Pre-cured Tread Building Procedures
by courtesy of the Retread Manufacturers Association
This article is an excerpt from the RMA’s technical training manual, “Retread Process – Workshop Practice”. To find out more about the full contents of this invaluable technical work, click here.
Pre-cured retreading differs from mould cure retreading in that the tread is already vulcanised to shape with the tread pattern defined. As for conventional retreading preparation may be manual, semi-automated or computer controlled. The cemented casing which should be tacky is rim mounted and inflated to approx 1.5 bar. The cushion may be attached to the tread prior to building or applied directly to the prepared casing.
Uncured cushion rubber is applied between the two cured surfaces of the tread and casing and is cured (vulcanised) to bond tread and casing together. Cushion usually has a high natural rubber content providing good tack to enable the tread to be instantly attached to the casing ensuring maximum cured adhesion.
The cushion is the most important material in pre-cured retreading and must be carefully checked before use. It must be fresh, with close attention paid to shelf life. Over-age material must be treated with suspicion, as it has fast-curing characteristics. Shelf life is usually short. It should be stored in cool, dry conditions, as higher temperatures will cause premature curing, known as set-up, which will prevent cushion flow resulting in poor adhesion and protective backing should be complete and easily removed without tearing or brittleness.
Cushion must not be:
- Dry (lacking in tack)
- Tough or nervy
- Contaminated by dirt, dust or foreign material
Other than endless tread bands where appropriate automation is provided for application, treads may be pre-cut to length before application. The tread should be cut to the same length, depending on pattern match, as the measured buffed circumference, which will allow for a slight tension during application and ensure that the shoulders will grip into the cushion. A tread, which is cut longer than the measured circumference, would result in air trapping. A tread, which is cut too short will increase the tension and could result in the splice opening. It is necessary to match the tread pattern.
If more than one join is required to allow for use of tread ends, the joins should not be too close to each other.
The tread ends should be cut perfectly square with the sides, and be perpendicular to the base. After cutting the tread ends are textured by buffing to remove all surface glaze. A sanding or finishing machine may have to be used to remove all surface glaze from the bonding side of the tread and provide a fine even, broken texture. Any areas not properly sanded must be treated separately by hand or the tread given a further pass through the machine. Excessive sanding results in reduction of undertread thickness.
The areas of thinnest section, those under the tread grooves, offer little resistance to sanding and frequently emerge from the machine still bright and shiny. If not treated such areas may cause local tread separation. After sanding look for an evenly textured surface, free from cuts and depressions.
A coat of cement is applied to the freshly treated surface including the cut and textured tread ends and if the tread is to be stored, the cemented surface is covered with a protective backing.
Apply and stitch on to the leading end a strip of cushion , 25 mm wide x 1.5mm thick, so that the cushion is flush with or slightly below the tread base, never above, to ensure there are no voids when the splice is closed.
The tread protective backing is gradually removed as the tread is applied, starting at the lead end, which is centred on the buffed/cemented surface. Press down the leading edge by hand.
Keeping tread on centre, continue application to trailing edge so that splice just closes. The tread may need occasionally to be removed from the casing and reapplied to ensure an even lay and correct splice closure. Place trailing edge in contact with leading edge to form a solid butt splice, aligned at base and, pattern if necessary matched at top. Trim excess cushion above splice and push any remaining cushion in grooves down to the base of the grooves.
Staple splice ends together, about 3 staples per 25 mm. This is a precaution against splice opening before and during the curing operation.
Stitch tread to casing from centre outwards. Care is required, the tread and the cushion being squeezed on to the casing and trapped air removed.
Facilities should be provided for the warming of cold tread stock, particularly in winter months, so that it is pliable. The tread should be at factory temperature or warm depending on the process adopted. Built tyres are best hung by their beads or horizontally stored on portable racks. Tyres standing under their own weight may be become distorted leading to problems during the curing process.