Carin, Aurélien (2005) Etude expérimentale et modélisation thermomécanique de l'étape de calibration dans le procédé d'extrusion de tubes en polyamide 12. PhD thesis Sciences et Génie des Matériaux, ENSMP - CEMEF Centre de Mise en Forme des Matériaux, ENSMP.

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Abstract

In polyamide 12 tube extrusion, calibration - or sizing - is the key step that affects subsequent mechanical properties. In external calibration, the extruded tube is pulled through a cylindrical calibrator located in a water tank under vacuum. The level of vacuum (DP) is adjustable. A water flow rate (DSU) is applied at the calibrator inner side creating a lubricant water layer at the polymer outer surface. Influence of calibration is twofold. Since it combines a quenching and a mechanical drawing of the tube external surface, high molecular orientation is generated in this zone. Simultaneously, the surface state of the tube can be affected by local catching to the calibrator surface. On-line video recording and lubricating water layer thickness measurements have been carried out to monitor the lubrication dependence of the polymer drawing kinematics. We have shown quantitatively that increasing the water layer thickness leads to a diminution of the draw ratio in calibrator, DrCAL (ratio between line velocity and velocity at the calibrator entrance). Lubrication and surface state have been correlated through microtopography. Tube molecular orientation was characterized by X-ray diffraction, light microscopy and birefringence measurements. Excellent correlation has been found between tube superficial crystalline orientation (DnSKIN) and DrCAL: a reduced elongation of the polymer in the calibrator leads to less superficial orientation. A thermomechanical model was also used to predict temperatures in the tube thickness and to show how drawing forces, concentrated at the external skin during the crystallization, lead to orientation. Besides, elongation at break has been found to strongly depend on DnSKIN and on the surface defects resulting from calibration. By showing the relationships between process parameters, tube structure, and mechanical properties, this study enables us now to fit the calibration parameters to improve lubrication and reduce the draw ratio in calibrator, in order to obtain strongly better properties.

Table of content

Introduction
chapitre 1: caracterisation du materiau
i. generalites sur le polyamide 12
ii. etude de la cristallisation de l'aesno tl
iii. comportement rheologique du polyamide 12
iv. caracterisations complementaires
conclusion
chapitre 2: caracterisations experimentales et modelisation du procede
i. analyse bibliographique de la calibration
ii. partie experimentale
iii. resultats obtenus
iv. modelisation thermomecanique du procede
conclusion
chapitre 3: caracterisation des tubes
i. techniques et demarches experimentales
ii. microstructure
iii. etat de surface
iv. proprietes mecaniques
v. tubes flammes et tubes bicouches pa/pe
conclusion
chapitre 4: de la calibration aux propriÉtÉs
i. influence phenomenologique de la calibration sur les proprietes
ii. interpretation microstructurale des proprietes
iii. mecanique de la rupture
conclusion
conclusion generale
annexes

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