High Temperature Non Aqueous Volatile Acid Titration in Recycled Polyethylene
High temperature non-aqueous titration strips volatile carboxylic acids from hot PE melt into organic media to quantify corrosion risks and thermal stability.
Melt Flow Instability
Meaning
Disturbing the uniform flow of molten plastic during extrusion creates surface irregularities and structural weaknesses in the finished profile. Such melt flow instability typically occurs when the shear stress at the die wall exceeds a critical value, causing the polymer to slip or fracture.
Shear Response
Distortions such as sharkskin or orange peel appear when the outer layer of the melt cannot relax quickly enough as it exits the die. If melt flow instability intensifies, the entire extrudate may exhibit a helical or gross fracture pattern that renders the material unusable for high precision applications.
Elastic Recovery
Elastic energy storage within the polymer chains contributes to the sudden release of stress at the die exit. This aspect of melt flow instability is more pronounced in resins with high molecular weight or broad molecular weight distributions.
Processing Boundary
Adjusting the die temperature or the extrusion speed moves the operation away from the unstable regime. Processors often introduce processing aids, such as fluoropolymers, to coat the die surface and reduce the friction that triggers melt flow instability at high throughput rates. Maintaining a consistent temperature profile across the die face is necessary to prevent localized flow disruptions.
Constant monitoring of the pressure at the die head allows for the early detection of these fluctuations before they affect the final part geometry.