Plastic Hardness Testing to Chinese National Standards (GB): Methods, Samples, and Applications

Plastic hardness testing to Chinese national standards (GB) is a fundamental procedure for verifying the mechanical quality, consistency, and regulatory compliance of plastic materials and finished products. This article provides a comprehensive technical guide to the principal GB hardness test methods, including Shore indentation hardness (GB/T 2411), ball indentation hardness (GB/T 3398.1), and Rockwell hardness (GB/T 3398.2), covering sample requirements, test procedures, typical applications, and frequently asked questions for manufacturers, quality engineers, and sourcing professionals.

Technical Overview

Hardness is one of the most frequently measured mechanical properties of plastics, defined as the ability of a material surface to resist indentation, penetration, or scratching by a harder indenter under a defined force. For plastics, hardness correlates closely with modulus, wear resistance, scratch resistance, and overall structural consistency, making it a fast and economical indicator of material quality in production control and incoming inspection.

In China, plastic hardness testing is governed by a family of national standards (GB/T) that are technically aligned with corresponding ISO standards. The three core standards are:

• GB/T 2411-2008 – Plastics and ebonite – Determination of indentation hardness by means of a durometer (Shore hardness), aligned with ISO 868.
• GB/T 3398.1-2008 – Plastics – Determination of hardness – Part 1: Ball indentation method, aligned with ISO 2039-1.
• GB/T 3398.2-2008 – Plastics – Determination of hardness – Part 2: Rockwell hardness, aligned with ISO 2039-2.

These GB methods are broadly comparable to internationally recognized procedures such as ASTM D2240 (Shore durometer hardness of plastics and rubbers) and ASTM D785 (Rockwell hardness of plastics), allowing test results to support product certification, export documentation, and supplier qualification. Testing is typically performed by accredited third-party laboratories using calibrated durometers, Rockwell hardness testers, and ball indentation instruments under standard laboratory atmospheres.

Testing Samples

Suitable test specimens may be prepared in the form of compression-molded or injection-molded plaques, sheet cuttings, or sections taken directly from finished products. Specimens must have a flat, smooth surface free of sink marks, flash, scratches, and visible defects, and must be conditioned to equilibrium at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with GB/T 2918 before testing.

Thermoplastic Molding Materials

Compression- or injection-molded plaques of polypropylene (PP), polyethylene (PE), polystyrene (PS), ABS, rigid PVC, and PMMA are the most common specimens. These materials are typically tested with Shore D durometers or Rockwell scales depending on stiffness.

Engineering Plastics

Higher-modulus materials such as polyamide (PA), polycarbonate (PC), polyoxymethylene (POM), polybutylene terephthalate (PBT), and polyetheretherketone (PEEK) are usually evaluated using Rockwell scales (for example, scale M or R) or ball indentation hardness for precise comparative measurement.

Thermosetting Plastics

Phenolic, epoxy, melamine, and unsaturated polyester moldings, including laminated sheets, are tested to verify the degree of cure and crosslinking consistency. Rockwell and ball indentation methods are well suited to these rigid materials.

Elastomers and Soft Plastics

Thermoplastic elastomers (TPE, TPU), soft PVC compounds, and rubber-plastic blends are measured with Shore A durometers. Very soft gels and foams may require special care regarding specimen thickness and support.

Finished Products and Components

Pipes, fittings, sheets, films, profiles, gaskets, seals, cable sheaths, equipment housings, and medical device components can be tested directly on flat regions, provided the wall thickness and surface condition satisfy the requirements of the applicable method.

Testing Items

Depending on the material stiffness and the specified product standard, the following hardness parameters are commonly reported:

• Shore A hardness – for soft plastics, elastomers, and flexible compounds (typical measuring range 20–90 Shore A).
• Shore D hardness – for harder plastics beyond the upper practical limit of the Shore A scale.
• Rockwell hardness – expressed on plastic scales R, L, M, E, or K, selected according to the anticipated hardness range of the material.
• Ball indentation hardness – expressed in megapascals (MPa), primarily used for rigid and semi-rigid thermoplastics and thermosets where a metric engineering value is required.
• Hardness uniformity assessment – comparison across a specimen or batch to evaluate mixing, curing, or molding consistency (reported as median and range of replicate readings).

