Roto-molded coolers are made from a special process called roto-molding. It works by first placing a powdered plastic poly-material into a heated mold. This mold is then rotated( hence the name) until the plastic completely melts.
What you get is a product with no imprecisions, the plastic has no voids(outside or inside). This technique is used to create things like kayaks. But recently companies have begun creating rotomolded coolers as well