When I first learned rubber molding, I thought vulcanization was the final step. Then I discovered post-curing — the quiet hero behind the stability and purity of silicone and rubber products.
Post-curing is a secondary heating process performed after the main vulcanization to remove residual chemicals, improve mechanical properties, and stabilize rubber or silicone materials.
This step may sound optional, but it determines how long a gasket keeps its elasticity, how clean a hose smells, and how safe a medical-grade part is.
Why Is Post-Curing Necessary?
Many engineers ask, “If rubber is already cured, why cure it again?” The answer lies in chemistry.
During vulcanization, not all curing agents or volatiles react completely. Post-curing removes unreacted residues and drives the crosslinking reaction to completion.
When rubber is first molded, small traces of peroxide by-products, moisture1, and plasticizers remain inside the part. Without post-curing2, these can cause odor, blooming3, or changes in hardness4 over time.
What Happens During Post-Curing
| Stage | Description | Effect |
|---|---|---|
| 1. Heating | Rubber parts are heated in an oven at controlled temperature5 | Removes volatile residues6 |
| 2. Crosslinking | Additional chemical bonds form between polymer chains | Increases strength and elasticity |
| 3. Stabilization | Molecules align into a stable network | Improves heat and chemical resistance7 |
In short, post-curing “cleans” and “strengthens” the material. I’ve seen parts that skipped this step turn yellow and brittle after a few weeks — proof that vulcanization alone is not enough.
How Is Post-Curing Performed?
The process looks simple — just heat and wait — but precision matters. Time, temperature, and airflow determine quality.
Post-curing is typically done in a hot-air oven between 150°C and 250°C for several hours, depending on rubber type and part thickness.
In our factory, we place molded silicone parts on metal trays, ensuring no contact or stacking. Air circulation removes volatile by-products continuously.
Typical Post-Curing Conditions
| Material | Temperature (°C) | Duration | Notes |
|---|---|---|---|
| Silicone Rubber (Peroxide-Cured) | 200–250 | 2–4 hours | Removes volatile peroxide residues |
| Silicone Rubber (Platinum-Cured) | 150–180 | 1–2 hours | Mild bake to remove moisture |
| EPDM / NBR / FKM | 150–200 | 2–3 hours | Used to improve compression set8 |
| Fluorosilicone (FVMQ) | 180–200 | 2–3 hours | Stabilizes chemical resistance7 |
Each material reacts differently. Silicone needs high-temperature baking to remove acetic acid or benzene derivatives. Fluoroelastomers, on the other hand, need controlled heating to avoid surface cracking.
What Are the Benefits of Post-Curing?
Post-curing adds value to the final part in many ways. It’s not just about cleanliness — it improves performance across mechanical, thermal, and chemical aspects.
Post-curing improves tensile strength, hardness, compression set, and resistance to heat, solvents, and aging, while eliminating odor and surface residue.
Main Advantages of Post-Curing
| Property | Before Post-Cure | After Post-Cure | Improvement |
|---|---|---|---|
| Hardness (Shore A)9 | 50–55 | 55–60 | +5 points |
| Tensile Strength (MPa)10 | 6–8 | 8–10 | +25% |
| Elongation at Break (%)11 | 250–300 | 350–400 | More flexible |
| Compression Set (22h @150°C)12 | 40% | 20% | Doubled life |
| Odor / Volatiles | Noticeable | None | Cleaner, safer |
When I first introduced post-curing into our EPDM gasket production, customer complaints about odor dropped to zero. It’s now a standard requirement for every high-spec order we produce.
What Materials Require Post-Curing?
Not every elastomer needs it — but those cured with peroxide or used in hygiene-sensitive environments almost always do.
Post-curing is essential for peroxide-cured silicone rubber, fluoroelastomers (FKM), and medical-grade materials where purity and performance are critical.
Common Materials That Benefit from Post-Curing
| Material | Curing System | Post-Curing Purpose |
|---|---|---|
| Silicone (VMQ) | Peroxide | Remove volatile residues and improve clarity |
| Silicone (LSR) | Platinum | Stabilize surface and transparency |
| FKM (Viton) | Peroxide | Improve heat resistance and compression set |
| EPDM | Peroxide | Enhance long-term elasticity |
| Fluorosilicone (FVMQ) | Peroxide | Improve solvent and fuel resistance |
| HNBR | Peroxide | Enhance thermal performance and aging resistance |
For medical and food-grade silicone, post-curing is non-negotiable — it ensures compliance with FDA 21 CFR 177.2600, LFGB, and ISO 10993. Without it, the material could fail extraction or odor tests.
