Plastic Hardness Testing Methods: Standards, Procedures and Technical Guidelines

Plastic hardness testing is a fundamental material characterization technique used to evaluate the resistance of polymeric materials to indentation, scratching, and deformation under applied loads. This technical guide provides a comprehensive overview of internationally recognized plastic hardness testing methods, including Shore durometry, Rockwell, ball indentation, and Barcol hardness testing, along with relevant ISO and ASTM standards, sample preparation requirements, and practical applications across industries.

Technical Overview

Hardness is one of the most frequently measured mechanical properties of plastic materials, defined as the resistance of a material to localized plastic deformation caused by mechanical indentation, scratching, or abrasion. Unlike metals, plastics exhibit viscoelastic behavior, meaning their hardness values are strongly influenced by test duration, temperature, humidity, and the magnitude of the applied load. Consequently, standardized testing conditions are essential for obtaining reproducible and comparable results.

For plastics, hardness is typically measured using indentation-based methods, in which an indenter of defined geometry (a sharpened cone, a steel ball, or a pyramid-shaped diamond) is pressed into the specimen surface under a controlled force for a specified dwell time. The hardness value is then derived from the indentation depth, the indentation footprint area, or the degree of elastic recovery observed after load removal. Because different methods measure different physical responses, hardness values obtained by different methods are not directly interchangeable.

Plastic hardness testing serves multiple purposes in material development, quality control, and product qualification. It provides rapid comparative data for material selection, helps verify curing or processing conditions in thermosets, supports incoming material inspection, and contributes to regulatory compliance documentation. Major standardization organizations, including ISO, ASTM International, and GB (Chinese national standards), have published well-established test methods specifically adapted to polymeric materials.

Testing Samples

Thermoplastic Materials

Common thermoplastic specimens include injection-molded plaques or compression-molded sheets of polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polycarbonate (PC), polymethyl methacrylate (PMMA), acrylonitrile butadiene styrene (ABS), polyamide (PA), and polyoxymethylene (POM). Specimens should have smooth, flat surfaces free from sink marks, flow lines, and surface contamination that could distort indentation measurements.

Thermosetting Materials

Thermoset samples include phenolic molding compounds, epoxy resins, unsaturated polyester, melamine-formaldehyde, and polyurethane systems. For these materials, hardness testing is frequently used to verify the degree of cure, as under-cured thermosets exhibit significantly lower hardness values. Post-curing effects and mold surface conditions must be documented when reporting results.

Elastomers and Flexible Materials

Soft, rubber-like materials such as thermoplastic polyurethanes (TPU), flexible PVC, silicone rubbers, and ethylene-propylene-diene monomer (EPDM) compounds are typically tested using Shore A durometers. These materials require thicker specimens because the indentation depth under Shore A conditions can be substantial, and backing support effects must be avoided.

Specimen Geometry and Preparation

Minimum specimen thickness requirements depend on the selected method. For Shore A testing, a minimum thickness of 6 mm is generally recommended, achieved by stacking plies if necessary, provided they are in full contact. For Shore D testing, a minimum of 3 mm is typical. For ball indentation and Rockwell tests, thicknesses of at least 4 mm are commonly specified. Specimens must be conditioned at standard laboratory atmosphere (typically 23 ± 2 °C and 50 ± 10% relative humidity) for a minimum of 40 hours prior to testing, unless otherwise specified by the relevant material standard.

Testing Items

Hardness test programs for plastic materials typically include the following measurement items:

• Shore hardness (Shore A, Shore C, Shore D) in accordance with ISO 868 and ASTM D2240
• Rockwell hardness (scales M, L, K, R, E) in accordance with ISO 2039-2 and ASTM D785
• Ball indentation hardness in accordance with ISO 2039-1
• Barcol hardness in accordance with ASTM D2583
• Vickers microhardness in accordance with ISO 6507 (adapted for plastics)
• Knoop microhardness for thin sections and coatings
• Hardness retention after environmental conditioning (heat aging, UV exposure, chemical immersion)
• Hardness uniformity mapping across molded parts

Testing Methods

Shore Durometer Hardness Testing (ISO 868 / ASTM D2240)

The Shore durometer method is the most widely used hardness test for plastics and elastomers. A spring-loaded indenter is pressed against the specimen surface, and hardness is read on a scale from 0 (full indentation) to 100 (no indentation). Shore A employs a truncated 35-degree cone indenter with a 0.79 mm diameter flat tip and is suitable for soft, flexible plastics and rubbers. Shore D employs a 30-degree sharp cone with a rounded 0.1 mm radius tip and is suitable for hard plastics. The test force is applied for a standard dwell time, commonly 3 seconds (immediate reading) or 15 seconds, and the timing must be reported with the result. Measurements should be taken at least 12 mm apart and at least 12 mm from the specimen edge, with a minimum of five readings averaged to obtain the reported value.

Rockwell Hardness Testing (ISO 2039-2 / ASTM D785)

The Rockwell method for plastics measures the net increase in indentation depth between a preliminary minor load and a total major load, after a defined dwell period. Common scales for plastics include Rockwell R (60 kgf major load with a 12.7 mm steel ball), and scales M, L, and K (100, 60, and 150 kgf respectively with a 6.35 mm ball). The dwell time under major load is critical for plastics due to creep behavior; ASTM D785 specifies that the reading is taken 15 seconds after the major load is applied. Rockwell hardness is best suited for relatively rigid plastics where the scale discrimination is adequate.

