From a great many raw materials, of which the rubber polymers are just one part, thousands of products are manufactured. The areas of application for the products are numerous and very different. What we produce is not primarily determined by material or processes; our goal instead is to resolve our customers’ problems. From this point of view, rubber technology is only one part of the solution.
1) CHECKING AND STORAGE OF RAW MATERIALS
After receiving the raw materials, random samples are taken for comparison with certificates from the supplier. It can happen that, for example, viscosity or moisture has changed due to incorrect transportation. It is important that correct storage conditions are adhered to, in order to avoid later problems in the process. Most of the raw materials can be stored at normal temperatures but a few need a cold storage area.
Each product has a compound made from a special formula, and therefore the number of raw materials used is fairly high. From storage bins or direct from bags, the ingredients are weighed according to the formula, with high accuracy. At this stage if a mistake is made it could be fatal for the whole process. Each bag of raw material is properly marked to identify ingredients and weight.
Mixing is carried out either in open mills or in an enclosed chamber containing rotors. In both cases, the aim is to distribute and disperse all the ingredients in the rubber through mechanical processing.
4) STORING THE COMPOUND
Every batch is controlled against a specification, and a maximum storage time is also specified.
5) THE MOULD
The first step is to make a drawing of the mold. The mold is made from the drawing of the product, taking into account the bigger heat shrinkages of rubber than steel. In such cases, the designer must know in advance how the compound is going to be used. For bigger parts, the molecule orientations in the rubber can influence the shrinkage. The mold is normally manufactured of hardened steel to obtain an optimally solid mold. In many cases, a first mold with less cavities is made for prototype manufacturing.
6) BONDING OF RUBBER TO OTHER MATERIALS
One important property of rubber is the opportunity it offers for bonding to other materials. This may be to metals, plastics, glass fiber, natural and synthetic fibers and types of rubber. Bonding feasibility depends on the rubber type and the bonding materials used.
RUBBER TO METAL BONDING
The metal gives the required stiffness and opportunities to mount the product, while rubber gives it elasticity and damping properties.
The metals giving the best results are steel and aluminum. For alloys like brass and bronze, the result depends very much on the composition of the alloy. When manufacturing, it is important that the metal is smooth and free from rust and other impurities. The metal is prepared by degreasing and sandblasting.
Finally, before molding, a bonding agent is sprayed onto the metal and, after drying the metal, the rubber is placed into the mould for forming and curing.
RUBBER TO TEXTILE BONDING
Many of our products are reinforced with textile, either as fabric, cord fabric or yarn. The fiber type may be polyester, polyamide, aramide, or glass and, to a certain extent, cotton is also used, as is steel wire. The textile cord or metal wire in tires is the part that gives the tire its mechanical strength.
At the start of the rubber industry in the 1800s, waterproofed textiles were in great demand, and the spreading technique developed early. A rubber compound is dissolved in an organic solvent and spread onto the textile with a knife. When the solvent has evaporated, a rubber film is formed on the textile, and the coated fabric is cured in a hot oven.
7) KINDS OF RUBBER CURING
Long length products such as profiles and tubes are formed by extrusion. The extruder used has the same principle as a meat-grinder, with a screw that feeds the compound through a die. The compound is heated by friction. When the profile has passed the die, it will be cooled in water if it is going to be vulcanized in an autoclave. Another method is to vulcanize continuously in either a microwave-heated oven, or in a salt bath.
The production of rubber sheets can be achieved by calendering technique. Typical products are parts for tires, conveyor belts, hoses and sheeting and mats.
A calender is a roller mill, with two or more rolls. The number of rolls and the configuration depend on the type of operation.
The compound is pre-heated in a mill or in an extruder and fed to the calender. As the rolls rotate in different directions, the compound is pressed and directed forwards. When calendered to final thickness, the material is air-cooled before being wound onto rollers or milled in smaller sheets.
The oldest method is compression molding, where a mold consisting of two parts is mounted in a press. A preformed rubber block is placed inside the cavity and when the press is closed, the rubber fills the cavity in the mould. To make sure that the product is perfect there must be some overflow. The curing time is rather time-consuming and generates substantial flash. The advantage is in the simplicity of the method and it is well suited to larger products. The requirements of the compounds are less than in other methods.
Some of the drawbacks with compression molding are overcome with transfer molding. The compound is placed into a third part of the mold – a heating chamber – and by closing the press the rubber is injected into the cavity with a piston. The pre-heating gives a shorter curing time and the risk for porosity, etc., is low. The flash can be minimized by maintaining the correct injection pressure. The bonding is imposed on rubber-metal parts by transfer molding. The molds are more complicated and therefore more expensive.
With injection molding the compound is extruded with a screw into a closed cavity. The extrusion pre-heats the rubber and shortens the curing time. The process places high requirements on the compound in terms of flow properties and curing behavior. The biggest advantage associated with injection molding is the possibilities it offers to automate the process and the control of flash. An injection machine is rather complicated and the whole process is more technically developed.
Rubber products cool down slowly, and the vulcanization continues even after the product has been de-molded. The postcuring process is normally around 10% of the total curing time. A few special compounds require postcuring in hot air for several hours, sometimes up to as much as 24 hours.
9) FINISHING OPERATIONS
We have now described the mixing of a compound, the forming of the product and curing. To finish the manufacturing process, some products have been cut, stamped or ground, for example. Other products will be assembled with other materials, or with rubber components.
All molded parts have some type of flash. The excess rubber is taken away either mechanically or manually by cutting or tearing. The rubber strings deriving from the flow of rubber between the cavities in transfer and injection molding also have to be removed. One method used for the deflashing of small parts is to freeze the rubber with liquid nitrogen, and deflash them by barrel polishing.
10) SURFACE TREATMENT
In some cases, the surface is treated to reduce friction. Other treatments may be necessary to bond the rubber to other materials. Chlorination and UV-light are other treatment alternatives.