Retreading is the process by which the worn tread of a tyre is removed and replaced by a new one. It allows the tyre to be used again, as it now has the same qualities as a new one. The process is carried out in several steps, which are presented below.
The tyre must not have suffered any irreparable damage during its life, such as running flat or under-inflated, overheating due to excessive speeds or overloading. etc. tyre
The retreading potential must of course be taken into account from the tyre’s design stage. This is because the endurance of the casing is the primary guarantee of a quality retread. All earthmover tyres and tyres for construction applications produced by MICHELIN are technically retreadable.
If carried out in accordance with recommendations, retreading extends the tyre’s wear life, thereby reducing its cost /hour or km.
Retreading also contributes to environmental conservation by postponing the scrapping of used tyres.
Note: Retreaded tyres may be mounted at either the front or the rear of a vehicle.
Certain tyres, in particular those for road cranes, can be regrooved. Regrooving carried out correctly does not affect the potential for retreading a tyre.
The different retreading methods
General principle: application, in a single operation, of an uncured tread, followed by the curing and moulding of the tyre.
The tread, and possibly the protection ply (fig. 1), can be replaced, or all of the belt plies can be replaced at the same time as the tread (fig. 2).
- Buffing: special machine-tools are used to remove the tread band by rasping. The precision machining is defined by geometric features and dimensions, and provides a a predefined roughness characteristics to the surface of the original tread rubber which provides maximal cohesion between the casing and the original tread rubber.
- Brushing/rubberizing: the tyre is then buffed to remove the oxidized parts and damage exposed previously.
Any perforations are also prepared and cleaned. Next, the casings are repaired; all parts previously removed are replaced by new materials where necessary (suitable rubbers, patches and steel plies).
- Build-up: this is the actual retreading operation, where an uncured tread is applied to the prepared tyre.
- Curing: this step ensures the correct chemical cohesion between the tread and the casing. It is during this phase that the tread acquires all of its physical characteristics. It is an irreversible physical-chemical operation whose aim is to:
- give the tread its final shape;
- ensure the cohesion between the different components;
- bond the different rubbers.
Three parameters are of prime importance for completing this operation:
1. the pressure, responsible for:
- giving the tread its pattern;
- creating the adhesion of the interfaces;
- eliminating any gases and air bubbles;
- ensuring the proper penetration of the cables into the interior;
2. the temperature, responsible for:
- the cross-linking of the products;
- a change of physical state of the uncured products;
3. the time required for the thermal and mechanical effects to take place successfully.
There are 2 curing systems: by press or by autoclave.
Curing with the aid of a press: the pressure is obtained by means of a curing tube placed inside the tyre and inflated with hot water under pressure.
The temperature is produced by steam from a boiler, or by means of electric resistances. Curing with the aid of an autoclave: the pressure comes directly from the autoclave the temperature is produced by steam from a boiler, or by means of electric resistances
- Controls: Permanent controls are carried out at every stage of production.
The pre-cure method consists of curing a length of tread, applying liaison ruber to a prepared casing followed by the pre-cured tread. The assembly is then cured.
- Buffing: after the casing has been cleaned and dried,a special machine is then used to remove the tread.
- Brushing / rubberizing: the tyre is then buffed to remove any rust and damage uncovered when the tread was removed. The affected areas are filled with rubber.
- Build-up: after the defects have been filled, the original shape of the tyre is restored by applying a layer of liaison rubber of the required size.
The procured tread strips are then applied.
- Precuring: the tread bands are cured in a special press applying very high pressure. This produces tread bands with extremely dense rubber, providing excellent resistance to wear.
- Curing: the following step in the process consists of enclosing the casing in an envelope and the tread components into an autoclavewith no applied pressure
- Controls: after the curing process, the tyre is removed and the curing is inspected.
Pre-cured retreading must be done when the remaining tread depth is 10% of the original depth.
MICHELIN has developed a support and advisory service which can help you to define your retreading policy.
Your usual MICHELIN representative would be happy to present it to you.