Hardness decides seal force, comfort, and fit. Too soft leaks. Too hard cracks. Tolerance tells you how much drift is allowed.
Typical rubber hardness tolerance is ±5 Shore A for general parts. Tight programs use ±3 Shore A or ±3 IRHD with strict test control. Method, thickness, and operator technique affect every reading.
I run a factory that measures hardness every shift. I see how setup, dwell time, and sample prep move the numbers. In this guide, I explain how to set realistic tolerance, how to measure it, and how to keep it stable from trial to mass production.
How do we measure rubber hardness—Shore A or IRHD?
Choice of method changes the number you see. The same part can read different results if you change the scale or the instrument.
Shore A uses an indenter and a spring on a durometer. IRHD uses a defined force and time on a thicker specimen. Shore A is common. IRHD is preferred for tighter control and lab consistency.
🧪 What each method does
- Shore A1 (ASTM D2240 / ISO 48-4, micro for thin parts): A spring-driven indenter presses into the surface. The scale runs 0–100. Readings depend on surface, thickness, curvature, and dwell.
- IRHD2 (ISO 48-2, ISO 48-3 micro-IRHD): The instrument first “seats” a minor load, then applies a major load and reads indentation after a set time. IRHD reduces operator influence and works well in labs.
📏 Specimen rules that protect your reading
- Thickness should be ≥6 mm for standard Shore A; stack plies to reach thickness if needed.
- Test on a flat, smooth3 area. Avoid ribs, draft, and gates.
- Keep dwell time4 consistent (e.g., 3 s for Shore A unless you specify otherwise).
- Condition samples at room temperature and test at a stable 23 ± 2 °C when possible.
✅ When I choose one over the other
- Shore A for routine in-process checks and most seals, gaskets, wheels, and cushions.
- IRHD when the spec is tight, parts are thick, or audits demand higher repeatability.
- Micro-IRHD when the part is thin or small and a standard indenter would bottom out.
What hardness tolerance should I place on my drawing?
Buyers want stability. Engineers want function. You must balance both with a clear tolerance5.
Most industrial rubber drawings use ±5 Shore A (or ±5 points). For critical parts, ±3 Shore A or ±3 IRHD is realistic with strict test control. Very soft grades (≤30 ShA) and very hard grades (≥85 ShA) need extra care.
📊 Typical tolerance windows I use
| Material family | Common spec | Typical tolerance | Notes |
|---|---|---|---|
| EPDM / NBR6 (industrial) | 50–80 ShA | ±5 ShA | Best balance of cost and control |
| VMQ Silicone7 (general) | 40–70 ShA | ±5 ShA | Color and post-cure can shift readings |
| VMQ Silicone (food / medical) | 40–70 ShA | ±3 ShA or ±3 IRHD | Needs strict oven logs and timing |
| FKM / FPM8 | 60–80 ShA | ±5 ShA | Higher filler content can raise scatter |
| Sponge / Foam (Shore 00) | 20–70 Sh00 | ±8–10 Sh00 | Use Shore 00 or Asker C, not Shore A |
🧭 How hardness range affects tolerance
- Very soft parts (20–30 ShA): Indentation is deep; support from the fixture, thickness, and dwell swings the number. Keep ±5 and control test setup.
- Middle range (40–70 ShA): Most stable. ±5 is practical; ±3 needs controlled test method and trained operators.
- Very hard parts (85–95 ShA): Small change in penetration equals big points swing. Use ±3–4 and a bench stand9.
🧰 Practical drafting rules I add to the print
- “Hardness: 70 ShA ±5, per ASTM D2240, 23 °C, 3 s dwell, test on 6 mm stacked pads.”
- “If thickness <6 mm, use micro-IRHD10 per ISO 48-3.”
- “Record three readings ≥6 mm apart; report average.”
These lines remove debate later. They also protect you in audits.
Do Shore A and IRHD match? Can I convert them?
People ask me for a “conversion.” There is no perfect one. The two methods measure different responses. Still, a rough equivalence helps planning.
Shore A and IRHD do not convert exactly. You can use an approximate mapping for early design, but you must verify on your compound.
🔁 Approximate mapping (use for guidance only)
| Shore A (approx.) | IRHD N (approx.) | Comment |
|---|---|---|
| 30 | 35–40 | Soft parts show larger spread |
| 40 | 45–50 | |
| 50 | 55–60 | |
| 60 | 65–70 | |
| 70 | 75–80 | Common sealing range |
| 80 | 85–90 | |
| 90 | 94–96 | Very firm; alignment critical |
I always run a dual-method check on first articles. We test Shore A and IRHD on the same batch and log the natural offset for that formula. The offset often stays stable run to run.
🧪 Why the numbers drift between methods
- Indenter geometry and force law are different.
- Viscoelastic recovery continues after indentation; dwell time matters.
- Surface vs bulk: Shore A is more surface-sensitive; IRHD probes deeper bulk response.
- Fillers and plasticizers change time-dependent behavior and skew the match.
✅ Best practice when customers use mixed methods
- Put both on the print if your customers test the other method.
- Define which method is acceptance and which is informational.
- Keep a conversion table for your approved compounds in your PPAP11 pack.
What causes hardness variation12, and how do I control it?
Hardness is a system property. The tool, the oven, the operator, and the bench all touch it.
Variation comes from cure state, post-cure, temperature, sample thickness, dwell time, and operator technique. You can cut the scatter by fixing the method and the timing.
