Polyethylene comes out of the reactor as granular powder, which is then melted and flows through a film extruder. The flow rate required to make the polymer flow is determined by solving a boundary value problem(BVP), which comes from the momentum balance.

Originally, bags were made directly from tubular film by wedding one end, but the tendency now is to make wide film on large machines and then make the bags by weilding and cutting. (click tubular for a quick look at a tubular film extruder, or click flat for a flat film extruder)

In the tubular film process, the bubble diameter may be up to 2 meters for general purpose packaging, and larger for heavy gauge industrial film. Additives are important in this process. Without them, the pressure of the windup rollers on the warm film forces the surfaces into such close contact that subsequently it may be impossible to separate them. Very fine silica can be used as an anti-blocking agent to overcome this problem. It roughens the surface on a submicroscopic scale without significantly affecting the optical properties. Slip agents can be added to reduce the friction between the surfaces.