Cleaning Methods for Twin Screw Extruder Screws

In the production of modified plastics, screw cleaning is a crucial step to ensure product quality. Failure to clean the screws, leaving behind residues such as carbon deposits, pigments, or cross-contamination between different materials, can lead to product defects like black spots and streaks, resulting in economic losses. This article analyzes the principles, operational key points, and precautions of six mainstream cleaning methods for twin screw extruder screws, providing practical reference for industry practitioners.

1. Resin Cleaning Method

The resin cleaning method, which uses polyester resin, epoxy resin, or dedicated screw cleaning agents, is a common method for cleaning, suitable for the initial cleaning of new equipment or routine maintenance cleaning.

Working Principle:
The resin cleaning method utilizes the physical displacement and chemical compatibility between the molten cleaning resin and the residues inside the equipment. Through mechanical scouring and interfacial peeling action, contaminants adhering to the screw and barrel surfaces are gradually carried out. The cleaning process is considered complete when the extrudate has completely transitioned from the processed resin to the cleaning resin.

Cleaning Steps:

1. Set the temperature of all heating zones on the barrel to 200°C. Once the barrel reaches this temperature, cleaning can begin (For low-pressure die applications, it is generally not necessary to remove the die for cleaning. For high-pressure die applications, it may be necessary to remove the die beforehand to reduce the risk of excessive pressure at the extruder front).
2. Adjust the screw speed to 15-25 r/min and maintain this speed until the flow of residual melt from the die front stops.
3. Manually feed cleaning resin from the hopper until the color of the extruded strands matches that of the cleaning resin. Stop feeding, evacuate the barrel as much as possible, and then shut down the machine.
4. To better ensure the cleaning effect, it is sometimes necessary to disassemble and clean the extruder die.

Main Limitations：

Directly using ordinary resin for cleaning may sometimes face issues of insufficient cleaning power and material waste, especially for stubborn carbonized residues which are often difficult to remove completely.

2. Flame Cleaning Method

Using fire or flame to remove plastic solidified on the screw is one of the most common and effective methods in plastics processing.

Working Principle:

This method involves heating the solidified plastic residue on the screw with a high-temperature flame, causing it to carbonize, decompose, and detach from the metal surface. The high temperature can rapidly decompose polymer chains, making stubborn residues brittle and easy to remove.

Cleaning Steps:

1. After pulling the screw out of the barrel, immediately clean it using a high-temperature flame. Since the screw retains heat from the processing run, its heat distribution remains relatively uniform at this point.
2. Keep the flame moving to ensure even heating of the screw and avoid localized overheating.
3. Use a copper scraper or copper brush to quickly clean the exposed residue from the screw.           

Main Limitations:

1. This method carries relatively high risks if not operated properly, requires high operator experience, and is not suitable for all types of screws.
2. May cause screw annealing or deformation.
3. Acetylene torches must never be used for cleaning screws. Acetylene flame temperatures can reach 3000°C; using an acetylene torch will not only destroy the metal properties of the screw but also significantly affect its mechanical tolerances.

3. Water Jet Cleaning Method

The water jet cleaning method utilizes the rotational kinetic energy of water for comprehensive screw cleaning, representing an environmentally friendly and efficient modern cleaning solution.

Working Principle:

Fully automatic screw water jet machines use the rotational kinetic energy of water, combined with the reactive force from screw rotation, to achieve 360-degree blind-spot-free peeling. Through precisely controlled water pressure, temperature, and flow rate, various polymer material residues are effectively removed without damaging the screw’s physical structure.

Cleaning Steps:

1. Confirm whether the material and structure of the screw to be cleaned are suitable for water jet treatment.
2. Select water jet equipment that matches the screw dimensions and cleaning requirements.
3. Adjust parameters such as water pressure, temperature, and action time based on the nature of the residue.
4. Dry the components promptly after cleaning to prevent rusting of metal parts.
5. Appropriately treat the wastewater generated from cleaning to meet environmental requirements.

Main Limitations:

The core limitation of the water jet method lies in its potentially incomplete cleaning effectiveness for certain stubborn residues, particularly in the complex intermeshing zones and narrow gaps of twin-screws, where rinsing dead spots may occur, leading to incomplete residue removal.

4. Grinding Wheel Polishing Method

The grinding wheel polishing method uses machine tools or handheld grinding wheels to polish away residues directly from the screw surface.

Working Principle：

Residues on the screw surface are gradually ground away through direct contact with a high-speed rotating grinding wheel.

Cleaning Steps:

1. After pulling the screw out of the barrel, remove large chunks of residue while it is still hot.
2. Allow the screw and stubborn surface residues to cool slightly, then use a grinding wheel polisher for even grinding and polishing.                                               

Main Limitations:

1. Relatively high danger, requiring experienced operators.
2. Can cause screw wear, dimensional changes, and shorten screw service life.    

5. Sandblasting Cleaning Method

The sandblasting cleaning method uses specialized sandblasting equipment to impact and remove residues from the screw surface.

Working Principle:

Compressed air is used to propel media such as quartz sand at high speed onto the screw surface, utilizing the impact force to knock off surface residues.

Cleaning Steps:

1. After pulling the screw out of the barrel, place it on the sandblasting equipment, ensuring a safe surrounding work environment.
2. Wear proper personal protective equipment, start the sandblasting equipment, and precisely remove stubborn residues from the screw surface.                                               

Main Limitations:

1. Requires preparation of specialized sandblasting equipment and experienced operators.
2. Can affect the surface finish of the screw, making it rougher.                                                              

6. Dry Ice Blast Cleaning Method

The dry ice blast cleaning method uses specialized cold blast equipment to blast and peel residues from the screw surface.

Working Principle:

Compressed air is used to propel dry ice pellets at high speed onto the screw surface. The micro-explosions created by the sublimation of the dry ice peel the residues from the screw surface.

Cleaning Steps:

1. After pulling the screw out of the barrel, place it on the ground and remove large chunks of residue while it is still hot.
2. Allow the screw and stubborn surface residues to cool slightly, then use dry ice blast equipment to peel away stubborn surface residues.                                              

Main Limitations:

1. Dry ice blast equipment has a high cost, and the consumable dry ice also incurs significant expense.                                          

Selecting the appropriate screw cleaning method requires comprehensive consideration of production material characteristics, equipment condition, product quality requirements, and cost-effectiveness. In practical applications, multiple cleaning methods can be flexibly combined to achieve optimal results.

Regardless of the cleaning method used, standardized operation and regular maintenance are key to ensuring cleaning effectiveness and extending equipment life. Establishing a scientific cleaning cycle and standard operating procedures can not only improve product quality but also significantly reduce production costs, creating greater economic benefits for modified plastics production enterprises.