TL;DR
Extruder surge is typically caused by inconsistent feed from the hopper, worn screw, or incorrect screw temperature profile. Verify feed zone temperature and throat cooling before adjusting screw speed.
What you might see
- output flow rate oscillates cyclically
- melt pressure fluctuates above and below setpoint
- downstream product dimension varies in a periodic pattern
- drive motor load varies rhythmically
Likely causes
Feed zone temperature too high causing premature melting and bridging at the feed throat
Throat cooling water flow insufficient allowing the feed pellets to bridge
Worn screw with reduced channel depth in the metering zone creating unstable throughput
Inconsistent bulk density of the feed material from mixed regrind ratios
Required tools
- Melt pressure gauge or transducer readout
- Thermometer for water temperature
- Flow rotameter
- Screw flight clearance gauge (for screw inspection)
Safety first
- Extruder barrel zones operate at temperatures above 200 C. Never touch barrel surfaces or the die head without heat-resistant gloves.
- Melt pressure transducers and pressure relief systems are safety-critical. Do not block or remove pressure relief devices.
Procedure
- 1
Monitor the melt pressure gauge or transducer readout and time the surge frequency. Record the period in seconds.[1]
- 2
Check the feed throat cooling water flow at the cooling water rotameter or flow indicator. Increase flow if below the minimum specified.[1]
- 3
Verify the feed zone setpoint temperature (Zone 1). For many polyolefins, the feed zone should run 20 to 30 C below the rear barrel zone to prevent premature melting.
- 4
Check the regrind blend ratio in the feed hopper. Regrind higher than 30 percent can change bulk density and cause feed instability.
- 5
Run the extruder at a reduced screw speed for 15 minutes. If surge reduces or disappears at lower speed, the issue is in the metering zone, likely screw wear.
- 6
Pull and measure the screw flight clearance if the extruder is scheduled for a cleaning run. Clearances above twice the original design value indicate significant screw wear.
- 7
Document the melt pressure trace before and after changes to confirm improvement.
Sources
Davis-Standard Davis-Standard MEDD / Super Blue Single / Twin-Screw Extruder general technical documentation, Davis-Standard
Davis-Standard extruder operator and process troubleshooting documentation (general)
More guides for Davis-Standard Davis-Standard MEDD / Super Blue
How to correct melt temperature deviation on a Davis-Standard MEDD / Super Blue extruder
Melt temperature above setpoint is usually excessive shear from screw speed or a barrel zone that has drifted. Melt temperature below setpoint usually means a barrel heater or thermocouple failure.
How to diagnose thrust bearing noise on a Davis-Standard MEDD / Super Blue extruder
Axial clicking or rumbling from the gearbox area indicates thrust bearing wear from high back pressure or lubrication starvation. Check gearbox oil level and back pressure setting before scheduling a bearing replacement.
Stop fixing the same fault twice.
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