Manufacturing Processes
GENERAL PRINCIPLES
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.
The common part is that the products are based on rubber polymers. To go from raw material to product needs two phases, one being a typical process operation and the second being more of a workshop type. In the first phase, the raw materials are mixed together to form a rubber compound. Each compound is made according to a specific formulation and the mixing is done according to such specifications as temperatures, time, and so forth.
The second phase is more like a kind of workshop, where the compound is formed, extruded, calendered etc., and finally ends up as a finished product through vulcanization. The methods vary from product to product, but all start with a compound, steel or textile when needed, and end up with curing.
Carbon black was used by the Chinese 500 years ago to make ink and lacquer. The Romans also used it – for wall paintings. In rubber compounds, carbon black is used to improve strength and abrasion resistance. Particle size is important since smaller particles provide better reinforcement but also more difficult processing. An increased particle size improves the processing.
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.
Preparations before mixing
Natural rubber may need to be decristallized before it is used in the mixing operation. In this case, it is heated up in a “heating room,” normally at a temperature of 95-104 F. For special raw materials, a check will be made to ensure that no changes have occurred during storage, and then the mixing can be started.
MIXING OF THE COMPOUND
The mixing operation can be compared with other mixing processes, as for example in a bakery. All ingredients are weighed according to an established recipe and are then mixed together according to specified parameters, like time, temperature and energý in the mixer.
Weighing
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. If a mistake is made here, it is too late to correct it later on in the process. Each bag of raw material is properly marked to identify ingredients and weight.
Mixing
Mixing is carried out either in open mills or in an enclosed chamber containing rotors. In both cases, the basic idea is to distribute and disperse all the ingredients in the rubber through heavy mechanical processing.
Storing the compound
Every batch is controlled against a specification, and a maximum storage time is also specified.
The first trace of rubber used in Europe shows up in coal deposits in Germany
The raw material was not from the rubber tree, and the material, believed to be 55, 000 to 60, 000 years old, still had some elasticity.
EXTRUSION
Long length products such as profiles and tubes are formed by extrusion. The same method is used for making other products, such as treads for tires. 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 and the extruder is cooled with water. 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.
SPREADING
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.
CALENDERING
In production of sheets or textiles coated with a rubber that needs to be thicker than can be achieved by spreading, a calendering technique is used. 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. 4-roller mills are common for coating of textiles and for precision-calendering of rubber.
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. When coating textiles, the fabric is fed between the last two rollers.
MOULD CURING
When making parts with complicated designs and small tolerances, mold curing is used. Example of the end products are seals and other components for the automotive industry. Several principles are used. But generally the compound has to fill a cavity and the curing is made directly in the mold by heating it up. As the rubber has about 2% higher heat shrinkage than steel, the cavity must be bigger than the finished part to give it the correct dimensions. The cavity will be slightly over-filled and the material forms a flash on the outside of the product. For economical reasons, the flash has to be minimized.
Compression moulding
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.
Transfer moulding
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 method cannot be used if fabric has to be inserted in the products. The molds are more complicated and therefore more expensive.
The mould
The first step is to make a drawing of the mold, today normally via by CAD (computer-assisted design software). 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. To get the best finish on the products, the mold has to be cleaned from time to time. A glossy surface helps to demold the products. Many molds are used continuously, which makes it essential to arrange for fast mold changes.
Injection moulding
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.
Postcuring
Rubber products cool down slowly, and the vulcanization continues even after the product has been demolded. 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.
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.
DEFLASHING
All molded parts have some type of flash. The excess rubber is taken away either mechanically or manually by cutting, tearing or stamping. 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.
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.
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. In the development of new products, this must be considered when selecting the most appropriate materials.
Rubber to metal bonding
When manufacturing paper, rolls are used to squeeze the water out of the pulp. The roll has a metal core, but to make it function efficiently, the core is lined with rubber. Other products to which rubber and metal are bonded include machine parts. The metal gives the required stiffness and opportunities to mount the product, while rubber gives it elasticity and damping properties.
The table on “Rubber Types” shows which rubber type gives the best bonding to metal. 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.
TESTING TO SECURE QUALITY
“Since the hose was not flexible in cold weather, it burst and oil poured out on the ground.” “The profiles did not seal, and the heat leaked out from the house.” “The road tape did not bond well.” “After a few days it had come off the road and two cars nearly crashed.”
These are a few typical examples of situations that can, unfortunately, happen. When a fault is not discovered before using the products, it can lead to a catastrophe. If a problem occurs during manufacture, it can lead to delays and increased costs. If a problem is found and corrected early in the process, it will reduce both problems and costs. The principle of “Do it right the first time,” should characterize our entire business. If the raw material is not correct, a perfect process will not help, and for just that reason all raw materials are always carefully checked.
If, for example, a material should be used in a cold climate, a test of its resistance to cold is essential. One type of low-temperature test is to extend a test piece, and thereafter freeze it at a very low temperature. The temperature is then slowly increased and a note is made of when the rubber starts to be flexible again. Other typical tests are hardness, tensile strength and elongation.
Quality assurance
To be able to compare the results, standardized test methods are available. Temperature speed, pressure and all kinds of parameters must be constant. All results are carefully measured and recorded and if a material does not correspond to specification, it is corrected or removed.
An international standard has been devised, giving the requirements of how a certain quality should be guaranteed in a process. Many customers require that the supplier should be certified in accordance with the ISO 9000-standard.