Troubleshooting Flow Surging in Single-Screw Extruders

Flow surging is defined as the oscillatory change in the rate of the extruder while maintaining constant set point conditions. Moreover, process parameters such as motor current, barrel temperatures and screw channel pressures can also oscillate. Flow surging can originate from many different sources, including improper solids conveying, melting instabilities and improper control algorithms. Surging, in most cases, results in lower production rates, higher scrap rates, material degradation and higher labor costs.

The most common root cause for flow surging is poor solids conveying due to improper temperature settings or controls on either the screw root in the feed section, feed casing or first barrel zone temperature. For smooth-bore extruders, solids conveying depends on the combination of the forwarding forces at the barrel wall and pushing side of the feed channel and the retarding forces at the root of the screw. Successful solids conveying will occur when the forwarding forces are greater than the retarding forces.

These forces depend mainly on the surface temperature of the screw and barrel, and to a lesser extent on the screw speed and channel pressure. Proper temperature control of the surfaces will provide acceptable forwarding forces while minimizing the retarding forces. Flow surging can also occur due to improper melting, but this form of surging is very rare.

The forwarding forces and retarding forces are directly proportional to the stress at the interface. The stress at the interface is a strong function of the metal surface temperature, as shown by Figure 1 for a PS resin. The forwarding forces at the barrel wall will be the highest at temperatures near 150^oC. Thus, the best solids conveying will occur when the barrel wall temperature is near 150^oC. Minimum retarding forces will occur at the screw surface at temperatures less than 90^oC for PS resins. Low retarding forces can also occur at temperatures greater than 180^oC, but if a catastrophic shutdown were to occur, the resin would melt and adhere to the screw. To avoid resin melting on the screw, the screw surface should be maintained at temperatures less than 90^oC.

Metal surface temperatures are rarely known. Instead, temperature sensors are positioned in the barrel wall away from the surface. The sensor temperature and the barrel wall temperature can be significantly different. The feed casing is typically cooled using water, and most of the time the water temperature is not monitored. Screw temperature control or screw cooling is typically not used, especially for extruders with diameters less than 6 inches. Many processes, however, could benefit by using screw cooling. The benefits can include higher specific rates and more stable operations. If any of these temperatures get out of the normal operating range, flow surging can occur.

The likely cause for flow surging for smooth-bore extruders is improper temperatures in the solids conveying section of the extruder.

An 8-inch diameter two-stage extruder was operating stably for a PS resin for the first 400 minutes of data collection, as shown by Figures 2 and 3. A gear pump was positioned between the extruder and the downstream equipment, and screw cooling was used. At about 400 minutes, the extruder started to operate unstably with large oscillations in the motor current and screw speed. There was a pressure sensor in the first-stage metering section.

The pressure was fairly constant for the first 400 minutes, but then it started to decrease due to poor solids conveying. The controller for the gear pump increased the screw speed from 100 to 160 rpm to maintain the inlet pressure at the set point pressure of 9 MPa. The screw speed cycled about every 25 minutes between 100 and 160 rpm while maintaining the inlet pressure to the pump essentially constant. If the gear pump would not have been there, the line would not have been operational. Here, the specific rate ranged from about 14 to 23 kg/(hr. rpm). The specific rate is simply the rate divided by screw speed.

Cooling water to the screw was determined to be inadequate for the process. That is, the water flow was too low and the cooling hole in the screw was not long enough, causing the screw to be too hot in the solids conveying section. The cooling hole should extend to the end of the solids conveying section. The hole was 4 diameters past the pocket of the screw, and the solids conveying section was 7-diameters long. The hole was short by 3 diameters.

To fix this process, the cooling hole was extended to the end of the solids conveying section and a water pump was installed upstream of the cooling device. The cooling of the screw was improved, and the flow surging problem never happened again on the line due to a hot screw.

When flow surging occurs, the feed casing should be checked first to see if it is hot.

A few years later, this same extruder began to become unstable and flow surge for the same PS resin, but the oscillations were completely different, as shown by Figure 4. Like before, the gear pump maintained the rate to the downstream equipment nearly constant while the screw speed and specific rate varied widely.

The root cause for the surging was a feed casing that was too hot. The cooling water channels had occluded with minerals and rust, limiting the amount of cooling water flow. The feed casing was flushed with an acidic aqueous solution. A pump was installed on the cooling water flow such that the cooling level to the feed casing was high. This high level of cooling solved the flow surging problem. In general, the feed casing should not be hot, but warm to the touch.

Many materials behave like the PS resin used for these cases, including ABS, SAN, PC and PET to name a few. When flow surging occurs, the feed casing should be checked first to see if it is hot. Next, if screw cooling is used, the troubleshooter should check on the flow out of the cooling lance. A simple way is to connect a garden hose to the outlet water flow and let the discharge water go to a floor drain. It will be obvious if the water is flowing and if the water is very hot. Sometimes disconnecting the return flow from the collection manifold and relieving the pressure with an open discharge garden hose will increase cooling to the screw and mitigate flow surging.

The likely cause for flow surging for smooth-bore extruders is improper temperatures in the solids conveying section of the extruder. Hot feed casings and hot screws are the most likely root causes. Troubleshooting the problem should focus on the feed casing and screw temperature. The feed casing is simple to check as it should not be hot to the touch. Methods were described in this article to troubleshoot a hot screw.

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 Analyzing and Troubleshooting Single-Screw Extruders with Gregory Campbell. Contact: 989-636-9849; maspalding@dow.com; dow.com.