The engineering behind a tennis ball is both historical and functional. The main component of a tennis ball core, vulcanized rubber, is often manufactured in Asia. The cover is soft and fuzzy and made to provide durability. With that in mind, you might be surprised just how they look inside.
What They Are Made Of
Tennis balls consist of two main parts, the cover and a hollow molded rubber core. Each hollow ball is filled with either a mix of sodium nitrate and ammonium chloride or compressed air to maintain outward pressure that maintains its spherical shape. Cut one open and you'll usually find the smooth rubber core on the inside of the cover is either black or a light brown. Most balls are manufactured with rubber that originates in Asia, where natural rubber is plentiful. The second largest producer of rubber is Indonesia. The thickness of the rubber will affect the ball's bouncing properties, which can vary by ball type and brand. Because of the rubber's porous nature, the gases eventually escape the ball, hindering its ability to keep shape over time through extended use. For this reason, many players will quickly discard them.
Tennis Ball History
Modern tennis is derived from two similar games, lawn tennis and royal or real tennis. Real tennis balls originally had a leather or cloth cover and were stuffed with horsehair. They were different from lawn tennis balls, which were made of rubber. Real tennis balls were hand-stitched while lawn tennis balls always have had some sort of rubber seam. Older tennis balls were either black or white, depending on the color of playing surfaces they were used on. According to the International Tennis Federation, "In 1972, the ITF introduced yellow tennis balls into the rules of tennis, as research had shown that they were more visible to television viewers." Today's tennis ball technology allows for the balls to be used on multiple surfaces.
The Core
Originally the rubber cores were made by stamping a sheet of rubber into a clover leaf shape. A machine would fashion the sheet into a rough spherical shape while chemicals were introduced. By placing it into a heated mold, the chemicals would give off gasses to inflate the ball. However, this method created inconsistencies in each ball and as the game progressed, a more uniform ball was required. Modern cores are molded from two hemispherical shapes that are pressed together. Once pressed and pressurized, they are buffed to a rough surface that will help a coat of heat-activated rubber to adhere to its surface. The buffed cores are put into a tumbler to receive a uniform coating that will stick to the cover when heated.
The Cover
The cover is usually made up of wool or needle cloth. The cloth comes in rolls and stamps are used to cut out a bell shaped blanks. Two blanks will be used to make up the cover. The backside of the cloth is covered with a solution that will help the cover stick to the core. The ball parts are placed in a machine that places the cover on the core and then into a heated molding press that cures the heat-activated rubber and seals the cover to the core. The ball is then placed in a steam tumbler that fluffs up the fibers of the cover to give it that fuzzy look and feel.
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