����ý

Some compounding lines feature a twin-screw extruder or continuous mixer for the compounding operation followed by a melt-fed, single-screw extruder for material pressurization for pelletization. A typical configuration is provided by the schematic in Figure 1.

Here, the individual feedstocks are metered to a conveyor belt and then fed to a twin-screw extruder or a continuous mixer for compounding. The rate of the process is controlled by the feeders, while the maximum rate is controlled by the compounder. That is, the rate is typically increased until the compounder is operating near the maximum torque limit.

Featured Content

How Vacuum Drying Is Revolutionizing Plastics Processing READ MORE

A Purge Solution for the Blow Molding and Compounding Industries READ MORE

A Simpler Way to Calculate Shot Size vs. Barrel Capacity READ MORE

Next, the discharge from the compounder is fed to the feed port of a single-screw extruder via gravity. The single-screw extruder is operated starve-fed such that compounded material never accumulates in the feed hopper. The pressure increases down the length of the screw, creating enough pressure to operate the pelletizer. The single-screw extruder should never be the rate-controlling segment of the line.

The single-screw extruder has zero pressure at the feed port and a relatively high discharge pressure to run the pelletizer. Thus, the extruder develops a positive axial pressure gradient. This gradient will reduce the specific rate of the process. The specific rate is simply the rate divided by the screw speed — for example, lbs/(hr rpm). The ability of the extruder to generate pressure while maintaining the rate depends mainly on the metering channel depth, the pressure gradient and the viscosity of the resin.

The main problem with melt-fed extruders that are starve-fed is setting the depth of the metering channel. A compounding line with a 15-inch diameter pressurizing, single-screw extruder will be described here. This extruder had a screw with a metering channel that was 1.73-inch deep. The channel was too deep to pump and pressurize the resin for pelletization.

For this screw, the extruder was only capable of operating at 3,280 lbs/hr at a screw speed of 15 rpm for a specific rate of 219 lbs/(hr rpm). The low specific rate caused the discharge temperature to be too high, causing a flame-retardant chemical additive to degrade. The maximum acceptable discharge temperature for this resin and flame-retardant chemical is about 180°C.

The problem is shown in Figure 2. For metering channel depths between 0.7 and 1 inch, the specific rate increases nearly linearly with increasing channel depth. This is because the specific rotational rate increases linearly with channel depth. The specific rotational rate is the specific rate of the screw due just to the rotation of the screw. That is, there is no imposed pressure gradient. The specific rotational rate is known historically as the specific drag rate.

As previously presented, the screw channels have an imposed positive axial pressure gradient. This is because the material entering the feed channel is at zero pressure and the discharge pressure is relatively high due to the requirements of the pelletizer. This pressure gradient decreases the specific rate of the extruder. Moreover, the specific rate decreases to the cube of the metering channel depth. That is why the specific rate has a maximum at a channel depth of about 1.18 inch.

At deeper channel depths, the specific rate decreases at a high rate with increasing channel depth due to the cube functionality of the channel depth. Figure 2 was created using numerical simulation at a rate of 8,500 lbs/hr and a discharge pressure of 3,000 psi for a polyethylene (PE) compounded with a flame-retardant chemical additive.

The discharge temperature of the material is also provided in Figure 2. Here, the temperature had a minimum value of 179°C near a metering channel depth of 1.30 inch. Recall that the maximum specific rate occurs at a channel depth near 1.18 inch. Discharge temperatures typically respond to the specific rate. That is, as specific rate increases, the discharge temperature decreases. At the very deep and very shallow ends of Figure 2, the discharge temperatures were higher at 184 and 182°C, respectively. At 182°C, some of the flame-retardant chemical was degrading.

Next, a new screw was built that had a shallower metering channel at the optimal depth of 1.18 inch. A simulation of the screw design providing the axial pressure and temperature profiles is shown in Figure 3. The simulation indicates that the new screw should be able to pump 8,500 lbs/hr at a screw speed of 15.5 rpm for a specific rate of 548 lbs/(hr rpm). This rate is more than 2.5 times the rate of the original screw.

As shown by Figure 3, the pressure was zero at the feed opening of the screw and the discharge pressure was 2,400 psi, creating the positive axial pressure gradient. As previously stated, this positive pressure gradient along with the channel depth sets the specific rate for this resin. The discharge temperature was simulated at 174°C, a temperature low enough to prevent the flame-retardant chemical from degrading.

Melt-fed extruders used for pelletization should never be the rate-limiting step for a process. Instead, the torque on the rotors of the compounding process should be the rate limiting step. The melt-fed, single-screw extruder example presented here was the rate-limiting step because the discharge temperature had to be less than 180°C. At higher temperatures, the flame-retardant chemical started to degrade. The optimal channel depth is typical where the specific rate is the highest. Most screw designers understand how to design for this optimal metering channel depth.

Melt-fed extruders used for pelletization should never be the rate-limiting step for a process.

The lead length of the screw can be increased slightly to provide a higher specific rate without making the metering channel depth too deep and sensitive to the positive axial pressure gradient. For example, the lead length was equal to the diameter for the original 15-inch diameter screw, and it was increased to 1.2 times the diameter for the optimized screw. This lead length increase provided an 18% increase in the specific rotation rate.

Moreover, the metering channel depth for optimized channels will typically be between 6 and 8% of the diameter, depending on the viscosity of the resin, the axial length of the metering channel and the discharge pressure. For example, the channel depth of the original screw was 11.8% of the diameter while the optimized screw has a depth of 7.8% of the diameter.

The optimized screw presented here requires additional torque from the motor. If the process cannot supply additional torque, the optimization cannot be performed. Most screw designers understand this problem and routinely check torque requirements.

About the Author: Mark A. Spalding is a fellow in Packaging & Specialty Plastics and Hydrocarbons R&D at Dow Inc. in Midland, Michigan. During his 39 years at Dow, he has focused on development, design and troubleshooting of polymer processes, especially in single-screw extrusion. He co-authored  with Gregory Campbell. Contact: 989-636-9849; maspalding@dow.com; .