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Partially filled screw discharge sections can result in erratic output. A screw backfills for a certain distance to develop the pressure necessary to overcome the resistance caused by dies and other elements beyond the barrel. If the backfill is not adequate, the screw output will be unstable, and even the smallest variation in output entering the discharge section will cause the screw to vary in its fill length.
In the last two columns we discussed surging due to feed-related issues and those related to melting limitations. There is a third type, caused when the discharge end of the screw is only partially filled.
This type of surge happens most frequently with two-stage screws when the second stage has far more capacity than the first stage. It can also happen when a screw is limited in feeding or melting, causing a partially filled metering section. For example, a poorly designed barrier screw can restrict the output of the barrier or melting section, leaving the metering section partially filled. Feed restrictions that occur when using high percentages of low-bulk-density regrind can leave even a well-designed screw with a partially filled discharge section. Very low discharge pressures also make such situations more likely.
Since there is always a die and/or other apparatus on the end of the barrel in a production situation, the screw necessarily backfills for a certain distance to develop the pressure necessary to overcome the resistance caused by these elements. The amount of backfill depends on the pressure, viscosity of the polymer, output rate, and screw design.
If the backfill is not adequate, the screw output will be unstable and even the smallest variation in output entering the discharge section will cause the screw to vary in its fill length. This sets up an oscillation, where the screw increases or decreases its backfill causing a change in pressure-generating capability, resulting in a temporary change in output. That, in turn, causes a change in fill length, and unless some change is made to increase the fill length, the output will have an almost perfectly harmonic surge forever.
This situation has frustrated many processors over the years because of its persistence. It does not show up as a variation in motor load, and its pressure cycle is usually a perfect sine wave. This is often seen today because of the wide use of melt pumps to reduce the pressure at the screw discharge. This reduces the fill length necessary to overcome the resistance of the forward apparatus and can send the screw into a harmonic surge that no amount of temperature adjustment will correct.
The solutions are first and foremost to increase the discharge pressure on the extruder. This can be done by adding screen packs when a melt pump is not used. If using a melt pump, simply increase the suction pressure. Alternatively, adjustments can be made to the screw design to better fill the discharge area for more fill length.
The fill length is pretty easy for a screw designer to calculate. All that’s required is the discharge pressure and the viscosity of the polymer at the discharge, along with the screw design. Often these data can be reasonably estimated to make the calculations, particularly on a new application where there is no actual data. By experience I have found that a fill length of less than two turns of the discharge section will result in some instability.