Processing of bioresorbable and other polymers for medical applications - Durability and Reliability of Medical Polymers - page 60

via evaporation. Melt spinning is, however, the most common commercial
process. Fibers solidification occurs either in a water bath or for forced
convection. Final properties of the filament, such as breaking strength,
Young Modulus and flex loss, are determined by the spinning process.
When the molten filament exits from the spinneret to a take-up roll, it is
simultaneously stretched and cooled, and these phenomena orient polymer
chains and induce crystallization in the polymer. The spinning process is
thus not only a shaping process but also a structuring process (Tadmor and
Gogos, 2006, Osswald and Hernández-Ortiz, 2006).

3.5.2 Cast film extrusion
In cast film extrusion a thin film is extruded through a slit. It passes through
a chilled turning roller where it is quenched from one side. The film is sent
to a second roller in order to cool the other side. Finally, the film is wound
in a roll by passing through a final system of rollers (Fig. 3.10; Osswald and
Hernández-Ortiz, 2006). Film thickness depends on roller speed.

3.5.3 Film blowing
In film blowing, a tubular film is extruded upwards. It is blown upwards, with
air introduced below the die, into a larger tubular film which is then picked
up by a pair of nip rolls that seals the bubble (Fig. 3.11; Han, 2007). An
external stream of chilled air cools and solidifies the film at a certain point
called the freeze line, where the temperature of the film is equal to the melting
temperature. A feature of this process is that the film is stretched biaxially,
improving mechanical properties. Tangential circumferential stretching
depends on blow-up ratio, i.e. the ratio between the tubular film diameter
after air introduction and the initial tubular film diameter. This parameter is
determined by the pressure level within the bubble. axial stretching depends

￿ ￿ ￿ ￿ ￿ ￿

Die inlet

Cast film die Stripping roll

Edge trim slitter

Pull rolls

Air gap
with
molten
web

Idler roll

Wind up rolls

Casting roll

3.10 Schematic of film-casting (from Osswald and Hernández-Ortiz,
2006).