Definition
An electric pencil hardness tester is an instrument used to evaluate the scratch resistance of coatings, plastics, metal coatings, and other materials. It uses an electric motor to drive a pencil refill of a specific hardness and strokes it on the surface of the specimen with a standardized load and speed to determine the surface hardness level of the material. This device is an automated upgrade of the traditional manual pencil hardness tester, improving the repeatability and ease of operation of the test.
Principle
The core working principle of the instrument is based on the correspondence between the Mohs hardness scale and the pencil hardness scale. A set of standardized drawing pencil refills, the hardness of which is marked from soft to hard is 6B to 9H, forming a continuous hardness scale. During the test, the instrument mounts the selected pencil refill on the pen holder at a fixed angle (usually 45°) and is driven by a motor to move at a uniform speed across the specimen surface. The vertical load applied to the specimen surface by the tip is precisely controlled by the weight, and the common standard load is 750 gram-force (about 7.35 Newtons) or 1000 gram-force (about 9.8 Newtons). Determine the highest pencil hardness rating a material can withstand by observing whether the surface is scratched or creates permanent indentations.
Measurement method
The measurement process follows a standardized operating procedure. First, the pencil refill is peeled to the specified length of cylindrical tip and smoothed on sandpaper. The specimen is flattened to the test platform. Choose a pencil that is expected to be similar in hardness to the specimen to begin the test, usually starting with a softer hardness and working your way up to a harder hardness. After starting the instrument, the refill is driven by a motor to stroke the specimen surface in a straight line, typically with a stroke length of 6.5 mm to 10 mm. After each stroke, wipe off the graphite traces left on the surface with a soft cloth, and check whether there are scratches on the surface of the specimen under lighting observation conditions. The highest pencil hardness value that does not cause visible scratches is used as the pencil hardness test result for the specimen.
Influencing factors
The accuracy and repeatability of test results are influenced by a variety of factors. The preparation status of the pencil tip is the key, and the flatness and edge sharpness of the pen tip must strictly meet the standards. The applied load must be precise and stable, and load deviations can cause hardness deviations. The uniformity of the test speed affects the dynamics of the nib interacting with the material. Ambient temperature and humidity can affect the physical condition of some coatings. In addition, the flatness and cleanliness of the specimen surface, as well as the lighting conditions and viewing angles when observing scratches, can affect the final decision. The operator's experience also plays a role in identifying minor scratches.
Applications
Electric pencil hardness testers are widely used in industry and research and development where the scratch resistance of surfaces needs to be quantified. In the coatings and inks industry, it is used to evaluate the curing hardness and wear resistance of coatings such as varnishes, colored paints, and printing inks. In the field of plastic products, it can be used to test the surface hardness of plastic sheets, films and molded parts. In the automotive industry, it is commonly used for quality control of interior parts surface coating and plastic parts. In addition, the instrument is also a commonly used testing tool in product development and quality inspection in industries such as metal surface treatment, wood paint, electronic product shell coating, glass coating, and building materials (such as flooring, aluminum alloy profile coating).
Selection considerations
When choosing an electric pencil hardness tester, it is necessary to comprehensively consider the testing needs and technical parameters. First, the test standards to be followed, such as ASTM D3363, ISO 15184, JIS K 5600-5-4, etc., should be clarified to ensure that the instrument design meets the requirements of the corresponding standards for load, angle, speed, and testing process. The load accuracy and range of the instrument, and the stability and adjustability of the walking speed are the core performance indicators. The degree of automation, such as whether it has automatic pen clamping, automatic stroke, and multi-point continuous test functions, can improve the testing efficiency. The overall rigidity of the instrument, the flatness of the platform and the convenience of fixing the specimen affect the reliability of the test. In addition, standard hardness pencil kits, calibration services, and technical support provided by the supplier are also aspects to consider during the selection process.