Gaelle Bellet

External Calibration in PA12 Tube Extrusion

Abstract In PA12 tube extrusion, calibration or sizing is the key step of the process that affects subsequent mechanical properties. The extruded tube is pulled through a cylindrical calibrator under vacuum. A water flow rate is applied at the inner side of the calibrator, creating a lubricant water layer at the polymer outer surface. The scope of this article is to show how this lubrication influences the elongation at break of tubes through drawing kinematics of the polymer. Lubricant water layer thickness measurements and on-line video recording have been employed to monitor the lubrication dependence of the velocity profiles from the extruder die to the end of calibrator. Velocities were measured through three independent innovative methods and thirty-two calibration conditions have been carried out to validate our work. Three main calibration parameters were found to determine the water layer thickness: the level of vacuum applied in the calibration tank, the water flow rate at the calibrator entrance, and the line speed. The influence of each parameter on lubrication level was found out. Simultaneously, the draw ratio in the calibration tank was deduced from velocity profiles. This parameter was found to affect tensile properties and to depend strongly on the level of lubrication during calibration. We showed quantitatively that rising the water layer thickness leads to a diminution of the draw ratio in the calibration tank and an increase of the elongation at break. This implies that we are now able to optimize tensile properties by fitting the main calibration parameters to improve lubrication and restrict draw ratio in the calibration tank.

External calibration in PA12 tube extrusion - Part II: Relations between molecular orientation, tensile properties and polymer drawing during calibration

Abstract Elongation at break is one of the major end-use properties of polyamide 12 extruded tubes. It is strongly affected by the tube microstructure and the molecular orientation resulting from extrusion conditions. Molecular orientation was characterized by X-ray diffraction and birefringence evaluation in light microscopy. Measurements were carried out on (r, z) sections obtained by polishing and microtoming. On the other hand, polymer drawing was measured on line by tracer techniques. Calibration stage was determined as the key step of the process that generates orientation in tubes: as the tube is drawn through a cylindrical calibrator under vacuum and cooled from its outside surface, calibration leads to a highly oriented zone in the twenty external microns. Calibration conditions and elongation at break have been connected through orientation level in this region. Molecular orientation was found to strongly depend on the draw ratio in the calibration tank. Finally, birefringence of the tube external layers and elongation at break were successfully correlated. Elongation at break can be enhanced by reducing orientation resulting from calibration conditions.