Melt-flow oscillation

Scorim process See injection-molding 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.]...

Molecular Mechanism for Oscillating Capillary Melt Flow.259... 

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. 

Evidence for wall slip was suggested over thirty years ago [9,32,63]. One of the first attempts at a slip mechanism was the performance of a Mooney analysis by Blyler and Hart [32]. Working in the condition of constant pressure, they explicitly pointed out melt slip at or near the wall of the capillary as the cause of flow discontinuity. On the other hand, they continued to insist that bulk elastic properties of the polymer melt are responsible for the flow breakdown on the basis that the critical stress for the flow discontinuity transition was found to be quite insensitive to molecular weight. Lack of an explicit interfacial mechanism for slip prevented Blyler and Hart from generating a satisfactory explanation for the flow oscillation observed under a constant piston speed. 

One leading explanation attributes the anomalous melt flow behavior (i.e., flow discontinuity and oscillation) to constitutive instabilities [65]. In other words, the anomalies would be constitutive in nature and non-interfacial in origin. Such an opinion has not only been expressed phenomenologically by Tordella [9b] and many other rheologists but found support from several theoretical studies [65-67]. However, these theories only attempt to describe inherent bulk flow behavior. Thus, a connection between the anomalous flow phenomena and constitutive instabilities was often explored without any account for possible molecular processes in the melt/wall interfacial region. 

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]. 

As discussed in more detail below, recent experiments convincingly showed that the flow oscillation in capillary extrusion of LPE is interfacial in nature due to a reversible coil-stretch transition at the melt/die wall boundary. Pressure oscillation phenomenon has also been reported in extrusion of other polymer melts. In particular, there are well-defined oscillations in controlled-rate capillary flow of PB that were found to arise from the same interfacial molecular instability [62]. 

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. 

Assuming that the melt flow is laminar, its flow rate through a multilayer filter does not depend on time. But in the case when a molten metal contains dispersed particles with a size less than the section of the channel, the flow rate becomes dependent on time mainly due to the adhesion of the particles to the channel walls. With this, those particles which have the size larger than that of the capillary channel section are retained at the entrance of the filter in a form of a cake which increases the apparent length of the channel and decreases the active surface of the filter. The input of intensive ultrasonic oscillations in the mode of developed cavitation results in the appearance of active acoustic streams near the filter surface and in washing-out the cake. In the ideal case, the value of the flow rate through the filter can be sustained constant for a sufficiently long period of fine filtration due to the action of acoustic cavitation and streams. 

To produce a high quality surface (10-12), it is necessary to avoid a discontinuous flow of the melt through the profiling channel. A high quality profile with a polished surface is produced with degradation of the polymer surface as a consequence of the effect of the magnetostrictive transducer. The ultrasonic oscillations reduce the toughness of the polymer and the resistance of the wall affect the melt flow at the channel boundary. 

The effect of compatabilizer (MAPE) to eliminate discontinuity in the flow curve of the thermoplastic vulcanized (PE/PB) was studied. Various concentration of the compatabilizer (0, 2, 4, 6, 8 and 10 wt/wt%) were tested. It was shown that the complete elimination of discontinuity which caused by the pressure oscillation inside the die during the polymer melt flow. The presence of the MAPE will act as compatabilizer between the two polymer chain and both the 8% and 10% can achieved the complete chain interaction and the blend will be completely miscible and act as one phase, while the 8% cannot offer the sufficient adhesion forces between the polymer and the die... 

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