Testing standard
ISO 6502-3
Rubber — Measurement of vulcanization characteristics using curemeters — Part 3: Rotorless curemeter
At a glance
- Test type
- Cure & rheology — how a rubber compound cures is tracked as it happens
- Published by
- ISO
- Edition
- ISO 6502-3:2023
- Material
- Rubber, elastomers & foams
- Runs on
- MDR
What the test does
Uncured compound is sealed inside a cavity between two heated dies rather than gripped. The lower die oscillates through a small fixed rotary arc while the torque transmitted through the rubber is recorded continuously. As crosslinks form the compound stiffens in shear and the torque rises, so the trace of torque against time is the cure curve for that compound at that temperature.
What it measures, and why it matters
The curve yields minimum torque (ML), maximum torque (MH), scorch time (ts), the time to ninety percent of torque rise (t'c(90)) and a cure rate index. ML flags a mis-mixed or ageing batch before it reaches the press. Scorch time sets the safe mould-filling window; t'c(90) sets the press cycle, since demoulding early leaves an undercured part. MH tracks crosslink density and therefore service stiffness. Together they are the routine batch-acceptance test for a rubber compound.
Specimen
No dumbbell is cut and there is no gauge length. A blank of uncured compound of roughly 4 to 5.5 cm³ — about 5 to 6 g depending on compound density — is taken from a milled sheet so that it slightly overfills a cavity of approximately 4.5 cm³. The closing dies form the final geometry across a nominal 0.45 mm gap, so no precision preparation is needed. Interleaving PET or nylon film between compound and die faces is routine housekeeping to keep the dies clean, not a requirement of the method. One blank gives one cure curve. Because the compound is reactive, the time and temperature between mixing and testing shift the result, so laboratories fix a rest period. A blank that fails to fill the cavity, or that has picked up contamination on the mill, is discarded rather than reported.
What the machine must be capable of
Nothing is pulled, so no force capacity and no force-accuracy class applies; the measured quantity is torque. A soft uncured compound sits at a fraction of a decinewton metre at ML and a stiff vulcanisate typically reaches 10 to 30 dNm at MH, against instrument spans quoted at roughly 0.001 to 250 dNm — so clean resolution at the bottom of the range matters more than headroom at the top. There is no crosshead motion and no extensometer: the test is isothermal and the kinematics are fixed. The conventional setting is oscillation at 1.667 Hz, one hundred cycles per minute, through ±0.5° of arc, approximately ±7 % shear strain; compliant instruments offer ±0.5°, ±1° and ±2°, some a much wider ±0.01° to ±20°. Both frequency and amplitude move the torque values, so both are held and reported rather than left to the operator. The fixture is the method: two biconical dies forming a sealed cavity closed under positive pneumatic pressure, their faces cut with radial grooves. Without the grooves and the closing force the compound slips at the die face and the recorded torque understates the compound. Die temperature is the other half — set points from about 100 °C to 200 °C, held to roughly ±0.3 °C with quick recovery after loading. No humidity control applies; the cavity is sealed.
What goes wrong in practice
Die slippage is the quiet one: worn or fouled grooves let the blank rotate against the die face and torque reads low across the whole curve. An underfilled cavity reaches the same place by a different route, giving low and unstable ML and MH. Scorch in storage advances the compound before it reaches the instrument, shortening ts and flattening the early curve. Die contamination from cured residue or torn interleaving film builds up over weeks and drifts MH downwards, which reads as a formulation change and is not.
Related and equivalent standards
ISO 6502-1 introduces the series and ISO 6502-2 covers the older rotor-type oscillating disc cure meter; rotor and rotorless instruments do not produce the same torque values, so historical oscillating-disc data cannot be read across to this part. ASTM D5289 is the direct counterpart for rotorless instruments and is routinely cited alongside it, though the two texts are maintained separately, so a compound specification should name one. ISO 2393 supplies the preparation and mixing procedures for test mixes.
Running ISO 6502-3 on the MDR
Both instruments run a sealed, pressurised cavity that prevents compound extrusion and holds the test volume exactly, so the torque trace reflects the material.
| The method asks for | Dak supplies | |
|---|---|---|
| Torque & frequency | Nothing is pulled — the instrument reads torque, not force: a soft uncured compound sits at a fraction of a dNm at the minimum (ML) and a stiff vulcanizate typically reaches somewhere between 10 and 30 dNm at maximum torque (MH), inside an instrument span that commercial MDRs quote as roughly 0.001 to 250 dNm. | Torque upto 20 N·m; the RPA Ultimo measures from 0.0001 N·m. MDR: ±0.5° arc as standard, at a fixed 1.667 Hz. RPA Ultimo: 0.0016–50 Hz, 0.05–90° arc. |
| Test cavity | Sealed biconical die cavity with an oscillating lower die (rotorless curemeter / MDR) | MDR: sealed biconical rotorless dies, closed pneumatically. RPA: a sealed, pressurised cavity |
| Temperature | Isothermal heated dies, working range about 100 °C to 200 °C, die temperature held to roughly ±0.3 °C | Room temperature to 230 °C; on the MDR, upper and lower dies are controlled separately |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.