Retreading has long served as a sustainable way to get more life out of truck and bus tyres, and this process has certainly yielded benefits for Michelin. The company has recently marked its 100-year anniversary of retreading with a claim that this process has given a new lease of life to around 30 million worn tyres at its plants in Stoke-on-Trent in the UK and Hamburg, Germany.
At the Stoke-on-Trent factory, retreading operations commenced in 1968. The site produces Remix and Encore tyres (see box, p12) from 17.5in to 22.5in and has benefited from more recent material handling innovations such as robots, while also relying on automated conveyors to transport tyre casings between different posts in the factory. On its own, the site has apparently produced 10 million tyres and saved 50,000 tonnes of raw materials. Around 60% of retreads produced in Stoke-on-Trent are destined for fleets operating in the UK and Republic of Ireland, with the remainder exported to mainland Europe.
It is at this location that Michelin celebrates its century-long journey by offering an insight into how the retreading is carried out.
To provide some context, Michelin manufactures a variety of truck and bus tyres for long distance to regional operation, urban to on/off road. From new, this tyre may cover thousands of kilometres during its first life of service. When the tread depth has worn to around 3-4mm, the tyre can be removed from the wheel and regrooved by a technician for a second life in service, following an approved regroove pattern. This pattern of regrooving a tyre is expected to increase the potential life of the tyre by around 25% in its most fuel-efficient state, meaning it is around 5% more fuel efficient than a new tyre.
A regrooved tyre is considered worn out when its tread reaches the legal wear limit of 1mm, which is when the dealer would send it to Michelin for retreading.
Once it has been retreaded as a Michelin Remix tyre, it is ready for its third life in service. (Michelin and industry recommendations call for new tyre fitment on steer axles; most operators also fit new tyres to the rearmost trailer axle because of the extent of wear from tyre scrub.) When the tread depth has worn to around 3-4mm again, the Michelin Remix tyre can also be regrooved.
According to Michelin, only 20kg of raw materials need to be added to the casing to manufacture a Michelin Remix tyre. This means that a retread truck or bus tyre consumes up to 70% less raw material than a new tyre.
Once trained, factory staff will inspect and verify every tyre before the retread process begins. All casings pass through a visual, tactile and X-ray inspection process upon arrival at the factory.
The tyre company claims that up to 90% of truck and bus tyres delivered for retreading meet the criteria of the initial inspection. Whether or not the tyre has been regrooved has no impact on its acceptance for retreading.
After this verification, the casings receive new treads and sidewalls before being cured for around 90 minutes in a mould at temperatures of up to 200°C, which gives them their final tread pattern. The amount of rubber is identical to those used to make new tyres, ensuring a constant thickness of rubber between the bottom of the tread pattern and the protective layers.
All tyres are then subject to a final quality inspection, which involves inflation to 140psi to confirm the product’s integrity. Tyres that have had any casing repairs carried out also go through a secondary test, which sees 50kV passed through the tyre to detect any penetrations to the casing.
Tyres that fail to make the cut are recycled into rubber granulate and steel. The rubber granulate is primarily used in sports surfaces and road construction, as well as in door seals on vehicles. The steel within the casing is also recycled and further processed into steel.
As part of a collaboration with Swedish start-up Enviro, research is underway to recover various raw materials such as soot, oil or steel from old tyres and used to manufacture rubber-based products. Enviro uses a patented process to recover up to 90% of the raw materials.
process improvements
Michelin’s new figures underscore the benefits of retreading, but there was a time when some people may not have been convinced. Around ten years ago, any driver travelling down a motorway and coming into visual range of tyre strips littering the hard shoulder would be forgiven for assuming retreads are nothing more than a cheaper alternative to buying a new tyre. But not today.
Factory manager Vincent Gridel explains how Michelin’s retreading process continues to improve. He says: “I think that the number of quality checks that we make to ensure the final product is perfect is the best way to manage this type of issue.” He adds that Michelin’s retreading process “provides the trust and removes the last obstacle to buying a retread tyre”.
At the factory are 180 workers working 24 hours a day spread across three shifts, five days a week. The staff receive training on the job, which can take six months to acquire the necessary skills. The trainees are required to learn all the potential defects that could be found on the casing to determine whether the casing is acceptable or not.
