lines, which are often also trimming marks.
A cross is formed from four crosses printed with four colours (for triad printing). Ideally, this should not be noticeable: if the colours are precisely matched, they will lie flat on top of each other without separating.
Each of these elements is usually reproduced separately for each ink.
What else is important?
As already mentioned, the most important element that the printer controls first and foremost is the 100% ink margins. Actually, these elements regulate therein process: with each revolution, the printing press ink unit transfers a certain amount of ink from the ink box to the plate. It should be enough to create a uniform density on the sheet surface. In other words, normal conditions for colour combination must be ensured with a minimum amount and sufficient ink “dryness” to fix itself on the printed sheet within a reasonable period of time. If the ink layer is too thick, it will not dry well and will begin to smear.
If less ink is fed to the machine rollers, it will simply not be enough to cover the entire surface evenly, and the print will “fade” from one edge to the other.
The values of optical densities for offset printing, dot gain, etc. are recommended by the international standard ISO 12647-2.
There are many different control scales designed for alternative types of printing: e.g. digital, screen, intaglio, etc. They have their own specific elements to take into account. For example, non-triadic, black and white printing does not require grey balance, trapping. It does need an element to control gradation transfer. However, most of them are the same as standard control scales for offset printing. The principles of control do not differ from those described above.
A system of standards has so far been developed only for offset printing. In other areas, such as flexography, there is no uniform international standardisation and the printer essentially relies on his own experience.
Control of platemaking processes
To describe the scales of formant control it is necessary to go back in time when an intermediate medium, photographic film, was used to make the plate. The veil area increased or decreased when the development parameters were changed. Because of this, the raster dot could also increase or decrease. A special optical wedge could help to control the raster dot.
Control scales for the prepress process are designed to identify problems typical of photographic film output and plate making, such as incorrect gradation, loss of fine detail, etc. Problems are usually caused by incorrect exposure times or developing modes.
To determine the optimum exposure time, a tonal optical wedge is usually used, each field having an optical density greater than the previous field. The constant of the wedge is equal to the root of 2, i.e. the illuminance behind the first field is 1.4 times greater than behind the second field, and 2 times greater than behind the third field, etc.
This distribution of densities is convenient for determining exposure.
And now there is a good news: the modern CTP plate has what is called a square point, which has no veil area at all. Therefore, the optical wedge is no longer used. For printing plates, a high-contrast copy layer is used. Only two levels of gradation are formed on it – either the illumination has occurred completely, or it is not present at all.Therefore, when exposing such a test object to the plate material, we will get as on the following picture.
The location of the transition boundary determines the threshold exposure, exceeding which will result in backlighting. Having carried out several backlightings, the exposure time that brings this boundary as close as possible to its ideal location (0.6-0.8) is selected.
The control of manifestation consists in controlling the reproduction of small elements in lights and shadows. These are usually positive and negative stroke elements of various thicknesses. The fields contain raster elements with relative areas of 1%, 2%, 3%, 4%, 5%, 95%, 96%, 97%, 98% and 99%.
If the plate is underexposed, fields with large raster dots will be perceived as dies, and if it is overexposed. The same trouble will happen to fields with small raster dots.
In addition, control scales for platemaking processes may contain elements for controlling gradation transfer and slip.
Control of postpress processes
These are individual elements. The most important of these are the trim marks. These are crosses, lines along which the cutting equipment is aligned. To control folding and correctness of the picking order, control marks are used in the form of numbered elements. On each next signature it is shifted downwards relative to the previous one. That is why, in case of correct picking, a “slide” of such elements moving downwards is formed on the spine of signatures. If the signatures are selected incorrectly, it is immediately noticeable by “springboards” on the “slide”.
When foil stamping, the process is usually controlled with a foil die.
Literature
(c) Methodological manuals of the Moscow State University of Printing, 1994-1999
(с) Marogulova N., Stefanov S. Consumables for Offset Printing. Moscow, 2002.
