Plastic Hardness Testing to GB Standards: Shore, Ball Indentation and Rockwell Methods Explained

Plastic hardness testing to GB (Chinese national) standards provides manufacturers with a reliable, internationally recognized way to quantify the indentation resistance of polymer materials and finished parts. The core GB standards—GB/T 2411 for Shore durometer hardness and GB/T 3398.1 and GB/T 3398.2 for ball indentation and Rockwell hardness—are technically aligned with ISO 868 and ISO 2039-1/2. This article explains the scope, sample requirements, test items, procedures, and typical applications of each method to help quality and engineering teams select the right hardness test.

Technical Overview

Hardness is one of the most frequently measured mechanical properties of plastics. It describes a material's resistance to localized deformation under a defined indenter and load, and it correlates broadly with stiffness, scratch resistance, and wear behavior. In China, plastic hardness testing is governed by the national standards system known as GB (Guobiao), in which the suffix "T" indicates a recommended (voluntary) technical standard. The principal GB standards covering plastic hardness are:

• GB/T 2411 — Plastics and ebonite: Determination of indentation hardness by means of durometers (Shore hardness), technically aligned with ISO 868 and comparable to ASTM D2240.
• GB/T 3398.1 — Plastics: Determination of hardness, Part 1: Ball indentation method, technically aligned with ISO 2039-1.
• GB/T 3398.2 — Plastics: Determination of hardness, Part 2: Rockwell hardness, technically aligned with ISO 2039-2.

The choice of method depends primarily on the stiffness of the material: Shore A is used for soft, rubber-like plastics, Shore D for rigid thermoplastics, ball indentation for general engineering plastics, and Rockwell scales (R, L, M, E, K) for harder semi-rigid and rigid materials. Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity before testing, and tests are performed in the same environment.

Testing Samples

Shore Hardness Specimens (GB/T 2411)

Shore hardness testing requires specimens with flat, smooth, parallel surfaces. Key requirements include:

• Specimen thickness of at least 4 mm; thinner sheets may be stacked, but in no more than three layers, and stacked results should be interpreted with care.
• A test area large enough that indentation points sit well away from edges (at least 9 mm in ISO/GB practice) and far enough apart that adjacent indentations do not influence each other.
• Surfaces free from sink marks, flash, scratches, and texture, since durometer readings only reflect the surface layer.

Ball Indentation Specimens (GB/T 3398.1)

Ball indentation testing is more demanding on specimen quality. The surface contacting the indenter must be smooth and flat, the opposite face must rest firmly on the anvil, and the thickness must be sufficient that no mark or bulge appears on the supporting side after testing. Specimens should be free of internal stresses, voids, and machining damage, and the indentation depth is controlled within a defined range to ensure valid hardness calculations.

Rockwell Hardness Specimens (GB/T 3398.2)

Rockwell specimens must have two parallel, flat faces so that the minor and major loads are applied along a well-defined axis. Sufficient thickness is required to prevent the anvil from affecting the result, and surfaces should be free from curvature and visible defects. Molded plaques, machined slabs, or the functional surface of finished parts can all be tested if these geometry requirements are met.

Testing Items

Typical hardness items measured under the GB standards in accredited third-party laboratories and in-house quality labs include:

• Shore A hardness (Shore A) — for soft plastics and elastomeric materials such as flexible PVC, TPE, TPU, and soft sealing compounds.
• Shore D hardness (Shore D) — for rigid and semi-rigid plastics such as PP, PE, PS, PMMA, and rigid PVC.
• Ball indentation hardness (N/mm²) — reported at a specified test load, used for general engineering thermoplastics and thermosets.
• Rockwell hardness scales R, L, M, E, and K (for example HRR or HRM) — for harder plastics where durometer methods lose sensitivity.
• Hardness comparison testing before and after heat aging, UV exposure, or chemical conditioning, where hardness change is used as a durability indicator.

Testing Methods

Shore Durometer Method (GB/T 2411)

The Shore method uses a spring-loaded indenter pressed into the specimen by a presser foot. A Type A durometer has a truncated-cone indenter for softer materials, while a Type D durometer has a sharp conical indenter for harder plastics. When a test stand is used, a calibrated mass is applied (commonly 1 kg for Type A and 5 kg for Type D). The general procedure is:

• Condition the specimen at standard laboratory atmosphere and place it on a rigid, flat surface.
• Apply the durometer perpendicular to the surface without shock and hold the specified load.
• Read the hardness value after the defined dwell time, with 15 s being the reference condition in ISO/GB practice unless a shorter instantaneous reading is agreed.
• Take at least five valid readings at well-spaced points and report the median as the Shore hardness.

