Plastic fluidity refers to the ability of molten plastic to fill the mold cavity under temperature and injection pressure, which is a core indicator of injection molding that directly determines molding success rate, product quality and production efficiency. It is determined by molecular structure, molecular weight and additives, and affected by processing parameters and mold structure.
Core Measurement Standard of Plastic FluidityThe industry commonly uses Melt Flow Index (MFI/MI) to evaluate fluidity, which is the weight of molten plastic passing through a standard capillary in 10 minutes under standard temperature and pressure, in g/10min. Higher values mean lower viscosity and better fluidity; lower values mean higher viscosity and poorer fluidity. PP and PE are high-fluidity materials, while PC, glass-filled PA and POM have relatively low fluidity, which further decreases after modification.
Impact on Appearance and Internal Quality of Plastic PartsUnbalanced fluidity is the main cause of defects. Insufficient fluidity leads to slow filling, rapid cooling, high viscosity, and frequent defects such as short shots, obvious weld lines, sink marks, bubbles and streaks, especially in thin-walled, deep-cavity and complex parts. Insufficient fusion also reduces weld line strength, leading to cracking. Excessive fluidity causes flash from parting surfaces, slider gaps and ejector pins, increasing post-processing costs. Over-filling creates high internal stress, resulting in warpage, deformation and dimensional deviation.
Impact on Injection Process Parameter SettingFluidity is the basis for process parameters. Materials with poor fluidity require higher barrel and mold temperatures to reduce viscosity, and higher injection pressure and speed to fill the cavity, but this increases energy consumption and cycle time, and may cause thermal degradation. Materials with good fluidity have a wider processing window, allowing lower temperatures, pressures and speeds to avoid flash and internal stress, and shorter cooling time to improve efficiency.
Impact on Mold Design and Structure OptimizationMold design must match plastic fluidity. For materials with poor fluidity, runners should be short, thick and straight, and gates larger to reduce pressure loss. Wall thickness must be above the minimum molding thickness. High-fluidity materials support fine runners, small gates and thin-wall molds, saving mold materials, but require precise parting surfaces, vents and ejection systems to prevent flash.
Impact on Production Efficiency and CostMaterials with moderate and stable fluidity involve fewer testing times, faster debugging, lower rejection rates and shorter cycles. Materials with poor fluidity are difficult to test, require high-tonnage machines, and increase energy consumption and rejection costs. High-fluidity materials reduce equipment requirements but have higher material prices and may increase post-processing costs.
