It may seem like a very obvious question, but do you know how important a reliable contract proof is? Contract tests are the subject of interest for many printers and plate suppliers. The objective of these tests is clear: They must reliably simulate the real result of our sample printed on the machine.
But the reality is very different. The majority of contract proofs analyzed are far from a result we can define as optimal. What happens if the proof is not reliable? We lose time on the machine, we waste film and ink, we need to image a new set of plates and, most importantly, we do not comply with a delivery deadline agreed upon with our client.
The growing popularity of expanded gamut (EG) printing increases the degree of quality demanded in our contract proofs. This is because when we print in EG, we convert spot and brand colors into versions made from up to four superimposed process colors, which can also be present in other parts of the design.
Press Color Profile
Many types of color management and proofing software have a common element: in order to carry out a contract proof that simulates the press result, the color profile of our press is required, as well as the color profile of our proofing device. Afterward, press and proof are linked in order for our proofing system to simulate our printing machine. Thus, our press color profile becomes our printing standard.
The press color profile is obtained from a spectrophotometer measurement of the color chart(s) placed in our “fingerprint” colorimetric test. This means that the way we print our fingerprint will condition the later quality of our proofing system. It is usual to print the fingerprint in very optimal conditions and with a very thorough control of all the printing parameters. Unfortunately, in the majority of cases in our real production, we do not reach the density in the process colors that we obtained during the printing of the fingerprint, because real printing conditions are not as optimal as we would like. If this is the case, it means our fingerprint must be reprinted under more realistic and repeatable conditions.
Verifying Contract Proofs
If we want to guarantee our contract proofs are reliable, we must verify them. We have to add a control strip, which we will compare to our press profile, based on established quality tolerances. The most common and recommended practice for CMYK printing is using the Ugra/Fogra Media Wedge CMYK V3.0 control strip, a control tool recognized worldwide for contract proofs. It includes 72 patches with color scales for CMYK, trapping elements and gray balance. None of these patches contain any CMYK value less than 10 percent, and this implies that we cannot verify whether our proof loyally renders our minimum dot. We must do so with additional and/or customized control strips.
Verifying a printing proof for EG printing (e.g. CMYK+OGV) implies that the verification process must be adapted. This implies using a control strip that contains all our process colors and specifying the customized tolerances for these additional colors.
Verifying our result on the press is as important as verifying our color proof. There are various standard color strips we can use for the verification of our prints. FTA’s Flexographic Image Reproduction Specifications & Tolerances (FIRST) methodology suggests a control strip that contains a scale for each color in the CMYK process, trapping elements and gray balance. It would be advisable to place these control strips at both sides of the print, although depending on the design and web width, this is not always possible. These control strips must be compared with our printing standard. It is important to understand that a contract proof must simulate our standard way of printing, instead of modifying our standard way of printing in order for the sample to look like our proof.
Often, quality controls in the press are carried out by means of densitometric functions such as solid ink density (SID) and tonal value increase (TVI). It must be said that it does not guarantee the result, as we can find that, even though we have an adequate density and dot gain, the colorimetric deviation can be very high. It is advisable to always carry out quality checks with colorimetric functions.
We should also keep in mind that a press color profile is nothing but an electronic file that contains information about how our press renders colors for a very specific set of printing conditions. This means that the contract proofs attempt to simulate these printing conditions, and that any alteration of said printing conditions will affect the reliability of our proof.
Comparing Expanded Gamut & Direct-Color Printing
Earlier this year, Comexi, Apex, Esko, Sun Chemical, DuPont and Cauchos Puntes organized a technical day at Comexi CTec on March 21 to compare an EG printing system with direct-color printing. Ten experts from six different partner companies joined in a practical session that consisted of printing a set of planned jobs continuously, as if to simulate a work shift.
All these jobs were originally printed with spot colors, but for this exercise they were converted into EG color separation. The target of this exercise was to print in an efficient way and under fixed quality standards, monitoring step by step relevant specific data in terms of job cost: changeover time, waste material, and ink and solvent consumption.
As a summary of the most relevant data, the experiment achieved a 60 percent reduction in changeover time, a 50 percent reduction of waste material and 20 percent decrease in inks cost.
About the Author: Albert López’s professional background began in 1999 as an engineer in paper production. From 2000-2006, he worked as a production manager in an offset commercial plant. At the end of 2006, he joined Comexi as project manager of flexographic projects and in 2011 was part of the team that created the new Technological Center of Comexi (Comexi CTec), of which he is currently the executive manager.
In the last few years, Comexi has developed a deep knowledge of color management for the flexible packaging field. The group has promoted new applications, as well as new advisory and training services, making it a top reference center in the treatment of color, and thus improving the productivity of its customers. From the end of 2012, Comexi has integrated color and its management as part of the company’s knowledge. In recent years, a wide range of services and equipment focused on color management have been developed and made available to the group’s customers.
In 2015, CTec technicians were certified in Esko’s FlexProof/E and Color Engine, certifications that, together with complete training in Equinox modules, have allowed Comexi to offer a wide range of products, services and technical support in color management, plates creation and optimization of work processes.