TYRE CURING

Curing recipes and settings

The curing recipe determines the settings to be used for the curing process. The curing recipe contains:

information on the mould, bladder, flanges as well as the tyre markings that vary by type (ply rating, LI, DOT etc.)

press settings (press type, bushing etc.)

shaping information (loading heights, shaping time)

curing program

shaping time

bladder steam / hot water filling time

bladder steam / hot water circulation time

pressure and time of dome steam applied

release time

cooling time

press opening

The operator follows a checklist to make sure all changes have been made and settings changed, and with his signature acknowledges that the setting procedure has been completed. The markings and dimensions are checked on the first tyre completed from each production series so as to get the final affirmation of the correctness of the settings.

Curing Time of a Tyre

Curing time = State of cure definition + Safety factor + Pressure release time + Press opening time

Safety Factor

Deviations in compound (reaction time)

Deviations in tyre thickness

Deviations in temperatures

Steam temperature

Hot water temperature

Variation of the temperature of the green tyre

Mould temperature

Deviations in bladder thickness

Postcuring inflation

The postcuring inflation stage involves mounting the green tyre on the flanges, inflating it and cooling it according to a predetermined and preselected procedure.

Conditions for Post Curing Inflation

Immediately after release from the press, the green tyre is mounted on the flanges either automatically or manually

The postcuring inflation pressure has to be about 20-35 % higher than what the normal inflation pressure of the tyre would be.

The inflation time has to be long enough (at least one curing cycle) depending on the size of the tyre (thickness of materials).

The distance between the flanges needs to be same as the width of the recommended rim for the tyre.

Purpose of Post Curing Inflation

• Stabilize the shape of the tyre.
• Postcuring inflation will help eliminate the influence of hot shrinkage on the tyre -> the tyre dimensions will not increase under actual operating conditions.
• Postcuring inflation is recommended especially tyres of nylon carcass construction.

Differences in the curing of radial and cross ply tyres

Cross Ply Tyres

• Cylindrical bladder is used.
• Green tyres are preshaped in a press.
• Before curing, the inside of the tyre carcass is painted - or alternatively the bladder may be processed or painted - which helps the tyre carcass slide better against the bladder during shaping.
• Considerable deformation of green tyres is possible during curing.

Radial Tyres

• Loading requirements are more precise than with cross ply tyres.
• A so called B-type bladder is used.
• Deformation during curing is small.

The most common curing faults according to the FMEA analysis

• Tread bareness
• Damaged tread
• Shoulder blisters
• Sidewall bareness
• Sidewall blisters
• Sidewall damage (steam or water leak)
• Toe cuts
• Inside bareness
• Bladder break
• Wrong green tyre
• Undervulcanized tyre
• Curing failures
• Dirty mould
• Damaged mould
• Distorted stretch
• Defective mould equipment
• Mould extraction split
• DOT missing/wrong
• Wrong post inflation pressure

Main phases of the curing process for cross ply tyres

The curing process can be divided into:

• preshaping
• loading the green tyre onto the press
• shaping: the shaping bladder is stretched evenly inside the green tyre
• curing and cooling
• offloading the tyre from the press
• postcuring inflation (if needed)

Curing press

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• The tyre is vulcanized in a curing press, which receives the energy required for the vulcanization process through the dome and bladder.
• The steam building up within the dome transmits the energy through the mould.
• Energy is transmitted from the hot water circulating in the shaping bladder through the bladder into the tyre carcass. The pressure of hot water forces the tyre carcass against the mould, which process causes the tyre tread impression to be moulded into the surface of the tyre and determines the final shape of the tyre.

The press requires four types of motive power:

• electricity (control commands, press motion open/closed)
• compressed air (lubricators, confirmation of the control commands, other compressed air powered equipment)
• hydraulic pressure (tyre loading/offloading equipment, bladder motion etc.)
• dome steam and hot water (vulcanization energy)

Curing Press Types

1. Bag-O-Matic, BOM type, the most commonly used curing method
2. AF-type press
3. Autoclave curing method

Common press sizes are 36", 40", 55", 60", 63,5", 75", 85", 88", 91", 104" and 114".