Читать книгу The Rheology Handbook - Thomas Mezger - Страница 208
5.1.2.1.85.2.1.2Examples of the behavior of VE liquids in practice 5.1.2.1.9a) Die swell, or post-extrusion swelling effect (see Figure 5.3) 5.1.2.1.10Experiment 5.2: Extrusion, using a small toy extruder, producing spaghetti-like strands
Оглавление1 Die swell does not occur when extruding plasticine, (or wax). Immediately after the extrusion, the plasticine strands exhibit a stable form. Therefore, this material shows behavior of a (viscoelastic) solid.
Figure 5.3: Die swell of a viscoelastic material after an extrusion (with pressure p, diameter d of the die, and diameter increase Δd of the extrudate)
1 Silicone polymer (uncrosslinked PDMS) clearly displays post-extrusion die swelling. After a certain time when at rest, the extruded strands are slowly flowing and spreading, penetrating one another finally. Therefore, the silicone is showing behavior of a (highly viscous, viscoelastic) liquid.
Due to the high pressure in the extrusion die, the polymer molecules are deformed and oriented into shear direction. As soon as leaving the die, the molecules of a visco-elastic material are recoiling since they are in a stress-less state then. As a consequence, re-formation in shear direction occurs, and in order to compensate this, they are expanding into other directions, for example, right-angled to the direction of the extrusion. Therefore, the extrudate shows an increased diameter finally (see also Chapter 11.4.2.2b). Further information on die swell effects, including images, can be found in [5.3] [5.4] [ 5.5] [ 5.6].
