Dr.S.N.Chakravarty shared this informative article on Manufacture of Cold Retreading Material.

Here is Part 1 of this two-part series.

Introduction

Commercial tire retreading provides an economical means of extending the asset utilization of worn-out tires. The worn-out tire, referred to as the casing, is a valuable resource that is often not utilized to the fullest potential. By replacing the worn tread with a new one, the retreaded tire provides performance similar to that of the new tire at a fraction of the cost.

Retreading often offers a less expensive alternative to the purchase of a new tire. For the truck tire customer, however, retreading is an integral part of a tire maintenance and purchasing programme.  A truck tire casing must be able to be retreaded two to three times.

Considering that only about one-fifth of a tire is worn out in service, it makes economic sense to retread the tires for multiple use. Although both cross ply and radial tires can be retreaded. 4 to 8 ply bias tires can be the most easily retreaded. However, the structural performance may fall short of the new tires and service conditions may have to be more closely controlled.

Tire Retreading

Tire Retreading can be done by the two processes (a) conventional ‘hot’ capping and (b) pre-cured tread rubber process.

The two primary methods currently used to retread tires, include – mould cure and pre-cure processes. The preparation of the casing is essentially the same in both cases.

In the mould cure process, the tread rubber is applied in the uncured state to the tire casing, and the product is cured in much the same manner as new tires.

In the pre-cured technology, the tread is supplied to the retreading factory cured, with the tread pattern already in place. The treads are supplied in rolls of various lengths (typically 3.5 to 10 meters in length), or as rings that can be stretched onto the casing. In the application of both pre-cured methods, a bonding material, the cushion, is applied to the tread or the casing prior to the application of the tread.

The majority of the tires retread today consists of truck tires, produced using the pre-cured technologies. A description of this method is given below.

1. Conventional Process (also known as ‘mould cure’ or ‘hot cure’ process) – In this process an un-vulcanized rubber strip is applied on the buffed casing of the tire. This strip takes the pattern of the mould during the process of vulcanization.
2. Precure Process (also known as ‘cold cure’) – In this process a tread strip, where the pattern is already pressed and precured is applied to the casing. It is bonded to the casing by means of a thin layer of specially compounded uncured rubber (known as cushion or bonding gum) which is vulcanized by the application of heat, pressure and time.

In the pre-cured rubber (cold process) retreading, better mileage is obtained than with the hot capping process.

The tire is inspected for cuts, ply separations, etc. and after inspection, it is repaired with repair compound or patches, and then buffed under inflated conditions to facilitate bonding. Thereafter, the tire is again inflated on an expandable hub and coated with vulcanizing cement on its buffed surface. A layer of bonding / cushion gum is applied around the surface of the tire. The trapped air between the layers is removed and the vulcanized tread strip is applied, its ends spliced and stitched.

After this operation, the expandable hubs are collapsed and the tire is deflated and removed for vulcanization. The tire is fitted on suitable rims and inflated with the use of a tube. The inflated tire is then slipped into an envelop and vulcanized in a curing chamber or autoclave at lower temperatures than is normally used for new tire vulcanization.

Tire Preparation

The buffing process provides a contaminant-free surface of uniform texture to allow adhesive of the new tread. During the buffing process, the casing is also brought to a uniform circumference, with the correct thyroidal radius and width, to accept the proper tread size for the casing.

After buffing and skiving the casing, a thin layer of contact adhesive is applied to the casing. This material usually referred to as cement, aids in the retread tire fabrication process and helps provide increased adhesion between the casing and the cushion. The cementing of the casings is optional, but widely used in retreading. Cements are solvent-or water based materials.

It is at this point that the majority of the repair to the casing is performed. The main purpose of repairing the casing is to restore the ability of the casing to maintain air pressure and to return mechanical properties of the casing to a level high enough to endure at least the next use life.

After the completion of the repairing of the casing, the tread can be applied. The application of the tread is referred to, as building the tire, and there are a number of variations to the building process.

