Rubber Vulcanization Principle & Tips for Temperature and Time Control

Vulcanization (also known as curing) is the core manufacturing process that converts plastic raw rubber into elastic finished rubber. With heat and pressure applied, vulcanizing agents create cross-links between linear rubber polymer chains. The linear molecular structure transforms into a stable 3D network structure. After crosslinking, rubber no longer flows plastically and gains essential properties: high elasticity, tensile strength, abrasion resistance and chemical stability.

Main crosslinking systems in industry: Sulfur-based system (general-purpose rubber), peroxide system (EPDM, silicone rubber), metal oxide curing system (neoprene CR).

Temperature supplies activation energy for crosslinking reaction.

• Higher temperature speeds up vulcanization and shortens curing cycle.
• Overhigh temperature → Overcure: Rubber becomes stiff and brittle, elasticity declines. It may also cause premature scorch before molding.
• Too low temperature → Under-cure: Incomplete crosslinking, poor mechanical properties and large permanent deformation.

Reference curing temperature range:

• NR, SBR, NBR (sulfur system): 145℃ ~ 165℃
• EPDM: 150℃ ~ 170℃
• Silicone rubber: 160℃ ~ 185℃
• FKM Fluororubber: 170℃ ~ 190℃

Time required to achieve adequate crosslinking under constant temperature. Setting depends on product thickness, thermal conductivity of rubber compound and curing temperature. As a general rule following Arrhenius principle: Curing time can be halved when temperature rises by 10℃.

• Under-cure: Insufficient crosslinking, high compression set, poor oil resistance.
• Over-cure: Polymer degradation, loss of flexibility and higher cracking risk.

Pressure expels trapped air, enables complete mold filling, strengthens interlayer adhesion and eliminates bubbles and surface defects.

1. Calibrate actual cavity temperature Do not trust machine display values blindly. Use temperature probes to measure real mold cavity temperature as formal process standard.
2. Adjust curing time based on product thickness Heat transfers slowly inside rubber. Curing time must be extended for thick-wall items. For thick rubber parts: Adopt moderate temperature with longer holding time to avoid surface overcure and inner undercure.
3. Differentiate parameters by product structure Thin small rubber parts: Higher temperature + shorter cycle for higher productivity. Solid thick rubber products: Medium temperature with extended curing duration.
4. Set baseline via Moving Die Rheometer (MDR) Take t90 (optimum cure time) from vulcanization curve. Actual production time = t90 plus 10%~20% safety allowance.
5. Avoid premature scorch Control stock temperature during open milling, extrusion and preheating. High temperature ahead of molding triggers early partial crosslinking.
6. Apply post-curing for special elastomers Silicone rubber and fluororubber require secondary oven post-curing after mold vulcanization. Post-curing removes volatile small molecules and optimizes compression set and thermal stability.
7. Maintain stable temperature Sharp temperature fluctuation of vulcanizing press leads to inconsistent quality between batches. Keep steady temperature during continuous production.