A two-roll mill mixes sulfur, zinc oxide and fillers into raw rubber by passing it repeatedly between two steel rollers of 150-300 mm diameter turning toward each other at a speed ratio of about 1.2 to 1, with the gap set between 1 and 5 mm. A batch takes 15-30 minutes. The nip between the rollers pulls in fingers, hands and arms faster than anyone can react, so the mill needs a stop bar the operator can reach with the free hand that releases the drive, and the operator feeds material with a wooden paddle, never with fingers.
Part of Rubber Processing
Blending rubber with its curing ingredients on a two-roll mill, and the safeguards the machine requires.
The Mill Works By Shear And It Cannot Tell A Hand From Rubber
The rubber is dragged through the narrow gap between two rollers that run at slightly different surface speeds, so it is sheared rather than just squeezed. That shearing is what breaks up lumps of sulfur and filler and spreads them through the rubber. The same shearing is what happens to anything else that enters the nip.
The carrier’s specification:
| Item | Value |
|---|---|
| Roller diameter | 150-300 mm |
| Roller length | 300-600 mm |
| Speed ratio (friction ratio) | about 1.2 to 1 |
| Gap | adjustable, 1-5 mm |
| Mixing time | 15-30 minutes per batch |
What You Need
- Two steel rollers, mounted in a rigid frame with an adjustable gap.
- A drive, hand crank or a line shaft; see Power Transmission, which covers overhead line shafting, and the speed ratio set by gearing or pulley sizes.
- A stop (see below).
- A bamboo or wooden paddle for feeding.
- Weighed ingredients from Compounding Recipe.
Method: A Mixing Batch
Step 1: Set the gap at the wide end, within 1-5 mm.
Step 2: Feed the raw rubber through; it wraps around the front roller.
Step 3: Add the ingredients in stages, cutting and re-rolling until the compound is even. The carrier names sulfur, zinc oxide and fillers.
Step 4: Narrow the gap as the batch softens.
Step 5: Mix for 15-30 minutes, then cut the sheet off and cool it.
Step 6: Test a strip by curing it; see Curing Temperature and Time.
Worked Example
How much does the roller itself store? Treat a roller as a solid steel cylinder (the corpus gives no hollow design). With steel at about 7,850 kg/m³ (recalled), a roller of 150 mm diameter and 300 mm length has volume π/4 × 15² × 30 = 5,301 cm³ and mass 5,301 × 7.85 g ≈ 41.6 kg. A roller of 300 mm by 600 mm has volume π/4 × 30² × 60 = 42,412 cm³ and mass about 333 kg.
A rolling cylinder stores kinetic energy of ¼ × mass × (surface speed)². Taking an assumed surface speed of 0.5 m/s (the corpus gives none): ¼ × 41.6 × 0.5² = 2.6 J, and ¼ × 333 × 0.5² = 20.8 J. Those are tiny. The roller’s own inertia stops it almost at once if the drive is released. What does not stop is the drive: the hand crank, belt or line shaft that is still pushing the rollers. The stop therefore has to release the drive, not just the rollers.
Reach and time: at 0.5 m/s a hand in the nip is drawn 0.5 m in one second, which is the length of a forearm, so even a one-second reaction time is too slow without a stop within reach.
Power: the corpus gives no power figure for the mill. A throughput or batch rate should only be quoted after running the real mill on its real drive and measuring.
Common Mistakes
- Feeding with fingers. Always the paddle.
- A stop at the drive end of the shop. A trapped worker cannot reach it.
- A stop that stops the drive but not the line shaft’s belt, so the belt drags the rollers on.
- No measured coast-down. Time it from release to stillness and write it on the frame.
Safety
The nip: no stop reachable from where you stand means no stop
The danger is not the roller; it is the drive you cannot stop from where you stand. Fit a bar across the whole front of the mill at hand height so that a trapped arm or the free hand can strike it, linked by rope or lever to a belt shifter that throws the belt onto a loose pulley, or to a clutch. The carrier also calls for the stop to reverse the rollers, which frees a trapped hand; a plain stop leaves it pinched but not drawn deeper. On a hand-cranked mill the crank is the drive: let go of it at once, and keep a second person beside the work. Time the rollers from release to rest and post the time; never run the mill with one person alone. Keep sleeves buttoned, hair tied back and no rings or chains.
What’s Next
- Learn next: Sulfur Vulcanization — curing the mix
- Combine with: Compression Molding — shaping and curing it