Silicone hardness test is a critical quality control and R&D tool for silicone products—from soft baby bottle nipples to hard automotive gaskets. It measures a silicone’s resistance to indentation, directly reflecting its flexibility, durability, and suitability for end-use. This article breaks down the two most common test methods (Shore and Rockwell), key influencing factors, and practical applications—with clear comparisons, step-by-step guides, and tips to ensure accurate, reliable results.
1. Common Silicone Hardness Test Methods
Silicone’s elastic properties require specialized testing methods. Below are the two primary techniques, along with their principles, procedures, and use cases:
| Test Method | Core Principle | Step-by-Step Procedure | Key Parameters | Ideal Applications |
| Shore Hardness Test | A needle of specific shape (conical for Shore A, spherical for Shore D) is pressed into the silicone surface under a fixed load. Hardness is determined by indentation depth—shallower = harder material. | 1. Sample Prep: Cut the silicone into flat, uniform specimens (surface roughness Ra ≤ 1.6μm). Ensure no bubbles or defects in the test area.2. Setup: Place the specimen on a rigid, flat surface (e.g., marble table) to avoid deformation during testing.3. Testing: Align the Shore hardness tester’s needle vertically with the specimen (no tilting). Apply pressure evenly at a speed of 1–2mm/s until the tester’s base touches the silicone surface.4. Reading: Hold the tester for 5–10 seconds (to account for silicone’s elasticity) and record the value. Repeat 3–5 times at different locations (20mm apart) and take the average. | – Shore A: For soft silicone (0–100 HA); indentation depth ~0.1–1mm.- Shore D: For hard silicone (0–100 HD); used when Shore A reads >90 HA.- Load: 1kg (Shore A), 5kg (Shore D). | – Soft silicone: Baby bottle nipples (20–30 HA), silicone phone cases (50–70 HA), medical catheters (30–40 HA).- Hard silicone: Automotive gaskets (80–90 HA → switch to Shore D), silicone cookware handles (60–80 HA). |
| Rockwell Hardness Test | A diamond cone (120° angle) or carbide ball (1.588mm diameter) indenter is pressed into the silicone using two loads: initial load (to seat the indenter) and main load (to create the indentation). Hardness is calculated from the difference in indentation depth before/after removing the main load. | 1. Sample Prep: Machine the silicone into specimens with flat, parallel surfaces (thickness ≥6mm; size ≥10mm × 10mm). Polish the test surface to remove scratches.2. Setup: Secure the specimen on the Rockwell tester’s stage. Select the indenter (diamond cone for hard silicone) and load (initial load: 10kg; main load: 60–150kg, depending on hardness).3. Testing: Apply the initial load first—this ensures the indenter makes consistent contact with the silicone. Then add the main load and hold for 15–30 seconds. Remove the main load (keep the initial load) and record the indentation depth difference.4. Calculation: The tester automatically converts depth difference to Rockwell hardness (HR) values (e.g., HRC for diamond cone, HRB for carbide ball). | – Indenter: Diamond cone (HRC scale) for hard silicone; carbide ball (HRB) for medium-hard silicone.- Loads: Initial (10kg) + Main (60kg = HRB, 150kg = HRC).- Accuracy: ±1 HR unit. | – High-precision industrial silicone: Aerospace seals (90–95 HA → HRC 20–30), high-wear silicone parts (e.g., pump diaphragms), and silicone composites (silicone + fiber reinforcement). |
Key Comparison: For most silicone products, the Shore Hardness Test is preferred—it’s faster, simpler, and causes minimal damage to specimens. The Rockwell test is reserved for hard, high-performance silicones where precise, load-based measurements are critical (e.g., industrial machinery parts).
2. Key Factors Influencing Test Accuracy
Even the best test method yields unreliable results if external factors are unregulated. Below are the three most critical variables and how to control them:
2.1 Specimen Thickness & Size
Silicone’s elasticity means thin specimens deform under the tester’s load, leading to false low-hardness readings.
- Thickness Requirement: For Shore A tests, the specimen must be at least 10 times the indentation depth (e.g., if indentation is 1mm, specimen thickness ≥10mm). For Rockwell tests, thickness ≥6mm (to avoid the indenter touching the test surface below).
- Size Requirement: The specimen must be large enough to avoid “edge effects”—test points must be at least 20mm away from the specimen’s edges (edges deform more easily, skewing results).
- Example: A 5mm-thick silicone phone case specimen will read 5–10 HA lower than its actual hardness (30 HA → 20–25 HA) because the thin material bends under the Shore tester’s load.
2.2 Test Environment (Temperature & Humidity)
Silicone’s molecular structure is sensitive to temperature and moisture, which alter its hardness:
- Temperature:
- Effect: As temperature rises, silicone softens (hardness decreases). A 10°C increase can lower Shore A hardness by 3–5 HA (e.g., 50 HA at 23°C → 45–47 HA at 33°C).
