KENTREDDER EXPLAINED by A. J. P. Wilding, A.m.i.mEcH.E.
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Piri HE Kentredder system used by Bere Regis has been developed by Kentredder Ltd., 32 and 33 The Parade, St. Helier, Jersey, and Retreading Equipment Ltd. is an associate and the U.K. franchise holder. Retreading Equipment was formed about two years ago to introduce the system to the U.K., one of its principal applications being for bigger fleet operators—with 100 vehicles or more -and apart from Bere Regis a plant has been installed in the workshops of a national bakery organization. The other application is for use by tyre distributors and so far 28 plants are in operation at tyre distributors' premises in the U.K., the retreads—called Kentred—being marketed locally in commercial-vehicle as well as ear sizes.
Although it is most unusual for an operator in this country to retread his own tyres, it is apparently not uncommon in America, where the latest development in retreading is the attachment of pre-cured treads to tyres by low-temperature vulcanization methods. This last method was, in fact, investigated by Kentredder but was considered to give an unreliable bond when compared with the normal-temperature vulcanization commonly employed in tyre retreading.
The Kentredder system produces what is generally called a re-cap tyre. That is, where the new rubber applied is a parallel strip and does not extend down the sidewall. The equipment needed to retread tyres is relatively simple and consists essentially of a buffer and an electrically heated ring mould.
The Kentredder buffer has been specially designed so that an operator with short experience can obtain a R*21
contour of the correct shape across the tread, and so that the surface textUre of the old tread rubber is automatically of the type to ensure a perfect bond between the old and the new rubber. As well as being used for buffing, the machine also incorporates equipment for spreading the tyre (and so reducing the outside diameter) so that it can be inserted in the mould. Instead of the conventional method of applying the new tread to the casing before insertion in the mould, with the Kentredder the tread is placed in the mould by hand, after being cut off to the correct length. The width of tread rubber varies according to the tyre size, of course, and after it is placed in the mould the spread tyre is inserted and the spreader mechanism released so that the buffed surface expands to locate on the new rubber. The location of the tyre is automatically taken care of and, whilst the tyre is still partly spread, a pressure bag and retaining shell are placed inside the tyre.
Tyre Located With the tyre finally located against the new rubber, the bag is positioned correctly around the inside of the tyre and the shell, which consists of hinged segments, is fully expanded. The air bag is then inflated and with the electric connections for the heating elements made, compressed air is connected to the bag and the cure begins. The length of cure depends on the particular grade of tread material used and includes a period in which the rubber flows before the curing begins. This simplifies the operation and ensures perfect bonding between the new rubber and old casing.
When curing is complete, the retaining shell and pressure bag are removed from the casing and the tyre is spread to remove it from the mould.
Although, as it will be seen, the Kentredder system is relatively simple, this simplicity does not in any way result in a poor job being done. In fact, the standard of retread is very high and the equipment, which is well designed, goes a long way to reducing the chance of failure. It will be noted that no solution is used between the applied rubber and the easing. This is no detriment, for the adhesive used in other forms of retreading often does little more than hold the new tread rubber on to the casing prior to curing. The main bond is obtained during the curing and it is said there is a chance that the solvents used in adhesives can result in an imperfect final bond in places if the process is not carried out exactly as laid down. Tests have shown that the bond strength between the new and old rubber in the Kentred product is actually stronger than the bond betmfeen the original tubber and the casing cords.
The major advantage to an operator using the Kentredder system is undoubtedly cost saving. Kentredder plant is supplied on a rental basis and for a fleet operator would probably consist of a buffer/spreader unit, two control panels, probably three moulds to suit different sizes of tyres, tyre spreaders, and bags and so on. Including an annual service charge, the annual outlay would be in the region of £650. Approximate production costs for a 9.00-20 retread, allowing for a daily production of three covers and the wages of one man (he would not be fully employed at this rate of production) and the cost of tread rubber, electricity and so on, would be a little under £6.
This shows a minimum saving of at least 50 per cent on the normal remould cost for a 9.00-20 tyre, even allowing for full trade discount. And if more tyres were produced with this equipment, the cost would be reduced, of course. Retreading Equipment takes care of staff training at no cost, and a man with no previous experience will be doing a first-class job within a couple of weeks.
Because the equipment is rented, no capital outlay is involved and another advantage is that little floor space is needed. For the plant described, 12 ft. by 15 ft. would be ample and many operators will be able to install the plant in a corner of their existing tyre shop. The all-electric operation means that the complex, steam-generating apparatus normally associated with retreading is not needed, and because the tread rubber is supplied in widths to suit different size moulds there is negligible wastage.