Four Screw Cleaning Methods for Twin-Screw Extruders in Modified Plastics Production: Principles, Operations, And Limitations

In the production of modified plastics, screw cleaning is a critical step in ensuring product quality, yet it is often overlooked by manufacturers. Residual carbonitides, pigments, or cross-contamination between different materials can lead to defects such as black spots or streaks in the products, resulting in significant losses.

This article provides an in-depth analysis of the principles, key operational considerations, and precautions associated with four common cleaning methods for twin-screw extruders, offering practical guidance for industry professionals.

01

Resin cleaning method

The resin cleaning method is a commonly used technique for machine cleaning, employing polyester resin, epoxy resin, or specialized screw cleaners; it is suitable for the initial cleaning of new equipment or for routine maintenance cleaning.

operational principle:

The resin cleaning process utilizes a cleaning resin that, in its molten state, undergoes physical displacement and chemical compatibility with residues remaining within the equipment; through mechanical scrubbing and interfacial desorption, contaminants adhering to the surfaces of the screw and the barrel are gradually removed. The cleaning process is considered complete when the extrudate transitions entirely from the processed resin to the clean resin.

operating process:

Turn off the feeding panel and close the discharge port at the bottom of the hopper.

Reduce the screw speed to 15–25 rpm and maintain this speed until the melt flow at the front end of the die head ceases.

Set the temperature of all heating zones of the barrel to 200°C.

Add the washing material through the feed inlet until the color of the extruded strips matches that of the granular washing material.

Stop feeding, drain the material, and cease the rotation of the double-screw extruder screw.

Open the mold head, rotate the screw, remove the orifice plate, and discharge any residual cleaning material from the blast chamber.

matters need attention:

Speed control: Throughout the cleaning process, the screw speed should be maintained within 20 r/min; when performing the final removal of residual cleaning resin, the speed should be controlled at approximately 10 r/min.

Pressure monitoring: Closely monitor the extrusion pressure to ensure that system pressure does not exceed safe limits; if necessary, remove the die to mitigate the risk of excessive pressure at the front end of the extruder.

Temperature control: strictly adhere to the recommended temperature settings to prevent resin degradation caused by excessively high temperatures.

Effectiveness evaluation: Assess the degree of cleaning by observing color changes in the extruded strips to ensure complete conversion.

Key limitations

Using conventional resins directly for cleaning may, in certain cases, result in inadequate cleaning efficacy and material waste, particularly when it comes to stubborn carbonized residues that are often difficult to completely remove. To address this, some users opt for specialized cleaning agents; these products typically enhance cleaning efficiency through mechanisms such as softening and debonding, although their effectiveness can still vary depending on the condition of the equipment and the operational parameters employed.

2

Disassembly and Cleaning Method

The disassembly and cleaning method involves completely disassembling the extrusion unit components and performing thorough manual cleaning; this approach is suitable for applications requiring deep cleaning or the replacement of specialized materials.

operational principle:

This method employs physical contact cleaning to directly remove stubborn residues from the surfaces of components such as screws, orifices, and mold heads. Using specialized tools, operators can access every nook and cranny of the equipment, ensuring comprehensive cleaning without dead zones; this approach is particularly suitable for handling severe carbonization or resin residue.

operating process:

Stop the screw rotation, open the extruder die, and begin the gradual disassembly.

Rotate the screw of the twin-screw extruder, remove the orifice plate, and discharge any residual cleaning material from the extruder barrel.

Thoroughly clean any residues from the orifice plate to ensure that the flow passages remain unobstructed.

Stop the rotation, remove the screw, and inspect whether it has been thoroughly cleaned.

Use appropriate tools to manually remove residual material from the screw.

Reassemble the screw in the correct sequence, then add new material to flush out any residual cleaning residue from the barrel.

Install the orifice plate and the die head to complete the entire cleaning operation.

matters need attention:

Disassembly sequence: Disassemble strictly according to the sequence specified in the equipment manual to avoid damaging precision components.

Tool selection: Use specialized tools such as copper scrapers and copper brushes to prevent damage to the screw and the barrel surface.

Thoroughness of cleaning: Ensure that all residues on the surfaces of all components are completely removed, particularly in threaded areas and gap zones.

Reassembly: During reinstallation, inspect all seals to ensure assembly precision and equipment hermeticity.

Main limitations:

This method requires operation by specialized technical personnel and may damage the screw and the barrel, potentially causing irreversible damage. Additionally, this method cannot clean the inner walls of the barrel.

