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.
Consider the various buffing finishes and suitability of casings for further processing.
The following procedure considers the two variations in buffing:
Bead to bead where tread and sidewall are removed during the buffing process in accordance with detailed specifications.
Removal of tread rubber only to specification, principally for pre-cured retreading.
The texture of the buffed surface is important and the chart below indicates an acceptable range of buffing textures.
A buffed surface having deep cuts or lacerations is not suitable and will result in marginal adhesion. Most buffing machines inflate casings to between 1 to 2 bars. This is essential to provide firmness when the tyre is in contact with the rasp, influencing the texture and profile obtained and preventing scorching.
Scorching is identified by a smooth, tacky surface, often with a bluish tinge to the rubber. White smoke emanating from the buffing surface is also an indication of potential scorching.
The principle of the buffing machine is to cut or rasp the outside circumference of the casing to a true circle. If any degree of out-of-roundness exists in the casing an uneven thickness results causing an out-of-balance condition.
When cross buffing the rasp is moved backwards and forwards smoothly across the casing surfaces avoiding gouging and grooving. Well-chosen rasp blades and spacer combinations as determined for the particular machine provide the desirable texture. As a general rule the more blades used and the faster the casing revolves, the better the surface temperature. Deep scoring and lacerations must be avoided.
Rasp blades require changing before they become dull or blunt when they will tend to produce a smooth or slick texture by scrubbing off and scorching rather than cutting off the old tread, resulting in poor adhesion of new tread material.
During the buffing operation, the casing is profiled to a balanced (symmetrical) shape so that it will properly fit the matrix in which it is to be moulded. Dimensions required are pre-determined and made known to the operator. Some machines incorporate a template or computer guided rasp to assist in obtaining the profile whilst others depend upon the operator to provde the profile. Whichever machine is used profile gauges may be used to ensure consistency. It is sometimes necessary to re-buff a casing in order to meet dimensional requirements and sometimes necessary to reject a casing if outside acceptable dimensional tolerances.
For pre-cured retreading the casing shoulder areas may need to be slightly tapered to match the width of the tread to be applied. Treads, which are too narrow for a buffed width so that a prominent step is apparent not only detract from appearance but can create a hinge point in service, which may result in failure. Treads with excessive overhang i.e. too wide for the buffed surface, may have loose edges, which are easily torn off in service.
The casing is buffed to a point where all effort must be made to ensure the entire old tread pattern is removed. Great care is necessary especially on car casings to avoid buffing away the rubber insulation over and between the cords or cutting into the body plies and cords. Due to a lack of old undertread rubber it is sometimes almost impossible to completely remove the tread pattern. Any remaining localised spots should be removed to produce an even surface by finishing on a hand operated flexible buffer. If not removed tread separation may eventually result due to contaminated oxidised rubber or air trapped in the groove base.
The most critical area of any casing is the ‘corner’ formed where the crown turns down over the shoulder. A narrow surface is presented to the rasp and any tendancy towards excessive pressure will force the rasp deeper than necessary and lacerate the surface.
For casings to be remoulded bead-to-bead, the buffing operation is extended down the sidewall to a point about 18mm from the edge of the bead for car casings and about 30 to 35 mm for commercial vehicle casings. Level off as much old sidewall styling and lettering as is practical to provide an even surface. Uneven or poorly buffed surfaces produce defects during curing e.g. localised separations and cracking of veneer around lettering.
Every effort must be made to maintain buffed tyres in a clean and fresh condition. If there is no alternative to stacking tyres on the floor, provide protection and keep away from dust and dirt.
Do not store buffed casings, because the surface may become contaminated or oxidised. Only buff sufficient casings needed for immediate production.
Casings should be built with the least possible delay after they have cooled.