Extruder shark-skin surface

Shark skinning Surface defect of a regular ridged surface distortion, with ridges running perpendicular to the extrusion direction Rapid acceleration of the surface layers of the extrudate when the polymer exits the die. This can be cansed by too high extrusion velocity, or too high a extrusion temperature... 

The extent to which die-wall slip is involved can be assessed by using dies of different lengths and diameters. An important characteristic that can be observed in the extrudate is its quality in terms of surface structure. Harrison, Newton, and Rowe, have shown how this can vary from a smooth, regular surface via a rough, shark-skinned extrudate. There is obvious need to prevent this phenomenon if extrudate of the correct quality is to be produced. The occurrence of the surface defects is associated with both the composition of the material and the operating conditions (e.g., die length and diameter and the rate of... 

Processing aids are often called on to solve problems having to do with poor melt stability ("shark-skinning") or with stray material buildup on extruder surfaces, which causes excessively long start-up times or downtimes. 

Melt fracture is generally defined as surface roughness on the extrudate,but may include distortion of the entire body of the extrudate (Fig. 2.28). It is most often observed in high-pressure extrusions, like blown film. The appearances of various forms of melt fracture have led to names such as shark skin, orange peel, bamboo, and ripple. Unlike die swell, which is expected and accounted for in the die design, melt fracture should be avoided entirely. This defect is the result of incorrect die design, improper processing conditions, and/or poorly matched material properties. 

Sharkskin shark- skin (1851) n. (1) An irregularity of the surface of an extrudate in the form of finely spaced sharp ridges perpendicular to the extrusion direction, believed to be caused by a relaxation effect on the melt at the die exit. (2) A hard-finished, twill fabric, woolen or worsted, made of... 

It is not a surface defect like shark skin, but is associated with the entire body of the molten extrudate. However, many workers do not distinguish between shark skin and melt fracture, but lump all these flow instabilities together under the term melt fracture. There is a large amount of literature on the subject of melt fracture (e.g., [152-164]). Despite the large number of studies on melt fracture, there is no clear... 

The mechanism of shark skin is postulated to be caused by the rapid acceleration of the surface layers of the extrudate when the polymer leaves the die. If the stretching rate is too high, the surface layer of the polymer can actually fail and form the char-... 

Melt fracture was discussed in Section 7.5.3.2. It manifests itself as extrudate surface roughness, shark skin, orange peel, and other distortions. Melt fracture can be reduced or eliminated by ... 

Figure 8 Scanning electron micrographs showing an example of (A) smooth extrudate and (B) extrudate having surface impairment, or shark-skinning.

Rheology of the molten fluoropolymers is of critical importance in the extrusion of these polymers. Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin in flow exceeds the critical velocity, the point where the melt strength of the polymer is surpassed by internal stresses. The critical shear rate of most fluoropolymers is usually much lower than most thermoplastics. Parts extruded in a process that exceeds the critical shear rate have poor quality. Typical symptoms of melt-fracture include a rough surface shark skinning), and frosty or cloudy surface. Section 7.9 provides detailed information about the rheology of fluoropolymers. 

Extrusion outputs are sometimes limited by flow instabilities producing rough surfaces (shark skin) or periodic fluctuation, but the cooling of thick extrudates is often the major limiting factor because of the low thermal conductivity of PE. 

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