Rheological Determination of Ultra High Molecular Weight Tails in Bimodal Resins
Low-frequency storage modulus and creep recovery accurately quantify ultra-high molecular weight tails in bimodal resins, ensuring stress crack resistance.
Stick-Slip Flow
Meaning
Periodic oscillations in pressure and flow rate occur when a polymer melt rapidly alternates between adhering to and sliding along a die wall. Stick-slip flow is a type of extrusion instability that leads to severe surface defects and cyclic variations in part dimensions. This phenomenon typically happens at a critical shear stress that depends on the molecular weight and the temperature of the resin.
Surface Defect
Visual evidence of this instability appears as a regular pattern of ridges or distortions on the extrudate, often called sharkskin or bambooing. When stick-slip flow occurs, the surface of the part becomes rough, which can compromise both the aesthetics and the mechanical seal of the product.
Critical Stress
Resistance to flow at the wall increases until the force exceeds the adhesion of the polymer to the metal. At this point, the melt slips forward, the pressure drops and the stick-slip flow cycle begins again.
Processing Correction
Moulders reduce this instability by increasing the melt temperature or by using processing aids that coat the internal surfaces of the die. These additives lower the friction during stick-slip flow by allowing for a stable slip condition at higher shear rates. Adjusting the die geometry to include a more gradual exit can also help prevent the onset of these flow oscillations.