Tips and Tricks for Final Printing Approval
The quality of your print products is largely dependent on how
they are realized on the press.
Even when you are not an printing expert, you should nevertheless be able to judge the quality of printed sheets according to selected features. This allows you to have a more target-oriented discussion with your print provider and make the right decision straight away at the printers' - even when pressed for time.
The aim of quality assurance for printing is to achieve a correct and constant color reproduction over the entire print run. In addition to the ink and colorfulness of the printing stock, the most important influencing factors are the density of the ink layer, the screen value, the color balance as well as ink acceptance and color secquence.
Here we present you some selected tips & tricks:
Complete print copies
The following documents will be provided to you by your print provider (or rather should be available for printing approval):
- Forme proof or hand sample
- Color proof of the images (proof with Ugra/Fogra media wedge)
- Color sample of sepcial colors (e.g. Pantone or HKS Fan)
- Proof from the running press
Color sequence
In order to get an optimal grey gradation, the color sequence on the print sheet must be observed. A different seqeunce influences the ink acceptance behaviour (trapping). It reveals how well the ink is accepted on an already pre-printed color compared to pritning on the clean printing material.
For reproductions with high grey and silver proportions, the following color sequence is advisable: cyan before magenta; when using magenta before cyan, the grey axis can shift into the reddish area.
Checking for completeness
As simple and trivial this aspect might be, the conseqeunces are just as fatal: check the print sheet for completeness of all elements to ensure that no element is missing on your print object.
Producing identical proof sheets
The proofs approved ahead of print production are the references for judging the ready-printed sheets. Normally, the images on the proofs are arranged in a completely different seqeunce or alignment than on the ready-printed sheet. Comparing a number of proofs with one print sheet is very confusing and can result in errors on the print sheet being overlooked.
You will not always have a proof of the complete print forme. It is therefore advisable to produce a "proof sheet" which is arranged just like the print sheet that will come off the press. This can be easily done by gluing the individual text and image elements cut out from the proof to a print sheet.
Register control
The rule is: register control before color corrections! The hues are preceived through the overlapping of individual dots. Misregisters cause a different ratio of dot overlapping. As a consequence, our eye perceives the color differently.
The register marks indicate whether the individual colors will be printed overlapping exactly or whether one color will "peep out". The register marks must lie congruently on top of each other. An effective control is only possible with a linen tester magnifier. You can also notice register differences in individual image elements, when, for example, a color "peeps out" at a black-and-white edge. Even slight register differences can led to color shifts, especially in the grey scales.
Visual control
The physical explanation for the influence of the ink layer density (amount of ink on the sheet) on the optical appearance is as follows: printing inks are not opaque but transparent. Light penetrates the ink. When passing through the ink, it gets into contact with pigments that absorb specific wavelengths of light to a greater or lesser extent. A thick ink layer absorbs a larger proportion of light and reflects less than a thin one; the viewer thus sees a darker and more saturated hue. Therefore, the amount of light that reaches the eye forms the basis for judging the respective color.
Control strips
For the purpose of technical quality measuring, color control strips offered by various research institutes and suppliers are produced during printing. These allow you to determine the color density and dot gains (for two screen values normally 40 and 80 percent) of the process colors. For special colors, it is advantageous to have a full-tone color measuring strip running along the entire width of the print sheet. The color values can then be measured and the hue can be optically compared to a color sample. The color density and dot gains should conform to ISO 12647-2 standards.
Quality control for the entire print run
In order to make a more secure quality assessment of the entire print run in the case of longer print runs, you should repeat the quality control after approx. 500 sheets.
Defining samples
Once the control has been carried out successfully, you should select a print sheet that will be used as a binding sample for the entire print run. Roughly every 500th sheet should be optically and metrologically compared to the sample sheet and corrected when necessary.
