Shore hardness test is typically used for measuring different materials, especially the softer ones like rubbers, elastomers, and certain softer plastics, such as vinyls, polyolefins, and fluoropolymers. This method evaluates how resistant a material is to indentation, providing valuable insight into its flexibility, surface durability, and potential performance under load.
This testing helps to determine whether a material is tough enough to endure certain exposure. The next section outlines the Shore test method, its measurement process, and how to interpret the results.
Here, Qualitest will walk you to understand Shore tests through its method, formula, and its diagram. Let's dive into the explanation below.
What Is Shore Hardness Testing?
Take a look at manufacturing shoe soles. To ensure comfort, flexibility, and durability, the materials (often rubber or thermoplastic elastomers) must strike the right balance between softness and resistance.
If the sole is too soft, it may wear out quickly. While, if it’s too hard, it could feel uncomfortable or even crack under stress. This is where the Shore hardness test becomes important, since it helps quantify just how firm or flexible these materials are.
Shore hardness testing is a standardized method used to measure the resistance of elastomers, soft plastics, and polymers to indentation. It tells you how a material responds when pressure is applied, whether it yields slightly or remains firm. The result is given as a number from 0 to 100, with higher values indicating a harder material.
Related article: Durometer: Accurate Hardness Testing for Rubber and Plastic
Shore Hardness Testing Method
The Shore hardness test measures the resistance of a material to indentation using a calibrated durometer. The measurement can be taken either immediately upon contact (initial indentation) or after a specified dwell time, depending on the material behavior and testing objective.
Some durometers include a maximum reading indicator to capture peak values, but it's important to note that these indicators may result in slightly lower readings compared to real-time indentation measurements.
Standard Shore durometers are used for general-purpose testing on flat specimens, typically with a thickness of at least 6 mm. For thinner or smaller samples, especially those with a cross-sectional diameter as small as 1.25 mm, Type M (micro) durometers are employed.
These are specifically designed to test small or narrowly shaped specimens that conventional durometers cannot accurately assess.
Here’s how the procedure works:
- Sample Preparation: For standard tests, specimens should be at least 6 mm thick. Type M allows testing on thinner materials, down to 1.25 mm.
- Test Execution: The indenter is applied perpendicularly to the sample using spring-loaded force. The type of indenter and force level depend on the Shore scale being used (A, D, or M).
- Reading the Result: Hardness is displayed on the durometer as a value from 0 to 100. The reading may reflect either the instantaneous indentation depth or the value after a defined dwell time, offering flexibility in evaluating viscoelastic materials.
All tests should be conducted in a controlled environment, as variables like temperature, surface texture, and sample orientation can significantly affect the results.
Shore Hardness Testing Formula
Unlike conventional hardness tests that rely on mathematical formulas involving load and area, Shore hardness testing does not use a direct algebraic formula. Instead, it is based on a mechanical relationship between the depth of indentation made by a calibrated indenter and the spring force applied to the material.
The Shore hardness value, ranging from 0 (deep penetration) to 100 (no penetration) serves as an index that reflects the resistance of a material to indentation under a specific force.
Each Shore hardness scale (A, D, M, OO, etc.) corresponds to a different indenter geometry and spring calibration, which means the same numerical reading across two different scales does not reflect the same force or material behavior.
According to the ASTM D2240 standard, this relationship is governed by precise calibration tolerances. The spring force exerted by the durometer must align with the expected indentation resistance for each scale. For example:
- On the Type A scale (used to measure rubber hardness, elastomers, and soft plastics), a hardness value of 50 corresponds to an applied force of 4.3 N.
- On the Type D scale (for hard plastics and dense polymers), the same value (50) represents a force of 22.225 N, due to the stiffer spring and sharper indenter.
- For microhardness testing using Type M, which is designed for very small or thin specimens, the forces range much lower, with a value of 50 equating to 0.544 N.
