Retreading Standard Operating Procedure

Tyre Retreading 
What is Retreading ?
RETREADING IS RECYCLING 
Standard Operating Procedure (SOP)
Retreading is the process where selected and inspected worn tyres, called casings, receive a new tread. Only sound and carefully inspected tyre bodies are used in retreading.

The worn tread is buffed away and a new tread is bonded to the tyre body in a process very similar to the manufacture of new tyres. There are several different process techniques, but the ultimate objective is the same, affixing a new tread through the application of heat, time and pressure.

Tyre retreading is an established industry and a growing one since the early 1990s. Today, there are approximately 1400 retread plants throughout the North America. These plants are of various sizes, from small operations producing 20 retreaded tyres per day to the large plants processing 2600 or more retreads per day.

Today’s retreads are produced in very modern plants operated by trained specialists. Professional retreaders adhere to the stringent industry standards at every step in the retread process and each retread product can be traced back to the facility that produced it. Only the best worn tyres are used for retread.

Why Retread?

• It would cost less as compared to producing new tyres (up to 30%-50% lower).
• The aircraft industry (military and commercial) saves 80 million a year.
• It’s completely safe. All commercial airlines, as well as military jet aircrafts, uses retread tyres. Approximately 80% of all aircraft tyres now in service in the US are retreads. In 1993, on the Boeing 727 aircraft alone, 28,000 retreads were used, with an average of 200 take-offs and landings per tread life.
• Retread tyres are proven to be as safe and durable as compared to new tyres. Professional retreaders adhere to stringent industry standards at every step of the retreading process.
• Many truck fleets plan their new tyre purchases with the intention of having worn casings retreaded two or more times as a routine part of their tyres’ budget.
• It conserves oil. The synthetic rubber components in a new passenger tyre contain 7 to 8 gallons of oil. Retreading the same tyre uses only 2 to 3 gallons of oil! The manufacture of new medium truck tyres require 22 gallons of oil, but only 7 gallons are required to retread.

 Retreading Standard Operating Procedure (SOP)

Grazing Light Inspection 

The first step is to verify the condition of the incoming casing. A sound, stable sidewall is essential and MRT uses a unique grazing light technique for inspecting sidewalls from shoulder to bead. This helps technicians detect subtle irregularities, which may indicate potential problem areas.

If any concerns are raised about the shoulder, bead, or sidewall, MRT verifies the condition of the steel cords utilizing x-ray. All MRT Retreading Plants have specific and measured lighting standards for inspection and trained technicians to accurately access each casing. Typical retreading shops do not require or do not enforce lighting standards

Electronic Liner Inspection

Inner liner penetrations too small to spot visually can cause big problems. Leak-free casings means you can maintain optimal inflation pressure and avoid casing damage from air infiltration. This means longer casing life and lowered downtime. Powerful electronic currents pinpoint an otherwise invisible injury and the technician marks the spot for repair. 

Advanced X-Ray System

Suspected steel damage, identified during Grazing Light Inspection, is examined by MRT's X Sight™ X-Ray. This advanced x-ray system reveals the status of the steel belts and cables hidden within the casing. It positively identifies suspected conditions such as zippers, road hazard damage, run-flat abuse, and verifies the condition of prior repairs.

Although not all casings are required to undergo x-ray inspection, this is required equipment in all MRT plants. Typical plants do not require any x-ray inspection. 

Buffing

Precise buffing is critical to retread performance. Proper buff texture strengthens the tread to casing bond. A correct undertread contour promotes long, even treadwear. Also, an ideal undertread thickness is key to cool operation. MRT uses an automated computer-controlled radial buffing system guided by a casing specification database, continuous undertread measurement, and handheld electronic Undertread Proximity Sensor to ensure optimal undertread depth. 

All MRT plants also utilize a buffer mounted undertread measurement device to make sure proper undertread is achieved. Typical industry practices use semi-computer controlled or physical templates, a circumferential buff, and pilot skives to measure undertread thickness. 

Casing Integrity Analyzer® (CIA)

The CIA, Casing Integrity Analyzer®, uses laser light to map the inner contour of the casing. Then the procedure is repeated under vacuum. If there are separations in the casing, the contour will change. 

A computer analysis of the difference between the first and the second images by ACE, MRT's proprietary Automatic Casing Evaluation software, determines if the casing contains hidden separations. A thorough image library of conditions is available for operators to reference. CIA is required in all MRT plants and every casing is inspected with this procedure. 

CIA is also available for after cure inspections. In typical plants not all casings are inspected. For those that are, casing separation detection requires the operator to continuously observe and interpret moving screen images. 

Repairs

Sound repairs begin with the proper preparation of the repair area and correct placement of the repair unit. All tools are inspected and replaced as needed. Repair size measuring templates assist the technician in the correct repair selection and help prevent undersizing of repair patches. 

Heat cure repairs have replaced chemical cure repairs, resulting in increased adhesion and improved tear resistance. Additionally, heat cure repairs complement the use of the double envelop in the curing process. 

Pre-Mold™ Cushion Extrusion and Tread Building

The MRT cushion application system extrudes hot cushion gum onto the casing to promote a strong tread-to-casing bond. Hot cushion gum automatically fills skives and buzzouts up to 25 mm wide and 5 mm deep while applying an ideal bonding layer. MRT using a computer-controlled Tread Builder to ensure the correct positioning of the new tread. 

The computer reduces tread-building variables with the tread automatically cut to the correct length to optimize compression of the tread at the joint. 

Pre-Mold™ Builder Balance Improvement 

This patent pending innovation works to compensate for casing construction and expandable rim variability. Prior to tread building, the Builder Balance Improvement device locates the sector of the tire with the least amount of undertread. 

The tread and cushion joints are then positioned in this sector. The tread and cushion joints are heavier, and when positioned at the low point, radial runout is improved and less balance weight is needed when the tire is mounted on the vehicle.

This indexing of the tread and cushion joints helps to ensure a better, smoother ride with less irregular wear. The Builder Balance Improvement device is proprietary to the MRT process. 

Enveloping

MRT Pre-Mold retreads are double vacuum enveloped. Double enveloping means that all surfaces of the casing and tread - inside and out, bead-to-bead - are subject to uniform mechanical pressure which promotes full integration of repairs. Enhanced temperature distribution reduces “cool” spots and greatly reduces over-curing. 

MRT Pre-Mold retreads are cured to Michelin protocols for time, temperature, and pressure. Typical pre-cure plants use only a single outer envelope and sealing rings around the beads, with large temperature variations inside the curing chamber. 

Pre-Mold Curing

Enhanced temperature distribution reduces “cool” spots and greatly reduces over-curing. MRT Pre-Mold retreads are cured to Michelin protocols for time, temperature, and pressure. 

Typical pre-cure plants use only a single outer envelope and sealing rings around the beads, with large temperature variations inside the curing chamber.

Final Inspection

Finished retreads do not leave the retread plant without undergoing a final hand inspection. The crown, sidewalls, beads, and interior are examined by a skilled technician to ensure your new retread adheres to Michelin's stringent quality standards. If needed, CIA and x-ray are available to verify repairs and audit the final retread. 

MRT requires that all tires undergo this detailed final inspection process. Typically, competitive retread plants will not have x-ray available.
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