The majority of modern cars are the road use pneumatic tires, primarily composed of rubber, with additional chemicals added to the process. With over a billion tires made each year in factories across the globe, it is obvious that the general manufacturing process is going to be similar for all tires. Assembly line production is standard practice in the tire industry worldwide. While must of the manufacturing has moved overseas, there are still a number of companies operating tire manufacturing plants in the United States. Each finished tire is a combination of a number of individual components. A tire is made up of the bead, the body plies, a belt package, sidewall, inner tuber or liner, tread, apex, and cusion gum. Differences in each of these components determines the ultimate type of tire being produced. For example, a belt package that includes steel belts below the tread of the tire creates a steel-belted radial tire. Differences in the dimensions of the tire will affect performance and the tread will be adjusted to suit different driving conditions. While tires are mostly rubber, the actual materials used will vary between manufacturers and tire models. Common materials used in tire production include both natural and synthetic rubber , carbon black, silica, sulphur, organic compounds known as vulcanizing accelerators, activators like zinc oxide, and antioxidants and antiozonants. All the components are produced individually and allowed to cure for a specified period of time. The finished components are then assembled on a drum. The tires are then subjected to pressure and heat in order to bond all the components together in a process known as vulcanizing. The curing machine will also set the traction pattern and imprint all the markings for the finished tire. Depending on the manufacturer, there may be additional finishing steps or inspections or the tire may be ready for sale and use.