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Extruder-melt bambooing

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. 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.
The form taken by the distortion varies between types of polymer but generally it is helical. With polyolefins a feature resembling the thread of a screw may appear, and with polystyrene the extrudate may form a spiral with other melts, ripples or repetitive kinks like bamboo may be seen. For all melts, at rates well above the critical point, the helical nature becomes obscured by severe distortion which looks quite random. [Pg.168]

Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein... Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein...
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. ... [Pg.269]

With the increase of flow velocity, polymer melt extruded in the tube will become unstable due to the stick-slip transition near the tube wall, which makes the extrudate shows wave-like, bamboo-Uke, or spiral-like distortions. All these phenomena are known as melt-broken phenomena. In these cases, the shear rate suddenly rises, as illustrated in Fig. 7.17, thus this behavior is also called capillary-jet phenomenon. The string-Uke shark-skin phenomenon upon the extrusion of polyethylene melt can be attributed to the intermittent stick-slip transition near the tube wall of the exit (Wang 1999). [Pg.143]

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. [Pg.55]

Melt fracture is a severe distortion of the extrudate, which can take many different forms spiraling, bambooing, regular ripple, random fracture, etc. see Fig. 7.118. [Pg.432]

Melt fracture Severe distortion of the extrudate leading to defective appearances of spiraling, bambooing, ripples, and random fractures Occurs when a critical shear stress is exceeded in the die. Causes can be too small, an entry angle in the die or pressure oscillations... [Pg.287]

Sometimes confused with melt fracture (also known as elastic turbulence, bambooing, and distortion) is sharkskin. In thermoplastics this occurs as tiny transverse ridges on the surface of an extrudate. It is found to occur above a critical linear output rate. The writer has suggested that this must be due to tearing of weak elastic melts as the surface of the extrudate accelerates in velocity, relative to the centre, as it leaves the die. Most formal studies have been made on thermoplastics but the phenomenon, or something very much like it, is observed with rubbers. [Pg.96]

There are basically five types of melt fracture sharkskin, ripple, bamboo, wavy, and severe. These types of melt fracture are shown in Figures 7.8, 7.9, and 7.10. Sharkskin is shown in Figure 7.8 for a LLDPE. At the lowest apparent shear rate the extrudate is smooth but at ya = 112 s, the... [Pg.204]

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. [Pg.235]


See other pages where Extruder-melt bambooing is mentioned: [Pg.106]    [Pg.249]    [Pg.350]    [Pg.352]    [Pg.106]    [Pg.249]    [Pg.350]    [Pg.352]    [Pg.223]    [Pg.227]    [Pg.250]    [Pg.260]    [Pg.261]    [Pg.264]    [Pg.270]    [Pg.271]    [Pg.223]    [Pg.320]    [Pg.822]    [Pg.223]    [Pg.294]   
See also in sourсe #XX -- [ Pg.249 ]




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