Selection & Manufacturing

2.1 Practical Measures and Procedures involved in selection

2.1.1 Tyres are an essential component for all vehicles and therefore must meet stringent specifications as well as legal requirements.

2.1.2 New tyres must pass type approval tests, which involve passing endurance tests, commonly called drum tests, at their nominated load and speed rating. This is covered in Regulations ECE 30 for passenger vehicle tyres and ECE 54 for commercial vehicle tyres. Retread tyres must now pass the same tests to conform with stringent new European Regulations - ECE108 (car tyres) and ECE109 (commercial vehicle tyres) which became mandatory in the UK on 1st January 20041, 2. These tests also define the testing procedures to regulate consistent safe use. The importance of these Regulations is expanded in Section 3.

2.1.3 Casings are collected by retreading companies and, in some cases, supplied by tyre collection firms Whatever pre-selection has been done at the basic collection stage, the collected tyres must be quality sorted and selected for retreading. To be suitable for retreading a tyre must:
· Be structurally sound
· Be of an inherent quality suitable for retreading ("80% of some types of car tyres are not suitable for retreading)3
· Need to be of the right size for the retreaders' markets

2.1.4 Once the tyre has been selected, it will undergo continuous visual inspection as it moves through the retreading process, until as a finished tyre it is subjected to the final quality checks and put into stock. Different methods are employed for the selection process and the final quality check including manual, visual, ultrasonic, laser shearography and inflation testing. The choice of test depends to some extent on the throughput of the process.

2.1.5 Manual/visual selection is the conventional process for acceptance, and is a highly successful method, carried out by well-trained inspectors. Tyres are subjected to visual and tactile inspection, which can discern component fractures and delamination between tyre layers. However, manual testing used as an acceptance criterion of casings for retreading has some limitations. Smaller delaminations cannot necessarily be detected at this stage, but they can grow during the curing process to a discernable size and are subsequently detected. Manual/visual inspection is well suited to car tyres, due to the ease of manipulating lighter structures and the ability to detect defects by feel.

2.1.6 Ultrasonic methods are usually used as acceptance tests, and measure how high frequency sound is transmitted through the tyre. Damaged areas absorb the sound, which can be detected from data scans. Ultrasonic testing is excellent for revealing delaminations within the structure.

2.1.7 Laser shearography measures the shape changes of the tyre when under stress. The technique is usually used as a final inspection test, but is also under consideration as an acceptance test due to its high productivity. Although the stiff rubber of the tyre holds it to shape when free standing and uninflated, it is only the steel reinforcement which keeps it in shape under the large forces imposed when inflated and loaded under an axle.
In the test, the tyre is subjected to an internal laser scan when at ambient pressure and then under vacuum. Any asymmetry in the shape will be detected by comparing the two scans and therefore the extent to which anomalies occur in the inflated shape. These anomalies will highlight the integrity of the controlling steel reinforcement compared with its original condition. Damage to the reinforcement such as gross distortion or fracture is easily revealed, providing a robust check on the critical strength-giving elements of the tyre.

2.1.8 Inflation testing is also a part of the process, related to manual inspection. Here the tyre is inflated to a third to a half of working pressure and the tyre is manually and visually inspected again. Defects in the structure are revealed under the inflation pressure and can be felt or seen. The tyre is inflated to full working pressure as a quality check after retreading, and offers an effective final proof test of the tyre structure. Some retreaders inflate to pressures in excess of operating levels under appropriate conditions.

2.1.9 The inspection processes used by retreaders currently depends on individual choice, but from December 2003 it will be compulsory for them to use "Non Destructive Testing" on their finished products. The inflation test qualifies under this heading, and represents a good baseline for quality.
Many retreaders will use the more sophisticated processes available to help improve productivity; to bring finished product quality to levels comparable with new tyres; and to avoid putting work into unsuitable casings. Laser shearography is capable of a high degree of automation which can lead to cost reductions in tyre selection for retreading. For hot cure retreading, final ultrasonic testing is especially effective for picking up porosity (mould filling) faults.
It is likely that whatever inspection procedures are employed during the process, a final inflation test at the full specified pressure will always give positive benefits as a final proof test of the soundness of the tyre structure.

2.2 Commercial Vehicle Tyres: Selection and Manufacturing

2.2.1 Sector experts acknowledge that most commercial vehicle tyres from reputable manufacturers are produced with a structure that is suitable for retreading over several lifetimes. This retreadability is part of the quality and value offered when the tyres are sold and most new tyre manufacturers are also involved in retreading.

