Extrusion swelling

Viscoelastic Measurement. A number of methods measure the various quantities that describe viscoelastic behavior. Some require expensive commercial rheometers, others depend on custom-made research instruments, and a few require only simple devices. Even qualitative observations can be useful in the case of polymer melts, paints, and resins, where elasticity may indicate an inferior batch or unusable formulation. For example, the extrusion swell of a material from a syringe can be observed with a microscope. The Weissenberg effect is seen in the separation of a cone and plate during viscosity measurements or the climbing of a resin up the stirrer shaft during polymerization or mixing. 

In a similar example (Fig. 27) based upon an SBR formula, extrusion swell is substituted in place of running temperature. 

Here, again, at fixed particle size, modulus is a function of both iodine number and structure, but extrusion swell is a function of structure only. The cross-hatched area is the quality control target area for that particular black to be in specification. 

Figure 27. Relationship of Iodine Number Void Volume, SBR Modulus and Extrusion Swell...

In the processing equipment of thermoplastics, many kinds of complicated flow configurations exist. The flow in a tapered die, for example, produces three components of deformation that due to flow from the reservoir into the die that due to telescopic shear within the die that to extensional flow within the die. These may be assumed separable and the separately calculated pressure drops may be added to give the total pressure drop. But in addition, each deformation mechanism contributes to post extrusion swelling. The components due to simple shear and extension at the die exit determine the swell ratio. 

The phenomenon of post-extrusion swelling or "ballooning" has been discussed in Chap. 15. It is related to the so-called Barus effect, according to which the diameter of polymer extrudates is larger than the capillary diameter when the melt is forced through an orifice. All materials with any degree of melt elasticity display this effect. The origin of the effect is related to the elastico-viscous nature of polymer melts, thus to Ni=Tu — T22. 

S17. SiEGLAFF, C. L. Rheological properties of polyvinyl chloride. Part I General flow properties. Part II Unstable flow. Part III Post extrusion swelling. SPE Trans. 4, 1 —138 (1964). 

Figure 9 is a plot of post-extrusion swelling versus residence time, as calculated from data given in Table V. As can be seen, the... 

With respect to Figure 7, it is clear that with short dies, the post-extrusion swelling includes the entrance effect contribution and the shear response in the viscometric region. Consequently, the extrudate swell not only depends upon the residence time within the die, but also upon the transit time within the entrance zone. Therefore, it is possible to define a correcte residence time, t, according to ... 

When plotting the extrudate swell versus the corrected residence time tj.j, an excellent fit is obtained for the four dies (x), as can be seen in Figure 10. This demonstrates that with the short dies of practice, the entrance converging flow region plays a very important role in post-extrusion swelling. 

Post-extrusion swelling in fact, is not really a processing problem in itself, as long as it remains constant since, in this case, one can always manage it to control extrudate dimensions. The real problem is the extrudate swell variations arising during the process. 

The above considerations explain why with the short dies of industrial practice, rubber extrudates frequently exhibit swelling variations which are actually compensated for by adjusting the takeaway conveyor speed. This is however not always satisfactory and the results presented here suggest that a suitable design of the entrance geometry could reduce post-extrusion swelling variations. 

The increasing volume of polymer production in the chemical industry and the wide range of items being manufactured require an increase in the capability for the production and quality of the products produced. It has been established that an imderlying problem is the distortions introduced as the melted polymer flows through the extrusion channels. Analysis of the extrusion heads, has allowed identification of certain parameters which can positively be controlled to improved performance reduction of the pressure required leading to a decrease in cost, increase of output of the flow, increase in the hardness of extrudate and decrease the post-extrusion swelling. 

The Weissenberg effect is clearly manifested in the increase in diameter of extruded profiles of a variety of molten polymers. The extrusion swelling (Figure 3.32), more commonly known as die swell, arises probably due to a combination of normal stress effects and a possible elastic recovery consequent to prior compression before the melt or liquid enters the die. 

Optical Properties of Blown Films and Extrusion Swelling and MI if Fresh and Processed Materials... 

TABLE 22 Optical properties of blown films and extrusion swelling and of fresh and processed materials (LDPE)... 

Bagley E B, Storey S H and West D C (1963), Post extrusion swelling of polyethylene , Journal of Applied Polymer Science, 7,1661-1672. 

Cure Rate 150°C, T90 minutes Mooney Scorch 121°C, T5 minutes Viscosity, MLIOO Garvey Die Extrusion Swell Rating... 

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