Cure Time and Temperature: Reading a Rheometer Curve
How to find t90, how thick sections change the required cure, and why cure time from a data sheet is not a production setting.
The cure time on a data sheet is measured on a thin test specimen at a stated temperature. Production parts are thicker, and thickness changes everything about the cure you actually need.
Reading the curve
- •ML — minimum torque, a rough indicator of compound viscosity before cure.
- •MH — maximum (or plateau) torque, related to crosslink density.
- •ts2 — scorch time, when cure effectively begins.
- •t90 — time to reach 90% of the torque rise; the conventional reference for cure time.
Thick sections
Rubber is a poor thermal conductor. A 5 mm test piece reaches mould temperature in a fraction of the time a 40 mm section takes. The centre of a thick part cures much later than the surface, which is why cure times for thick mouldings cannot simply be taken from a data sheet. Overcuring the surface while waiting for the core is a common cause of reversion in thick natural rubber parts.
What to ask for
Ask for the rheometer curve at the temperature your mould runs at, and for the t90 measured at that temperature. If your part is thick, ask the supplier for a recommended cure time based on section thickness, or request a cure simulation. State the thickest section in your part when you request a quote — it is the single most useful piece of information for a correct cure recommendation. ## Need the documents or today's price? Send us the grade, quantity and destination port — we return a quotation with COA and MSDS attached. Email manager@fipolymer.com or use the inquiry form on this site.