Ball Indentation Hardness Testing (ISO 2039-1)

Ball indentation hardness is determined by pressing a hardened steel ball of 5 mm diameter into the specimen surface under a defined test force, maintaining the load for 30 seconds, and measuring the indentation depth. The hardness value is calculated from the applied force divided by the projected area of the indentation, corrected for the depth of the recovered indentation, and expressed in Newtons per square millimeter (N/mm²). This method is common in European practice and provides results in engineering units that correlate with other mechanical properties such as compressive modulus.

Barcol Hardness Testing (ASTM D2583)

The Barcol impressor is a portable instrument originally developed for cured reinforced thermoset composites. It uses a sharp steel indenter actuated by a spring-loaded system and reads hardness directly on a 0 to 100 scale. It is widely applied to fiberglass-reinforced polyester panels, gel coats, and other rigid reinforced plastics, particularly for field and in-process quality checks. The instrument must be verified against reference blocks before each series of measurements, and a minimum of ten readings is recommended.

Microhardness Testing (Vickers and Knoop)

Microhardness methods use a diamond indenter applied at very low loads (typically 10 gf to 1 kgf) to measure the hardness of thin films, coatings, individual material phases, or specific zones within a plastic part. The Vickers indenter is a square-based pyramid, while the Knoop indenter is an elongated rhombic pyramid suited for shallow indentations. The recovered diagonal lengths of the indentation are measured microscopically, and hardness is calculated from the applied load and contact area. These methods are valuable for research and failure analysis, although load dependence and viscoelastic recovery must be carefully controlled.

Method Selection Criteria

The appropriate method should be selected based on the expected hardness range of the material, the specimen geometry and thickness, the required precision, and any contractual or regulatory requirements. As a general guideline, Shore A is used for soft elastomers (values below approximately 90 Shore A), Shore D for harder plastics (values below 90 Shore D), and Rockwell or ball indentation for rigid engineering plastics. Conversions between scales are approximate and should only be used for estimation, never as a substitute for direct measurement.

Applications

Plastic hardness testing is applied extensively across a broad range of industries and product categories:

• Automotive: hardness verification of interior trim components, elastomeric seals, gaskets, tires, and under-hood plastic parts
• Consumer goods: quality control of handles, footwear components, sporting goods, and flexible PVC products
• Electrical and electronics: qualification of connector housings, cable insulation and jacketing compounds, and switch components
• Construction: assessment of PVC pipes and fittings, flooring materials, and window profiles
• Medical devices: hardness specification of tubing, catheters, and instrument grips, often tied to biocompatible elastomer grades
• Composites manufacturing: cure monitoring and release readiness of thermoset laminates, gel coats, and fiberglass-reinforced panels using Barcol instruments
• Material research and development: comparative evaluation of polymer formulations, filler content effects, and post-processing treatments
• Failure analysis: hardness mapping to detect degradation, incomplete cure, or material substitution

FAQ

Can Shore hardness values be converted to Rockwell hardness values?

No exact conversion exists between Shore, Rockwell, and ball indentation hardness scales because each method measures indentation resistance under different loads, indenter geometries, and dwell conditions. Approximate comparison tables are available in the literature, but they should be used only for estimation. For specification and compliance purposes, the hardness must be measured directly using the method stated in the applicable standard or drawing.

Why is the dwell time important in plastic hardness testing?

Plastics are viscoelastic materials that continue to deform under a sustained load due to creep. As a result, the indentation depth increases with time, and the measured hardness value decreases with longer dwell times. Standards therefore specify exact timing conditions, such as 3 seconds or 15 seconds for Shore durometer readings, and these conditions must always be reported together with the numerical result.

What temperature and humidity conditions are required for testing?

Unless otherwise specified, specimens should be conditioned and tested at the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity, as defined in ISO 291 and ASTM D618. Temperature has a pronounced effect on polymer hardness, particularly for materials near their glass transition temperature, so deviations from standard conditions must be documented.

How many measurements are required for a valid hardness result?

For Shore durometer testing, a minimum of five readings at well-separated locations is standard practice, with the median value reported. For Barcol testing, at least ten readings are recommended. For Rockwell and ball indentation testing, multiple indentations spaced sufficiently far apart are required to avoid interaction between adjacent indentation zones. Accredited third-party laboratories typically apply these repetition requirements along with instrument verification procedures to ensure statistical validity.

What is the difference between Shore A and Shore D hardness?

Shore A and Shore D differ in indenter geometry and applied spring force. Shore A uses a blunter truncated cone indenter suited to soft, flexible materials such as rubbers and flexible PVC, while Shore D uses a sharper conical indenter suited to rigid plastics such as PMMA, PC, and PS. As a practical rule, materials reading above 90 Shore A should be retested on the Shore D scale, and materials below 10 Shore D should be retested on the Shore A scale.

How does specimen thickness affect hardness readings?

If the specimen is too thin, the indenter may sense the underlying support surface (anvil effect), producing artificially elevated hardness values. For Shore A testing, a minimum total thickness of 6 mm is recommended, and thin sheets may be stacked to reach this thickness without trapped air. Specimen thickness requirements vary by method and must always be verified against the governing standard before testing begins.