For many years, the film industry has used pencil hardness measurements to determine the scratch hardness of specific coated films. The test for pencil hardness is fairly simple, but results may vary depending on the equipment and procedure used for the test. Every effort should be made to standardize testing, equipment and techniques to obtain comparable results.
What is "Pencil Hardness"?
In the film coating industry, hardness is the ability of a given coated surface to resist scratching, marring or gouging. In expressing the measurement of pencil hardness, we do so with values ranging from 6B, the softest, to 9H, the hardest (see Figure 1-1 for the full scale). Typically, films such as polyester and polycarbonate fall in the range of B to 4H, depending on whether the film is uncoated or coated, and if coated, what properties the coating provides. This value is recorded as the hardest pencil that will not scratch or damage the coated surface.
How do you measure pencil hardness? Standardized procedures.
There are two main standardized tests for pencil hardness: ASTM D3363 and JIS K-5600-5-4. For in-depth, step-by-step procedures, please refer to the respective websites. For our purposes, we will give a general overview.
Before getting started, you will need the following supplies:
Pencil Hardness Tester - Wolff-Wilborn is a common brand (ASTM D3363 allows manual testing without a stand, but for good and consistent results you should use a Pencil Hardness Tester)
Pencil sets designed for pencil hardness testing - common brands include Mitsubishi UNI
400 grit sandpaper
Pencil sharpener
90° tool
Glass plate test surface
First, place the film to be tested on the glass plate test surface. Then you need to hone in on the proper pencil for testing. Sharpen the pencil, then load the pencil into the 90° tool, place on 400-grit sandpaper, and recoat the pencil. Mount the pencil in the pencil Hardness Tester and using the appropriate weighted gram load (Note: This depends on the test you are following. Different weights can be used for each test. The weight used needs to be defined in the results), at 45 ° angle, so that the pencil gently falls to the surface of the film sample. Push the Tester ½" forward. Rotate the pencil 90°, to the unused edge, and repeat the test parallel to the first test, at least 1/2" apart.
Check 2 strokes to determine if the surface of the coated film sample is scratched by lead. If neither stroke scratches the surface, the film passes. If one or two strokes scratch the film, the test fails. Repeat the test with different pencils to determine the highest level of pencil hardness the film passes.
Common Pitfalls of Pencil Hardness
The biggest flaw of the pencil hardness test is the various testing procedures used to determine the pencil hardness value of the film. Although there are two standardized procedures, many companies will perform their own testing methods to determine the "pencil hardness" of their films. A common way to change the test is to lower the gram load weight used with the pencil Hardness Tester. The less pressure applied to the pencil, the less likely the lead will scratch the film, thus producing a "higher pencil hardness value". When comparing the pencil hardness of different films, be sure to check the test procedure to make sure you are comparing "apples to apples".