- Softer materials such as gels and foams require even gentler measurement tools, such as Type OO, OOO, or OOO-S, with corresponding forces calibrated between 0.167 N and 1.932 N depending on the scale and reading.
Here’s a breakdown based on the durometers calibration chart:
Shore Type | Force Range (0–100) | N/Unit | Spring Tolerance |
Type A, B, E, O | 0.55 N – 8.05 N | 0.075 N | ± 0.075 N |
Type C, D, DO | 0 – 44.45 N | 0.4445 N | ± 0.4445 N |
Type M | 0.324 N – 0.765 N | 0.0044 N | ± 0.0176 N |
Type OO, OOO | 0.203 N – 1.111 N | ~0.009 N | ± 0.0182 N |
Type OOO-S | 0.167 N – 1.932 N | 0.01765 N | ± 0.0353 N |
Although there’s no "formula" to calculate a Shore hardness value post-test, the calibrated mechanical system within the durometer ensures that the dial reading correlates with a specific indentation resistance.
This makes it a reliable and repeatable method for comparing the hardness of materials (from thick rubber pads to small polymer components) without requiring additional calculations.
Shore Hardness Machine Specification
Shore hardness testers commonly referred to as durometers. Depending on your application needs, these machines range from handheld analog durometers for field inspection to advanced digital benchtop models for quality control labs.
Though designs may vary slightly between brands and models, most Shore hardness testing machines follow these core specifications:
1. Measurement Scale Support
Shore hardness testers are categorized by scale types, each optimized for specific material groups:
- Shore A: For flexible rubbers, elastomers, and soft plastics (e.g., tires, shoe soles).
- Shore D: For harder plastics (e.g., ABS, polycarbonate).
- Shore M: Micro-scale testing for thin or small specimens.
- Shore OO, OOO, OOO-S: For extremely soft materials like gels and sponge foams.
Some benchtop systems are multi-scale compatible. It allows interchangeable indenters and spring modules to test across several Shore types.
2. Indenter and Spring Force Calibration
The core principle of Shore testing is to measure the material's resistance to indentation under a calibrated spring force. Each Shore type has a distinct indenter shape and spring force range, calibrated as per ASTM D2240:
- For instance, Shore A applies up to 8.05 N at 100 hardness, while Shore D applies up to 44.45 N.
- Shore M uses significantly smaller forces, ideal for testing specimens down to 1.25 mm in thickness.
Durometers must maintain calibration within strict tolerances—such as ±0.075 N for Type A or ±0.0176 N for Type M—to ensure repeatability and accuracy.
3. Measurement Resolution
Most modern Shore hardness testers provide digital or analog readings from 0 to 100. Digital models often feature 0.1 unit resolution, auto-hold functions, and max reading indicators to capture peak values, especially useful for viscoelastic materials that change under prolonged load.
4. Display and Interface
Digital durometers come equipped with LCD or OLED screens, and many include USB/Bluetooth connectivity for data export and report generation. High-end lab models may integrate with quality control software for traceability and compliance tracking.
5. Sample Compatibility
Standard Shore durometers are designed to test materials with a minimum thickness of 6 mm. For thinner or more intricate parts, micro (Type M) durometers are used with precision anvils or clamps to secure small samples without distortion.
Optional accessories include:
- Custom test stands for consistent vertical application
- Time dwell control modules
- Fixtures for curved or irregular surfaces
6. Portability and Test Stand Integration
Handheld durometers are ideal for field testing or production lines, offering quick readings with minimal setup. For higher precision, testers can be mounted on a mechanical or motorized test stand, which ensures consistent force application, perpendicular alignment, and optional dwell-time control.
7. Compliance and Standards
Reliable Shore hardness testers conform to ASTM D2240 and ISO 868, ensuring consistency and compatibility across different testing labs and industries. Calibration should be verified regularly using standardized test blocks with known hardness values.
