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Editor’s Pick: Manufacture of Cold Retreading Material (Part 2)

This is a continuation to Part 1 of this informative article on Manufacture of Cold Retreading Material shared by Dr.S.N.Chakravarty.

Cold Retreading Material - Part 2

Advantages of ‘Pre Cured’ Process:

  1. Precured rubber generally gives more mileage than the conventional rubber due to richer compound, denser tread and flatter profile;
  2. For radial tires, pre-cure retreading is the most ideal process. Radial tire, in its construction, has an inextensible belt. In the conventional retreading process the matrix (rigid mould) should have exact dimensions matching the dimensions of a built radial tire.  Even a small difference in the matrix dimension affects the ultimate performance of the retreaded tire.  But in the case of pre-cure retreading, radial tires are cured in a chamber and not confined to a rigid matrix or mould, hence there is no distortion.
  3. Longer casing life because tire is retreaded in inflated normal road running condition. Thus the casing is not put under tension and over stressed as it happens inside a rigid mould.  (Due to distortion of casing, the number of times a tire can be retreaded with conventional retreading is always lesser than with pre-cure retreading).
  4. Better balancing, due to uniform thickness of tread and better buffing and building techniques.
  5. Better traction due to flatter profile of buffing.

Advantages of Conventional retreading process

  1. More economical (compared to cold cure retreading).
  2. Comparative advantage ( pre cure retreading due to difficulties in setting up franchisees everywhere in case of latter – high investment).
  3. Better aesthetics (compared to pre cure retreading since better finish given to the tire sidewall also).

Tire Retreading Processes 1

Tire Retreading: Salient Features

  1. Since labour is one of the main components of tire retreading in the organized sector, it is more profitable to undertake high value addition business in the organized sector (i.e. retreading of truck and bus, light truck, jeep etc.). Hence, pre cure retreading of other categories of tyres (especially farm, two wheelers etc.) is not as popular.
  2. Retreading, as per international experience, finds greater and ready acceptance in the commercial segment since the main objective is ‘savings’ whereas in the passenger segment the focus is ‘safety’ followed by ‘aesthetics’. Moreover, savings are also very low in passenger segment.
  3. In India, trucks carrying loads above 16 ton and plying on long distance routes do not find operational economy in using retreaded tires. However, trucks and buses plying short distance routes (and loading pattern upto 12 ton) find retreading a more economical option.
  4. The current trend is going in favour of pre-cured primarily due to the following reasons:
  • Better road conditions (with resultant less damage to tire casing)
  • Better driving habits (getting more mileage even from retreaded tires)
  • Overloading is comparatively less
  • Presence of large number of ‘job shops’ for retreading with each specializing in a special part of the process – repairing, buffing, curing etc. being handled by different persons/processors who, over the years, have gained sufficient experience and expertise in conventional retreading.

 

Pre-cured Tread Manufacturing Process

Compound Mixing

Typical Compound Formulation of Pre-cured Tread, Solution and Cushion Gum

 Ingredients Pre-Cured Tread Compound

(phr)

Solution Compound

(phr)

Cushion Gum Compound

(phr)

Natural Rubber ( RMA 4 ) 70 100 100
Polybutadine Rubber (High cis type) 30 0 0
Rubber crumb ( 40 mesh ) 5 0 0
 WT Reclaim Rubber 5 0 10
Peptizer 0.15 0.2 0.3
Zinc Oxide (White Seal) 4 4 5
Stearic Acid 3 1.5 1.5
Antioxidant TDQ 1 1 1.5
Antioxidant 4020 1 0 0
MC Wax 0.8 0 0
Carbon Black N 339 / 220 65 0 0
Carbon Black N 550 0 35 30
Rubber Process Oil 710 12 12 0
Pine Tar 0 0 12
Wood Rosin 0 6 5
PF Resin 0 2 4
MF Resin 0 2.8 0
Insoluble Sulfur 0 0 3
Sulfur 2.3 3 0
Accelerator  NOBS 0.8 0 1
Accelerator TMTD 0 0 0.25
Retarder PVI 0.1 0 0.15
200.15 167.50 173.70

Rubber compound is prepared by mixing rubber with different ingredients like fillers, process oil, activators, accelerators, curing agents, antioxidants etc. In order to achieve desired level of properties of the product. It is necessary to reinforce rubber with different fillers and vulcanize with sulfur with the help of accelerator etc.

Mixing and mastication are carried out in an Internal Mixer or Kneader or Two Roll mixing mill.

Rubber compounding is one of the most difficult and complex subjects to master in the field of Rubber Technology. There is no simple mathematical formulation to help the compounder. That is why compounding is so difficult a task. (More on this subject)

Kobelco Make Mixers

L&T Marketed Kobelco Make Internal Mixers

Principles of Mixing 

Vulcanizable polymers cannot be used without compounding. Various additives like curative system, protective system, reinforcing agents, cheapeners and other process aids have to be mixed to the polymer or polymer blend ‘to make a coherent homogeneous mass of all these ingredients, which will process satisfactory and on Vulcanization will give the product capable of giving the desired performance, all with the minimum expenditure of machine time and energy.’ (More on this topic)

Extruder and Extrusion Process

Extruders are machines, which shape rubber to a profiled strip by forcing it through a die. (More on this topic)

The rubber compound is passed through a hot feed or cold feed extruder to produce rubber blanks of suitable size (width and thickness) for use in the next manufacturing step i.e. curing in a hydraulic press.

