Melt shear effect

Due to the plastic s viscosity, a melt shear effect is developed throughout the process. This shear is of prime importance between the calender rolls. The calender forms the web as a continuous extrusion between the rolls (Chapter 1). Unlike when processing just through a conventional extrusion line, the plastic mass cannot be confined when being calendered. Because of the lack of confinement, the shear effect and a broad melt band are essential aspects of calendering. 

A study was made of nucleation in PE foams produced in a counter-rotating extruder using talc powders and masterbatches as nucleating agents and CFG and HCFC blowing agents. The results indicated the importance of shear force in nucleation, as proposed in the lump cavity nucleation model and demonstrated by melt temperature effects observed in these experiments. It was shown that shear enhancement via lump break-up was not a vital mechanism in nucleation. 6 refs. 

Le Meins JF, Moldenaers P, Mewis J (2002) Suspensions in polymer melts. 1. Effect of particle size on the shear flow behavior. Ind Eng Chem Res 41 6297-6304... 

Possible changes in the molecular structure through a reduction of the chain length or degradation are not discussed here. The factors responsible for this are residence time in the barrel, melt temperature, and an intense shear effect in the runner system during injection. 

An extrudate jet swells owing to normal stresses generated in a moving melt (Weissenberg effect) under shear stresses (71,74). Sheared flow of polymer melts under shearing forces is accompanied by forced changes in macromolecule conformations as compared with the equilibrium condition. This results in creation... 

A wide variety of plastics can be used although about 80 percent is PVC and 15 percent ABS. (Calendering consumes about 6 percent of total plastics consumption for all processes.) Other plastics used are HOPE, PP, and styrenes. The basic plastic limitation of the calendering process is the need to have a sufficiently broad melt index to allow a heat range for the process. This permits the material to have a relatively high viscosity in the banks of the calender (banks indicating where two rolls meet, or the nip of the rolls). As a result of the viscosity, a shear effect can be developed throughout the process, and especially between the calender rolls. Thus, the calender forms... 

Figure 58 Schematic illustration of the occurrence of melt fracture (a) at die walls due to shear effect and (b) at die entrance due to elongational effects.

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