Plastics Machine's Screw

Conventional "square pitch" versus a "variable pitch screw" versus say a "Barrier Screw"

Chromed screws, tools are generally used with materials that degrade and burn causing caustic damage to steel tooling when they move from being cohesive to adhesive - aka PVC

Bi-metallic screws generally are steel screws that have one or more hard materials actually welded on and ground away (generally to improve material handling and wear characteristics). It is generally done to the tops of screw flights and then ground to final shape to give the flight tops better wear resistance to the interaction with the barrel.

Just my two cents -

Jim Wilson