Extrusion instabilities

The limiting factor in the extrusion rate of polymeric fluids is the onset of a low Reynolds number instability called melt fracture. The onset of melt fracture leads to varying degrees of imperfections which may only affect the clarity of a material on one hand, while on the other may be so severe as to reduce significantly the physical properties. We first discuss the nature and origin of melt fracture and then what can be done to alleviate or at least mitigate the problem. 

In order to reduce the detrimental effect of melt fracture through die design or polymer modification, it is important to know the origin of melt fractiue. The major sources for melt fracture are the die entry region, the die land, and the die exit. For a polymer such as LDPE fracture originates at 

FIGURE 7.7 The ratio of entrance pressure drop to wall shear stress versus apparent shear rate, Ys- ( ) PP (D) PS (o) LDPE ( +) HDPE ( ) 2.5% PIB in mineral oil (X) 10% PIB in decaUn (A) NBS-OB oil. (Reprinted with permission of the publisher from White, 1973.) 

FIGURE 7.8 LLDPE extrudates obtained from a capillary at different apparent shear rates, Yu - from left to right the values of are 37, 112, 750, and 2250 s . (Data from Moynihan, 1990.) 

Variations in extruder performance is perhaps the most frequent problem encountered in extrusion. One possible reason for the frequent occurrence of instabilities is the fact that they can have a large number of causes, some of which are  

Proper instrumentation is vitaiiy important to be abie to diagnose a problem quickly and accurately as discussed in Chapter 1. A prerequisite for stabie extrusion is a good extruder drive, good temperature controi system, good take-up device, and 

The extruder drive should be able to hold the screw speed constant to about 0.1% or better the same holds true for the take-up device. However, this is not always the case on actual extrusion lines. The extruder should be equipped with some type of proportioning temperature control, preferably a PID-type control or better. On-off temperature control is inappropriate for most extrusion operations. 

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M. M. Denn, Extrusion Instabilities and Wall Slip, Annu. Rev Fluid Mech., 33, 265-287 (2001). 

In the film blowing process where a continuous stable parison is blown from an annular die, it is crucial that the molten polymer exhibits certain elastic extensional properties and it is here that the viscoelastic nature of the polymer is beneficial.If, however, the manufacturer is concerned with profile and surface finish of an extrudate, viscoelastic effects of the polymer may well present difficulties. Both die swell and most polymer extrusion instabilities are linked to viscoelastic effects and as such different levels of viscoelasticity give rise to different extrusion characteristics. 

In the last twenty years, major advances in the characterisation of polymer melt viscoelasticity has taken place and in addition applied mathematicians have produced numerical codes that enable viscoelastic fluids to be modelled for processing conditions. Within the last few years it has become possible to reasonably accurately predict the way in which a viscoelastic polymer will flow into, within and out of an extrusion die. The accurate prediction of die swell is nearly possible and advances are being made to predict the onset of extrusion instabilities. 

Extrusion instabilities] screw speed too high/screw temperature too high/barrel temperature at delivery end too high/channel depth too high in the metering sec-... 

In blends, fluoropolymers are used in small quantities to enhance throughput, reduce the frictional properties, and increase the wear resistance. Blends comprising 0.3-50 wt% of a low molecular weight PTFE (T , < 350 °C) with engineering resin showed improved antifriction properties (Asai et al. 1991). LLDPE generally exhibits sharkskin melt fracture, but the use of fluoropolymer additives, such as the copolymer of vinyUdene fluoride and hexafluoropropylene, can help to eliminate the extrusion instability (Hatzikiriakos and Migler 2005). 

Denn, M. (2001). Extrusion instabilities and wall slip. Annu. Rev. Fluid Mech. 33, 265. 

The dynamic behavior of pressure transducers is of particular interest in the analysis of extrusion instabilities. The procedure for dynamic testing described in VDMA 24456 was used to test various commercial pressure transducers. Puetz [4] published some dynamic test data for different pressure transducers (see Table 4.2). 

Dekker [3] studied the effect of various flight geometries on solids conveying performance. He proposed that many extrusion instabilities might be due to internal deformation of the solid bed. Internal deformation is more likely to occur when the internal coefficient of friction of the polymer particles is low. Spherical particles tend to have a lower internal coefficient of friction than non-spherical (e.g. cylindrical) particles and are, therefore, more susceptible to internal solid bed deformation. This may explain the often observed difference in extrusion behavior between strand pelletized and die-face pelletized material. 

Various workers [68, 69] have classified extrusion instabilities based on the time frame in which they occur. The frequency of the instability is often an indication of the cause of the problem. Most of the earlier works distinguished only three or four types of instabilities based on the frequency. However, it is probably more appropriate to distinguish at least five types of instabilities ... 

There are many different causes of extrusion instabilities. Even though the mechanism of the instability is not always clear, the following measures often reduce extrusion instabilities ... 

Delaunay D, Le Bot PH, Fulchiron R, Luye JF, Regnier G (2000b) Nature of contact between polymer and mold in injection molding. Part II Influence of mold deflection on pressure history and shrinkage. Polym Eng Sci 40 1692-1700 Denn MM (2001) Extrusion instabilities and wall slip. Annu Rev Fluid Mech 33 265-287 Denn MM (2004) Fifty years of non-Newtonian fluid dynamics. AIChE J 50 2335-2345 Dinh SH, Armstrong RC (1984) A rheological equation of state for semi-concentrated fiber suspensions. J Rheol 28 207-227... 

The question then is what might be the mechanism for an entry instability One possibility is a tensile failure in the extensional flow in the die entry. Figure 12.15 shows a section of an extrudate of an LLDPE that appears to have been ruptured cohesively and then welded back together, which would be an indication of tensile failure, but such data are limited and the connection to a flow instabihty is tenuous. (The flow was stable when this sample was extruded.) Another possibility follows from the observation that noticeable shp occurs at stresses close to, and perhaps just below, the onset of extrusion instabilities. As noted in Section 12.5, shp in a converging region would lead to curved streamlines, and curved streamlines in highly elastic liquids appear to drive flow instabUities. 

There is an overview of wall slip and extrusion instabilities in polymer melts, with extensive... 

There are extensive discussions of extrusion instabilities in the two Annual Review of Fluid Mechanics chapters cited at the beginning of this section. Comprehensive reviews that focus on the earlier literature are... 

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