Plastic Hardness: Testing Methods, Standards, and Applications

Plastic is a typically soft material, prone to scratches or wear when in contact with harder or more complex surfaces. Hardness is one of the most commonly measured properties and is frequently reported in plastic datasheets. Analyzing hardness is critical for Injection Molded Parts Suppliers, as surface wear from friction or contact can significantly affect performance.

For injection-molded plastic automotive parts, hardness plays a crucial role in the basic properties of plastics. particularly ductile materials, and often shows a positive or negative linear correlation with yield strength and tensile strength.

However, hardness is not always a reliable indicator of other properties, such as wear resistance, strength, or scratch resistance. Therefore, it should not be considered in isolation during product design.

Types of Hardness Tests

Shore Hardness

Shore hardness is measured using a Shore durometer, which is categorized into three types: A, C, and D. The principle is the same for all types, but the needle size, especially the tip diameter, differs. Shore hardness is the most commonly used hardness measurement in Injection Molded Plastic Automotive Parts.

• Type A: Suitable for general rubber, synthetic rubber, soft rubber, greases, leather, wax, etc.
• Type C: Suitable for microporous materials made from foamed plastics or a mix of rubber and plastic.
• Type D: Suitable for hard rubber, resins, acrylic, glass, thermoplastic rubber, printed circuit boards, and fibers.

Note: For Type A durometers, readings below 10HA or above 90HA may be inaccurate. It is recommended to switch to Type C or D when necessary.

Measurement Principle: The durometer needle penetrates the material surface, and the hardness value is displayed on the dial.

Rockwell Hardness

Rockwell hardness is used for thin or small parts, or materials with thin or moderately thick hardened layers.

• N Scale: Suitable for materials similar to HRC or HRA.
• T Scale: Suitable for materials similar to HRB.

Measurement Principle: A penetrator (diamond cone or hardened steel ball) is pressed into the surface in two steps. After applying the main force and holding for a specified duration, the residual indentation depth under initial force is measured to calculate hardness.

Sample Requirements

• The surface of rubber and plastic samples must be smooth, flat, and free of defects, mechanical damage, or impurities.
• Rubber Samples: Minimum thickness 6mm, width 15mm, length 35mm. If thickness <6mm, stack up to 4 layers of the same film, ensuring parallel surfaces. Testing temperature: 18–28°C, sample storage ≤5 hours.
• Plastic Samples: Square, 50mm x 50mm x 6mm, or 50mm x 15mm allowed. Testing temperature: 20°C ±2°C, samples conditioned for 16 hours.

Application Scope

• Rockwell Hardness:Suitable for harder plastics like polycarbonate (PC), polystyrene (PS), and polyamide (PA).
• Shore Hardness:Used for elastomers, rubber, and softer plastics like polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC).

Relevant Standards

• GB/T 2411-2018:Plastics and hard rubber — Shore hardness measurement.
• GB/T 3398.2-2008:Plastics — Rockwell hardness measurement.
• ISO 2039-1 / 2039-2:Plastics hardness tests — ball indentation and Rockwell hardness.
• ASTM D2240-2021:Standard test method for rubber hardness.
• ISO 48-4-2018:Shore hardness for vulcanized rubber and thermoplastic plastics.

For a full list of plastic hardness testing standards, reply “Hardness Standards” for free access.

Conversion Between Shore A and D

Shore A is used for materials below 90HA, and Shore D for harder materials. For HD readings below 20, use Type A. Approximate conversion formulas:

• D = A – 50
• A = 0.173 × D + 86.58

Note: These formulas provide only an approximate conversion. Actual testing should use the appropriate durometer type for accurate results.

Conclusion

For Injection Molded Parts Suppliers, plastic hardness is a key material property widely used to evaluate surface resistance and mechanical behavior. Shore and Rockwell hardness tests are the most common methods, each suited to different plastic and rubber materials.

While hardness often correlates with tensile and yield strength, it should not be used as the sole criterion in material selection. Choosing the correct testing method, sample preparation, and standard ensures accurate results and reliable product performance.

FAQ

What is plastic hardness?

Plastic hardness refers to a material’s resistance to surface indentation, scratching, or wear. It is important because it affects surface durability, appearance retention, and suitability for industrial and construction applications.

What is the difference between Shore hardness and Rockwell hardness?

Shore hardness is mainly used for rubber, elastomers, and softer plastics, measured with a durometer (Shore A, C, or D). Rockwell hardness is suitable for harder plastics and thin components, using an indenter under controlled loads to measure indentation depth.

 How do I choose between Shore A and Shore D hardness tests?

Shore A is typically used for materials with hardness below 90HA, such as rubber and soft plastics. For harder materials above 90HA, Shore D provides more accurate results. When Shore D readings are below 20, Shore A testing is recommended instead.

Can Shore A hardness be accurately converted to Shore D hardness?

Shore A and Shore D hardness values can only be approximately converted because they use different test methods and indenter geometries. Conversion formulas may provide estimates, but for accurate results, the appropriate durometer should always be used for direct testing.