How to Choose Silicone Hardness: A Shore A Guide for Buyers
Quick answer: how do you choose silicone hardness?
Choose silicone hardness by starting with the product's function, then comparing candidate materials in the intended product geometry. Specify the hardness scale, material grade, test method, specimen condition and acceptance limits together. Approve the finished product's performance as well as its hardness reading before mass production.
Shore A is a material indentation measurement—not a complete specification for how a finished product should feel or perform. A purchasing decision should therefore combine material data, controlled testing, approved samples and application-specific checks.
This guide from WQ Silicone explains how buyers can turn a request such as “soft but supportive” into a clearer development and inspection brief. It does not prescribe universal hardness values for product categories or replace a qualified laboratory's test procedure.
What does Shore A hardness actually measure?
A durometer measures a material's resistance to indentation under defined conditions. ASTM D2240 covers several durometer types, including Type A, and describes testing for elastomeric materials and other materials within its scope. Different scales and methods should not be treated as interchangeable.[1]
ASTM's publicly accessible explanation of the method also notes that the result depends on the indenter, applied force and the material's elastic and viscoelastic behavior. It cautions against assuming a simple relationship between indentation hardness and a fundamental material property.[2]
For a buyer, the practical distinction is:
- Hardness: a measured response to indentation using a specified method.
- Product feel: an assessment of the finished design under a particular handling or use condition.
- Product performance: whether the finished item meets its agreed functional requirements.
Use all three in development. Do not turn one hardness value into a promise of tear resistance, comfort, durability or suitability for food contact.
Why can products with similar hardness readings feel different?
A hardness comparison does not hold every design variable constant. When evaluating samples, record the wall thickness, shape, support, surface details and the way the product is loaded or handled.
If those conditions differ, do not conclude that an unexpected feel proves the supplied material is wrong. Compare the material measurement and the finished-product behavior separately.
Use a controlled sample comparison to answer design questions:
| Buyer concern | What to keep comparable | What to evaluate on the finished product |
|---|---|---|
| A flexible section feels too difficult to bend | Geometry, test setup and handling method | Effort required and return to the intended shape |
| A product feels insufficiently supportive | Wall design, support conditions and intended load | Stability during the agreed use scenario |
| A fitted component is difficult to assemble | Mating component, dimensions and assembly method | Fit, installation effort and retention |
| A folding product does not behave as expected | Fold geometry and the agreed test sequence | Folding action, recovery and visible damage |
These are proposed evaluation questions, not tested performance claims for WQ Silicone products. Final test conditions and acceptance criteria should be agreed for the actual design.
What can a material data sheet tell you—and what must you still confirm?
Read the full material description rather than selecting a grade from its hardness label alone. Check the stated condition of the material and whether the supplier provides a typical value, a range or a contractual specification.
Two manufacturer documents illustrate why context matters:
- WACKER's ELASTOSIL eco LR 5040/70 A/B page explicitly states its hardness range for the condition without post-curing.[3]
- WACKER's LUMISIL LR 7601/70 A/B page explains that its stated hardness refers to non-post-cured articles and that post-curing can increase hardness depending on processing conditions and part geometry.[4]
These are grade-specific observations. They are not a rule for all silicones, a recommendation to use these grades, or evidence that WQ Silicone uses them.
Before comparing supplier samples, establish whether the reported results describe the same material condition. A comparison between differently processed samples may not answer the purchasing question you intended to ask.
How should buyers select hardness during product development?
Start with the required function
Describe what the product must do before choosing a target hardness. Explain the intended user, contact conditions, handling, cleaning and the behavior that would make the product unacceptable.
Replace “make it premium and soft” with an observable requirement. For instance, identify the part that must flex, the part that must remain supportive and the fit that needs to be maintained. These are design objectives to evaluate—not performance results already achieved.
Agree on candidate materials and sample conditions
Ask the supplier to propose material candidates with supporting technical documents. Keep a record of the material grade, drawing revision, color, processing condition and sample identification.
Where possible, compare candidates in matching geometries. If geometry or processing must change, document it so that a difference in product behavior is not automatically attributed to hardness alone.
Evaluate function alongside the hardness result
Use a comparison sheet with separate sections for:
- Hardness results and the associated test conditions.
- Dimensions and fit.
- Handling or functional observations.
- Surface and appearance requirements.
- Application-specific safety and compliance evidence.
Do not use a passing hardness result to waive an unsatisfactory functional test. Equally, do not use a pleasant hand feel as a substitute for an agreed material inspection.
Approve the specification and reference sample together
A reference sample helps communicate an intended result, but it should be connected to written requirements. Record its identifier, drawing revision, material specification, approval condition and the documents that control acceptance.
Retain the approved reference and establish which changes require renewed approval. The goal is a traceable purchasing agreement rather than a subjective instruction to “match the sample.”
What should a silicone hardness specification include?
