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

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: 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)

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.

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.

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.

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

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

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