Selection principles
The primary task of Shore hardness tester selection is to determine the hardness range and mechanical properties of the material to be measured. Hardness models (such as A, D, AO, AM, etc.) correspond to different needle shapes and spring forces, and the measurement range must match the rebound characteristics of the material, otherwise it will lead to data distortion or pin damage. Selection should follow the general guideline that "measurements fall within 20%–80% of the model's full scale" to ensure linearity and repeatability.
Model and range
The various Shore hardness models and their typical measurement ranges are shown in the table below. The table lists commonly used models and corresponding hardness ranges for preliminary screening.
| model | Applicable hardness range (Shore units) |
| Type A | 20–90 |
| Type D | Above 90 (when Type A cannot be covered) |
| AO type | Below 20 (soft foam, gel) |
| AM type | Below 20 (microporous material) |
| Type E | 10–50 (Medium Soft Elastomers) |
| Type O | 20–85 (soft rubber) |
When the hardness of the material is exactly at the junction of the two models (e.g. 80–90 A), the harder model should be preferred and the measurement value should be confirmed to be within the effective range. If the expected hardness is less than 20 A, AO or AM type should be used; If it is higher than 90 A, it is necessary to switch to type D.
Matching calculations
The measurement range matching can be quantitatively verified by the relationship between spring force and pressing depth. The conversion formula between the Shore hardness value H and the indentation depth d(mm) is:
H = 100 − d / 0.025 (for Type A, 0–2.5 mm press-in depth for full scale)
For Type D, the formula is:
H = 100 − d / 0.025 (but the spring force is more than 5 times that of type A)
Therefore, it is necessary to ensure that the material does not produce plastic deformation or penetration under the test force. For example, the spring force of type A is about 0.55 N (20 H) to 8.05 N (90 h), and if the material is too soft (such as a sponge), this force may cause the needle to sink completely, so the AO type (lower spring force) must be used.
Guide the process
The selection can be carried out by following these steps:
Step 1: Estimate the hardness of the material. Refer to historical data of similar materials or simple contact pressure judgment.
Step 2: Select a model that may be applicable to the table above and ensure that the estimated hardness falls in the range of 20%–80% of the model's full scale.
Step 3: If you cross the range, choose a model that is closer to the median (around 50%).
Step 4: Conduct pilot measurements to observe numerical fluctuations and indentation rebound. If the value is constant and there is no slippage, it is considered a match.
Common misleading
A common misconception is that harder materials can only use Type D. In fact, materials with hardness of 85–95 A can be used either Type A (only slightly over-scale) or Type D (low-scale end). In this case, Type A readings should be preferred, but it should be noted that the data may be in a non-linear region. Another misconception is that soft materials are directly used with type A, resulting in pointer runout or zero shift, which is a typical mismatch between spring force and material resistance.
Quote the instructions
1. Hardness tester standards: ISO 7619-1 and ASTM D2240 respectively specify the technical parameters and test conditions of type A, type D, AO type and other models, which are the basis for type selection.
2. Mechanical matching principle: based on Hooke's law and the theory of the relationship between indentation depth and hardness in material mechanics, see the relevant chapter of "Rubber Physical Test Methods".
3. Summary of practical experience: quoted from the technical reports of a number of testing laboratories on the hardness testing of soft foam, superelastomer and other materials.