Extruder Screw Maintenance Made Easy

Screw Cleaning for Extruders

When changing resins, materials, or colors, it is sometimes necessary to clean the screw. Generally, this can be done either by using a purging agent or by removing the screw from the cylinder and cleaning it manually using a brush, chemicals, or by burning off residues with a torch. Over time, carbonized residues can build up on the screw, causing negative effects on production.

Tips for Cleaning Screws

The best way to thoroughly clean a screw is to remove it from the cylinder and clean it manually.
If removing the screw is not possible, a dedicated purging agent is typically used, though it tends to be expensive.
A more cost-effective method, often used by recycling companies and compounders, involves cleaning with HDPE resin.
HDPE has a high viscosity and can effectively remove stuck resin, burns, and charred material from the screw as it moves forward through the extruder.
After running HDPE through the machine, the screw often becomes noticeably cleaner.
Thus, using HDPE instead of an expensive purging agent is considered a handy trick.
However, for thorough cleaning, it is still recommended to either remove the screw for manual cleaning or use a proper purging agent. The HDPE method should be regarded as a temporary, quick solution.

Screw Wear in Extruders

Screws are typically treated with nitriding and surface polishing to enhance their wear resistance.
Nevertheless, depending on the materials being processed, the screw and cylinder can still suffer from wear.
For example, adding calcium carbonate or talc (essentially stone powder) to resin significantly accelerates screw wear compared to processing pure resin.
Similarly, glass fibers (GF) used for resin reinforcement are known to cause considerable damage.
In recycling operations, soil or sand left on materials—even after washing—can also promote wear.
Large amounts of cellulose (paper) can have a similar effect.
When screws wear down, production output gradually decreases. If production volume drops compared to when the machine was new, screw wear might be the cause.
Other forms of damage include corrosion from aggressive gases, which require corrosion-resistant materials for protection.
In recycling facilities, metal contamination—like bolts, nuts, and even hammers—can enter the extruder, causing significant internal damage.
Screws and cylinders can also get damaged if they scrape against each other due to friction.

How to Remove Screws from Extruders

There are times when screws need to be removed for cleaning, maintenance, or removing foreign objects.
Special tools are used for this purpose.
Since removing a screw may require moving downstream equipment, the process can take an entire day.
If the screw is stuck due to severe metal contamination, it is advisable to call a specialist.

Screw Materials for Extruders

Screw materials are carefully selected for wear and corrosion resistance, often involving nitriding and surface polishing.
When processing particularly abrasive materials like calcium carbonate, talc, or glass fibers, screws with higher wear resistance and special surface treatments are necessary.
For processes that generate corrosive gases, corrosion-resistant materials must be used.
The choice of screw material depends on the type of resin, fillers, and gases involved.
If higher production output is desired, selecting a screw with a surface that allows resins to flow more easily can help increase extrusion rates.
While extruders are mechanically simple, their simplicity allows for deep optimization.
Screw design involves countless combinations of material, length, diameter, groove depth, and other features.

Generally, screws for extruders can be categorized into single-screw and twin-screw types.
Surface treatments like nitriding and plating are commonly applied to further enhance durability.

Extruder Screw Q&A

1) How is an extruder screw structured?
An extruder melts resin by efficiently performing three functions:
① Feeding materials consistently at the inlet,
② Melting the resin using shear heating,
③ Pressing the molten resin forward toward the die.

2) What is a "dalmage"?
In addition to the three basic functions, a "dalmage" refers to a section of the screw designed with vertical grooves to enhance mixing when better kneading of the resin is needed.

3) How can production output be increased using the screw?

Screw length: Increasing the screw length and rotation speed can raise output. However, simply increasing the speed may cause unmelted resin to reach the die.

Motor power: For highly viscous resins (with low MFR), upgrading to a higher horsepower motor can help increase output.

Groove depth: Deeper screw grooves allow more resin to be fed into the screw, boosting production.

4) Is a thicker screw better?
Thicker screws can produce more material per hour.
However, thicker is not always better—higher output also means higher production loss.
If the production lot size is small, using a large-diameter screw may result in frequent material changes and reduced efficiency.
In plastic recycling, ensuring a steady supply of scrap is key.
For frequent material changes, a thinner screw is preferable for flexibility.

5) Do screws wear out?
Yes.
However, when processing 100% pure resin, screw wear occurs very slowly.
The problem arises when fillers like calcium carbonate, talc, or glass fibers are mixed in, which significantly accelerates wear.

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