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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

Since regrind comes from many different products and is resized by any one of a handful of particle-reduction devices. A typical screw can handle a range of bulk densities, allowing for low levels of regrind with minimal changes in performance. But when the overall bulk-density variation exceeds 20-25%, the variation becomes impossible for the screw to handle. For a given screw design, the level of discharge pressure determines how far back from the tip the screw is filled. With varying feed rates, the fill will change constantly and the length of fill affects the output at that given instant. By raising the pressure, the fill is increased so that greater fill length results in smaller percentage changes in the discharge pressure. In addition to the proportional band, there are a lot of other adjustments that can be made for further stability, such as band width, ramping, and filtering, all of which are built into modern PID controls.


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: 2010

The melt pump is the extruder's best friend as it precisely controls the output of melt and eliminates many of the variables that impact extruder throughput stability. The pump compensates for poor or erratic temperature control, feed interruptions, changes in the polymer mix and surging due to imprecise screw design. With many output stability problems out of the equation, the operator is free to focus on the die and downstream equipment. Even though the pump will continue to discharge a constant volume of polymer, melt temperature will vary as the screw changes speed to try to maintain constant suction pressure. Melt pumps have bearings that are lubricated by the melt itself, there is designed leakage from the discharge of the pump to the suction side through the bearings. The melt pump provides several other important advantages in addition to discharging a constant volume of polymer. It is a more effective device for raising pressure than is an extruder screw.


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: 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: 2015

A number of barrier screws are available equipped with high-shear mixers, when their need has theoretically been eliminated from barrier screws. Adding a dispersive or shear mixer after a barrier section would needlessly increase the melt temperature and reduce the output in most cases, due to the added pressure drop and result in greater energy requirements during both the extrusion and the subsequent cooling processes. A distributive mixer would be a better choice to homogenize temperatures, colors, or additive distribution, while providing minimal additional increase in melt temperature and reduction in output.


Frankland J.,Frankland Plastics Consulting LLC.
Plastics Technology | Year: 2015

Dies should be designed to deliver polymer from their entrance to all points at their exit with the same pressure drop. This results in a shape exiting the die at the same velocity at all points in its cross-section to form the desired shape. In order to accomplish uniform exit velocity, it is necessary to design the entire length and cross-section of the die while considering the viscosity at all points to calculate the pressure drops from entrance to exit. In many extruded parts, changes in die-flow pattern can also adversely affect physical properties. Polymers exhibiting the least non-Newtonian behavior have the greatest range in output variation because their change in viscosity is less affected by shear rate. That property can be defined by the power law coefficient, which is essentially the slope of the relationship between shear rate and viscosity. There is no universal or general-purpose die for polymer extrusion. A great deal of expertise goes into the design of most dies.


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

There are certain applications where double-flighted feed sections are suitable, while in some other applications these are unsuitable. A double-flighted feed section is best used for either grooved-barrel extruders where the extreme pressure buildup needs to be balanced around the screw or where the screw is going to be subjected to a side load from a feeder, crammer, or stuffer, which is common in recycling applications. Double fights provide more consistent pressure around the screw circumference and more bearing or support area for the screw. This way the contact pressure between the screw and barrel is reduced, thereby relieving the condition where adhesive wear can occur.


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

Barrier screws eliminate the problems associated with solids-bed breakup that are typical in conventionally fighted screws. Solids-bed breakup can still be a factor in designing barrier screws. If the polymer in the solids channel somehow does manage to break up, it similarly becomes much more difficult to melt because it again becomes dependent on conduction for melting rather than on shear or viscous dissipation. Since polymers are effectively insulators, conductive melting is very inefficient and requires a high temperature differential between the surrounding melt and the unmelted portion to have any efifciency at all. As the solids channel cross-section is reduced by either depth, width, or both, the remaining solid-bed pieces collect again and completely fill the channel at some location. This creates an obstacle for further transport of the solids, and since the polymer is unmelted at that point and the channel is typically quite shallow, a very high localized pressure develops in the solid. This pressure is most often contained in approximately half of a turn.

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