⚠️ Common sources of error and the fixes
| Source of variation | What happens | How I fix it |
|---|---|---|
| Cure state (under/over) | Soft or brittle outliers | Hold press time/temperature; track mold temperature; use cureometers for setup |
| Post-cure timing13 | Hardness drifts upward after bake | Fix oven cycle; test at a fixed post-bake wait time (e.g., 24 h) |
| Temperature of sample | Reads softer when warm | Condition at 23 ± 2 °C before test |
| Thickness/support | Reads softer on thin or warped parts | Stack plies to 6 mm or use micro-IRHD; test on a flat spot |
| Dwell/reading time | Longer dwell lowers reading | Fix dwell (e.g., 3 s) and train operators |
| Operator angle/force | Handheld tilt causes scatter | Use a bench stand with a flat foot |
| Part geometry | Lips, ribs, and curves skew values | Define a test coupon or pad location |
| Aging and fluids | Hardness shifts with exposure | Define re-test after heat-age and fluid-soak |
🧾 My in-process control plan (simple and strong)
- At press: record cavity, press number, cure time, and mold temperature.
- After post-cure: hold parts 24 h, then test hardness and compression set14 coupons.
- Sampling: take 3 readings per lot, spaced across the pad; report average and range.
- Gauge control: calibrate durometers weekly; cross-check with IRHD monthly.
I add these rules to the control plan so the numbers do not bounce from shift to shift.
How should I write and verify hardness on my drawing?
Clear words save money. Good verification saves time.
State the scale, tolerance, method, dwell, temperature, and test location. Verify with a short PPAP: material cert, hardness study, and gage R&R if the tolerance is tight.
✍️ Clean drawing language I use
- “Hardness: 70 ShA ±5, ASTM D2240, 3 s dwell, 23 °C. Test on flat pad or stacked coupons (6 mm).”
- “Alternate: IRHD N 70 ±3, ISO 48-2, 30 s reading.”
- “Record 3 points; report average. Use bench stand.”
🔍 Small but powerful verification set
| File | Why it matters |
|---|---|
| Material COA with formulation code | Traceability and repeatability across lots |
| Hardness study (10 parts × 3 readings) | Shows natural spread and proves tolerance is realistic |
| Gage R&R (only if ±3 points) | Confirms instrument and operator are capable |
| Heat-age recheck (e.g., 70 h @ 125 °C) | Confirms drift stays within function window |
🧠 Buyer guidance for tolerance choice
- Use ±5 when cost and speed matter and the function is forgiving.
- Use ±3 only when the seal force window is tight or an audit demands it.
- For thin parts, specify micro-IRHD to avoid “bottoming-out” errors.
- For sponges, switch the scale to Shore 00 or Asker C.
FAQ that I answer every week
Can I get ±2 Shore A?
You can, but it will cost time and money. The instrument, method, and environment must be locked down. I only accept ±2 on thick pads or IRHD with a bench setup.
Why did my lab read 3 points softer than yours?
Different dwell, temperature, or thickness is the usual cause. Align the method on the PO. Test a shared coupon from the same batch.
Do colored silicones vary more?
Yes. Pigments and post-cure change the time-dependent response. I standardize the oven cycle and re-verify after a color change.
Does compression set relate to hardness tolerance?
They are linked by cure and formulation. If you chase very tight hardness while ignoring compression set, the seal can still fail. I test both.
Why buyers choose Julong Rubber for tight hardness programs
I run a high-mix, low-volume factory with in-house tooling and more than 40 vulcanization presses. My team controls cure, post-cure, and test timing. We supply HVAC gaskets, custom seals, wheels, hoses, and precision silicone parts to Germany, France, the Netherlands, Sweden, and the UK.
- ✅ Clear hardness specifications and test benches
- 🛠️ Shore A, IRHD, and micro-IRHD capability
- 📈 PPAP/FAI, AQL plans, and EN 10204 3.1 on request
- 🔄 Stable oven logs and traceable batches
- 🌐 B2B wholesale only, factory-direct communication
Explore more: www.rubberandseal.com
Send me your drawing and the function window for seal force. I will propose a hardness spec, a test method, and a control plan that meet your risk and cost targets.
Conclusion
Set hardness tolerance with method and environment in mind. Use ±5 for speed. Use ±3 with control. Write the test conditions on the print and your numbers will hold.
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Understanding Shore A hardness is crucial for selecting the right materials in engineering and manufacturing. ↩
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Learn about IRHD and its significance in ensuring material quality and performance in various applications. ↩
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Discover why surface quality is critical for accurate hardness readings and how to achieve it. ↩
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Explore the impact of dwell time on hardness measurements to improve testing accuracy. ↩
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Understanding tolerance helps in balancing functionality and stability in engineering designs. ↩
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Gain insights into EPDM and NBR to make informed decisions for industrial applications. ↩
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Learn about VMQ Silicone's unique properties and its applications in various industries. ↩
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Explore this resource to understand the unique benefits and applications of FKM/FPM materials in various industries. ↩
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Discover how a bench stand can enhance the accuracy and consistency of hardness measurements. ↩
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Understand the specific applications of micro-IRHD for testing thin rubber parts effectively. ↩
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Understanding PPAP is essential for maintaining quality control and compliance in production. ↩
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Explore the factors contributing to hardness variation to improve testing reliability. ↩
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Understanding post-cure timing is crucial for maintaining consistent rubber hardness and quality in production. ↩
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Understanding the link between compression set and hardness is vital for material performance. ↩