What Happens If Post-Curing Is Skipped?
Some manufacturers skip post-curing to save time or energy, but the risks are serious.
Without post-curing, residual chemicals may cause odor, discoloration, poor compression set, or even failure in sensitive applications.
Potential Problems When Skipping Post-Cure
| Problem | Cause | Impact |
|---|---|---|
| Odor / Taste | Residual volatiles | Unacceptable for food and medical use |
| Surface Blooming | Unreacted additives migrate to surface | Aesthetic and functional issues |
| Poor Compression Set | Incomplete crosslinking13 | Leaking seals and dimensional drift |
| Color Yellowing | Chemical instability | Poor appearance in transparent parts |
| Adhesion Failure | Inconsistent cure | Separation during bonding or coating |
I once tested an un-post-cured silicone gasket for a beverage machine. After two weeks of use, the customer reported a sour smell — that alone disqualified the part from FDA compliance14. After post-curing, the same batch passed both taste and odor tests.
How Is Post-Curing Controlled and Verified?
Post-curing is not guesswork. Every batch must be validated through time-temperature records and post-test analysis.
Quality control involves measuring hardness, compression set, and volatile content before and after post-curing to ensure process stability.
Quality Control Parameters
| Test | Method | Acceptance Criteria |
|---|---|---|
| Hardness (Shore A) | ASTM D2240 | ±3 points from target |
| Compression Set | ASTM D395 | ≤25% |
| Volatile Content | Gravimetric analysis | ≤1% weight loss |
| Odor / Taste | FDA / LFGB sensory test | No detectable odor |
| Color Stability | Visual / Spectrophotometer | ΔE < 2.0 |
We use digital oven controllers that log every minute of temperature data. This ensures traceability for clients in automotive, medical, and food-grade industries.
Post-Curing in Different Industries
Post-curing plays a key role across multiple fields — each with its own priorities.
Automotive, medical, and food processing industries rely on post-curing for safety, durability, and regulatory compliance.
Application by Industry
| Industry | Typical Parts | Post-Curing Benefit |
|---|---|---|
| Automotive | O-rings, turbo hoses, gaskets | Improves compression set and heat resistance |
| Medical | Tubing, seals, masks | Ensures sterility and odor-free material |
| Food & Beverage | Coffee machine gaskets, dairy hoses | Removes volatiles for FDA compliance |
| Electronics | Cable insulation, keypads | Enhances stability and flame resistance |
At Julong Rubber, we post-cure every peroxide-cured silicone and EPDM part before shipment. It ensures consistent performance whether the part ends up in a food line or an EV battery pack.
Conclusion
Post-curing is the final but crucial step in rubber manufacturing — the difference between a good product and a certified one. It purifies, strengthens, and stabilizes materials, ensuring long-term reliability and safety.
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Discover the impact of moisture on rubber quality and performance. ↩
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Understanding post-curing is essential for improving rubber quality and performance. ↩
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Understand the phenomenon of blooming and its effects on rubber products. ↩
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Learn about the methods used to measure rubber hardness and its importance. ↩
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Discover the optimal temperature ranges for effective post-curing processes. ↩
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Understand the significance of removing volatile residues for product quality. ↩
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Discover how chemical resistance affects the durability and application of rubber. ↩ ↩
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Gain insights into compression set and its impact on rubber performance. ↩
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Understanding Shore A hardness is crucial for evaluating rubber performance in various applications, ensuring quality and durability. ↩
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Understanding tensile strength is crucial for evaluating rubber performance in various applications, ensuring safety and durability. ↩
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Understanding elongation at break is crucial for evaluating rubber's flexibility and durability in various applications. ↩
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Understanding compression set is crucial for ensuring the longevity and performance of rubber parts in various applications. ↩
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Learn how crosslinking enhances the strength and elasticity of rubber materials. ↩
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Explore the FDA regulations that ensure safety and compliance in rubber products. ↩
