Color fidelity: inks form a critical part of the color management process which seeks to match the end user’s expectations for image fidelity and brand colors using the signed off proof as a reference. Testing for color fidelity also requires taking the target substrate into account.
Physical resistance of the print: the inks should be capable of resisting the environmental conditions under which the final product will be stored and used. For example, rub resistance and light fastness.
There are established testing procedures for both color management and physical resistance, and these are the subject of this article.
OFF-LINE QUALITY ASSURANCE
Off-line quality assurance involves checking the color and printability of inks before going to press. It is an essential element of overall process control and minimizing press downtime. It also allows checking of subsequent processing steps such as laminating. This workflow can be integrated into manufacturing quality control systems such as 5S or Six Sigma.
Off-line quality assurance systems come at a number of different levels of complexity.
The simplest method is the manual application of a coating with wire wound bars (see Figure 8.1).
This is a cost-effective method of making very simple, reproducible coatings.
Figure 8.1 Wire bars. Source- RK PrintCoat Instruments
The diameter of the wire is controlled very accurately, meaning the coating weight applied is repeatable from batch to batch. Wire bars are very easy to clean. The downside is that the results of hand-application depend upon the speed of drawdown and the pressure exerted by the operator.
A more accurate method is provided by automated machines where both speed and pressure are controlled, allowing much more repeatable samples to be achieved (see Figure 8.2).
These systems are typically used as part of a process control system in the ink manufacturing process.
Quality control of the equipment requires systems to control the bars and to make sure that the equipment is running effectively the whole time. The calibration of the wire bars’ diameter is a critical factor and is very accurately controlled.
Figure 8.2 Automated wire applicator. Source- RK PrintCoat Instruments
Despite being very consistent and repeatable, the main limitation of bars is they do not replicate the printing process. Where a bar is used, the entire surface of the material is flooded and the excess wiped off, leaving a repeatable film. But the substrate can have a big influence – if you have a more absorbent paper, the ink will soak into it much more quickly, which affects the color density.
The next step up are small hand roller devices which replicate elements of the flexo process. These use an anilox roll and rubber roll and, in some cases, a doctor blade to meter the ink onto the device (see Figure 8.3).
This again is a manual process, but it is very accurate and consistent drawdowns can be made.
Figure 8.3 Hand roller. Source- RK PrintCoat Instruments
Moving on from there, these hand-operated devices can be integrated into motorized systems where speed and pressure are automatically controlled (see Figure 8.4). This increases accuracy and repeatability in achieving the target delta E.
Figure 8.4 Motorized roller system. Source- RK PrintCoat Instruments
For the most accurate print prediction, converters should use the same substrate that will be used on the press. If a wide range of substrates are being used, an alternative strategy is to test the inks onto a standardized substrate.
Converters should be aware of the problems that arise where optical brighteners are used in label papers. These interfere with spectrophotometric readings, so converters should ensure there are no optical brighteners in the base substrate used for color matching.
The next step up the accuracy scale comes from flexo ink proofing devices which are more like a miniaturized printing press with a doctor blade and ceramic anilox (see Figure 8.5).
They use actual printing plates – the same printing plates as on the press, including cushion mount. The proof press uses a ceramic anilox carrying the same volume as the anilox on the press. The machines have variable speed and pressure adjustment, so the result is closer to a true flexo print.
Figure 8.5 A miniature flexo press. Source- RK PrintCoat Instruments
This means the accuracy becomes much greater: a delta E less than one is consistently achievable. Bigger converting groups may have multiple machines and operators and are looking to achieve consistently across their manufacturing base.
Indeed, so accurate are these systems, that the first, second and third prints can actually look like a production machine: the first print is lighter because the printing plate is not yet homogenous. As it is inked three or four times, the amount of ink that the plate will carry increases. For this reason, it is recommended that converters use either the third or fourth proof as their reference, in the same way you would on a production press.
These type of very accurate printing devices move beyond pure color matching, and allow other facets of flexo printing to be assessed. So, for example, different forms of ceramic anilox engraving or steel anilox can be custom made and converters can assess the differences achievable with these types of engraving before they are adopted for the production press. An example might be a GTT MaxFlow engraving compared with a standard hexagonal array for high coverage whites.
The proof press can also be used to assess the effect of texturized printing plates. This is where the solid print is no longer just a completely flat plate, but is textured. Converters can carry out this sort of research and development on these types of device.
When testing purely for color, an alternative method is to use a gravure technique to simulate flexo print (see Figure 8.6). With a gravure unit the printing plate can be engraved with different densities. So, for example, a plate with 16 different densities is roughly equivalent to a range of anilox volumes from four to 20 cubic cm/sqm. So, in one print, the converter can match any one of the aniloxes that might be used on a production press.
