This is a question I get asked very often. First, let’s point out that we are talking about offset here, as digital, wide format and flexo really require their own topic of discussion.
Let’s break it down into what could be the cause and what to check.
1. What is the colour target?
- Is it an inhouse colour target/standard or international standard or customer supplied target?
- Check to make sure that your selected target or standard contains the same components (substrate, inks, sequence, coating, etc.)
- Does the target have the correct instrument settings?
The colour target should include all of the information, in detail, outlined above.
- Substrate type, finish, coating, weight, L*a*b* values
- Measurement conditions (M0, M1, M2 or M3) and backing (white or black) and instrument type, geometry
- Solid CMYK L*a*b* and Overprints RGB L*a*b* and dE tolerances (and dE methodology)
- Calibration method used – CMYK TVI target and spot colour SCTV or G7 calibration
- Measurement made through coating/varnish?
2. Check the substrate (paper)…
- The substrate you are using – is it exactly same as specified in the target?
- Is the substrate the same type from the same supplier?
- Is there consistency and QC in the substrate from the supplier?
- With spot colours are the targets made from draw downs on the same substrate?
3. The effect of an in-line coating…
- If there a coating or varnish being applied, in line, on press it will have a large effect on the final results visually and with the measurements.
A coating or varnish will change the white point of the substrate, usually to a lower L* value and significantly alter the dot gain (visual appearance of tones) by increasing the ‘weight’ of tonal colour appearance.
- Sometimes the varnish is not applied to the colour bar on the sheet – but the target and measurements here are very important for machine colour process control.
- With the coating/varnish covering the majority of the ‘product’ area of the job the ‘contract proof’ should also reflect this.
i.e. The colour and the target of the ‘proof’ should include or simulate the colour as per the final production process.
4. Are the inks correct?
- Check with your ink supplier that the inks you are using are correct, as specified – for process and spot.
- If you are formulating inks internally do you have a procedure to receive a colour quality report that the ink is correct and within tolerance?
Ideally inks should be measured and checked before going onto press – especially for spot colours.
5. Watch out for contamination, drying issues, and opacity
- Thorough press clean-up is required when changing colours on press – if this is rushed and the necessary steps are missed it can lead to contamination
- Often overlooked – Opacity – this can be measured, for example with a Techkon SpectroDens Premium. This will ensure consistency and the correct print capabilities can be communicated between the printer and the print buyer/customer.
The above five points outlined should be documented as SOP (standard operating procedures) to ensure press operators, off siders, production and press maintenance personnel are all following the same procedures as communicated to all relevant staff.
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