Extrusion shearing

Holay, S. H. and Harper, J. M. (1982). Influence of the extrusion shear environment on plant protein texturization. J. Food Sci. 47,1869-1874. 

Screw speed is another variable that impacts extrudate quality via residence time. The effect of screw speed on extrusion shear stress is given (22) by the expression... 

Sharpe, I. D., Ismail, A. F., and Shilton, S. J. (1999), A study of extrusion shear and forced convection residence time in the spinning of polysulfone hollow fiber membranes for gas separation, Sep. Purif. Technol, 17,101-109. 

Crystallization of polymers takes place usually under inhomogeneous conditions as those related to the presence of temperature gradients or mechanical processing of the melt, such as extrusion, shearing or injection, which largely influence the kinetics. 

Braga RR, Ballester RY, Daronch M. Influence of time and adhesive system on the extrusion shear strength between feldspathic porcelain and bovine dentin. Dental Materials 2000 16(4) 303-310. 

Usually we refer to ductile fracture, underlining the role of plastic deformation which precedes failure of the part. The fracture occurs on the surfaces, for instance, in tensile test or in the inner (central bursting in extrusion). Shear stress could play a relevant role as it occurs in tension or compression tests in which the failure follows the surfaces characterized by the larger shear stress (Fig. 11). 

R. R. Braga, R. Y. Ballester, and M. Datonch, Influence of Time and Adhesive System on the Extrusion Shear Strength between Feldspathic Porcelain and Bovine Dentin, Dent. Mater., 16, 303-10 (2000). 

The life cycle of a polyethylene resin begins with polymerization in a reactor. When the reaction is terminated, molten or powdered product is passed through a melt blending extruder where antioxidants, processing stabilizers, processing aids and other additives are added. Thermal exposure and extrusion shear rates (< 500 s ) are generally kept low at... 

Extrusion. Like other thermoplastics. Teflon PEA resin exhibits melt fracture above certain critical shear rates. Eor example, samples at 372°C and 5-kg load show the following behavior ... 

Fig. 8. Sheet extrusion A, die inlet B, die C, three-roll finisher D, support roUers E, edge-trim cutter F, puU roUs and G, saw or shear (15).

Thus it is found that extrusion die swell, when compared at equivalent shear rates, tends to go down with an increase in temperature and go up with molecular weight. As shear rates are increased, polymer uncoiling is increased and die swell increases Figure 8.9). A point is reached, however, where the die swell no longer increases and in fact it can be argued that it goes down. At this point it is... 

In addition to elastic turbulence (characterised by helical deformation) another phenomenon known as sharkskin may be observed. This consists of a number of ridges transverse to the extrusion direction which are often just barely discernible to the naked eye. These often appear at lower shear rates than the critical shear rate for elastic turbulence and seem more related to the linear extrudate output rate, suggesting that the phenomenon may be due to some form of slip-stick at the die exit. It appears to be temperature dependent (in a complex manner) and is worse with polymers of narrow molecular weight distribution. 

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

The strong dependence of apparent viscosity on shear rate. This necessitates particular care in the design of complex extrusion dies. 

Structurally viscous grades are based on branched polymers (branching being effected by the use of tri- or higher functional phenols). These polymers exhibit a sharp decrease in viscosity with increasing shear rate which makes them particularly suitable for extrusion and blow moulding and also, it is claimed, in reducing drip in case of fire. 

Using this expression it is possible to make an approximation for the temperature rise of the fluid during extrusion through a die. If it is assumed that all the work is changed into shear heating and that all the heat is taken up evenly by the polymer, then the work done may be equated to the temperature rise in the polymer. 

Kalyan et al. [56] have also studied the effect of alpha-olefin comonomers on the rheological properties and processing of LLDPE. The characteristics of the resins are shown in Table 2. It is found that 1-octene-based LLDPE has the lowest shear viscosity as compared to 1-butene- and 1-hexene-based polymers (Fig. 9). Decrease in power consumption, pressure before the die, temperature in the die, and increase in output has also been found according to shear viscosities of the polymers during tubular film extrusion. 

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