Figure 8.6 Using a gravure coating technique to simulate flexo print. Source- RK PrintCoat Instruments
The limiting factor is that this has nothing to do with printability, because a gravure print has a different surface texture to a flexo print. But for looking purely at color, this can be a good short cut and a good system to do quick proofs for a flexo press.
REVERSE ENGINEERED PROOFING
Reverse engineering is a more recent concept developed/pioneered by proofing systems expert RK PrintCoat Instruments. It involves taking a press return ink, putting it onto the proofing device and getting the proofs as close as possible to the result just achieved on the printing press. This way the proofing device can be tailored to give results very similar to what is achieved on the press.
Testing dried ink properties
After the print has been made, there are a range of standard tests available to control different aspects of the dried print’s resistance to external forces. The technical committee of FINAT, the European label federation, has done significant work on standardizing these tests (FINAT Test Methods, or FTM), and these are referenced where appropriate below.
While some tests required specialist equipment – and adequate training – much can be discovered about the resilience of a piece of print by manually scuffing, stretching or scratching the material, providing this is done to a rigorous set of standards.
Surface tension of plastic films (FTM15)
This method covers measurement of the surface tension of plastic film surfaces in contact with drops of specific test fluids. Surface tension (wettability) of plastic films is one of the properties used to judge surface characteristics related to printability and adhesion of coatings or adhesives. Although the print key or adhesion property is not dependent on the surface tension only, this method is often used as a quick, practical method of assessing these characteristics.
It is important to note that wettability characterizes can change during storage.
The surface tension is measured by applying to the surface of the plastic film a test fluid of known surface tension and recording the time taken for the continuous film of the fluid to break into droplets. The surface tension is defined as the level at which a continuous film of the test fluid remains intact for two seconds.
Fluorescence and Whiteness (FTM20)
Fluorescent brighteners impart increased brightness or whiteness to label papers. Assessment of whiteness requires a specified standard light spectrum including a defined range and intensity of ultraviolet radiation.
Ink Adhesion (FTM21)
This method allows rapid assessment of the degree of adhesion of a printing ink or lacquer to a filmic label stock.
The printing ink or lacquer is applied to the substrate and cured on the printing press or using a standard method appropriate for the type of ink.
The ink adhesion is then estimated by the amount of ink that can be removed when adhesive tape is applied and peeled off.
The resistance of the ink to mechanical removal is also measured by scratching the ink and by deformation under pressure.
FTM22 allows assessment of the degree of adhesion of a printing ink or lacquer to digital print media.
Ink rub test for UV printed labels (FTM27)
This test method is suitable for checking rub resistance of printed UV ink: ink surface against substrate or ink surface against ink surface. Converters should note that in difficult cases bad adherence and bad rub resistance can occur together, so this test should be conducted in tandem with ink adhesion tests such as FTM21 above.
Rub resistance is usually a question of through curing or reactivity of an ink film. Rubbing off or scratching replicates the damage caused to an ink surface by mechanical influence – for example two labeled bottles rubbing against each other during transport. Scratch or rub resistance describes the mechanical resistance of an ink or varnish surface.
While this FINAT test procedure is intended for UV inks most commonly used in the narrow web labels industry, rub tests are equally valid for other ink chemistries. In all cases, the print sample is placed face down onto an unprinted sample of the same substrate and rubbed under controlled pressure for an agreed number of cycles. The degree of transfer of color to the unprinted sample is inversely related to the rub resistance.
This type of testing is most relevant for printing on paper rather than flexible films.
In all cases the ink needs to be fully dry. With paste inks it is usual to test after 24-48 hours.
Evaluation of bleeding through paper (FTM28)
This procedure describes the spectrometric assessment of the migration of a label adhesive through the paper substrate at elevated temperature.
Scratch resistance of a UV ink film (FTM29)
This test method describes how to test the scratch resistance of a UV ink film applied to different printed surfaces. For this test to give reliable results it is important that the ink has been cured to a sufficient level, and this can be checked in advance using a quick thumb smear test.
This test method is also suitable for water-based and solvent based-inks and varnishes, although the original test development has been done with UV inks.
The scratch resistance of an ink is normally dependent on the level of curing of the ink film both at the surface and within the bulk of the ink. The scratching of the ink film describes the damage of an ink surface through mechanical means – for example by the edge of a metallic spatula.
Adhesion and flexibility
These are the tests for films which correspond to the rub resistance test for papers. The most common test for adhesion is the Scotch Tape test. A length of tape is placed on the print and smoothed down with a finger. It is then slowly pulled back half its length before being snatched back rapidly for the remainder.
Both print and tape are examined for any sign of ink transfer.
Flexibility is tested by manually flexing the material and examining it for signs of ink removal. If any examination of scuff resistance is required, the material is repeatedly scuffed with the back of a finger and examined for any marring of the print.
