Cleaning the plasticizing screws and cylinders in extruders and injection molding machines is a fundamental operation to maintain production process efficiency, prevent material contamination, and ensure the quality of the final product. It is also essential for good preventive maintenance.

The cases in which cleaning is needed are mainly two:

• “Out-of-machine” cleaning: when the screw is removed from the cylinder for inspection, regeneration, or replacement.
• “In-machine” cleaning: when switching production and color, all the residues from the previous batch need to be removed before starting with the new material to avoid contamination.

At Viplas, we carry out cleaning of screws from residues and carbonized deposits as a preparatory step for inspections and regenerations. Sometimes, screws arrive so clogged with material that without thorough cleaning, it is impossible to assess the wear condition of the screw.
“In-machine” cleaning is a process performed by the customer, and Viplas can provide consulting regarding suppliers of purge compounds.

For cleaning, different materials and methods are used depending on the type of plastic processed and the needs of the plant. Let’s take a look at the types of materials and methods for cleaning screws and cylinders:

Thermoplastic Cleaning Materials (Purging Compounds)

Purging compounds are materials specifically designed to clean machines during production changes or after long periods of inactivity. These materials can be used while the extruder is still operating, without needing to disassemble parts.

Features:

High thermal compatibility with the plastics to be removed.

• Ability to remove carbonized residues and contaminants.
• Used for both amorphous materials (e.g., ABS, polystyrene) and semi-crystalline materials (e.g., polypropylene, polyethylene).

  Types of Purging Compounds:

• Mechanical: Contain additives that increase friction and abrasion, mechanically removing residues.
• Chemical: Utilize chemical reactions to degrade plastic residues.
• Hybrid: A combination of mechanical and chemical methods for more efficient cleaning.e.

Examples of commonly used products for removing residues during color or material changes: ASACLEAN, Z CLEAN®, Bamberko®, Ultimax®, DPE®, Purgex™, PolyPurge, Polyram, Dyna-Purge®, Ultra Purge.

Mechanical Cleaning Methods

These methods require disassembling the screws and cylinders from the extruder and using specific tools to manually remove plastic residues.

• Metal brushes and hand tools: Used to remove stubborn residues from the screw and cylinder surfaces. Generally made of stainless steel or brass to avoid scratching the surfaces.
• Sandblasting or grit blasting systems: Use abrasive particles to eliminate carbonized and plastic residues.
• Abrasive materials: Sand, metal or plastic granules, depending on the hardness of the residues and the sensitivity of the surfaces.
• Ultrasonic baths: Used to clean small components like nozzles or check valves of screws. Ultrasound creates microbubbles that implode, removing residues from hard-to-reach areas.

Pyrolysis Ovens

Pyrolysis is one of the most effective solutions for removing polymerized or carbonized plastic residues.

How do pyrolysis ovens work?

• Particularly useful for cleaning screws and cylinders used for high-temperature or heat-resistant materials, such as PVC or cross-linked polyethylene.
• They operate at temperatures between 400°-500°C in an oxygen-free atmosphere to avoid direct combustion.
• Plastic residues are broken down into gases and ash, leaving the metal surfaces clean.

Chemical Cleaning Materials for Manual Cleaning or Chemical Baths

In some cases, chemical solvents or cleaning additives can be used to dissolve plastic residues without damaging the metal parts.

Chemical solvents:

• Made from strong solvents or weak acids capable of dissolving polymer residues.
• Commonly used for thermoplastic materials like nylon or polycarbonate.

Alkaline or solvent-based baths:

• Parts are immersed in hot chemical solutions that dissolve plastic or facilitate mechanical removal.
• Limitations: Some solvents may be corrosive and should be used with caution, especially on special steels or parts with coatings.

Factors to Consider When Choosing Cleaning Materials

• Cleaning frequency: The choice of method may depend on the frequency of production changes and the types of materials used. Plants that process different materials frequently may require quick and versatile cleaning solutions like purge compounds.
• Type of plastic material processed: Polymers with different melting points or thermal properties require different cleaning techniques.
• Surface sensitivity: Some cylinders and screws have surface coatings (e.g., nitriding, chrome plating) that may be damaged by overly abrasive or chemically aggressive methods.
• Safety and environmental impact: Chemical solvents and abrasive methods must be handled appropriately to avoid environmental damage or safety risks to operators.