Transient Entanglement Disentanglement Kinetics in Deep Draw Thermoforming under Multiaxial Strain Gradients
Matching plug speed to polymer disentanglement kinetics prevents corner thinning and optimizes wall thickness in deep draw thermoforming.
Chain Slip
Meaning
Molecular disentanglement at the boundary of a flow channel occurs when the shear stress exceeds a critical threshold during melt processing. Such boundary slip, commonly called chain slip, causes a sudden drop in flow resistance as the polymer molecules lose their grip on the metal wall. The phenomenon governs the onset of surface irregularities in linear low-density polyethylene and structured resins during high-shear extrusion.
Melt Behavior
Interfacial sliding occurs when polymer chains disentangle from the absorbed layer on the die wall under high shear rates. Under normal conditions, polymer molecules remain anchored to the metal surface while the bulk melt flows through shearing action. Once shear stress reaches a specific value, chain slip dominates the flow profile.
This transition reduces flow resistance but creates unstable velocity variations in the extrudate.
Processing Impact
Extrusion instability in the die land represents the primary consequence of excessive flow rate. Extruders must operate below the critical shear rate to prevent chain slip from inducing cyclic pressure fluctuations. If the extrusion speed passes this threshold, the polymer exits the die unevenly, causing dimensional variance.
This flow transition dictates the maximum safe output rate for narrow-spec packaging films.
Surface Defect
Melt fracture appears as a visible distortion on the surface of the finished part or film. The periodic alternation between slip and stick states at the exit of the die generates surface roughness known as sharkskin. This cosmetic defect reduces the clarity of clear films and creates weak points in moulded thin-wall containers.
Operators typically resolve the issue by adding fluoropolymer processing aids or reducing extrusion speed. In injection moulding of linear polyethylene, the same phenomenon can lead to gloss variations and visible flow lines across the surface of the part, compromising the mechanical integrity of the component in high-stress applications.