2.2.2 However commercial vehicle tyre retreaders admit that they have some problems with tyres being cherry picked by some collectors, and having to purchase the selected tyres outside of the normal population. It will also be difficult to use this free market system to formally tell a consumer that their make of end of life tyre cannot be retreaded, as this could have legal implications from budget tyre manufacturers and marketing problems at dealerships.
It is likely that at this stage only a graduated casing condition charge could be employed. An increasing quantity of commercial vehicle tyres are becoming part of a total supply and collection managed deal, with the tyre manufacturer or retreader being responsible for the tyre throughout its life

2.2.3 Some empirical information arises from an analysis of selection data for commercial vehicle tyres over a three month period at one major site. This provides an indication of the main reasons for rejection of tyres collected at end of operational life.

2.2.4 Of all tyres taken into their site, around 50% were rejected during acceptance inspection. 72% of these rejections were due to in-service damage to the tyre, and 22% were because of poor service repairs or casing integrity faults. Within both of these categories it is possible to identify specific causes.

2.2.5 Of the tyres rejected because of poor service repairs and casing integrity failures, around 64% were because of material failure making the casing unsuitable for further retreading. This included those tyres that have been retreaded as many times as is possible and served out their useful life. However, in excess of 35% of the repair and integrity rejects were classified as "unwanted", due to their size or load specifications or unsuitability of certain tyre brands for retreading. A substantial proportion of new tyres on the road are therefore not seen as being suitable to retread because the basic build is not sufficiently durable.
Only 29% of casing integrity/service repair failures were due to tyres having been retreaded to the limit of their useful lives. Improvements to the specification standards for tyres could therefore appreciably increase the number of retreadable tyres.

2.2.6 In the case of in-service damage, over 60% could be assigned to poor driving practice resulting in, for example, sidewall impact damage. Just under 20% of damage was due to unavoidable foreign object damage such as severe puncture and frontal impact etc. Hence improved driver awareness and training could lead to a significant increase in the number of tyres suitable for retreading, theoretically of up to 20%

2.2.7 The tyres that are found to be suitable for retreading have the remains of their old tread abraded away by a process called buffing. They are then inspected again to check that the casing is sufficiently robust for retreading. Some damages in tyres can be repaired prior to the addition of new tread. Other casings have to be rejected at this stage.

2.2.8 In retreading, a worn casing which is still of good structural quality in a given size is accepted into a process which completely renews the tread of the tyre and sometimes sidewall rubber is added. The rebuilt tyre is then subjected to a "Hot cure" or "Cold cure" retreading process.

2.2.9 In hot cure, the rebuilt tyre is then cured in a tyre moulding press. This press subjects the tyre to heat and pressure, forming a new tread into the uncured rubber, creating a chemical bond with the casing by vulcanising at about 160C.

2.2.10 In the "Cold cure" process, a pre-vulcanised tread, including tread pattern is made by external specialists. This tread is supplied to the retreading plant where it is applied to the casing and chemically bonded to the surface in a cold curing autoclave process at about 100C.

2.2.11 Both processes do produce excellent quality tyres.

2.2.12 Once suitability is confirmed, new tread can be added. The alternative hot and cold cure methods for doing this differ in the heat stresses applied to the tyre (100C for cold, versus 150-180C for hot). It is claimed by cold curers that the low temperatures of this process result in lower degradation of the structure during retreading. However expert opinion indicates that this would only be a marginal benefit even if it operates at all. Indeed the increased stresses of the hot cure process can be viewed as an extra proof test, revealing hidden flaws in the structure which cold cure is unable to do.
Selling the different remoulding practices against each other would probably have a negative image impact on the entire market. It is probably more the case that the low costs of cold cure processes suit them to local operations or to runs on small inventories of a particular tyre type where hot moulds are uneconomic.

2.2.13 Whatever process is chosen, tyres are checked at each stage, and stringent quality checks remove casings which are revealed to be unsuitable.

2.2.14 Before the tyres can be sold they are subject to further quality control checks to ensure that the factory finished product meets legal and customer requirements. Those companies interviewed for this study considered that about 5% of retreads are rejected at this stage to meet quality assurance criteria.3
Most of these rejects are due to the maturation of hidden damage to the tyre structure under the stresses of the retreading process. These process stresses are significantly higher than those of service, and therefore offer an effective proof test of the structure, a recognised method for ironing out hidden structural flaws.