Slip is critical for many post-processing operations, for example when the container will be run through high speed automated packaging equipment. If the slip is too low, the print could have problems passing over or past fixed elements on the packaging line. If it is too high, packs might slip during transport and storage. Correct slip coefficient is also critical when processing shrink sleeve labels. Measuring slip requires specialist equipment and training.
Filmic labels and flexible packaging (i.e. unsupported films) are usually re-reeled after printing and stored in the reel before being used. The high tension in the reel can lead to immense pressures on the print, and it is vital the print does not ‘block’, or stick to the back of the web that it is in contact with. This is tested by placing a sample of the print in contact with the back of a similar sample, immediately after drying, and putting them in a hydraulic ram at pressures of 750-3,000psi. This can be done at room temperature or at elevated temperature.
Common substances to test chemical resistance include soap, detergent, alkali, acid, wax, cheese, edible oils and fats, spices, water and solvents, always adopting a ‘worse case’ scenario. The customer may specify the use of alternative substances.
Lighfastness is a measure of how fast a color fades when exposed to sunlight. Inorganic pigments are in general more stable than organic ones, and black is the most lightfast of all the common inks. Blue and green pigments are more stable than red, and yellow is the least stable.
Lightfastness is specified in eight steps on the ‘Blue Wool’ scale, ranging from 8 (excellent) to 1 (very poor). Level 5 represents ‘good’.
The eight steps are a logarithmic series, so one step higher means the ink can tolerate double the amount of sunlight until it starts to fade. Level one represents lightfastness of around 20 hours and level 8 over 3,000 hours.
Lightfastness is tested by exposing a print to sunlight or to a xenon arc with a UV-filter, which has an emission spectrum similar to sunlight but is much stronger, accelerating the testing process.
It should be noted that the quality of sunlight – and hence its effect on ink – will differ depending on the light source. So, a print will last longer exposed for the same amount of time to light in northern Europe in winter compared to southern Europe in summer. It will also fade faster at higher altitudes, where light is richer in the UV end of the spectrum which carries the most energy.
Therefore, a sample print should be exposed to a light source alongside a series of standards.
A number of different heat resistance tests are used to assess performance in different high temperature conditions. Flexible packaging that is to be heat sealed, for example, requires testing in a heat seal tester at the appropriate temperature and dwell time while in contact with a piece of foil. After cooling, the film and foil should separate without any cling. A more stringent test is to use two pieces of print face-to-face.
Deep freeze resistance
A print subjected to deep freeze conditions may fail at one of two points: the print could come off in the freezer if adhesion and flexibility are inadequate at that temperature; or once the pack has come out of the freezer and is thawing, the water present can be the source of failure.
The test procedure is to place the print in a deep freeze for 24 hours before removing the sample and flexing it.
Then it is placed in water at room temperature for two hours before being flexed again. There should be no more than five percent removal of ink at either stage.
Laminating and varnishing
When laminating, varnishing or coating a label, there are two areas of concern. Will the ink film be properly wetted by the adhesive, varnish or coating so that satisfactory inter-film adhesion is maintained; and will any solvents present in the adhesive, varnish or coating attack the pigment, resulting in visible color change or bleed?
Once the print has properly dried, it is processed with the laminating adhesive, coating or varnish. Between one to seven days later the print is compared to an unprocessed sample to see if there has been any visible bleed or color change.
The bond strength of the laminate is tested by attempting to separate the two plies, and the adhesion of a varnish and coating is assessed ty the Scotch Tape test. A heat seal coating is tested in a similar way to the laminate.
Odor and taint
Inks for food packaging must be free of odor and taint if they are not to contaminate the contents of the package. The test procedure is to take samples of the test print, a standard print and the unprinted substrate and tightly seal them into separate odor-free containers, where they are stored for one hour at 50 degrees C. When removed, the samples are checked for residual odor before being rapidly resealed. They are examined again once the samples come back to room temperature. There should be no detectable difference between the unprinted substrate and the two printed samples. Other, more elaborate, tests are also available.
This lengthy list of test procedures demonstrates that converters need to ask their customers a lot of questions about how the labeled container will be processed, stored, used and disposed of. They can then add to the job bag an appropriate description for the QA department of the range of tests which need to be carried out, and defining who will carry out those tests. This will sometimes be a press operator, sometimes a lab worker operating specialist equipment. This will ensure that the correct tests are carried out on the printed label and it is fit for purpose for its full intended life cycle.
Chemical drum labels – used for identification, warning and instruction – intended for marine use are tested to British Standard BS 5609. This includes requirements for the label base material. It sets out various durability criteria including adhesive performance, print permanence and abrasion resistance, stipulating minimum standards for labels to be used in Marine Environments’.
BS 5609 testing is a requirement for self-adhesive drum labels needing International Maritime Dangerous Goods (IMDG) certification. Testing includes a three-month exposure of labeled test plates in salt water at mid-tide.