How To Optimize The Single-Screw Extruder for Melt Feeding When A Twin-Screw Extruder Is Followed by A Single-Screw Extruder?

Some blending production lines use a twin-screw extruder or a continuous kneader for blending operations, followed by a melt-feeding single-screw extruder to apply pressure and pelletize the material.

A schematic of the compound production line, using a continuous mixer for compounding and a melt-feed single-screw extruder to apply pressure to the pelletizer. Source (all images): Mark Spalding.

In this process, the raw material is first metered into a feeder, then fed into the twin-screw extruder or kneader for blending. The blending rate is controlled by the feeder, while the maximum rate is determined by the blending equipment. In other words, the rate usually increases until the blending equipment reaches its maximum torque limit.

Next, the material exiting the blending equipment is gravity-fed into the inlet of the single-screw extruder. The single-screw extruder uses a starved feeding method to ensure that material does not accumulate in the hopper. Pressure increases along the length of the screw, generating enough pressure to drive the pelletizer. From this stage, the single-screw extruder should never be the rate-controlling part of the production line.

In the single-screw extruder, the pressure at the inlet is zero, while the discharge pressure is relatively high, creating a positive axial pressure gradient. This gradient reduces the process's specific throughput rate. The specific throughput rate is the rate divided by the screw speed—for example, pounds per hour per RPM. The extruder's ability to maintain rate while generating pressure mainly depends on the metering channel depth, pressure gradient, and resin viscosity.

Therefore, the primary issue with melt-feeding extruders (specifically the single-screw part) is the setting of the metering channel depth. This article introduces a blending line with a 15-inch diameter, boost-type single-screw extruder. The screw's metering channel depth is 1.73 inches.

This depth is too large to effectively pump and pressurize the resin for pelletizing. For this screw configuration, the extruder can only operate at a rate of 3,280 lbs/hour, with a screw speed of 15 RPM, resulting in a specific throughput rate of 219 lbs/hour RPM. The low specific throughput rate causes the discharge temperature to rise too high, leading to the degradation of flame retardant additives. The acceptable maximum discharge temperature for this resin and flame retardant chemicals is around 180°C.

As shown in Figure 2, when the metering channel depth is between 0.7 and 1 inch, the specific throughput rate increases almost linearly with the depth. This is because the specific speed increases linearly with channel depth. Specific speed refers to the screw's rate generated by rotation, historically known as the specific resistance rate, which occurs in the absence of an applied pressure gradient.

The relationship between specific throughput rate, discharge temperature, and the metering channel depth in compounded PE.

As previously mentioned, the screw channel has a positive axial pressure gradient. This is due to the material entering the feed channel under zero pressure, while the discharge pressure is relatively high to meet the pelletizer’s requirements. This pressure gradient lowers the extruder’s specific throughput rate. Moreover, the specific throughput rate is proportional to the cube of the metering channel depth. This explains why the specific throughput rate reaches its peak when the channel depth is about 1.18 inches.

At deeper channel depths, the flow rate decreases significantly as the cube of the channel depth increases. Figure 2, created using numerical simulations, assumes a rate of 8,500 lbs/hour and a discharge pressure of 3,000 psi, suitable for polyethylene (PE) mixed with flame retardant chemicals.

Figure 2 also shows the material's discharge temperature. At a metering channel depth near 1.30 inches, the temperature reaches its lowest value of 179°C. Recall that the maximum specific throughput rate occurs at a channel depth near 1.18 inches. Discharge temperature generally responds to specific throughput rate; as the specific throughput rate increases, the discharge temperature decreases. At both the deepest and shallowest ends of the channel in Figure 2, the discharge temperatures are relatively higher—184°C and 182°C, respectively. At 182°C, some flame retardants begin to degrade.

Next, we manufactured a new screw with a shallower metering channel, optimized at a depth of 1.18 inches. Figure 3 shows the simulation of the screw design, providing axial pressure and temperature curves. The simulation indicates that the new screw should be able to pump 8,500 lbs/hour at a screw speed of 15.5 RPM, with a specific throughput rate of 548 lbs/(hour·RPM), more than 2.5 times the rate of the original screw.

As shown in Figure 3, the pressure at the screw inlet is zero, while the discharge pressure is 2,400 psi, forming a positive axial pressure gradient. As previously mentioned, this positive pressure gradient, along with the channel depth, determines the specific throughput rate of the resin. The simulated discharge temperature is 174°C, which is low enough to prevent the degradation of flame retardant chemicals.

Simulate the axial pressure and temperature of the new screw with a screw channel depth of 1.18 inches. The rate is 8,500 lbs/hour, the screw speed is 15.5 RPM, and the specific throughput rate is 548 lbs/hour RPM.

In melt feeding extrusion processes, the single-screw extruder should never become the limiting step of the process. Instead, the limiting step should be the torque on the blending process rotor. The melt-feeding single-screw extruder presented here becomes the limiting step because the discharge temperature must remain below 180°C to prevent the degradation of flame retardant chemicals. The optimal channel depth typically occurs at the highest specific throughput rate, and most screw designers know how to design this optimal metering channel depth.

Additionally, the screw's lead length can be slightly increased to provide higher specific throughput without making the metering channel too deep or being overly sensitive to the positive axial pressure gradient. For instance, the original 15-inch diameter screw has a lead length equal to its diameter, whereas the optimized screw’s lead length is increased to 1.2 times the diameter. This increase in lead length results in an 18% increase in specific throughput.

Furthermore, the optimized metering channel depth is typically 6% to 8% of the screw diameter, depending on the resin's viscosity, the metering channel's axial length, and discharge pressure. For example, the original screw's metering channel depth is 11.8% of the diameter, while the optimized screw's metering channel depth is 7.8% of the diameter.

The optimized screw presented here requires additional torque from the motor. If the process cannot provide the extra torque, optimization will not be possible. Most screw designers are aware of this issue and regularly check the torque requirements.

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