2.3 Car Tyres: Selection and Manufacturing

2.3.1 Compared with commercial vehicle tyres, initial manufacture quality of new car tyre structure has a much greater significance on the operational life. Significant numbers of cheap imported tyres are designed with structures and production qualities that mean the tyre lasts only as long as the tread, and consequently they are not suitable for more than one tread lifetime.
These tyres meet international standards for tyre quality, but have lower safety factors in terms of overall structural lifetimes, and cannot safely sustain an extended operational life. This shift away from "over engineered" tyres has been driven by cost reduction pressures to meet market demands. In addition, new low cost automated processes for tyre manufacture result in structurally strong tyres which are not easily subjected to a retreading process.
It is reasonable that passenger car tyres can be less robust than commercial vehicle tyres because they operate under completely different conditions. However, purchasers are certainly not aware that certain brands and types have an effective casing lifetime of 1.5 times the average tread lifetime, rather than 3 times.

2.3.2 Overall, tyre collectors indicate that of all the tyres they collect, only ~17% are suitable for retreading. This means that currently of the 34 million new tyres reaching "end of life" in the UK, about 6 million could be retreaded. This compares with the current car retreading level, and indicates that around 5 million more car tyres could be diverted from landfill (presently) or final disposal for one extra lifetime. Rejection rates are also partly influenced by market demand for specific sizes. Although good quality casings may be available, there may be limited demand due to shifts in market trends

2.3.3 No centralised data of the reasons for car casing rejection are available, however, audits of car retreading companies revealed overall rejection rates, by the retreader, of 70-90%. These values, however, reflect operational demands. Prior to the collapse in the home and export markets for retread passenger tyres, acceptance rates of around 45% were common.

2.3.4 Car tyre retreading is exclusively hot cure. Tyres are specifically selected for their intended market which increasingly tends to be higher value applications, for example, 4 x 4, winter tyres and rally tyres. The tread pattern and compound are selected for the specific market to ensure that each tyre has the right properties for its intended application. The new tread is applied as a continuous thin strip which is applied automatically to each tyre to ensure an even distribution. The new tread is then vulcanised so that it bonds to the casing.

2.3.5 It is clear that while the levels of stress and operational lifetimes for a commercial vehicle tyre are different to those for a car tyre, the types of forces and conditions encountered in service are broadly similar The materials and design principles are also broadly similar. Therefore broadly speaking the ways in which their structures respond, and the environmental economics of retreading, should ideally be similar between the two. However, the rise of the low value-added single use tyre has damaged the business case for car retreading.
As a generalisation retailers have decided to stock and promote these cheaper tyres because of the larger margins available to them and the attitude that new tyres are easier to sell. This is because few consumers purchase on life cycle costs, and the up front cost of a more retreadable casing, irrespective of source, provides little incentive to purchase. This may be addressed by forthcoming rises in tyre disposal costs, leading to a consumer-visible differential between the end-of-life costs for retreadable and non-retreadable tyres. However, a simple market solution like this could be contested by new tyre manufacturers unless the use of disposal costs is sufficiently transparent.

2.3.6 A tyre disposal charge is now becoming an accepted feature on the bill for new tyres at several large car tyre fitting chains. While this charge varies from dealer to dealer and from area to area, it currently stands at about £1.00 per car tyre including vat, but is the same for all tyres irrespective of make and quality. This is a small figure, but can be expected to rise as landfill disposal is eliminated. The obvious way to exploit this to the benefit of retreads is to make this end of life market more efficient, as is done for commercial vehicle tyres.

2.3.7 Disposal charges are likely to become more of a controlling issue with car tyres for large scale purchasers of tyre services, due to the heavy "duty of care" legislation under which they operate. This will exert pressure not only financially, but also through Corporate Social Responsibility, ISO 14001, and Greener Government policies. In these areas, the market has potential to become significantly more efficient, as the power of used tyre purchasers will be easier to wield. Here casing dealers should be able to operate as for the commercial vehicle sector, where they can modify the price paid for the worn casings dependent upon their brand and quality composition. One major question is that of how much disposal charges will rise in future, and whether their magnitude will be sufficient to materially influence purchasing decisions.

2.4 Conclusions:

2.5 References:

1. ECE 108 prepared for the Retread Manufacturers' Association, 4 February 1999. Document specification E/ECE/324. E/ECE/TRANS/505, November 3, 1998. Uniform provisions concerning the approval for the production of retreaded pneumatic tyres for motor vehicles and their trailers.

2. ECE 109 prepared for the Retread Manufacturers' Association, 4 February 1999. Document specification E/ECE/324. E/ECE/TRANS/505, November 3, 1998. Uniform provisions concerning the approval for the production of retreaded pneumatic tyres for commercial vehicles and their trailers.

3. Environment Agency 1998 Tyres in the Environment. Environment Agency