Thermoplastic elastomers (TPEs) are widely used in various fields with their dual properties of rubber elasticity and plastic processability. Extrusion granulation is a crucial step in preparing TPE materials into granules, and high-quality granules are the foundation for subsequent molding processes.
Advantages of Using TPE Granules
- More Convenient Feeding Method: Granular materials have good flowability and are easier to feed into the extruder through feeding devices, avoiding issues such as bridging and caking that can occur with powders. This is especially beneficial in continuous production, eliminating the need for complex forced feeding systems.
- Higher Product Quality: The bulk density of granules is typically higher than that of powders. This means that for the same volume, granules can provide a more uniform and stable material supply, contributing to the production of final products with consistent density and quality.
- Fewer Defects: Granules contain relatively fewer volatile substances and air compared to powders. This helps reduce the formation of bubbles during extrusion, thereby improving the surface quality and internal properties of the products.
- Easier Dispersion of Functional Additives: When specific functions need to be imparted to TPE materials, pre-compounding functional additives with the TPE matrix to create functional masterbatches before granulation allows for more uniform dispersion of the additives within the matrix. This improves the stability and reliability of the modification effects.
Key Influencing Factors in the TPE Extrusion Granulation Process
1. Influencing Factors in the Extrusion Process (TPE Extrusion)
Temperature Control
- Reducing Melt Viscosity and Improving Flowability: Appropriately increasing the extrusion temperature can significantly reduce the viscosity of the TPE melt, improving its flowability. This makes it easier for the material to be conveyed and mixed within the TPE extruder, while also reducing the extruder’s power consumption.
- Optimizing Plasticization and Improving Product Performance: A suitable temperature helps ensure thorough plasticization of the TPE material, promoting uniform mixing between components and thus improving the strength, gloss, and transparency of the final products.
- Avoiding Parison Sagging: However, excessively high melt temperatures will reduce the melt strength, causing the extruded parison to deform under its own weight (commonly known as “sagging”), resulting in uneven longitudinal wall thickness.
Melt Pressure Control
Melt Conveying Speed Control
2. Influencing Factors in the Pelletizing Process
Cold Cutting Method
- Sheet Cold Cutting: Suitable for applications where extrusion requirements are not high but large sheets are needed. The material is mixed through a kneader or Banbury mixer, then calendered into sheets on a two-roll mill, cooled, and then pelletized. This method is relatively simple but has low efficiency.
- Extruded Sheet Cold Cutting: The mixed material is plasticized by an extruder, extruded into sheets, then air-cooled or naturally cooled, and finally fed into a flat-die pelletizer for pelletizing. This method has higher production efficiency than sheet cold cutting and is suitable for some applications where the shape of the granules is not critical.
- Strand Cold Cutting: This is the most common granulation method for thermoplastic TPE. The material is plasticized by an extruder and extruded into round strands, which are then air-cooled or water-cooled and cut into cylindrical or elliptical granules by a pelletizer. This method has relatively simple equipment and process, low cost, and a wide range of applications.
Hot Cutting Method
- Dry Hot Cutting: A rotating cutter is pressed tightly against the die face of the TPE extruder, directly cutting the extruded hot round strands into granules. This method has high production efficiency and directly yields granules, but it is prone to issues such as granule sticking and usually requires subsequent cooling and separation treatment.
- Underwater Hot Cutting: An advanced granulation technology, particularly suitable for polyolefin TPE. The die head and cutter operate in circulating warm water. The extruded molten material is directly cut and cooled in the water, producing regular-shaped, smooth-surfaced granules and effectively preventing sticking.
- Air Hot Cutting: Similar to dry hot cutting, to prevent the freshly cut hot granules from sticking together, cold air is usually blown or warm water is sprayed inside the pelletizing hood to quickly cool the granules. This method is suitable for applications requiring high granule shape and dispersibility.
Common Quality Issues and Solutions for TPE Extrusion
1. Long Strands Larger Than Normal Granules
2. Agglomerated Modified Material Granules
3. Caterpillar Strands
4. Black Spots on or Inside the Granules
5. Color Change
6. Poor Vacuum (Vacuum Eyes or Foaming on Granule Surface)
7. Iron Filings or Other Metal Particles on or Inside the Plastic Granules
8. Poor Plasticization
9. Excessive Moisture
Conclusion
