Plates

Looking at the effects that plate making can have on the finished Job.

When a set of four colour plates are made they should have a set of colour control strips on them. The strips help the plate-maker and the press operator produce the job that the customer desires. In this article we look at what effects a wrongly produced set of plates has on the finished job. For the purposes of this article we will be using the Gretag CMS-3™ Colour Control Strip, both jobs used the same strip that was stripped into each job. The Print Room also has System Brunner™ strips available to use and they produce similar results to the Gretag strip.

The first visual check is the step wedge continuous tone strip, located in the centre of the sheet. The strip is made up of a four-step strip. The aim of the plate maker is to expose the first step so it is no longer visible and half expose the second (on a positive working plate). It is an effective visual check on the finished job because all of the four colour strips sit next to each other on the sheet. If any step is out on any colour then it will indicate a problem with the colours on the job. Our first example is from a job for Coca-Cola, printed on January 4^th 1999. You should notice that there are only three steps visible as the fourth (0.6D) has been exposed away. Of the three that are left the top one (0.9D) has been partially exposed. All four colours are at similar levels of exposure, so from this we can say that this set of plates has been processed in the correct way, no more can be expected of the planner/plate maker than producing a set of accurate plates, or can it...

Our second example, produced on 28^th January 1999. You will immediately notice that the levels are different. Yellow and magenta are under exposed (the fourth step has fallen off the tail edge of the sheet), cyan is about right and black is over exposed. This gives the press operator three different levels of exposure to deal with. This will affect the screen percentages of all colours. A 50% tone in yellow or maganta will print much higher while cyan will be around 50% and black much lower. On this cover which was mostly grey (grey being an equal mix of all colours) the colours produced were difficult to maintain and were not very sharp. The operator achieved the desired density of colour but the plates were over inked on yellow and magenta and under inked on black. While this step wedge can identify the problem, it cannot signify the percentage that the plates were wrong so a correction in printed ink density cannot be made.

The next visual check is to use the micro-lines on the colour control strip. The purpose of the micro lines is to determine the resolution that is usable on the press and is a happy, usable medium. If the resolution is too high then the plate will produce a lot of spots from dust and dirt in the plate making room, if the resolution is too low then screen values will be affected. The micro-line exposure is used to fine adjust the exposure on the plate.

A lot of changes can be read on the micro-lines without the step wedge being affected. You will see (if you look closely, this is the Coca-Cola job) that all of the micro-lines up to 15µ have been exposed away for each colour, with the possibility that the black has lost the 15µ micro-line. These results would produce a set of plates that would be low in resolution and would print 'clean'. That means that you wouldn't see and spots, sellotape marks or scratches because that level of detail has been removed. A clean printing plate can have side effects, which we will look at later. The optimum resolution is 6µ but that is difficult to achieve and maintain.

Our second example shows that the exposure is of higher resolution on the magenta and yellow and is of low resolution on the cyan and black. This is a result of exposure time that we have already discussed. You should note that micro-line are very accurate in what they can read. If the plates were viewed on the cyan and black plates then you probably would see traces of the 15µ lines but they have not printed, likewise, the yellow and magenta would probable show traces in the 8µ area.

The Other areas on the Colour Control Strip

The CMS-3 strip has areas that look like this, obviously this is the black patch. There is a solid area an 80% screen and a 40% screen. Using the densitometer (our one is a Gretag D19C) we can determine how everything is performing on the day that the job was run (pre-press and press). Many factors affect dot gain and the 80%/40% screen patches can be used to calculate the dot gain on the press. Using the other areas of the CMS-3 strip the operator can determine whether an inaccurate dot gain reading is the fault of the press or the plates, or a combination of plates that have not been made with the relevant press in mind. Each press behaves differently and its condition will affect a reading. If it is dirty and has dead blankets then the dot gain would probable read higher because more impression pressure is needed to produce an acceptable printing result. Below is a table of measurements from the Coca-Cola leaflet and the set of values that they should of read.

Targets

Readings

You can see that the density values are very close to the targets, magenta is a little high but still within the tolerated range (all values are for matt-coated paper). The press operator printed the job with the correct ink film weight but you will see that the dot gain values are wildly inaccurate. This would produce a job that would look 'wishy-washy' when compared to the desired effect. The set of plates, although produced to the correct step wedge values have not been produced to the correct resolution. This is born out by the 15µ reading on the micro line area of the CMS-3 strip. The situation is complicated further by the fact that the CMS-3 strip is exposed onto the plate at a different time than the image. If the image and CMS-3 strip is not exposed consecutively then all of the exposure readings are useless. The Heidelberg is a very accurate printing machine that produces a 9% dot gain in the mid tone area. The standard is 12% so you can see that we are three percent inside that range. This gives us the option of producing much higher quality halftones due to the fact that the press will accept a wider range of screens. In real life it will produce a screen range between 2% and 98%. Again this is 3% inside the standard range of 5%-95%. These figures should be used when exposing the plate to produce a screen range that is within the desired dot gain range.

The aim of this article was to highlight that the press is only as good as what has gone on before it. If corners have been cut and time saving measures have been put into practice that effect the cleanliness and efficiency of the plate making process then the finished job will suffer. If more time is put into producing a top quality set of plates then the finished job will reflect that effort. If the press has a poorly made set of plate to work with then the job will suffer as a result. Printing still follows the saying of:

'Crap in Crap Out'