The main components used in the tire building are the tread and cushion. The tread is supplied to the retread factory fully cured with the desired design and tread width. The back side of the tread is prepared at the manufacturer end by buffing with a wire brush drum and the application of a contact adhesive. A polyethylene film is placed on the cemented side of the tread to prevent contamination.

The cushion is supplied to the factory in either calendared sheets or as strip stock to be used in an extruder. The calendared cushion is supplied in different widths and thickness. As the first step of the building processes, the casing is placed on a builder machine equipped with an expandable hub, like that on the buffer. The hub is expanded and the leading edge of the tread is cut to provide a uniform uncontaminated surface.

Enveloping & Curing

The next step in the process is the placing on the “built” tire in a rubber containment device called an envelope.

The enveloped tires are placed in a curing chamber that is essentially a large autoclave, steam or electrically heated. The enveloped tires are suspended from a rail system inside the chamber and are connected to exhaust lines inside the chamber (via the valves in the envelope). The tire is then moved down the rail to the rearmost portion of the chamber. Once the desired number of tires is in the camber, the chamber door is closed and the heating and pressurization is started. During the pressurization, the air is allowed to evacuate from inside the envelope.

The operating pressure of the chamber is typically at least 0.3 MPa, with curing temperature between 100°C and 141°C. The cure time is dependent on the thickness of the treads and the composition of the cushion.

Earlier it was stated that cold retreading of tire using pre–cured tread gives better performance – higher mileage. Why ?

Because abrasion (wear) loss of the tread is much lower giving rise to higher mileage. This is because cold tread material is more compact due to much higher pressure (hydraulic) applied during curing of the tread in a hydraulic press compared to a new tire curing in tire mould where pressure is limited because of the steam / pressure relationship.

Cold Cure Process: Pre-requisites

It has been well established that heat is the most damaging cause of tire deterioration / ageing, and therefore, its eventual failure.

The critical temperature of rubber is 115ºC, beyond which ageing and deterioration of the tire casing is accelerated, resulting in premature reduction in body strength and leading to failure.  So, technically any retreading system with curing temperature lower than the critical temperature can only be genuinely called a ‘cold process’.

However, another view is that it is the pre-cured tread which is the difference, not lower temperature per se.

In the cold cure process,  factory-cured treads are dense, tough and are of uniform consistency and resilience, as they receive heat and pressure uniformly while moulding, unlike in the conventional retreading process.  The toughened, cured tread is bonded to the tires at considerably lower temperatures, compared to cure mould retreading.

Comparative Features: Conventional versus Pre-cured Retreading

Feature	Conventional	Pre-cured /cold
Mileage	Lower	Higher
Investment	Comparatively lower.  However, for comparable levels (as that of cold cure process), higher investment is required	Higher
Shelf Life	Limited shelf life for uncured tread strips	Long shelf life
Range	Besides truck and bus, larger tires like OTR tires can also be retreaded	Generally only truck and bus, LCV and  Passenger Car tires
Curing Temperature	Higher temperature 140ºC – 160ºC	Comparatively lower 100ºC-125ºC
Tread Composition- Process-Patterns	Natural rubber (NR) extruded unvulcanized strips used usage of different tread pattern is restricted	– Generally synthetic rubber (SR) or a blend or NR/SR with high quality carbon black -extruded and vulcanized (moulded with various designs) strips are used – flexibility in having different tread patterns
Range	Not suitable for radial tires	Ideal for retreading radial tires
Distortion	Tire undergoes distortion while curing in the mould due to variations in tire dimensions	No distortion in tires as no moulds are used
Cost	Lower cost	Marginally higher cost

All the operations such as buffing the tire, building the tread and curing while retreading are carried out in the inflated ‘road running condition’, without causing any distortion to the original casing unlike the case of mould retreading.

However, this is not in the case of smaller pre-cured retreaders not having proper equipment.

In Part 2 of this article, you will read on Pre-cured Tread Manufacturing Process.

Dr. Chakravarty can be reached on kpspltd@gmail.com

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