Offset printing tires are composed of several layers of fibers that are attached to each other by thin layers of rubber, which are the components of the tire body.
By adding special polymer materials, a rubber coating is formed on the outer surface of the tire, the thickness of which is about 0.5 mm. In total, the thickness of the tire, i.e. the outer cover plus its body, is between 0.76 and 1.9 mm, all of which depends on the number of layers that make up the tire.
Making cylinder rubber includes three main steps: 1. Polymer preparation 2. Fiber texture 3. Rubber body and surface production
Polymer preparation for rubber printing
In the past, offset tires were made of natural polymer materials, which had the property of accurate transfer without compound defects in low circulation. Unfortunately, the surface of the rubber quickly changes when in contact with the cleaning materials, and the materials used in the compounds and the drying materials in the compound also affect it.
Nowadays, synthetic polymer is used to increase the resistance of rubber against the mentioned cases, and with the developments that followed, synthetic polymer is now named as natural rubber.
The cylinder rubber must have characteristics such as hardness and resistance to cleaning agents and solvents, which are of great importance in the rubber formulation. Various materials such as sulfurs, oxides, plastic materials and color are added to the main polymer formula to produce cylinder rubber with the desired color and required properties.
Each tire manufacturer uses its own instructions for the production of cylinder rubber. So, the mentioned materials and the amount of their consumption in the production of rubber will definitely be different for each manufacturer.
Weaving fibers for printing rubber
Cylinder rubber fibers are woven from the best thread, which is usually made of linen thread due to its strength and resistance. The yarns are woven lengthwise and folded in two layers to have maximum resistance to tension.
After weaving, the yarn fibers are impregnated with various materials and the work is completed until the final stages. Then, the fiber stretching stage is done by stretching it in the direction of closing the tire around the cylinder to obtain the maximum tensile strength of the fibers.
Some manufacturers use colored threads on the back of the tire to show the direction of closing the tire around the cylinder, and some others print a special mark to specify the direction of closing the tire, which is the same direction of closing the cylinder.
Two types of fabric are used in making cylinder tires.
• One is very strong and strong, which is called Wigan.
• The other is a kind of soft fabric called (Cambric), which is a type of linen.
Wigan gives the necessary resistance in terms of elasticity and strength to the rubber and most of the tires are made of this type of fiber.
Cambric is usually used as soft and delicate fibers for the back of tires.
There are different methods to test the amount of tire tensile strength, two of the more common methods are mentioned here
a: The fibers woven to make rubber are soaked in water and passed under pressure around cylinders driven by steam.
B: The woven fiber is passed through boiling water under pressure and then passed through cylinders heated with steam to remove the water and dry it.
Making the body and cover of printed rubber
A spreader machine is used to pay the final coating of the rubber. The main building of this machine consists of a very sensitive object that is placed on top of a rubber roller. The surface of the cylinder rubber produced by passing over this roller and being placed under the blade is polished and then passes over a thermal plate. In this part, the moisture in the rubber is evaporated and at the end the rubber is collected again around a reel. Ken gets complicated.
This part of the work is very sensitive and all the mentioned operations must be done in a very clean and dust-free environment. To make the rubber body, several layers of fibers covered with a thin layer of special materials are used to place Layers of fibers on each other use a device called a duplicator. In this device, the mentioned fibers will be passed between two rolls that are under pressure and will be connected to each other.
By repeating this process, the appropriate number of layers are produced to make the tire body and they use the spreader machine or the tire surface polishing machine to polish the tire surface. This operation requires precision and precise control, it is better to use several thin layers of polishing material for a thick layer to polish the rubber surface.
The application of several thin layers ensures that all the chemical gases used in the construction of the body will be removed, the remaining chemicals and their non-evaporation will cause the gas to form in the rubber body and give it a spongy state. This is the reason why the polishing of the rubber surface should be done several times to ensure the evaporation of chemicals and uniformity of the rubber surface. Finally, by repeating the process of polishing the rubber surface, a layer with a thickness of 0.5 mm is added to the tire body. After finishing the polishing operation, the surface of the tire is coated with powder and cleaned with another machine equipped with a cleaning brush. When the excess powder is removed from the tire, the tire is covered with a special wrapper made of very soft paper. And the next stage is the process of curing the rubber, which is done in special thermal ovens with controlled temperature. After curing, the rubber surface, which was initially soft, becomes firm and resistant. After baking, the rolled rubber is opened and placed flat, and its surface is inspected and necessary sampling is done for quality control.
The most important features required in offset printing cylinder rubber
The most important characteristics required in an offset cylinder tire are classified as follows:
1- The fibers used in the cylinder rubber must be strong and its elasticity is very low. Of course, the cylinder rubber should be slightly stretchable to wrap evenly around the cylinder
2- The thickness of the cylinder rubber must be completely uniform.
3- The surface of the cylinder rubber must be free of any pinholes and obstructions. The surface of the rubber must be smooth and uniform and have a lot of resistance. The surface of the rubber must be resistant to ink, cleaning and varnishing materials and have the ability to absorb ink.
The resistance of the rubber surface should be such that it does not suffer from abrasion in contact with paper and cardboard, and it should also have the ability to quickly separate ink and paper from itself.
The hardness of offset rubber is between 60 and 85 Shore. Shore is the unit of measurement of the degree of hardness of rubber. Some manufacturers do not care about the degree of hardness of the rubber and believe that it will not play a role in its efficiency. The hardness of printed tires is between 72 and 84 degrees.
Measuring the hardness of the tire is done by a tool called a pressure gauge, which indicates the hardness of the tire against the needle sinking. This work includes rubber parts and rubber fibers.
Resistance of printing rubber to grease and cleaning agents
The power of absorbing and receiving ink and resistance to grease and cleaning chemicals seem to be two opposite factors, the ink carrier can also cause problems such as separation of paper from rubber. The compound carrier is selected in harmony with the polymer materials used in the rubber manufacturing to avoid these problems.
Petroleum materials have been used in making most of the compounds. KB grade is the abbreviation of Kauri Butanol and the chemical symbol to indicate their degree of solubility. The higher this degree is, the greater the impact of chemicals on the tire and, as a result, the rate of inflation of the tire.
Rubber is usually supplied in tubes in cardboard tubes. It is recommended that after receiving the tire, take it out of the cardboard tube and place it flat and flat, back to back or face to face, and it should be kept in a cool and dark place. Heat, moonlight, and sunlight can make the rubber shiny and hard and brittle.
If tire storage space is limited, it is recommended to store tires in cardboard tubes that do not put any pressure on the tire. It is better not to store the tire on the ground, it will be more suitable to hang it. Necessary instructions for maintaining tires have been provided by the manufacturers, whose compliance guarantees the maintenance of tires.