Irregular Flow or Melt Fracture

A true melt fracture by definition must involve some kind of bulk failure in melt flow. Either massive chain disentanglement or chain scission or both must occur in the bulk. Such a real cohesive breakdown away from the surface may also produce extrudate distortion. In general, other forms of irregular extrudate distortion do occur at high stresses. Only direct flow visualization may reveal the origin of such extrudate distortions. 

The flow of the polymer melt through the spinneret capillaries themselves constitutes still another portion of the pre-extrusion experience of the polymer. The complexity of this aspect can be demonstrated by considering the phenomenon of melt fracture. When a molten polymer is extruded, it can exhibit surface irregularities that culminate in the extrudate being twisted or actually broken (see Fig. 11-9). Three main theories have been proposed to try to explain this... 

In the case of the MD, variations in the flow rate due to pressure or temperature variations in the pumping device are the main cause of the irregularities. However, flow instabilities associated with the phenomena of melt fracture and draw resonance can lead to variations in the dimensions of the extrudate. These variations are closely connected to the rheological properties of the melt, but die design can at least alleviate the severity of the irregularities. 

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