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Frankland J.,Frankland Plastics Consulting LLC
Plastics Technology | Year: 2013

Most extrusion screw designers often do not design the feed section with the same rigor as other screw sections. Optimum channel depth is largely a function of the solid properties of the polymer, such as bulk density, frictional characteristics, and solids flow properties. However, the melt viscosity of the polymer also enters into the depth considerations the lower the melt viscosity, the greater the feed depth required to maximize melting rate. The compressibility of the resin infeed material also has to be considered. So compression ratio has more than one meaning, and consequently is not a positive indicator for feed depth. The best way to evaluate the solid properties is by relying on your experience with each polymer, along with some simple tests. Measuring the angle of repose gives a good indication of the internal friction coefficient of the material.


Frankland J.,Frankland Plastics Consulting LLC
Plastics Technology | Year: 2013

Jim Frankland shares his views on the cause of severe screw wear problems faced in recycling process. The localized forces acting in a single-screw extruder are enormous when the polymer is still in solid form. The screw is alternately full and partially full at different locations until compaction is fully completed farther down the screw when it is fed inconsistently, leading to severe wear problems. Areas that are full will develop pressure from the wedge action, as solid polymer does not slide easily on the barrel wall. That results in pressure in the polymer and an unbalanced force on the screw. This pushes the screw in the opposite direction and presses it against the barrel wall with enormous force due to the small resisting area of the screw flight. Wedging is found to be one of the main causes of screw wear problems faced in recycling processes. There is also a distinct difference between the wear caused by erratic feeding and the wear due to insufficient melting capability.


Frankland J.,Frankland Plastics Consulting LLC
Plastics Technology | Year: 2014

Die design is basically a procedure where the internal die configuration is shaped to deliver polymer from the entrance to the exit with the same pressure drop. That provides a balanced flow where all of the polymer is exiting at the same velocity and closely duplicates the shape of the die orifice. That task is complicated by the fact that polymers exhibit different degrees of non-Newtonian (shear-thinning) behavior when exposed to different shear rates in the die's varying cross-section. For example, having to make an annular die non-concentric to equalize the desired distribution suggests there is a flow issue. Correcting the weight by adjusting off-center will cause the flow to vary in velocity around the circumference as it exits the orifice. One of the problems in pursuing thermal die-flow balancing is that many dies are not properly set up for temperature balancing in the first place, leaving mechanical balancing as the only option.


Frankland J.,Frankland Plastics Consulting LLC
Plastics Technology | Year: 2014

The article examines the effect of the screw flight on melt temperature and energy use. The shear rate/viscosity curve in accompanying graph shows a ratio in viscosity between the channel and the clearance of about 3.5:1 based on the calculated shear rates, with resulting viscosities being 3500 poise and 1000 poise for the channel and flight clearance, respectively. The greater the ratio or the greater the slope of the curve, the lesser the effect that viscous dissipation will have in the clearance. The viscosity-thinning effect or the slope of the curve is typically described by the power law coefficient. This coefficient can range from 0.2 to 0.8 for common polymers. The lower the number, the more non-Newtonian the polymer and the more shear thinning will occur. Mixing elements in particular can have large areas with narrow clearances to the barrel that generate high levels of viscous dissipation.


Frankland J.,Frankland Plastics Consulting LLC
Plastics Technology | Year: 2010

Some of the significant types of mixing techniques employed in single-screw mixing are discussed. Dispersive and distributive mixing play a key role single-screw mixing process. Dispersive mixing is similar to putting two materials to be mixed between two plates and rotating one of them. The shear stress developed in the polymer between the plates will be proportional to the distance between them and the speed at which the plate is rotated. Distributive mixing involves putting the two materials in a bowl and stirring them with a spoon. The number and path of the spoon strokes will be proportional to the degree of mixing. Screws have been found to provide both dispersive and distributive mixing and more dispersive mixing occurs when the screw is shallow. Dispersive and distributive mixing increase significantly, as the polymer leakage over the flight increases with increasing barrel and screw clearance in a worn screw.

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