Quimica, Chemical: Flat Film

Flat film is also called chill roll film, roll cast film, cast film, slot cast film,
water chill film, etc. A process similar to flat film processing is the casting
process to produce film (and sheet) from highly fluid solutions. The usually
higher viscosity of TP melts results in differences in machine design
as well as in operational techniques. The most popular process used to
produce the flat film is with the chill rolls. Water chill or quench films is
also a popular process.
The flat film chill roll process usually has low melt viscosity melt extruded
through a coat hanger or T-shape slot die that
may be at least 3m (loft) wide. The extrudate is chilled below the melt
temperature (T,) or the glass transition temperature (T,) by passing it
principally over two or more chrome plated chill rollers (that have been
cored for water cooling) or a water bath,
the extrudate web from the die is made dimensionally stable by contacting
several chill rolls prior to being pulled by the nip or pull rolls and
wrapped around the windup roll.
Extrudates can leave the die downward as a highly viscous melt that
must be pulled away from the die. They may also be led directly
onto the upper surface of the chill roll. Cooling in contact with the polished
surface of the roll imparts a good surface to the film, while the other
surface relaxes as it cools in air. The roll forms a means of haul-off by
surface friction and its speed controls drawdown or neck-in and final film
thickness.
When the hot film is drawn down onto the cooled first chill roll, it will
shrink at the edges. The amount of neck-in and beading varies for the
different plastics. This neck-in is the difference
between the hot melt width at the die lips and the film width on the chill
roll. This diagram also indicates the distance between contact line and
frost line on the roll. With this neck-in, beading takes place. It is the
thickening at both edges of the film. Later in the line, this bead is trimmed.
The relatively high degree of drawdown over a short distance requires
high elongation with generally high melt temperatures. As many films are
manufactured from crystalline (or semicrystalline) plastics, the rapid cooling
tends to suppress crystallization. This molecular action is beneficial in
providing high elongation of the cooled film for subsequent drawing/
orientation processes.