Use the following as a discussion checklist when preparing the purchase specification. It is not a completed specification or a substitute for the chosen standard.
| Specification item | What the buyer and supplier should agree |
|---|---|
| Product identification | Part name, drawing revision and sample or lot reference |
| Material identification | Material grade and the formulation or color details relevant to the agreement |
| Hardness requirement | Named scale, target and permitted acceptance range |
| Test method | Standard and edition, with the applicable procedure identified |
| Specimen definition | Whether testing uses a suitable finished-part area, a test specimen or another agreed approach |
| Material state | Cure and post-cure condition, together with relevant conditioning information |
| Measurement setup | Instrument, verification or calibration records, support and measurement location |
| Reporting conditions | Applicable thickness, temperature, reading time and other conditions required by the selected method |
| Sampling and disposition | Inspection frequency, reporting approach, retest rules and treatment of nonconforming results |
| Functional approval | Separate checks for the intended finished-product behavior |
Do not adopt a tolerance from another product or supplier without review. Establish it from the chosen material, test method, manufacturing capability and functional requirements.
Which test conditions belong in the inspection report?
Ask for enough information to understand and reproduce the comparison. The report should identify the sample, scale, method and edition, material condition, instrument and relevant test conditions.
Specimen thickness, measurement location, temperature or conditioning, reading time and support are items for the laboratory to specify under the applicable procedure. This article deliberately does not prescribe universal values for them.
A thin, curved or otherwise difficult measurement area should be discussed with the laboratory before using a result for acceptance. Do not improvise a test setup and describe it as standard-compliant without checking the procedure.
ASTM D2240 expressly distinguishes its method from other indentation-hardness methods and instrument types.[1] If two reports disagree, compare their methods and conditions before deciding that a production batch has failed.
Does the right hardness prove that silicone is safe for the application?
No. A hardness result is not an application-specific safety approval. Material selection and finished-product compliance require their own evidence.
WACKER's ELASTOSIL eco LR 5040/70 A/B documentation illustrates this separation: it describes hardness and food-contact conditions separately, and places responsibility on the party placing the article on the market to verify relevant requirements for the finished article.[3]
For purchasing purposes, keep separate document groups for:
- Mechanical requirements, including hardness and functional performance.
- Material identity and processing information.
- Safety or regulatory requirements for the intended product and market.
Do not describe a material as suitable for an application solely because its hardness matches the drawing. A supplier's material statement also needs to be checked for scope and applicability to the finished product.
How do you keep the approved result consistent in production?
Connect the approved sample to a controlled specification and inspection record. Before production, agree on the material condition to be tested and how results will be recorded against the relevant lot.
If the material grade, formulation, processing condition or product design changes, review whether the original approval still applies. The grade-specific post-curing information discussed above is one reason to record processing state rather than compare unnamed samples.[4]
When a discrepancy appears, review the evidence in order: sample identity, instrument and method, specimen condition, drawing revision, material record and processing record. Decide on acceptance or corrective action using the agreed criteria—not a hardness reading detached from its context.
For a custom project with WQ Silicone, share the drawing or physical reference, intended use, target market and the behavior you need to achieve. An existing hardness target is useful, but it should be accompanied by its measurement method and approval conditions.
Frequently asked questions
Is softer silicone always better?
No universal answer applies to every product. Define the required flexibility, support, fit and use conditions, then evaluate samples against those requirements. Softness alone is not an acceptance criterion for the whole product.
Can I compare Shore A and Shore D as if they were the same scale?
No. Name the actual scale and method in the specification. ASTM describes different durometer types and cautions against treating different indentation methods or instrument types as equivalent.[1][2]
Will the same Shore A value guarantee the same hand feel?
No. Treat hardness and finished-product feel as separate evaluation items. Compare geometry, support and handling conditions, and confirm the actual result with product samples.
Can curing or post-curing affect the hardness result?
It can for particular grades and conditions. WACKER's LUMISIL LR 7601/70 A/B documentation provides a specific example.[4] Check the selected grade's documentation rather than assuming that all silicones change in the same way.
Does a material's nominal hardness define the permitted production tolerance?
Not by itself. Read the supplier's stated range and conditions, then agree on the acceptance requirement for the actual project. A value in a material name should not replace the purchasing specification.
What should I send with a silicone hardness inquiry?
Provide the drawing or reference sample, intended function, target market, critical fit or flexibility requirements and any existing material or test specification. Identify which requirements are fixed and which still need sample evaluation.
Next step: turn a hardness preference into an approval brief
A useful hardness decision ends with more than a number. It produces a documented material choice, a defined test procedure, an approved product sample and clear functional acceptance criteria.
For related manufacturing background, read WQ Silicone's LSR injection molding guide. To discuss a custom part, send your drawing and application requirements.
Sources and technical scope
- ASTM International — ASTM D2240-15(2021), Standard Test Method for Rubber Property—Durometer Hardness. Public scope covering durometer types, applicable material groups and distinctions from other methods. Read the standard's public scope.
- ASTM International — ASTM D2240-05(2010), public Significance and Use section. Used only to explain the measurement principle and its limitations. This is an older edition, not presented as the current acceptance procedure. Read the background explanation.
- WACKER — ELASTOSIL eco LR 5040/70 A/B product documentation. Grade-specific hardness condition and separate finished-article compliance considerations. Read the manufacturer's document.
- WACKER — LUMISIL LR 7601/70 A/B product documentation. Grade-specific relationship between processing state and hardness. Read the manufacturer's document.
Technical note: This article provides purchasing and development guidance, not laboratory instructions or an application approval. Public standard descriptions do not replace the complete applicable standard. Confirm the edition, procedure and acceptance limits with the supplier and qualified laboratory. Manufacturer examples are specific to the cited grades; they do not establish WQ Silicone's material inventory, test capability or certifications.
