Overview
Pencil hardness plans are a common method for evaluating the scratch resistance of lacquered coating surfaces. The program uses a series of pencils of known hardness to draw over the surface of the coating film under specific conditions to determine the pencil hardness rating of the coating film based on whether permanent scratches are formed. This method is widely used in furniture, automobiles, electronic product shells and industrial coatings, providing a reliable basis for the quantitative comparison of coating properties.
Test Principle:
The test is based on the principle of relative comparison of material hardness. Pencil leads are made from graphite mixed with clay in different proportions, and their hardness is graded from soft to hard. When the pencil is scratched by the coating film at a fixed angle and pressure, the softer film will be scratched by the harder pencil. The main reference standards include the international standard ISO 15184, the American Society for Testing and Materials standard ASTM D3363, and the Chinese national standard GB/T 6739. These standards specify the test instruments, pencil preparation, test procedures, and evaluation methods, ensuring repeatability and comparability of tests.
Pencil hardness grade
Pencil hardness grades are usually arranged in order from soft to hard. Common grading systems use "H" for hardness, "B" for blackness (softness), "F" for hardness between H and HB, and "HB" for intermediate hardness. For example, 9B is the softest, followed by 8B, 7B... B、HB、F、H、2H... 9H, of which 9H is the hardest. Test kits typically contain pencils ranging from 6B to 9H.
| Hardness symbol | General characterization description |
| 6B to B | The graphite ratio is high and the refill is softer |
| HB and F | Hardness and blackness are balanced |
| H to 9H | The clay ratio is high and the refill is hard |
The testing process is with
The test should be performed in a standard temperature and humidity environment. First, use a special sharpener to expose the pencil lead by about 5-6 mm and grind the nib on sandpaper into a flat, sharp-edged cylindrical surface. Hold the pencil on the tester at a 45° angle to the coated film sample. Push the pencil with a force of about 7.5N to line about 6.5 mm on the coating film. Pencils of each hardness class are tested at least three times in a new area. Start with a softer pencil and work your way up to a harder pencil until you find a pencil hardness that is just enough to scratch the coating and cause permanent visible scratches. The pencil hardness rating of the coated film is defined as the hardness of the hardest pencil that fails to scratch the coated film.
The result is determined
The result should be observed under the specified lighting conditions. Scratches are determined by whether there are irreparable grooves or damage to the coating. Factors that affect test results include coating thickness, degree of curing, substrate properties, ambient temperature and humidity, and uniformity of the force applied by the operator. To ensure accurate results, it is recommended to test the same sample multiple times with consistent results. The test report should document the test standard, pencil hardness range, final grade, and any observed anomalies.
Pencil hardness planning is an important part of coating research and development, quality control, and incoming material inspection. It helps engineers predict the scratch resistance of coatings during use and guides formulation adjustments. In program management, test instruments should be calibrated regularly, pencil hardness verified using standard reference plates, and operators should be trained to maintain long-term stability and consistency of the test. Pencil hardness data can be analyzed in conjunction with other mechanical property tests (e.g., adhesion, abrasion resistance) to provide a more comprehensive evaluation of coating system performance.
References
ISO 15184: Colored paints and varnishes. Pencil test method to determine the hardness of paint film.
ASTM D3363: Standard Test Method for Measuring Paint Film Hardness by Pencil Testing.
GB/T 6739: Colored paints and varnishes - Determination of paint film hardness by pencil method.