BHARAJ MAKE 200MM PIN BARREL EXTRUDER WITH TREAD DIE HEAD

BHARAJ MAKE 200MM PIN BARREL EXTRUDER WITH TREAD DIE HEAD

The extruded and cut rubber blanks are placed in tread dies and cured in a steam-heated hydraulic press at suitable curing temperature and pressure. After the curing cycle is completed cured treads with desired tread pattern are taken out and cooled.

Cushion Gum

A three roll calendering machine is used to prepare “cushion gum” i.e. uncured rubber sheet that acts as an adhesive layer between the pre-cured tread and tyre casing during the pre-cured retreading process.

Cushion Gum Roll

Cushion Gum Roll

The term “to calendar” is defined as “to press between rollers or plates in order to make smooth & glossy sheet”.

Calender can be of two bowl or three bowl or multiple bowl machines which is used for Calender rolls are not perfectly cylindrical but have different shape.

Solution

A solution churner vessel is used for the preparation of cement solution (contact adhesive) applied to the casing and helps provide increased adhesion between the casing and the cushion.

****

Here is a flow chart for you summarizing the production process.

Pre-cured Tread - Production Flow Chart

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


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Editor’s Pick: Manufacture of Cold Retreading Material (Part 1)

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.

Cold Retreading Material - Part 1

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.

Tire Retreading Processes

Pre-curing Method (Right) and Moulding Method (Left). Image from Web

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.

Tire_Buffing_Machine

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.

Precured Tread Application

Application of Pre-cured tread on the prepared casing

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.

Autoclave 1

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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An Introduction to Tire Buffing Machine

Tire Buffing Machine is a widely used tire retreading equipment. The other known names include Tire Buffering Machine, Tire Buffer, Tire Buffing Apparatus, or Raspers.

When you get a tire that passes all the initial checks and is certified as a worthy candidate for retreading, your first job is to buff this tire. Buffing is the process by which you remove all the old-worn tread on the tire, and prepare the tire casing for the application of a new tread. Buffing also trues up (the roundness) your tire casing. A buffed tire gives you a textured surface on the tire casing and this aids in proper adhesion with the new retread that you will later apply on this surface.

Tire_Buffing_Machine 1

An Image From The Web

Typically, a tire buffing machine has a rasp pedestal and a connected mobile assembly to swing the rasp pedestal perpendicularly. Each rasp pedestal has a rasp head and a texturing device. The rasp pedestal rotates about a vertical axis. A tire hub assembly on this machine rotates the tire casing. The tire buffer is controlled by an operator station through an operator console unit.

Tire_Buffing_Machine

You get tire buffing machinery to buff all the available tires – large or small size, radial or bias, two-wheeler or OTR. The speed of spindle varies according to different tire size. Usually lower speed is apt for bigger tires, and higher speed suitable for smaller tires. These are all free-standing machinery and equipped with a dust collection system for pollution control. 

Tire_Buffing_Machine 2

Image from the Web

Safety mechanisms to increase life of the grinder, automation of operations through computer-controlled programming, ancillary equipment like tire loaders, automatic lifting and centering of your tire, measuring device to ascertain steel wires (and thus prevent its damage), efficiency and precision of buffing, automatic blade-wear compensation, automatic reverse rotation of buffing head, blade cooling, sound-proof enclosure and overall user-friendliness will differ from manufacturer to manufacturer depending on the machine you select.

Tire_Buffing_Machine 6However, the operations are mostly similar and easy to visualize. First you mount the tire onto the expandable hub on the tire buffing machine. Then you inflate the this tire to its normal running shape. This action ensures that your tire is buffed to the correct profile and radius. When you delve deeper into this subject, you will notice that every tire buffing has its specific settings to be considered on the machine (either manually or automatically programmed). Few of these settings include Expandable Rim Number, Tread Size, Radius, Trim Angle, Cut Depth, Shoulder measurements (left, crown and right), and finished buffed circumference. Attention to these parameters ensure that your final retreaded tire provides its peak performance always.

Before you start buffing, care has to be taken that impregnated stones or foreign materials that could cause potential damage to the tire buffing machine needs to be removed by the operator. And always start buffing at the highest part of the crown of the tire. This prevents overloading of the buffer. Operate your tire buffer until all of the tread design is removed.Then you buff the shoulder and side walls as is appropriate to your retreading process.

If you leave much of under-tread on the rim, you will find it very difficult to skive the casing (for repair). Also excessive under-tread can cause high heat generation when the tire is put in use. This can lead to retread failure. On the other hand, if you do not have enough under-tread, this can affect the bond between the casing and the new retread you will apply on the textured surface.

Summarizing, tire buffing is part of the tire retreading operation and tire buffing machine removes the tread of your worn tires.

I look forth to your comments.


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