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A BEGINNER'S GUIDE TO RUBBER: RUBBER VULCANIZATION
Oct 18, 2018

There are many ways to form rubber parts, part 3 of this guide will discuss the remaining more popular processes used in the industry today. As mentioned earlier, the process for turning rubber compounds into usable finished products is vulcanization. There are various types of vulcanization, each producing a unique end product. There is open vulcanization which can take place in hot air or in steam. Vulcanization in hot-air ovens is not very efficient because of the poor heat transfer of hot air. Open steam vulcanization takes place in large containers called autoclaves with a heated jacket and a closed chamber in which articles are placed and the steam is introduced. Saturated steam has better heat transfer which results in shorter cure times at higher temperatures. Open steam curing is typically used in the production of extruded articles such as hoses, cables and strips. Continuous vulcanization is a single-line operation, the shaped uncured product is transferred along a curing medium such as liquid, hot air, steam, microwaves and infrared, and high-energy radiation, the rubber is then cut to produce extruded goods, coated wiring, conveyor belts and flooring. The continuous vulcanization in liquid baths is called LCM (liquid curing method).In this process, the extrudate is run through a suitable hot bath immediately after leaving the extruder. The bath temperature is usually 200-300 degrees Celsius consisting of salt mixtures, polyglycols, silicone oil and metal alloys. Hot-air tunnels can be used for continuous curing of thin articles. Continuous vulcanization in steam tubes is used mainly in the cable industry. Flat rubber goods such as conveyor belts and floor coverings are vulcanized continuously by the Rotocure method. This process involves the use of a wide steel band that presses the article against large, slowly rotating heated drums. Microwave energy is normally used to pre-heat the the rubber parts before it is finished with another medium such as hot air. Infrared is also used in continuous curing in which infrared bulbs supply the heat. High-energy radiation continuous vulcanization is achieved by using gamma radiation from cobalt 60 or electron beams. Cold vulcanization can occur at room temperature by exposure to sulphur monochloride vapors; howeever, this curing agent has been replaced by ultra accelerators that can cause the vulcanization at room temperature. Postcure is a process that may be used on some elastomer types to improve one or more properties of the vulcanizate (the just vulcanized rubber product). It requires circulation in hot-air ovens with a constant fresh air supply and can last several hours at a higher temperature than the press. Finishing operations Many rubber articles require deflashing after vulcanization. This is sometimes carried out by hand trimming using scissors or knives. A widely used method of deflashing involves exposing the rubber parts to low temperature (using liquid nitrogen or carbon dioxide) until the flash is frozen, then the parts are tumbled and the flash is cleaned away, this is called cryogenic deflashing. Some rubber parts may then be painted to give them an attractive finish, or more often, to protect against ozone, oils, acids, chemicals and etc. Quality control is necessary to ensure the quality of the final products. At various stages, incoming materials, in-process materials and finished products will be checked to make sure they are uniform and of adequate quality. Measurement of viscosity, scorch time and cure time are all checked for unvulcanized materials. Vulcanized materials are checked for measurement of the density, hardness, tensile modulus, tensile strength and elongation at break. Quality control of finished products involves visual inspection for defects, dimensional control as well as physical tests according to specifications such as hardness, compression set and density.