Numerical Simulation of the Wire-Pinning Process in PET Film Casting: Steady-State Results

摘要

Poly (ethylene terephthalate) (PET) film casting involves melt flow through a slot die, across a small span, and onto a fast moving quench drum. In the "wire-pinning" casting process, a thin electrified wire close to the line of contact with the drum creates a strong pinning force that delays air entrainment to higher line speeds. Nonuniform wetting of the die lips by the extruded melt is thought to be responsible for the formation of streaks, a defect in the machine direction. A finite element model of the film casting process with wire pinning was developed to assist in understanding what causes significant wetting of the lips and whether this can be avoided by electrostatic pinning. The solution of the governing equations provide the location of the static and dynamic contact lines, thus finding the wetting and pinning points in the process. The simulations investigate the sensitivity of the static wetting line locations on the die lands to the imposed values of static contact angle and die-lip gap. It was found that while the contact angle has a small effect on the extent of die-lip wetting within the parameter ranges examined, there is a considerable chance that the feed slot surfaces can be dewetted. This seems to be the greatest danger for causing streaks rather than excessive wetting of the die lands.