Extrudate roughness Melt fracture

A second consequence of the elastic behaviour of the melt is the occurrence of melt fracture. At high rates of extrusion the elastic deformation of the melt may become so high that it locally breaks, resulting in an irregularly shaped extrudate or in surface roughness. 

Varying Plastic Sources—Which to Choose for Composite Materials Movement of different fluid layers of the plastic-based composites sometimes has a different pattern with and without added regrind. It is often described as a difference in shear rate at the same viscosity. This might lead to a more narrow window for a proper flowability of the hot melt before a melt fracture is observed and result to a roughness, sharkskin effect, or other kinds of defective extruded profiles at conditions that would normally give good quality products. 

The processing window for the first four HOPE materials was wider in terms of extrusion speed, stress, and temperature. The last four HDPE materials were closer to melt fracture and the resulting sharkskin, roughness, tearing, and rupture of the extruded shape. The corresponding composite materials formulated with the last four resins had a lower throughput. 

ROUGH EXTRUDATE - MELT FRACTURE -HIGH SHEAR STRESS IN SPINNERET —... 

Thickness variation in extrudates can be caused by many trivial factors such as inhomogeneous materials, fluctuation in the feeding rate of the extruder or in the melt temperature, surging of the extruder and lack of mechanical stability and stiffness of the equipment. A more fundamental reason for surface roughness or thickness variations is melt fracture, which appears suddenly as soon as a certain critical extrusion rate is exceeded. 

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. 

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 ... 

For primary insulation or cable jackets, high production rates are achieved by extruding a tube of resin with a larger internal diameter than the base wire and a thicker wall than the final insulation. The tube is then drawn down to the desired size. An operating temperature of 315-400°C is preferred, depending on holdup time. The surface roughness caused by melt fracture determines the upper limit of production rates under specific extrusion conditions (83,84). Corrosion-resistant metals should be used for all parts of the extrusion equipment that come in contact with the molten polymer (85). 

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. 

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. 

The results of melt fracture and slip/stick are most commonly observed during extrusion, the effects being manifest as a nonuniform extrudate. The nonuniformity may take the form of periodic fluctuations of the cross-sectional area (sometimes referred to as bamboo ), helices, rough, highly erratic extrudate profiles, and, in extreme cases, fragmentation of the extrudate. Some of the manifestations of slip/stick and melt fracture are illustrated schematically in Figure 59. 

Figure 59 Extrudate profiles before and after the onset of sl.p/stick and melt fracture, (a) Smooth extrudate below the critical shear rate (h) periodic fluctuations of extrudate profile (bamboo) (c) helical extrudate (d) rough, irregular extrudate (e) fragmented extrudate.

Land fracture is a fine surface roughness on the extruded product. It is caused by friction between the melt and the wall (land) of the... 

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