But vials of acrylic plastic are a lot more common. Each vial station gets a pair of vials because each vial has a definite up and down. These vials are molded in sophisticated, fully automated presses in which molten acrylic is molded around curved cores. The hollow tubes are then filled with dyed mineral spirits or alcohol, capped and printed with reference lines. The process of making vials is far from simple, but the vials are fast and cheap to make.
Curved vials are traditional.
Just like the curved vials, these are made by injecting molten acrylic around a core inside a mold. The result is a curved hollow inside a straight tube. Unlike a curved vial, which must be set into a level in only one position, these vials can be read in any direction. Manufacturers like them because a single-barrel vial can replace two curved vials. Barrel vials are curved only on the inside.