Rubber Calender

Overview

A rubber calender forms warm rubber compound into a continuous sheet of controlled thickness, or laminates that compound onto fabric and cord. It is a precision machine despite its mass: rolls weighing several tonnes set a gap to a hundredth of a millimetre and hold it across a web up to 2.5 metres wide. Calenders sit downstream of an internal mixer and warm-up mills, taking soft stock and turning it into the sheet stock used for tyre plies, conveyor belts, hose, roofing membrane, and rubberised fabric.

The machine carries three or four heavy chilled-iron rolls in a stack. Common layouts are the inverted-L, the S or Z, and the four-roll arrangement that gives two independent nips. Rolls are heated or cooled, driven at controllable speed ratios, and pressed apart by enormous separating force that the frame and nip system must resist without losing gauge.

How it works

Warm compound arrives on the Feed Conveyor from a warm-up mill, sometimes sized first by a Strip Cutter. It is laid into the top nip of the Roll Stack, where it forms a rolling bank held to width by the Stock Side Guide. As the rolls turn, the bank is dragged through the nip and squeezed to a thin film on the roll surface.

The gap, and therefore the sheet thickness, is set by the Nip Adjustment Screw driven by the Screw-Down Gear Motor under each Bearing Chock. Separating force at the nip tries to spread the rolls and bow them in the middle, which would make the sheet thicker at the centre. The machine fights this two ways: the Roll Bending Cylinder applies bending moments to the roll necks, and the Roll Crossing Mechanism skews one roll axis slightly. A Nip Load Cell reads the actual separating force for overload protection and gauge correction.

When the calender coats fabric, cloth is fed into a nip alongside the compound so the rubber is pressed into the weave. A friction calender runs adjacent rolls at different surface speeds, set by the Friction Ratio Gear, so the faster roll wipes rubber into the fabric. After the last nip, the sheet passes the Doctor Blade and goes to the take-off section.

Rolls and frame

Each Calender Roll is chilled cast iron with a hard wear skin over a tougher core, ground true and slightly crowned to offset bending. The rolls turn on heavy Roll Neck Bearing units carried in the Bearing Chock inside the housing windows. Peripheral Roll Drilling drilling carries heating or cooling media close to the surface for even temperature, fed through a Roll Rotary Joint at the roll end.

The Frame and Housings takes the separating load. Two cast steel Side Housing members stand on a Bedplate, tied by Tie Rod members and stiffened by a Cross Beam pair. Frame rigidity is what lets the screw-down and bending systems hold gauge; any frame stretch shows up directly as thickness error.

Drive and temperature control

The Main Drive Motor feeds a reduction gear in the Gearbox Housing, then a Pinion Stand splits torque to each roll. A Universal Drive Spindle couples each pinion to its roll neck so the roll can move on its screw-down without losing drive. Roll temperature comes from the Roll Temperature Control system: a Roll Temperature Unit blends steam from the Steam Control Valve and water from the Water Control Valve, with a Roll Thermocouple closing the loop and a Condensate Trap returning steam condensate. Temperature sets compound tack and flow, so it is held tightly per roll.

Take-off, gauge, and wind-up

After the stack, the Take-Off Roll draws the sheet onto a bank of water-cooled Cooling Drum units, sometimes through a Cooling Festoon for extra cooling dwell, with a Dancer Roll holding line tension. A Thickness Gauge traverses the web on its Gauge O-Frame, and the Profile Controller feeds corrections back to the bending and crossing systems so the cross-web profile stays flat. The Edge Trim Knife knives clean the edges and a Trim Take-Up collects the scrap. Finished sheet is rolled on the Wind-Up Mandrel with interleaved liner cloth paid out by the Liner Unwind so the tacky sheet does not block.

Materials and applications

Compounds include natural rubber, SBR, EPDM, butyl, and nitrile, with carbon black or silica reinforcement. Calenders make tyre inner liner and ply skim, conveyor belt covers, hose sheet, single-ply roofing, and rubberised textiles for diaphragms and protective wear. Sheet gauge tolerance on a well-set machine reaches ±0.01 mm.

Safety, maintenance, and failures

Calender nips are among the most dangerous points in a rubber plant. Each roll carries a Nip Trip Bar across the in-running nip that crash-stops the line through the Safety Relay and Fast-Stop Brake, backed by guards and an Emergency Stop Station network. Maintenance centres on the Calender Roll surface, which is reground when crown or finish degrades, and the Roll Neck Bearing units, which are tracked by the Lubrication System system through its Lube Oil Cooler and Lube Oil Filter. Common faults are gauge drift from a worn Nip Adjustment Screw, temperature streaks from a fouled Roll Rotary Joint or scaled drilling, and sheet defects from a nicked Doctor Blade or uneven bank.