Inks: testing, proofing and quality assurance

• 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.

WIRE BARS

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. 

 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.

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.

MINIATURE PRESS

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. 

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. 

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.

MINIATURE PRESS

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.