Capillary Rheometry Shear Rate Correction Protocols for Bimodal High Density Polyethylene
Correcting capillary rheometry data via Bagley and Rabinowitsch protocols is mandatory to prevent up to 50 percent viscosity errors in bimodal HDPE die design.
Sharkskin
Meaning
A surface defect in polymer extrusion is characterized by a series of fine, transverse ridges or a matte finish on the surface of the extrudate. Occurring at the die exit when the outer layer of the melt is subjected to high tensile stress, sharkskin arises from the rapid acceleration of the polymer as it leaves the metal wall. This defect occurs when processing linear low-density polyethylene and other highly viscoelastic materials.
It restricts production rates by limiting the maximum extrusion speed before surface quality degrades.
Stress Generation
As the polymer melt flows through the die, it moves quickest in the center and is stationary at the metal wall due to friction. When the material exits the die, the outer layer must accelerate suddenly to match the velocity of the bulk stream. This transition generates intense tensile stresses that stretch the surface of the polymer beyond its elastic limit, causing it to tear and form a rough, repetitive pattern of ridges that compromises both the appearance and the mechanical strength of the product.
Processing Factor
Extrusion speed and die temperature influence the occurrence of this surface defect. High extrusion rates increase the shear rate at the die exit, which raises the tensile stress on the surface. Lower melt temperatures also exacerbate the problem by increasing the viscosity and lowering the elasticity of the polymer melt.
Remedial Method
Processors can eliminate the defect by adjusting the process temperature or using extrusion additives. Raising the die temperature lowers the melt viscosity at the exit, which reduces the shear stress. This adjustment helps to restore a smooth finish.