Views: 5 Author: Site Editor Publish Time: 2025-06-16 Origin: Site
Resin degradation byproducts often cause defects in finished products like polyethylene (PE) films, sheets, and molded parts. This degradation usually occurs when stagnation points in the process allow resin to remain at high temperatures for too long. Once degradation begins, even slight vibrations can cause the degraded material to detach from the screw and contaminate the product. This issue is common in many single screw extrusion operations and often results from subtle design flaws in the screw.
It is difficult to identify the exact location of degradation by analyzing screw diagrams or operation results. The best way is to heat-pull the screw from the extruder and inspect it for signs of degradation.
Removing the screw is straightforward, but disconnecting the transfer lines adds complexity.
Steps:
1. Run the extruder using the same base resin without any additives or colorants. Additives, especially colorants, make it harder to observe degradation. Continue until the extrudate is free of all additives.
2. Let the screw continue to rotate while stopping material feed so the machine runs empty. This purges as much resin as possible for easier inspection and cleaning.
3. Once no more resin exits the die, stop the screw rotation. Maintain or slightly lower barrel temperature to keep the resin molten and slow down degradation during removal.
4. Disconnect the transfer lines and use a hydraulic or screw jack to push the screw out of the barrel. Push it out to about four times its diameter. (Fig. 1 shows a 2.5-inch screw used for PE.)
Figure 1: A 2.5-inch diameter screw being hot-extruded from the extruder.
Figure 2: Close-up photograph of resin degradation at the screw radius in the metering section.
Image source: M. Spalding
5. Study and photograph the first 4-diameter section for signs of degradation. In Fig. 1 and the magnified Fig. 2, degradation appears in the thread corners. This is caused by Moffatt eddies, which lead to excessive residence time.
Moffatt Eddies form in sharp corners during cavity flow driven from the top. These slow vortex zones exist outside the main high-speed flow and cause resin to stagnate and degrade.
They often result from thread radii that are too small. Increasing the thread radius to 1.4–1.5× the local channel depth can eliminate these eddies and reduce degradation. (Fig. 3 compares small and large thread radii.)
Figure 3: Schematic of the screw thread radius in the metering section:
(a) A smaller radius tends to form Moffatt eddies, leading to resin degradation;
(b) A larger thread radius does not cause resin degradation.
Most screw manufacturers use a thread radius of 0.5× channel depth because it's easier to machine and standard in the industry. However, this often causes Moffatt eddies and degradation. Manufacturers generally accept requests for larger thread radii.
6.After inspecting the first section, clean it while still hot to expose the metal. Only use brass tools, which won't damage the screw surface. Never use steel tools or torches, which can alter the metal or damage hardened coatings.
7. Push out another 4-diameter section and repeat the inspection, photographing, and cleaning. Fig. 4 shows the next section of a 2.5-inch screw with a Maddock mixer. Degradation here is due to deep groove depths relative to their width.
Figure 4: Maddock mixer with resin degradation occurring inside its channels.
Groove width should generally be twice the depth. Common design mistakes include using too many groove pairs (making them too narrow) or cutting them too deep. Though thermally efficient, this leads to degradation at groove edges.
8. Continue pushing out the screw to reveal the metering section inlet and melt section outlet. Improperly designed spiral dam elements in this area can reduce specific throughput by up to 50%, causing poor feeding and degradation (Fig. 5).
Figure 5: Screw metering section with a poorly designed barrier thread entrance. The white material is freshly molten low-density polyethylene (LDPE), while the dark material is degraded, crosslinked resin.
This specific output drop and degradation result from restrictions between the solid feed and dam sections, limiting material flow.
Figure 6: Schematic of a helical dam element:
(a) Side view showing potential stagnation zones that may lead to resin degradation;
(b) Unwrapped view;
(c) Cross-sectional view perpendicular to the flight edge, showing the gap between the dam and the barrel wall.
Some screw elements—like spiral dam or gear-type mixers—are known for causing stagnation. Fig. 6 illustrates a spiral dam mixer with three-diameter length and a dispersive mixing blade. Stagnation can occur at the inlet and outlet of flow channels, especially if undercuts are missing. Many modern designs taper these flow paths and include undercuts to avoid this issue.
9. If an unplanned shutdown occurs and lasts more than 30 minutes, reduce barrel temperature. Shutdowns often last longer than expected. If the temperature remains at processing levels, degradation can occur throughout the metering zone.
Fig. 7 shows this type of degradation, which can affect the screw root and thread radius. It takes significant cleaning to remove degraded material, making restart difficult.
Figure 7: The screw shown above experienced multiple shutdowns, with the extruder remaining at operating temperature for extended periods. Although the extruder was purged before disassembling the screw, dark degraded resin still covered most of the metering section.
Though this case focuses on PE resin, similar degradation can occur with polystyrene, polycarbonate, polypropylene, and ABS. These problems also happen with injection molding screws, where the plunger pushes the screw out of the barrel—similar procedures and solutions apply.
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