Melt fracture gross distortion

Melt fracture and distortion. The phenomena occur in materials at higher stresses and can be an indication of slip/stick in the die. The extrudate can display a wide variety of distortions from simple sharkskin (a very rough surface on the strand) to twists and kinks in the strand (gross melt distortion). These distortions usually occur at the exit of the die and do not affect the viscosity curve as long as the material does not slip. 

The site of the sharkskin distortion is again the die exit, and so is the screw thread pattern. The site of, and the mechanism for the gross extrudate distortion are problems that have no clear answers. The work of White and Ballenger, Oyanagi, den Otter, and Bergem clearly demonstrates that some instability in the entrance flow patterns is involved in HDPE melt fracture. Clear evidence for this can be found in Fig. 12.18. Slip at the capillary wall, to quote den Otter, does not appear to be essential for the instability region, although it may occasionally accompany it. ... 

Gross extrudate distortions have constantly been linked to melt fracture. This alleged connection between a gross extrudate distortion and melt fracture re-... 

The term melt fracture has been applied from the outset [9,13] to refer to various types of visible extrudate distortion. The origin of sharkskin (often called surface melt fracture ) has been shown in Sect. 10 to be related to a local interfacial instability in the die exit region. The alternating quasi-periodic, sometimes bamboo-like, extrudate distortion associated with the flow oscillation is a result of oscillation in extrudate swell under controlled piston speed due to unstable boundary condition, as discussed in Sect. 8. A third type, spiral like, distortion is associated with an entry flow instability. The latter two kinds have often been referred to as gross melt fracture. It is clearly misleading and inaccurate to call these three major types of extrudate distortion melt fracture since they do not arise from a true melt fracture or bulk failure. Unfortunately, for historical reasons, this terminology will stay with us and be used interchangeably with the phase extrudate distortion. ... 

It is important to distinguish between melt fracture, which is a gross distortion or waviness, and a fine scale high frequency surface... 

It is inqHntant to distinguish between melt fracture, which is a gross distortion or waviness, and a fine-scale high-frequent surface roughness [40]. Ihe latter may commence at ouqtut rates below those at whidi melt fracture is observed and is called matte or mattness. The extreme case of mattness is referred to as shark skin. The distinction between sharit skin and melt fracture has been convincingly demonstrated [66], as shown in Fig. 2.11. 

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