Views: 0 Author: Site Editor Publish Time: 2026-08-17 Origin: Site
Most operators only realize their screw is worn out when severe product quality issues arise—such as obvious melt fracture (shark skin), localized degradation (black spots), or an inability to increase output regardless of RPM. By this point, the damage is already severe.
Screw wear essentially means the outer diameter of the screw flights has decreased, resulting in an enlarged radial clearance between the screw and the barrel wall. When this gap widens, the pressurized polymer melt flows backward, drastically increasing backflow and leakage. This directly leads to two major consequences in your plastic pelletizing machine:
Exponential Increase in Shear Heat: The melt is repeatedly sheared and heavily friction-rubbed inside the enlarged gap, causing localized temperatures to spiral completely out of control.
Inability to Build Melt Pressure: Especially in the metering and homogenization zones, the machine cannot compress the material. In a twin screw compounding extruder, this manifests as a steep drop in production output. In injection molding, dosing times become abnormally long, and the screw drifts forward during the holding pressure phase.
Since we are not dismantling the machine, we must rely on operational data, physical properties, and minimally invasive techniques to deduce the internal wear.
This is the most practical blind-testing method in the industry. While running the same material at standard temperatures, record the critical data.
For Extruders: Keep the feed rate and RPM constant, and compare the current die head pressure against historical data from when the screw was new. If you find that you must drastically increase the screw RPM to achieve the same output, and the die pressure has dropped by more than 15%, you can confidently conclude that the metering zone flights are severely worn. Additionally, the pressure curve will change from regular, minor oscillations to erratic, massive swings.
For Injection Machines: Watch the dosing (plasticizing) time. If it has noticeably lengthened, wear is likely. Also, closely monitor the final screw position during the packing phase; if the screw continues to creep forward slowly, the melt is leaking backward through worn clearances or a damaged check ring.
This method requires a brief pause in production but does not require dismantling the co-rotating twin screw extruder.
Fill the barrel entirely with melt. It is best to use a stable, standard test material, such as a high-melt-index polyolefin or a dedicated purging compound.
Stop the feeder and stop the screw rotation. Rely on the pressure sensor at the die head, or manually apply a reverse pressure block at the exit.
Monitor the rate of pressure decay. If the clearance is healthy and the melt seal is tight, the pressure will drop extremely slowly. If the screw is heavily worn, the melt will rapidly leak backward, and the pressure curve will show a steep, cliff-like drop.
Pay close attention to your control panel—your machine is actively trying to warn you.
Abnormal Drop in Torque and Current: If you are running the exact same material, temperature, and RPM, but the motor torque and current are noticeably lower than historical averages (e.g., dropping by 10% - 20%), do not celebrate. The machine is not saving energy; the enlarged clearance is allowing material to slip past, reducing the viscous drag and physical resistance against the screw.
Heaters Shut Down, Cooling Fans Max Out: Normally, the barrel requires the heater bands to maintain temperature. If wear is severe, the excessive clearance generates massive frictional shear heat. You may set the zone to 200°C, but the actual temperature stubbornly climbs to 215°C. The heater bands will show zero output, while the cooling fans or water valves run continuously at 100%. This thermal runaway is a classic symptom of extreme shear caused by screw wear.
If the data makes you suspicious but you want absolute confirmation without pulling the whole shaft, try this minimally invasive trick. While the machine is shut down but the material hasn't fully frozen, find an access window on your equipment:
Extruders: Remove a vent port, a vacuum housing, or unbolt the die head.
Injection Machines: Remove the nozzle assembly while it is still hot.
Insert a high-temperature industrial borescope (endoscope) into these cavities. Through the high-definition camera, you can directly inspect localized screw flights. On a severely worn screw, you will clearly see that the top of the flights have become rounded, blunt, or even exhibit visible metal flaking and deep gouges. This method requires a few tools but completely avoids the massive labor of pulling the screw.
Keep this logic handy on the factory floor:
Scenario 1: Output won't increase / Dosing time is too long. This usually indicates wear in the conveying or metering zones. Extrusion operators can calculate volumetric slip efficiency by comparing historical "RPM-to-Output" curves.
Scenario 2: Actual temperature drastically exceeds the set point in a specific zone. This almost guarantees extreme shear caused by worn flights in that specific barrel section. Check the heater current; if it reads zero while the zone is overheating, the diagnosis is confirmed.
Scenario 3: Wild die pressure fluctuations / Unstable holding position. This is a textbook sign of wear in the homogenization or metering sections, making it impossible to hold melt pressure. Use a borescope through the nearest vacuum port to confirm visually.
When output drops and temperatures drift, do not immediately assign a crew to pull the screw out of your polymer compounding equipment. Grab your maintenance log, pull up the data from three months ago, and cross-reference your torque, pressure, time, and heater outputs. The data will almost always reveal the truth long before your eyes do.