03

Fire-baking cleaning method

The flame cleaning method is a conventional technique that utilizes high-temperature flames to directly remove stubborn plastic residues from the surface of screws; it is widely employed in plastic processing facilities.

operational principle:

The plastic residues on the screw are instantaneously heated and cured by a high-temperature flame, causing them to carbonize, decompose, and detach from the metal surface. The high temperature rapidly breaks down polymer chains, rendering stubborn residues brittle and thus easy to remove.

Key Operating Points:

Timing selection: Perform cleaning immediately after using the screw; at this stage, the screw retains residual heat from the machining process, and the heat distribution is uniform.

Temperature control: Use a blowtorch for cleaning, and avoid excessive temperatures.

Uniform heating: Maintain the movement of the flame to ensure even heating of the screw and prevent localized overheating.

matters need attention:

The use of an acetylene flame is strictly prohibited: the temperature of an acetylene flame can reach 3,000°C, which may compromise the metallic properties of the screw and significantly affect mechanical tolerances.

Prevent changes in the metal structure: If the flame turns to a persistent blue color at a particular location, it indicates that the metal structure at that location has undergone a change, which will result in reduced wear resistance.

Prevent screw bending: localized overheating can cause one side of the screw to overheat, leading to bending deformation of the screw.

Material protection: Most screws are fabricated from 4140.H.T. steel, featuring tight tolerances (typically within 0.03 mm) and stringent straightness requirements (within 0.01 mm); improper heating may result in irreversible deformation.

Key limitations

If improperly performed, this method carries a high risk, requires considerable experience from the operator, and is not suitable for all types of screw materials.

04

Water washing method

The water-based cleaning method utilizes the rotational kinetic energy of water to achieve comprehensive cleaning of the screw, making it an environmentally friendly and highly efficient modern cleaning solution.

operational principle:

The fully automated screw-type washing machine utilizes hydraulic rotational kinetic energy, combined with the counter-force generated by the screw rotation, to achieve 360° complete material stripping without dead zones. By precisely controlling water flow pressure, temperature, and flow rate, it effectively removes residues of various polymer materials while preserving the physical integrity of the screw.

technological superiority:

Comprehensive cleaning: The rotating water flow reaches every crevice and complex structure, ensuring thorough cleaning.

Material Protection: Hydro-jetting does not damage the screw substrate, thereby preserving its original mechanical properties and dimensional tolerances.

High efficiency and energy savings: The cleaning process is rapid, making it suitable for forced debonding and removal of a wide range of polymer materials.

Environmental Safety: Using water as the medium, eliminating chemical contamination, and ensuring a safer and healthier working environment.

Comparison diagram of screw cleaning

Comparison images of mold head cleaning

matters need attention:

Applicability Assessment: Determine whether the material and structure of the screw to be cleaned are suitable for water-based cleaning treatment.

Parameter settings: Adjust parameters such as water pressure, temperature, and treatment duration based on the properties of the residue.

Equipment Matching: Select water washing equipment that is compatible with the screw dimensions and cleaning requirements.

Follow-up procedure: After cleaning, dry promptly to prevent corrosion of metal components.

Environmental considerations: Properly treat wastewater generated from cleaning operations to comply with environmental regulations.

Key limitations

The primary limitation of the water-washing method is that it may not thoroughly remove certain stubborn residues, particularly in the complex meshing zones and narrow crevices of twin-screw machines, where washing dead zones can occur, leading to incomplete removal of residues.

Selecting an appropriate screw cleaning method requires a comprehensive consideration of the characteristics of the production materials, the condition of the equipment, product quality requirements, and cost-effectiveness. In practical applications, various cleaning methods can be flexibly combined according to different production stages and levels of contamination to achieve optimal results.

Regardless of the cleaning method employed, standardized operations and regular maintenance remain critical to ensuring optimal cleaning results and extending equipment service life. Establishing scientific cleaning cycles and standardized operating procedures can not only enhance product quality but also significantly reduce production costs, thereby generating greater economic benefits for manufacturers of modified plastics.

Equally important is choosing screw and barrel components that are built to withstand demanding production conditions and maintain performance over the long term.

About SUPERIOR MACHINERY

Established in 1992, SUPERIOR MACHINERY has been engaged in the rubber and plastic molding machinery industry for more than 30 years. With an increasing market share, the company has established itself in the field of high-quality fully hardened screws and bimetallic barrels that are highly resistant to wear, corrosion, high pressure, and high speed.