- Control: Test at room temperature (23±2°C). If testing in hot/cold environments, let the specimen acclimate for 2–4 hours (until it reaches 23±2°C) before testing.
- Humidity:
- Effect: High humidity (>60%) causes silicone to absorb moisture, making it slightly softer (1–3 HA decrease).
- Control: Store specimens in a dry environment (humidity 40%–60%) for 24 hours before testing. For high-humidity regions, use a dehumidifier in the test area.
2.3 Tester Operation & Calibration
Human error and uncalibrated tools are major sources of inaccuracy:
- Vertical Alignment: The tester’s needle must be perfectly vertical (±1°). A 5° tilt can increase indentation depth by 10% (e.g., 1mm → 1.1mm), lowering hardness by 5–8 HA.
- Loading Speed: Apply pressure at 1–2mm/s. Loading too fast (≥3mm/s) causes over-indentation (false low hardness); too slow (≤0.5mm/s) lets silicone recover elastically (false high hardness).
- Calibration: Calibrate the tester every 3–6 months using a standard hardness block (e.g., 50 HA, 80 HA blocks for Shore A). An uncalibrated tester may read 10+ HA off (e.g., 60 HA actual → 70 HA uncalibrated).
3. Purpose & Significance of Silicone Hardness Testing
Hardness testing isn’t just a quality check—it guides every stage of silicone product development and production:
3.1 Quality Control (Production)
- Raw Material Inspection: Test incoming silicone raw materials to ensure they meet hardness specifications (e.g., a baby bottle manufacturer rejects silicone with 15 HA—too soft for safe use).
- Finished Product Testing: Sample 1–5% of finished goods to verify consistency. For example, a silicone gasket factory tests 10 gaskets per batch—if 2 read 85 HA (spec: 75–80 HA), the batch is reworked (adjust crosslinker dosage to soften the silicone).
3.2 Material R&D & Selection
- Formulation Optimization: R&D teams adjust silicone formulations (e.g., adding fillers like silica powder) and test hardness to achieve target properties. For example:
- Adding 5% silica powder increases Shore A hardness by 10–15 HA (30 HA → 40–45 HA), making the silicone suitable for cookware handles.
- Reducing crosslinker dosage by 2% lowers hardness by 5–8 HA (60 HA → 52–55 HA), ideal for flexible phone cases.
- Product-Specific Selection: Choose silicone hardness based on end-use needs:
- Medical gloves: 30–40 HA (soft for comfort, durable for repeated use).
- Automotive door seals: 60–70 HA (flexible to seal gaps, hard enough to resist wear).
- Silicone molds: 50–60 HA (balances flexibility for demolding and rigidity to retain shape).
4. Yigu Technology’s Perspective on Silicone Hardness Testing
At Yigu Technology, we’ve seen 70% of silicone product failures stem from overlooked hardness issues—e.g., a client used 80 HA silicone for a baby pacifier (spec: 25–30 HA), leading to safety recalls. Our key advice: Prioritize Shore A testing for most applications—it’s cost-effective and aligns with consumer product needs. For industrial clients making hard silicone parts (e.g., aerospace seals), we recommend combining Shore D and Rockwell tests to cross-verify results. We also emphasize environmental control: A client in a tropical region (30°C+ humidity) saw 5–7 HA hardness variations until they installed dehumidifiers and temperature controllers. Finally, train operators on vertical alignment and loading speed—even small mistakes (e.g., 3° needle tilt) cost thousands in rework. Hardness testing is the foundation of reliable silicone products; cutting corners here risks product failure and brand damage.
5. FAQ: Common Questions About Silicone Hardness Testing
Q1: Can I use a Shore A tester for hard silicone (e.g., 95 HA)?
A1: No—Shore A testers are designed for 0–90 HA. For silicone harder than 90 HA, use a Shore D tester—Shore A readings above 90 are inaccurate (the needle can’t indent deeply enough to measure true hardness). For example, 95 HA silicone will read ~92 HA on a Shore A tester but show the correct 50 HD on a Shore D tester.
Q2: Why do my test results vary by 5–8 HA even with the same specimen?
A2: Variations usually come from three issues: (1) Edge effects: Testing too close to the specimen’s edge (must be ≥20mm away). (2) Elastic recovery: Not holding the tester long enough (wait 5–10 seconds for silicone to stabilize). (3) Surface defects: Bubbles or scratches in the test area—use a new specimen or test a different location. Fix these, and variations will drop to ≤2 HA.
Q3: How thick should my silicone specimen be for a Rockwell test?
A3: Rockwell tests require a minimum specimen thickness of 6mm—thinner specimens let the indenter touch the test stage below, causing false high hardness readings. For example, a 4mm-thick specimen may read 10 HR higher than its actual value. If your silicone is thinner than 6mm, layer 2–3 specimens (each ≥2mm) with a thin layer of silicone glue (to avoid separation during testing)—ensure the glued surface is flat and free of air bubbles.