Analysis of Causes of Strand Breakage in Plastic Pelletizing

01. External Impurities

★ Check whether dead corners of equipment in mixing and feeding processes are properly cleaned and free of foreign particles.  

★ Minimize the use of recycled crushed materials, or manually pre-screen them to remove impurities.  

★ Increase the mesh size and number of filter screens.  

★ Cover potential openings where foreign materials may fall in (with solid covers or mesh covers).  

02. Internal Impurities  

Severe carbonization caused by poor processing or equipment can lead to carbonized particles being carried into the strands, which, under pulling force, result in breakage.  

03. Poor Material Plasticization  

★ Low extrusion temperature or insufficient screw shear leads to incomplete plasticization, forming lumps that cause strand breakage under pulling force.  

★ Low-melting-point additives (such as EBS or PETS) in the formulation may cause poor plasticization if the screw shear is weak or if there is excessive clearance between the screw and barrel, leading to breakage.  

04. Variations or Inconsistencies in Raw Material Properties  

★ Blended components may have large differences in flowability at the same temperature. If the materials are not fully compatible (physically or chemically), it can lead to “phase separation.” Although phase separation is more common in injection molding, if the MFR (Melt Flow Rate) difference is too large and the screw shear is weak, strand breakage may occur.  

★ Changes in component viscosity: For the same material, a decrease in MFR may indicate increased molecular weight, resulting in higher viscosity. If the original processing temperature and conditions remain unchanged, this can lead to poor plasticization and strand breakage. Increasing extrusion temperature or reducing screw speed can resolve this.  

05. Poor Venting  

★ Excessively high processing temperature, localized overheating, or strong shear may cause decomposition of flame retardants or additives, releasing gases that are not removed in time by vacuum systems, resulting in trapped air in the strands and breakage under pulling force.  

★ Severe moisture in materials causes steam to be trapped if not removed through natural or vacuum venting, leading to strand breakage.  

★ Ineffective natural or vacuum venting (e.g., clogging, leakage, gaskets too thick) can also result in trapped gas or vapor, causing breakage under tension.  

06. High Material Rigidity, Overcooling, or Poor Pulling Coordination  

Highly rigid materials, excessive cooling, or too much water contact can cause strands to harden rapidly after exiting the die. If the traction force is not well matched, this sudden stiffness can lead to strand breakage. This often occurs in materials with high crystallization speeds or rigidity, such as glass-fiber-reinforced PBT, PET, PC, AS, or ABS. This is especially problematic during small-scale lab experiments. To resolve it, raise water temperature and reduce water exposure to keep strands sufficiently soft before entering the pelletizer.

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