How bowling balls are made

Bowling balls are poured three times: once for the core, once to round it out, and once for the colored shell.

stations: 8
things upstream: 69
raw sources: 23
steps from the ground: 10

The short version

A bowling ball is a cast sandwich. A heavy core is poured from resin in a mold, surrounded with filler to make it round, then dropped into a second mold where liquid urethane is poured around it. The shell sets in about two minutes. The ball is then cut down, polished, engraved, and later drilled for a bowler's hand.

Batch the resin for cores

Raw materials are mixed in a sealed area to keep fumes away from workers. The result is a liquid resin that Storm's production chief compared to making a cake, with lighter or heavier ingredients chosen to set the weight.

Goes in
Plastic
Pour the core

The resin, at roughly 260 degrees, is poured into molds in many shapes. Each shape makes the ball roll differently, much as the head of a golf iron does. After about six minutes of cooling the core pops out.

about 260 degrees
Fill out to a sphere

The core is centered in a mold and surrounded with a mix of glass microbubbles and a denser filler. Different proportions change the ball's weight while it keeps the same 27-inch circumference.

Goes in
Glass
Pour the urethane shell

The round core goes into another mold, and liquid urethane is poured around it. A ball can carry up to three colors, and the swirl is unique because of how the mix fills the mold. Mixing generates heat, and the shell hardens in about two minutes.

Goes in
Polyurethane
Crack the mold and cool

The mold is cracked off like an egg and the balls cool on a rack overnight. A hole left by the pin that held the core is plugged, often with a color that tells workers which core is inside.
Cut, sand and polish

A machine cuts the shell down to size. The ball is then sanded for a matte finish or polished for a shine, and each finish rolls differently on oily lanes.
Engrave the logo

A machine rotates the ball while it engraves up to five points. Workers fill the engraved lines with paint so the logo stands out.

Goes in
Paint
Drill the finger holes

Holes for two fingers and the thumb are drilled to fit the bowler's hand. This is usually done in a pro shop, not the factory, and the rules limit how much material can be taken out.

End of the lineBowling ball

Worth repeating

One Utah factory can pour as many as 3,000 premium bowling balls in a day, and a ball takes about three days from start to finish.
A patented scented ball can be made in over 11,000 different fragrances, from bacon to baked goods.
Bowling balls were made of hardwood until rubber ones appeared in 1905, and plastic ones in 1959.