“Learning is just the beginning because workers do not have their book to hand when checking a tyre to determine whether the defect is acceptable, so they must develop this awareness as a reflex. Depending on the operation, workers will have to check a certain number of tyres correctly in a row in order to become qualified.”
But what makes a tyre irredeemable? “A lot of defects in the tread can be repaired, but the sidewall is more challenging because it is a more sensitive part of the casing because it is not supposed to be in contact with the road,” he adds.
– Ben Spencer
BOX: OPTIMISING TYRE LIFE
B2B sales director Andrew French says that good tyre pressure management is key. “We don't want under- or over-inflation. Also, the right tyre for the right application is crucial, and we will give technical advice on that. Finally, make sure it’s serviced by a reputable service provider. Those three elements will ensure maximising the use of the casing when it comes back to the factory.”
BOX: ENCORE
Another retreading operation that takes place at the Stoke-On-Trent factory is Encore. Similar to Remix, this process allows the potential to extract additional lives from a worn Michelin Remix casing. However, it can also reuse tyre casings originating from a selection of other manufacturers. Encore allows an operator to switch over to a Michelin policy from a competitor tyre brand. This is achieved by taking old tyres which pass Michelin’s quality standards and processing them as Encore, while also introducing Michelin new and Remix tyres.
COLUMN: A VISIT TO BANDVULC
Continental’s commitment to retreads was demonstrated some seven years ago when it acquired long-established British retreader Bandvulc and its Ivybridge, Devon, plant. Since then it has invested £1.5m a year in the West of England operation, creating one of the most automated factories in the sector.
Opting to use retread tyres is one way in which truck and bus fleets can demonstrate their environmental credentials, and save cash at the same time. Retreads typically cost 35% less than new tyres, says Continental, adding that 50% of UK supermarkets run their trucks on them. They also reduce consumption of valuable raw materials.
One of the big savings is in the re-use of the steel employed to construct the tyre’s casing, says Tony Mailling, Continental’s head of hot retread production for Europe, the Middle East and Africa, and plant manager for ContiLifeCycle Ivybridge and Stocken, the manufacturer’s retreading plant in Germany.
“Steel manufacturing is an energy-intensive process, and contributes significantly to a tyre’s environmental impact,” he points out.
Continental clearly views retreading as a key weapon in its onward march towards greater sustainability. Group-wide, the tyre and automotive components behemoth is targeting 100% carbon neutrality by 2050. By that stage its tyres should be made from 100% sustainable materials, it says.
In the meantime, Ivybridge has become a retreading technical innovation hub, with ambitions to increase an already-high level of automation in what has been traditionally largely a manual activity.
Now operating under the ContiLifeCycle banner, but still developing and producing Bandvulc products alongside the ContiRe retread range, the site retreads 3,500 to 4,000 tyres a week. It swallows half a million worn tyres annually which undergo intensive inspection before retreading begins.
Around 30% are rejected promptly because they are too badly damaged, while another 10% are set aside because defects are detected during the retreading process. Some 1% to 1.5% may fail final quality checks.
“As it’s impossible to see any issues lurking under the surface, shearography is used,” says Mailling. “This works like an ultrasound scan, analysing the tyre’s core structure. If a problem is found then the tyre will be rejected and recycled.”
Around 34kg of the rubber dust removed from scrapped tyres is reused in other industries.
All the retreads built at Ivybridge are hot-cure. Once the old tread has been removed from a casing, new tread rubber is applied in a bead-to-bead process which includes the renewal of the tyre’s sidewalls in one of the factory’s 54 presses.
ContiRe products are based exclusively on Continental casings and the company is eager to emphasise that choosing retreads need not mean compromising on day-to-day performance. Its ContiRe EcoPlus HT3+ 385/65 R22.5 trailer tyre can deliver a 20% reduction in rolling resistance when compared with rival products, it asserts. That spells enhanced fuel economy and a smaller CO2 footprint.
Bandvulcs use casings from other premium manufacturers in a line-up which includes the Wastemaster, designed for trucks used in the waste management industry. Again, there is no fall-off in performance, Continental states, with stones that could cause tread damage ejected by specialised tread grooves, and visual indicators to help alert drivers to any damage caused by kerbing when they carry out their daily walk-around checks.
– Steve Banner