Tests to check the degree of UV curing – printing

Posted: October 3, 2012 in UV Curing, UV Drying
Tags: , , ,

In addition to print tests done with a densitometer or a spectrocolorimeter, it is imperative to check the degree of UV curing.

In fact, a incomplete cure due to several different parameters (such as an excessive ink filmweight, no interdeck dryers, a failing dryer or non-optimised printing conditions) can cause the following problems:

• Excessive residual smell of the printed item;

• A tacky printed surface,

• Lack of substrate adhesion

• Poor mechanical resistance

• A risk of candling (or flaking)

• A high sensitivity to humidity

When they exist, the specified defects are often discovered too late. A second pass under the UV dryer rarely recovers the poor initial cure. To be able to guarantee the consistency of printing, it is essential to implement quality control tests of the print when it comes out of the press.

Obviously, tests such as the analysis of nonreacted monomers by chromatography in liquid phase or humidity resistance control by using a climatic oven, can only be carried out by specialised equipped laboratories. Nevertheless, some simple and efficient tests are available: their application helping in mastering UV printing, and in overcoming eventual curing faults.

The following tests are examples, which can be carried out at the end of the press.

Solvent resistance tests

These are designed to check the curing of UV inks and varnishes. Ethanol is used for testing the cure of UV inks; while Methyl Ethyl Ketone (M.E.K.) is used to test the curing of UV varnishes.

• Ethanol resistance test:

Principle:

Ethanol has a dissolving action, especially on the non-reacted components of the ink. This test assesses the degree cure of the ink film, and consists of rubbing over the print sample with an ethanol soaked cotton pad.

Operation mode:

• Soak cotton (or similar) with ethanol. Then place the cotton on the face of the print sample and rub backwards and forwards at constant pressure and at regular speed and amplitude.

• Count the necessary numbers of cycles needed to register:

• the beginning of the ink film deterioration,

• the complete destruction of the ink film.

The results of the test will depend on:

• the size of the cotton pad,

• quantity of solvent used,

• exerted pressure,

• deplacement speed on the print sample,

• ambient temperature (solvent evaporation).

A specification for the number of cycles to produce this deterioration (initial or complete) can be established after having performed the test on validated prints.

• MEK resistance test:

Like Ethanol, MEK has a dissolving power action on varnishes, especially on its nonreacted components. MEK is used to test the curing of UV varnishes only, since it would be too aggressive on UV inks (a test using MEK would not give conclusive results on UV inks). Refer to the Ethanol resistance test for the application mode.

It must be noted that a negative test result can not only be due to a lack of cure, but also due to an insufficient varnish coating weight.

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