The Mechanism of Melt Fracture/Super-shear Phenomena in a Single Screw Extruder
Yue Liang / Ohshima, Masahiro / Hashimoto, Iori / Komura, Yukio / Mikami, Toshihiro / Sakamoto, Kazuhide
When fluorocarbon resins are extruded, they show several die flow phenomena, such as sharkskin, melt fracture and super-shear phenomena. Occurrence of the melt fracture defines the upper limit of the production rate. In the region of melt fracture, however, there exists a narrow region of stable extrusion rate in which a smooth extrudate of fluorocarbon resin can be obtained again. This is called super-shear phenomenon.
In processing operations such as wire and cable coating, using a fluorocarbon resin, economics dictate operating in the so-called super-shear extrusion region to enhance productivity. The advantages of using the high extrusion rate are smooth extrudate surfaces and high productivity. In order to produce coated wire and cable stably in the supper-shear region, it is necessary to elucidate the mechanism of the super-shear pheonomenon and to find proper operating conditions.
From the experimental results, it was found that the super-shear phenomenon occurs at the end of a melt fracture which takes bamboo-like form. As a first step in elucidating the mechanisms of the melt fracture/super-shear phenomena, a Maxwell model was developed to describe the die pressure fluctuation during the occurrence of the melt fracture phenomenon. Then it was used to clarify the mechanism of the super-shear phenomenon.
It is assumed that the melt fracture/super-shear phenomena result from the "stick-slip" behavior in the interface region between the internal surface of the die and the polymer material. On the basis of this assumption, this paper presents a discussion of the mechanism of "slip" from the view points of viscous heating and the catastrophic failure of adhesion or cohesion. It was found that "slip" occurs from the catastrophic failure of cohesion rather than from viscous heating.
Key words: Extrusion process / Fluorocarbon resin / Super-shear extrusion / Melt fracture