Casting Doubt Over Accepted Print Film Tests

The universally adopted and accepted pencil hardness test is not as reliable, repeatable, or accurate as a measure of film properties as has often been believed.

The universally adopted and accepted pencil hardness test is not as reliable, repeatable, or accurate as a measure of film properties as has often been believed.

That is according to MacDermid Autotype, which has just announced the results of a gauge repeatability and reproducibility study on the effectiveness of the test.

Most importantly, the results of the test highlighted the implications for the film substrate industry.

For example, manufacturers need to agree on common standards for pencil hardness testing under controlled conditions so they can identify the most accurate and repeatable methodology.

In addition, customers should be made aware of the issues and be encouraged to work closely with manufacturers to decide which products meet their specifications, based on tests carried out in controlled circumstances.

MacDermid Autotype cited the Taber test as being more relevant to most display applications as it is a surface scratch test rather than a gouge test.

The Taber test has been examined and found to be objectively more reliable as well as being more relevant to the final application of film in the print and converting sectors.

The study, which was commissioned by MacDermid Autotype's research and development department to determine the effectiveness of the pencil hardness test, showed that variations of 1H can be expected from the test.

That is even when the test is carried out by highly trained individuals, with good eyesight, following a strict protocol and using the correct specification of pencil.

MacDermid Autotype said that the variations can be even greater under laboratory conditions, or when carried out by different suppliers or end users.

The company believes that there are a number of reasons why the test has so many variables.

For instance, it is common for the test to be carried out by hand by many different people, without a controlled holder, whilst testing is often carried out with the coated film placed upon a variety of surfaces, ranging from glass to rubber.

The pencil hardness test, which is used to determine how well a hardcoat film will survive when in use, is often used within the film manufacturing, printing and converting industries as it requires no expensive equipment and provides an immediate visual result, added the company.

The surface of the coating can be tested with a range of leads, 9H being the hardest and 6B the softest.

The tester performs a number of strokes, typically five, with the pencil on the surface of a film; if the surface is not scratched then the next hardest pencil is used, continuing until the sample is scratched and damaged by the pencil.

If a coating can survive scratching by a 3H pencil, or better, then it is generally considered to be a hard coating.

As part of the study, MacDermid Autotype examined a variety of pencils from different suppliers and discovered that they also displayed large variations in test performance.

For example, a Japanese manufactured 3H pencil can be very different from a European 3H one, which likewise can be different from an American 3H.

The company believes that the explanation for that is clear, yet often ignored, as pencils are designed for use by artists and draughtsmen and not for objective scientific tests.

MacDermid Autotype commented that ultimately, the study concluded that in today's highly competitive marketplace, with exacting quality standards, the pencil hardness test is an ineffective and unreliable mechanism on which to base development or purchasing decisions.

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