Shore Hardness Testing Diagram
Typical setup requires a standardized durometer stand. The figure below presents a schematic of the Shore durometer operating stand used for testing soft materials.
This diagram demonstrates how force is applied and controlled during the Shore hardness test using a mechanical stand. The Shore durometer stand minimizes user influence and improves the repeatability of measurements by standardizing test orientation, load application, and specimen support.
Durometer Testing Procedure (with Stand)
The Shore hardness test begins with the test specimen being placed on the specimen support table, located centrally on the base of the stand. The table height adjustment knob allows the operator to align the specimen precisely with the indenter tip of the mounted durometer.
Once aligned, the release lever is engaged, allowing the durometer to descend in a controlled and perpendicular motion guided by the mounting assembly guide post and table guide post. The indenter makes contact with the surface of the specimen under the combined effect of the durometer’s internal spring force and gravity-assisted alignment via the stand.
During testing, the indenter penetrates the material, and the depth of indentation is resisted by the material’s surface hardness. The dial gauge on the durometer registers the Shore hardness value, which corresponds to how much the indenter is resisted by the material. A higher value indicates a harder surface with less indentation.
Standardized Force Application
The force applied during testing is dictated by the spring calibration force within the durometer itself, which varies by Shore type. For example:
- Shore A uses a calibrated force of 0.075 N per unit, reaching 8.05 N at 100 durometer units.
- Shore M, intended for very thin materials, uses only 0.0044 N per unit, achieving 0.765 N at maximum scale (see Table 1 from ASTM D2240).
The operating stand ensures that the force is applied vertically and uniformly, reducing potential variability due to hand pressure or angle misalignment—an integral factor when testing materials sensitive to surface deformation or pressure duration.
Measurement Readout
The hardness value is displayed on the durometer dial, which is reset between each test via the durometer return mechanism. This return mechanism ensures that the indenter fully retracts, allowing for accurate baseline conditions before subsequent measurements.
Common Applications and Best Practices
1. Automotive Industry
In automotive manufacturing, elastomeric materials like seals, gaskets, bushings, and tires must meet stringent performance standards. Shore test helps manufacturers evaluate flexibility, compressibility, and wear resistance which are critical for components that must withstand high temperatures and dynamic stresses.
2. Medical Device Manufacturing
Devices such as catheters, syringes, and flexible tubing rely on specific tactile and mechanical properties. Shore 00 or Shore M scales are frequently used for these ultra-soft materials. It enables manufacturers to precisely control material softness for patient safety and comfort.
The test results help verify that polymers meet biocompatibility and flexibility standards, to minimize risks during clinical use.
3. Consumer Electronics and Wearables
Soft-touch buttons, silicone keypads, and wearable device straps must offer both comfort and durability. Shore A or Shore D testing is used based on the material type.
In this industry, Shore hardness test assesses how materials feel to the touch while ensuring resistance to compression or abrasion. The data collected from the test supports design decisions for optimal user experience while reducing product returns due to premature wear.
Related article: Elevating The Quality of Rubber Samples: Qualitest's Solution for Sika's Testing Needs
Get the Right Hardness Tester from Qualitest – Trusted by Industry Leaders Worldwide
Selecting Shore hardness testers is critical to maintaining product quality, meeting regulatory standards, and improving manufacturing efficiency. Qualitest, a global leader in material testing solutions, offers a comprehensive range of Shore durometers and automated hardness testing systems designed for diverse industrial needs.
Whether you're testing ultra-soft gels with Shore 00 or rigid thermoplastics using Shore D, Qualitest provides:
- Manual and digital durometers calibrated to ASTM D2240
- Precision durometer stands for improved repeatability and minimized operator bias
- Automated hardness testers for high-throughput production environments
- Custom testing solutions tailored for specialized materials or complex geometries
Investing in the right Shore hardness testing equipment from Qualitest means fewer production errors, better quality assurance, and long-term cost savings.
Discover our durometers catalog today and get ready to meet your most demanding testing challenges.
