Extrusion extruder output rate

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. 

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). 

Very high speed single screw extruders have been commercially available since about 2005-2010. This is one of the most significant developments in single screw extrusion over the past several decades. These relatively small extruders (50-75 mm) that run at screw speeds as high as 1,000 to 1,500 rpm and achieve output rate about an order of magnitude above the rate of conventional extruders. 

The supramolecular order for a part of the polymers was discussed specially at the extrusion of the polyethylene in melted state. Rudin and Schreiber [769] had been found that the output rate of the extrudate modified in time, even if the mechanical stress remains constant. 

Figure 121 Output rate and melt temperature versus screw speed for 38 mm extruder fitted with the A2-B2 mixer. Reproduced with permission from G.M. Gale, Distributive Mixing in Plastics Extrusion, Rapra Technology Members Report No.46, Rapra Technology, Shrewsbury, UK, 1980, Figure 13. 1980, Rapra Technology)...

In product extrusion it may also be necessary to assess how much better or worse mixing has become as a resnlt of making changes to the operating conditions or to the machine itself. For example, raising screw speed to increase output rate may delay completion of melting such that the mixing efficiency is impaired to the point where the extrudate is imacceptable. 

In blown film trials comparing a barrier screw and an energy transfer screw, and VB energy transfer screw, the latter outperformed the others in the extrusion of LLDPE and LLDPE blends with regards to both output rate and melt temperature. The extruder screw was 88.9 mm diameter, with 30 1 L/D. 

Surging n Any irregularity in the output rate of an extruder. Extrusion engineers have recognized two kinds long- and short-period surging. Long-period... 

Film extruders who routinely process LDPE must often make modifications to equipment, or, at the very least, the temperature profile, to produce films with good physical properties. Melt fracture, or sharkskin, can be a problem at high die shear rates. This can be eliminated by raising extrusion and die temperatures, by increasing the opening of the die to reduce shear, or by using polymer processing aids. The key extrusion parameters that affect final film properties are blow-up ratio (the ratio of the bubble diameter to the die diameter), die gap, output rate and so forth. 

The MWD has an important effect on the output rate, extruder torque, and pressure development in the extruder. Polyolefins with a narrow MWD have a higher output rate, extruder torque, and pressure development during extrusion than those with broad MWD. The die length has no significant effect on film structure and on factors such as tensile, tear, and impact properties. 

---