Fur further information, please contact:
Heidelberger Druckmaschinen AG
Print Media Academy
Kurfürsten-Anlage 52-60
69115 Heidelberg
Tel.: +49 (0)6221 92 24 01
Fax: +49 (0)6221 92 49 29
E-mail: pma-info@heidelberg.com
Even when you are not an printing expert, you should nevertheless be able to judge the quality of printed sheets according to selected features. This allows you to have a more target-oriented discussion with your print provider and make the right decision straight away at the printers' - even when pressed for time.
The aim of quality assurance for printing is to achieve a correct and constant color reproduction over the entire print run. In addition to the ink and colorfulness of the printing stock, the most important influencing factors are the density of the ink layer, the screen value, the color balance as well as ink acceptance and color secquence.
Here we present you some selected tips & tricks:
Complete print copies
The following documents will be provided to you by your print provider (or rather should be available for printing approval):
- Forme proof or hand sample
- Color proof of the images (proof with Ugra/Fogra media wedge)
- Color sample of sepcial colors (e.g. Pantone or HKS Fan)
- Proof from the running press
Color sequence
In order to get an optimal grey gradation, the color sequence on the print sheet must be observed. A different seqeunce influences the ink acceptance behaviour (trapping). It reveals how well the ink is accepted on an already pre-printed color compared to pritning on the clean printing material.
For reproductions with high grey and silver proportions, the following color sequence is advisable: cyan before magenta; when using magenta before cyan, the grey axis can shift into the reddish area.
Checking for completeness
As simple and trivial this aspect might be, the conseqeunces are just as fatal: check the print sheet for completeness of all elements to ensure that no element is missing on your print object.
Producing identical proof sheets
The proofs approved ahead of print production are the references for judging the ready-printed sheets. Normally, the images on the proofs are arranged in a completely different seqeunce or alignment than on the ready-printed sheet. Comparing a number of proofs with one print sheet is very confusing and can result in errors on the print sheet being overlooked.
You will not always have a proof of the complete print forme. It is therefore advisable to produce a "proof sheet" which is arranged just like the print sheet that will come off the press. This can be easily done by gluing the individual text and image elements cut out from the proof to a print sheet.
Register control
The rule is: register control before color corrections! The hues are preceived through the overlapping of individual dots. Misregisters cause a different ratio of dot overlapping. As a consequence, our eye perceives the color differently.
The register marks indicate whether the individual colors will be printed overlapping exactly or whether one color will "peep out". The register marks must lie congruently on top of each other. An effective control is only possible with a linen tester magnifier. You can also notice register differences in individual image elements, when, for example, a color "peeps out" at a black-and-white edge. Even slight register differences can led to color shifts, especially in the grey scales.
Visual control
The physical explanation for the influence of the ink layer density (amount of ink on the sheet) on the optical appearance is as follows: printing inks are not opaque but transparent. Light penetrates the ink. When passing through the ink, it gets into contact with pigments that absorb specific wavelengths of light to a greater or lesser extent. A thick ink layer absorbs a larger proportion of light and reflects less than a thin one; the viewer thus sees a darker and more saturated hue. Therefore, the amount of light that reaches the eye forms the basis for judging the respective color.
Control strips
For the purpose of technical quality measuring, color control strips offered by various research institutes and suppliers are produced during printing. These allow you to determine the color density and dot gains (for two screen values normally 40 and 80 percent) of the process colors. For special colors, it is advantageous to have a full-tone color measuring strip running along the entire width of the print sheet. The color values can then be measured and the hue can be optically compared to a color sample. The color density and dot gains should conform to ISO 12647-2 standards.
Quality control for the entire print run
In order to make a more secure quality assessment of the entire print run in the case of longer print runs, you should repeat the quality control after approx. 500 sheets.
Defining samples
Once the control has been carried out successfully, you should select a print sheet that will be used as a binding sample for the entire print run. Roughly every 500th sheet should be optically and metrologically compared to the sample sheet and corrected when necessary.
Fur further information, please contact:
Heidelberger Druckmaschinen AG
Print Media Academy
Kurfürsten-Anlage 52-60
69115 Heidelberg
Tel.: +49 (0)6221 92 24 01
Fax: +49 (0)6221 92 49 29
E-mail: pma-info@heidelberg.com