Testing Methods

Shore Durometer Method (GB/T 2411-2008)

The Shore method measures the penetration of a spring-loaded indenter into the specimen under a defined force applied for a fixed dwell time. The Shore A indenter is a truncated 35° cone with a flat tip of 1.25 mm diameter, while the Shore D indenter is a sharp 30° cone with a 0.1 mm radius tip. Key procedural requirements include:

• Specimen thickness of at least 6 mm; thinner sheets may be plied to reach this thickness, provided layers do not separate during testing.
• Support of the specimen on a rigid, flat surface at least 4 mm thick to prevent local deflection.
• Application of pressure foot force vertically without shock; readings are taken after a dwell time of 15 seconds (or as otherwise specified).
• Measurement points spaced apart from each other and from specimen edges to avoid interaction and edge effects.
• A minimum of five readings, with the median value reported, and temperature of 23 ± 2 °C maintained during testing.

Rockwell Hardness Method (GB/T 3398.2-2008)

The Rockwell method measures the net increase in penetration depth when a full test force is applied and then partially removed, relative to a preliminary force, using polished steel ball indenters. Commonly used plastic scales include:

• Scale R: 60 kgf total force with a 12.70 mm (1/2 in.) ball – for relatively soft plastics.
• Scale L: 60 kgf total force with a 6.35 mm (1/4 in.) ball.
• Scale M: 100 kgf total force with a 6.35 mm ball – widely used for general-purpose engineering plastics.
• Scale E: 100 kgf total force with a 3.175 mm (1/8 in.) ball.
• Scale K: 150 kgf total force with a 3.175 mm ball – for harder, highly filled materials.

The specimen must be thick enough that the plastically deformed zone does not reach the supporting anvil, and indentation points must be adequately separated. Because Rockwell hardness for plastics is time-dependent, the loading and recovery sequence specified in the standard must be strictly followed to obtain reproducible results.

Ball Indentation Method (GB/T 3398.1-2008)

The ball indentation method presses a hardened steel ball of 5 mm diameter into the specimen surface under a selected test force (typically 49 N, 132 N, 358 N, or 961 N) chosen so that the indentation depth falls within the calibrated range of approximately 0.15 mm to 0.35 mm. The ball indentation hardness is calculated from the test force divided by the corrected projected contact area and is expressed in MPa. This method provides an engineering hardness value suitable for material datasheets, research comparisons, and specification compliance, particularly for rigid thermoplastics and thermosetting compounds.

Applications

GB plastic hardness testing supports a wide range of industrial and regulatory applications:

• Automotive industry – interior trim, bumpers, instrument panels, seals, and under-hood components where hardness affects tactile quality, scratch resistance, and fit.
• Electronics and electrical equipment – housings, connectors, switch components, and cable insulation verified against material specifications and safety requirements.
• Pipes, fittings, and construction products – hardness checks on PVC-U, PE, and PP products as part of factory quality control and product certification.
• Medical and pharmaceutical devices – hardness verification of device housings, tubing, seals, and packaging components.
• Consumer goods and footwear – hardness grading of TPE and TPU soles, grips, and soft-touch overmolded surfaces.
• Material development and supplier qualification – comparison of resin grades, filler loading effects, curing studies, and incoming material inspection.

FAQ

How do I choose between Shore A and Shore D scales? Shore A is intended for softer elastomeric materials, generally below approximately 90 Shore A; harder plastics are measured on the Shore D scale. When readings approach the upper or lower practical limits of a scale, the adjacent scale should be used.

Can Shore, Rockwell, and ball indentation values be converted into one another? No exact conversion exists because each method measures indentation resistance under different geometries, forces, and loading times. Published conversion tables are only approximate; for specification and compliance purposes, the material must be tested directly by the method named in the relevant standard or purchase specification.

Why is specimen thickness important? Insufficient thickness allows the deformation zone to interact with the supporting anvil, producing falsely high readings. A minimum thickness of 6 mm is required for Shore testing, and Rockwell specimens must be thick enough to prevent anvil effects, often achieved by plying thin samples without gaps.

Do temperature and conditioning affect results? Yes. Plastic hardness is viscoelastic and time-dependent; specimens should be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with GB/T 2918, and testing should be performed at the same temperature for comparable, reproducible data.

How many readings are required, and how are results reported? For Shore testing, at least five valid readings are taken and the median is reported together with the scale and dwell time. Rockwell and ball indentation tests similarly require replicate indentations at well-spaced positions, with results reported per the format defined in each standard.

Are GB hardness results internationally acceptable? Because GB/T 2411, GB/T 3398.1, and GB/T 3398.2 are aligned with ISO 868 and ISO 2039-1/-2, results from accredited third-party laboratories are widely accepted in international trade, particularly when test reports clearly state the applied standard, scale, forces, and conditioning conditions.