Extrusion output rate

Die adjustments do not significantly influence the extrusion rate the lip opening, restrictor bar clearance, die heat, and overall die design have small to moderate effects on the back pressure. However, the extrusion rate is only slightly affected by substantial changes in back pressure. Thus attempts to open the lip or increase the die heat to increase the extrusion output rate can be exercises in futility, as well as the source of many additional problems. 

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. 

During extrusion blow moulding of 60 nun diameter bottles the extruder output rate is 46 X 10 m /s. If the die diameter is 30 mm and the die gap is 1.5 mm calculate the wall thickness of the bottles which are produced. The flow curves in Fig. 5.3 should be used. 

The scale-up factors depend on the specific event being scaled-up in extrusion. The cube rule for mixing (23) states that at constant screw speed, output and power consumption increase with ct when H/D ratio is constant. The square-root rule for conveying (24) of material states that when channel depth is increased and screw speed decreased with x/d, the output rate increases with S, while power consumption increases by One could obtain... 

All experimental results reported in this paper were obtained with molded samples for a theoretical reason. These experiments should be repeated with pellet samples so that the experimental results could be more directly applied to actual extrusion operations. The energy efficiency will become an important factor in the future in designing a new extruder for a given output rate as energy cost increases. 

The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure developed in the extruder system is largely a function of die resistance and dependent on die geometry and melt viscosity. Extrusion pressures are lower than those encountered in injection molding. They are typically 500 to 5000 psi (3.5 to 35 MPa). In extreme cases, extrusion pressures may rise as high as 10,000 psi (69 MPa). Variants on the single screw include the barrier or melt extraction screw and the vented screw (Chapter 3). 

Rod extrusion is a slow process because the plastic is a poor conductor of heat. Output rate may be about 9 to 23 kg/h (20 to 50 Ib/h). After extrusion, these rods are usually annealed for stress relief and stabilization of dimensions. If required by design, they can be centerless ground to final size. 

The rheological properties of molten polymers influence many aspects of processing. In injection molding and extrusion they affect screw pumping efficiency, generation of mechanical heat, and die pressure (and hence output rate). They also influence die and mold weld lines, surface finish, product dimensions, etc. 

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