Using ping pong balls as an example, Paddle Palace explains that the hollow celluloid balls used in ping pong are manufactured using a process that includes die cutting, molding, trimming, gluing, measuring and polishing. The entire process takes more than a week, and the final product isn't perfectly spherical.
Celluloid is a plastic which is derived from cotton. The celluloid material is cut into circular disks, which are treated with alcohol to soften the celluloid before molding. The treated celluloid is pressed into half spherical molds, and heated with boiling water before being removed from the mold and dried for two days to allow hardening. Paddle Palace asserts that, at this point in the manufacturing process, the half spheres are measured for thickness and paired with other half-spheres of similar thickness before being trimmed down to size by machine. They claim that yet another machine glues the two halves together in order to form a rough sphere, and the balls are left to dry for an entire week. Following this drying period, the balls are individually weighed to within 1/100th of a gram and grouped based on similarities in weight. The balls are then polished and sent to a second mold, which gives the balls a perfectly spherical shape. The final steps include quality checking and polishing the balls before selling them to consumers.