Extruder melt-flow oscillation

Fig. 12.17 Scanning electron micrograph of HDPE extruded at a shear rate slightly lower than the oscillation region, showing sharkskin. [Reprinted by permission from N. Bergem, Visualization Studies of Polymer Melt Flow Anomalies in Extruders, Proceedings of the Seventh International Congress on Rheology, Gothenberg, Sweden, 1976, p. 50.]...

Polymer melt rheologists constantly applied the notion of wall slip in describing a variety of polymer melt flow anomalies [ 10,11,14]. However, no explicit knowledge of a molecular mechanism for and microscopic origin of wall slip had been available. Therefore, for 40 years no explanation was given about why wall slip was pertinent and how it produced such melt flow anomalies as flow oscillations and sharkskin-like extrudate distortion. One objective of current research around the world is to explore the molecular nature of wall slip and establish a correlation between wall slip and some of the anomalous melt flow phenomena. 

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

Extrudate distortion has been viewed as explicit evidence for melt flow instabilities or melt fracture. This is another calamitously misleading assertion in the massive literature of over 3000 papers on the subject. Because extrudate distortion was also observed even without any signature of slip, it was concluded by Blyler and Hart [32] that the slip process is not an essential part of the flow instability. This dilemma, that the flow anomalies including flow oscillation cannot be accounted for in terms of either a constitutive instability or interfacial slip mechanism, has persisted until very recently. Denn coined this plight the paradox [10b]. 

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. ... 

We have surveyed the most recent progress and presented a new molecular level understanding of melt flow instabilities and wall slip. This article can at best be regarded as a partial review because it advocates the molecular pictures emerging from our own work over the past few years [27-29,57,62,69]. Several results from many previous and current workers have been discussed to help illustrate, formulate and verify our own viewpoints. In our opinion, the emerging explicit molecular mechanisms have for the first time provided a unified and satisfactory understanding of the two major classes of interfacial melt flow instability phenomena (a) sharkskin-like extrudate distortion and (b) stick-slip (flow discontinuity) transition and oscillating flow. 

For continuously extruded products, such as sheet and profile extrusions, thickness uniformity is a key quality factor, and it is directly linked to flow oscillations or flow disturbances at the die. Many sources of flow oscillations in the extrusion system are identified, and methods are shown on how to configure the melt delivery system to minimize their transmission to the die. These methods are illustrated with a model based on the theory of hydraulic transients. 

Thickness uniformity is a major concern for many continuously extruded products, such as sheet and profile extrusions. Any flow oscillation or disturbance at the die will manifest itself in a comparable thickness variation in the product. Therefore, diagnosing and eliminating flow disturbances in the melt delivery system is key to good thickness uniformity. This first involves identifying the sources of flow disturbances so that they may be eliminated or minimized. Secondly, a method is shown on how to design the delivery system to minimize the transmission of flow disturbances to the product. 

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...

Continuum Depiction Local oscillation of the melt-wall boundary condition in the exit region causes peturbations on the exit stress and die swell Oscillation of the overall stress due to unstable boundary condition produces cycles of melt compression and decompression in the barrel and fluctuations in the extrude swell The extrudate distortion arises from formation of secondary flow (vortices) in the barrel due to the strong converging flow near the die entry... 

A specific pattern of sharkskin depends on polymer chemistry, molecular weight and MWD, and polymer-wall surface interaction [50, 51]. With a further increase of the shear rate, a spurt flow can be observed that results from pressure oscillations in the extruder [52]. At higher shear rates, gross melt fracture eventually occurs, with the extrudate coming out with an irregular pattern [45],... 

Mathematical description of the process of polymer melting in the extrusion channel is complex when ultrasound is used. The description requires firstly, consideration of the mass flow of the polymer, knowledge of the flow characteristics of the melt, the temperature and pressure of extrusion, sizes of the channel and frequency of ultrasonic oscillations. Secondly, coefficient of swelling of the extrudate, effective viscosity of polymer, pressure of melt, and frequency of oscillations. 

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