The Elastic Melt Extruder

The elastic-melt extruder makes use of the Weissenberg or rod climbing effect—which is observed when an elastic fluid is sheared or rotated inside a container by a rod. Because of viscoelasticity, the fluid climbs the rod. 

The elastic melt extruder was developed in the late 1950s by Maxwell and Scalora [40, 41]. The extruder makes use of the viscoelastic, in particular the elastic, properties of polymer melts. When a viscoelastic fluid is exposed to a shearing deformation, normal stresses will develop in the fluid that are not equal in all directions, as opposed to a purely viscous fluid. In the elastic melt extruder, the polymer is sheared between two plates, one stationary and one rotating see Fig. 2.19. 

The detailed study by Fritz [44] concluded that transport by normal stresses only could be as much as two orders of magnitude lower than a corresponding system with forced feed. In addition, substantial temperature gradients developed in the polymer, causing substantial degradation in high molecular weight polyolefins. The scant market acceptance of the elastic melt extruder would tend to confirm Fritz s conclusions. 

Several modifications have been proposed to improve the performance of the elastic melt extruder. Fritz [43] suggested incorporation of spiral grooves to improve the pressure generating capability, essentially combining the elastic melt extruder and the spiral disk extruder into one machine. In Russia [47], several modifications were made to the design of the elastic melt extruder. One of those combined a screw extruder with the elastic melt extruder to eliminate the feeding and plasticating problem. Despite all of these activities, the elastic melt extruder has not been able to acquire a position of importance in the extrusion industry. 

Extruder, Hydrodynamic n A device similar to the elastic-melt extruder (see preceding entry) in that the plastic pellets are sheared between relatively rotating disks. However, the disks in a hydrodynamic extruder are shaped to provide positive driving force, whatever the properties of the melt. It, too, can provide efficient melting while developing little pressure. 

During the period in which these rather academic studies were going on another development was taking place that would have a profound affect on the future course of my research. In 1958 we began to wonder about the utility of the results of the studies on polymer melts. How could we apply the principles being developed The result was the elastic melt extruder Here... 

Another area of current research deals with combining the earlier work on production of polymer blends in the elastic melt extruder and the observation of the maximm in melt elasticity as a fimction of composition as fo md in the orthogonal rheometer studies on blends. It would be expected that as the melt elasticity increased the forces required for melt orientation would also increase. This has been confirmed in current studies of melt tension in fiber spinning experiments. The important point is that the composition of the blend can be used to control the molecular orientation. 

Scaling Up the Elastic Melt Extruder," S.P.E. JouAnoZ 26, June 1970. 

During extrusion of polymer melts with high throughputs, the elastic melt properties can also lead to elastic instabilities which can result in surface distortions of the extrudate. One example are wavy distortions also described as sharkskin. Depending on the polymer, this can also lead to helical extrudate structures (stick-slip effect) or to very irregular extrudate structures (melt fracture) at even higher throughput rates [10]. 

There are a number of machines that do not utilize Archimedean screw for transport of the material, but still fall within the category of continuous extruders. Sometimes these machines are referred to as screwless extruders or elastic melt extruder. These machines employ disks or drums to melt, mix, pump, and extrude the material and are based on viscous drag transport principle. They are not widely used in industry as forming machines, and will not be discussed here. 

Disk extruders. There are several types of screwless extruders. These machines employ a disk or a drum to plasticate, mix, and extrude. Most designs are based on viscous drag flow. They include Maxwell s elastic melt extruder, Westover s stepped disk and drum extruders, and Diskpack [39]. The Diskpack extruder has the capability of performing all the elementary steps of plastics processing by combination of differently shaped rotating... 

Most high polymers exhibit die swell on extrusion. Thermotropic polymers do not, and indeed values of the ratio of extrudate diameter to capillary diameter as low as 0.9 have been observed 104). Since die swell is associated with viscoelasticity in the melt, it is interesting to note that even in the isotropic phase little or no elastic behaviour is observed in a thermotropic polymer 105),... 

So far in this chapter we have looked into the viscous phenomena associated with the flow of polymer melts in capillaries. We now turn to the phenomena that are related to melt elasticity, namely (a) swelling of polymer melt extrudates (b) large pressure drops at the capillary entrance, compared to those encountered in the flow of Newtonian fluids and (c) capillary flow instabilities accompanied by extmdate defects, commonly referred to as 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. 

Two examples may be used to illustrate the complexity of problems of this kind. When film is made by extrusion followed by casting on chill rolls there can be a tendency for the extruded web to shrink inwards towards the centre of the rolls—the phenomenon known as neck-in . The edge of film concerned becomes thicker than the rest. It has been found that more elastic melts, capable of keeping a tension in the direction of extrusion, are less liable to exhibit this fault. 

The melt parison is extruded from an annular die. The wall thickness of the parison depends both on the annular gap setting and on the shear rate of the melt in the die. The melt will swell after the parison exits from the die and the die swell Increases with increasing shear rate. The die swell is also a function of the temperature, the type of polymer, and its elastic melt properties. Usually the shear rate in the die varies from 10 to as high as 700 s . As a rule of thumb one uses a die land length 8 times the annular gap. The parison should normally not be blown up beyond 3 1 (i.e., bottle parison diameter). 

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