Methodical Work Towards MS 224

Retread tyres need to be as safe as new tyres. We met with Kit Loong to fnd out how this can be ensured by having methods in place that ensure retreads meet the standards of MS 224.

Some have voiced out that MS 224 by Sirim certifcation for retreaded tyres is not only time consuming, but also costly. Companies wanting to use this certifcation as proof of the quality they provide would have to submit tyres for evaluation as soon as the tread pattern changes. And not only that, the re-assessment takes place twice a year. While some may see this as a burden, Kit Loong’s Kenneth Teh, managing Director, Kit Loong Commercial Tyre Group, hails this as a great tool to identify quality providers of retreaded tyres.

Why Retreading?

Teh explains that all tyres should be developed with a 3-R concept in mind, whereby 3-R stands for “Reduce. Reuse and Recycle”. He said that “One misconception is that retreading means recycling. However, that is not the case. Retreading is about the reduction of use of raw materials or reuse as you can retread them. Only when you really scrap the tyre, you recycle.” Reusing tyres by retreading makes a lot of economic sense as 80 percent of the initial cost of the tyre are held in the casing. “It is the casing that holds the air, thus supporting the weight of the vehicle and the load. Transporters really need to care about the maintenance of the casing.” There are limitations as to how much rubber one can add to a tyre as eventually, the treads will fex when in use, putting more stress to the material. According to him, there are two issues that impact the lifespan of a tyre the most: Road conditions and the weight of the load in relation to the pressure. In many cases, the trucks are actually not overloaded, but due to the tyre pressure being too low, the tyres wear out premature or are damaged premature. The truck may not be overloaded, but the tyre might run under overweight conditions.

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MS 224 vs ISO

While the ISO standards are widely accepted, they are a means of documenting processes, which are different in every company. “When you look at the MS 224 though, it is a very good tool to assess tyres as this certifcation requires everyone to comply to the same set of criteria.” As it all boils down to the casing if one wants to retread a tyre, it is not a question of “if” a tyre is inspected upon receipt for retreading, but “how” it is evaluated. There are hundreds of tyre brands and the price range is quite wide. Branding may have an infuence on the price, but mainly it is the methodology used to assess a tyre for retreading.

Loong is grouping the tyres in 10 categories. There are six for tyres approved under Sirim and another four, which are off-road applications not needed MS224 approval. The top six categories are the ones that are approved by Sirim under MS 224. When submitting tyres, Sirim will simply state pass or fail. Tyres approved to bear the MS 224 mark will be tested randomly and have to be re-assessed every six months. What Sirim is achieving with this is to ensure that tyres adhere to a minimum quality standard that is guaranteed. “With the MS 224 approval, buyers just need to look at your certifcate for any particular tyre and they know that the product is of a certain quality, therefore buyers should always ask for their supplier’s valid MS224 certifcation to prove their tyre MS224 is valid. This photocopied certifcate can be used during Puspakom inspection,” Teh said. A change of pattern would require a re-test.

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Assessment of Tyres

To achieve the desired result, tyres need to be assessed frst before they can be retreaded for a second or third life. This second or third life may not be the same application as the tyre was initially used for. As an example, Teh cites a scenario whereby a tyre was used in a long-haul operation, but the casing is no longer suitable for such purposes. The retreaded tyre may end up in an off-road application where speed and performance aren’t as crucial. To assess the casings prior to retread process, Kit Loong categorizes in 5 brands grouping, then in term of tyre casing condition, we split to 11 standards. It is here where we decide between good for retreading and rejected tyres.

Shearography

Following the selection of the casings suitable for retreading, the casings then undergo the process of Shearography. Using Shearography, bubbles hidden in the rubber are being detected. “Bubbles can lead to separation of the liner from the casing and if there are too many or too big bubbles in the tyre, we need to reject them at this stage” explains Teh. He said further “It is here where the grouping according to the technology matters. As an exaggeration, a bubble the size of a fve cent piece in a low end tyre may disqualify the tyre whereas a 50 cent-size bubble in a top tier brand tyre may still be acceptable. It all comes down to the technology used to build the tyre for the frst life.”

Buffing

Tyre buffing is a process of shaving a portion of tread off a tyre in order to gain increased traction in dry tarmac conditions. This works largely by removing tread ‘blocks’, which are the areas of rubber between tread grooves. It is done in accordance to international standards, specifying the texture and roughness of the rubber left on the casing. This is to ensure that the cushion gum and the treadliner will adhere to each other. The contour of the buff plays an important role and is also regulated. Contours may need to be adjusted according to the wear pattern. Kit Loong controls teh tim eafter the tyre being buffed, the reason being is to control the possibility of the exposed casing being contaminated by air moisture, thus causing the casing getting rusted.

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Undertread

This is a rubber layer provided for use in retreading the tyre and which provides adhesion of tread to the tyre carcass. If the undertread is too think, too much heat will be generated when running. The heat generated may be even more than in the original tyre. As heat is the enemy of all tyres, a tyre running too hot will not last as long. “Too thick is more dangerous than too thin. A thinner undertread results in a cooler tyre, but it will be more prone to penetration and damage to the casing.” Teh explains that most burst tyres are due to too much heat.

Skiving

This is the time where injuries to the tyre are being fixed. Most of the injuries cannot be seen. Only after buffing, one can see the damages done to the casing. Nail penetration may lead not only to wholes but also corrosion. These parts need to be patched up and it turns out that a good retread is one with a lot of repairs. Again, an initial standard regulates how much repair can be done to a tyre. Sometimes, the visual inspection does not reveal all the damages and it is in this stage only that the full extent of damages will come to light. As all these processes involve labour, it is crucial to identify as many failed cases early on. Clients will also be upset if a casing is initially accepted and then rejected after the buffing process.

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Cushion Gum and Liner

At this point, time is of the essence. Exposure to air, dust and moisture will cause corrosion. Kit Loong has developed software to track the timing the tyre is in production to ensure that the exposure to air is below the allowed time limit. Traditionally, pre-produced cushion gum is applied to the casing. “Although the technology is more expensive, we believe we achieve a better quality by using freshly extruded rubber. As it is still warm, it has better adhesion qualities” states Teh. In order to avoid corrosion, pre-cured liners are being applied immediately after the cushion gum has been ft. Kit Loong practices a “Zero-Work In Progress Policy”. While technologically possible, tyres that have been buffed can be stored overnight, Teh believes it is best to complete building any tyre on the day itself once it has been buffed. There is no tyre left to be completed the next day.

Curing

In simple terms, the tyre will be placed in an envelope whereby the air is being extracted, thus putting pressure on the liner to gel with the casing. The curing will take place in an oven-like tube. Hot air flow and the temperature at different times, as well as the thickness of the tread have to be taken into account. Various sizes of tyres may create turbulences inside the tube, affecting the curing process.

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Quality Control

Each of these steps is followed by a quality inspection. Finally, a high-pressure test is administered to see if the completed tyre is fully functional. Sometimes, this final test is where some tyres fail, even after all the effort put into them. Premium tyres may undergo another round of Shearography. In rare cases bubbles may have increased or other issues arisen due to the heating process. In summaryComing back to the premise of the MS 224 being a seal of approval and sign of a certain quality standard, Teh stresses that the process and methodology of producing retreads is crucial. “Retreading is a high-tech service. You need to have stringent processes in place and ensure that they are adhered to. What makes the difference is how the tire is being managed by the clients.