Ball Indentation Method (GB/T 3398.1)

The ball indentation method presses a polished steel ball of 5 mm diameter into the specimen. A small preliminary force of 9.8 N establishes the zero point, after which the test load is applied. The standard test loads are 49 N, 132 N, 358 N, and 961 N, and the load is selected so that the indentation depth falls within the range defined by the standard. The depth is measured after a 30-second holding period, and the ball indentation hardness is calculated from the corrected indentation depth, giving a result expressed in N/mm². This method provides a hardness value tied to the bulk behavior of the material rather than only the surface.

Rockwell Method (GB/T 3398.2)

The Rockwell method for plastics uses a ball indenter, a minor (preliminary) load of 98 N to seat the indenter, and a major load that defines the total test force. The hardness value is derived from the difference in indentation depth between the minor-load and total-load conditions; for the ball scales the value is calculated as 130 minus the depth value e, where e is expressed in units of 0.002 mm. The available scales are:

• Scale R — 12.700 mm ball, 588 N total load, for relatively soft plastics.
• Scale L — 6.350 mm ball, 588 N total load, for low-hardness rigid plastics.
• Scale M — 6.350 mm ball, 980 N total load, for harder engineering plastics.
• Scale E — 3.175 mm ball, 980 N total load, for hard thermoplastics and thermosets.
• Scale K — 3.175 mm ball, 1471 N total load, for very hard materials.

In practice, softer rigid plastics such as PE and PP are often measured in Rockwell R, while engineering materials such as PA, PC, and POM are typically measured in Rockwell M or by ball indentation. Above approximately 90 Shore A, testing shifts to Shore D, and above the upper end of Shore D, Rockwell or ball indentation methods provide better resolution.

Applications

GB hardness testing supports material qualification, production control, and product compliance across many industries. Common applications include:

• Incoming inspection and supplier quality control for resin grades, compounds, and semi-finished sheets.
• Automotive components such as interior trim, instrument panels, bumpers, and sealing profiles, where hardness affects tactile feel and durability.
• Pipes, fittings, and valves made from PVC-U, PE, and PP, where hardness relates to pressure resistance and long-term performance.
• Wire and cable insulation and sheathing materials, where hardness is balanced against flexibility requirements.
• Consumer electronics housings, appliances, and packaging components.
• Medical device components, footwear soles, sporting goods, and elastomeric parts.
• Research and development, including formulation screening, aging studies, and comparison of recycled content effects on material properties.

FAQ

What is the difference between Shore hardness and Rockwell hardness under GB standards?

Shore hardness (GB/T 2411) measures the penetration of a spring-loaded indenter and is best for soft to semi-rigid plastics, giving a dimensionless value from 0 to 100. Rockwell hardness (GB/T 3398.2) applies controlled minor and major loads with a ball indenter and is suited to harder plastics, giving values on defined scales such as R or M. The methods overlap only partially, and results are not interchangeable.

Which GB hardness method should be used for soft materials like flexible PVC or TPE?

Soft, rubber-like plastics are normally tested with a Type A durometer under GB/T 2411. If the material reads above approximately 90 Shore A, a Type D durometer gives better resolution and should be used instead.

Can hardness values be converted between Shore, Rockwell, and ball indentation scales?

No conversion is exact. Hardness scales measure deformation under different indenter geometries and loads, so published conversion charts are only approximate guides. For specification and certification purposes, the hardness must be measured directly on the scale stated in the product standard.

What is the minimum specimen thickness for GB hardness testing?

For Shore hardness, a minimum thickness of 4 mm is required, and thin sheets may be stacked in up to three layers. For ball indentation and Rockwell testing, the thickness must be sufficient that no deformation shows on the supporting surface, and the requirements of the respective standard must be verified.

Are GB hardness results comparable with ISO and ASTM results?

GB/T 2411 is technically aligned with ISO 868, and GB/T 3398.1 and GB/T 3398.2 are aligned with ISO 2039-1 and ISO 2039-2, so GB and ISO results are generally equivalent when test conditions match. Comparisons with ASTM D2240 or ASTM D785 should be made carefully, because dwell times and instrument details may differ.

How should GB hardness test results be reported?

A complete report should identify the test standard, the hardness scale or method, the number of measurements and the statistic reported (commonly the median of at least five readings), the dwell time, the test load where applicable, and the conditioning atmosphere. Testing performed by accredited third-party laboratories to these GB standards is widely accepted for supplier approvals and regulatory submissions.