Melt fracture is a common and quality-affecting problem in masterbatch extrusion molding. Melt fracture manifests as a rough, uneven extruded surface, sometimes exhibiting wavy, bamboo-like, or spiral distortions, and in severe cases, breakage. This phenomenon primarily stems from the shear stress experienced by the melt during extrusion exceeding its tolerance limit, leading to irreversible elastic deformation and resulting in flow instability. Effectively preventing melt fracture in masterbatch extrusion molding requires a comprehensive approach, encompassing process parameter optimization, equipment improvement, raw material selection, and standardized operating procedures.
Optimizing process parameters is crucial to avoiding melt fracture. Extrusion temperature directly affects the melt viscosity and flowability. If the temperature is too low, the melt viscosity becomes too high, increasing flow resistance and potentially causing shear stress concentration; conversely, excessively high temperatures may trigger melt degradation, similarly compromising flow stability. Therefore, it is essential to precisely control the temperature of each section of the extruder based on the specific composition and characteristics of the masterbatch to ensure the melt flows within a suitable viscosity range. Simultaneously, matching the extrusion speed and screw rotation speed is also critical. Excessive extrusion speed can exacerbate melt shear deformation, while excessively high screw speeds can lead to insufficient melt residence time in the barrel and uneven mixing. By gradually adjusting and finding the optimal parameter combination, the risk of melt fracture can be significantly reduced.
Equipment design and maintenance are equally important in preventing melt fracture. The screw structure of the extruder directly affects the plasticizing and conveying of the melt. Using a gradient screw or a barrier screw can optimize the shearing and mixing process of the melt and reduce local stress concentration. Furthermore, the die design must fully consider the melt flow characteristics. A streamlined die inlet area can reduce resistance when the melt enters, avoiding flow turbulence caused by sudden contraction or expansion. Regularly cleaning residues from the die and screw surfaces to prevent melt adhesion and slippage abnormalities due to increased surface roughness is also an important measure for equipment maintenance and preventing melt fracture.
Raw material selection and pretreatment play a fundamental role in preventing melt fracture. Different types of masterbatches have different molecular weights, molecular weight distributions, and additive compositions, and their sensitivity to shear stress also varies. Selecting masterbatches with a narrow molecular weight distribution and low melt elasticity can reduce the tendency for melt fracture. Simultaneously, the moisture and volatile substances in the raw materials must be strictly controlled. These impurities vaporize at high temperatures, forming bubbles that disrupt melt continuity and exacerbate flow instability. Therefore, the masterbatch must be thoroughly dried before extrusion to ensure its moisture content meets process requirements.
Strict adherence to operating procedures is crucial to preventing melt fracture. During extrusion, continuous and uniform feeding must be maintained to avoid sudden changes in melt pressure caused by feed fluctuations. Close monitoring of extruder current and pressure changes is also essential for timely detection and handling of abnormalities. For example, a sudden increase in current may indicate melt blockage or degradation within the barrel, requiring immediate shutdown and inspection. Furthermore, regular maintenance and upkeep of the equipment to ensure optimal operating conditions are also vital for preventing melt fracture.
Environmental factors are also critical. Fluctuations in workshop temperature and humidity can affect masterbatch performance and the stability of the extrusion process. For example, high temperatures can cause masterbatch to absorb moisture, increasing its moisture content; while excessive humidity can lead to equipment corrosion, affecting its precision and lifespan. Therefore, it is essential to maintain a constant workshop environment to provide stable external conditions for the extrusion process.
Formulation optimization is a deeper solution to avoid melt fracture. By adjusting the types and proportions of additives in the masterbatch, its processing performance can be improved. For example, adding an appropriate amount of lubricant can reduce friction between the melt and the equipment surface, reducing shear stress; while using external lubricants can improve the melt's release properties and prevent flow turbulence caused by adhesion. Furthermore, adding nanoparticles or fiber reinforcement materials can improve the melt's strength and toughness, enhancing its resistance to shear deformation.
