Formation of monofilaments involves two basic steps (Fig 3.113)
Step 1: Filament spinning- formation of the filament or “as-spun monofilament”, which is semi-crystalline, but nonoriented
Step 2- “cold drawing” or “drawing”, confers most of the orientation through a stretching, yielding, and drawing process that takes place in the solid state, but above the Tg of the crystallizing polymer
Monofilament in final form is “drawn filament”
Filament spinning, achieved in several ways
Chemical reaction during the fiber-forming stage, or
Transformation are only physical, involving heat and mass transfer
Wet spinning (Fig 3.114) involves the extrusion of a liquid-like fluid through small holes (orifices) of a spinneret in a bath containing another fluid with which the extruded strand interacts, either chemically or through molecular exchange
After sufficient interaction and residence time, the strand becomes solid and as-spun monofilament
Frothing method corresponds to 2-stage expansion .
Suitable low boiling point blowing agent is incorporated to the resin under pressure (4-5 atmospheres) (1 atmosphere = 14.69 psi) to prevent expansion
Pressure release at the exit of the dispensing nozzle causes the immediate formation of a froth (foamed cream) corresponding to a pre-expansion ratio of 10X. Subsequent expansion is associated with the curing reaction which causes the vaporization of the other blowing agent with expansion of 3X.
Pressure developed in a cavity and temperature variations are lower than in the case of direct liquid feeding and mush larger than by successive layer build-ups.