Pencil hardness fails the test
A
MacDermid Autotype product story
Edited by the Engineeringtalk editorial team
Mar 6, 2006
Study finds pencil hardness test, a universally adopted standard, is not as reliable, repeatable or accurate a measure of film properties as has often been believed.
MacDermid Autotype has announced the results of a repeatability and reproducibility study on the effectiveness of the pencil hardness test, which demonstrate that this universally adopted standard is not as reliable, repeatable or accurate a measure of film properties as has often been believed.
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.
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.
The study, which was commissioned by MacDermid'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, even when carried out by highly trained individuals, with good eyesight, following a strict protocol and using the correct specification of pencil.
These variations can be even greater under laboratory conditions or when carried out by different suppliers or end users.
There are a number of reasons why this 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, while testing is often carried out with the coated film placed on various surfaces, ranging from glass to rubber.
As part of the study, MacDermid examined pencils from different suppliers and discovered that these 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.
This is because pencils are designed for use by artists and draughtsmen and not for objective scientific tests.
The results of this test highlight the many far reaching implications this can have on the film substrate industry.
For example, manufacturers need to agree on common standards for pencil hardness testing under controlled conditions to identify the most accurate and repeatable methods.
In addition, customers should be made aware of these issues and be encouraged to work closely with manufacturers to decide which products meet their specifications, based on tests carried out in controlled circumstances.
Ultimately, the study concludes that in today's highly competitive market, with exacting quality standards, the pencil hardness test is an ineffective and unreliable mechanism on which to base development or purchasing decisions.
Alternative tests, the study found, such as the taber test, can be more relevant to most display applications; this is because the taber test is a surface scratch test rather than a gouge test.
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