Several process parameters affecting injection molding quality
Injection pressure is a key parameter affecting injection molding quality, directly determining whether the melt can fill the mold cavity and form a dense part. Its setting should be determined based on a comprehensive consideration of material flowability, part wall thickness, and mold resistance. Too low an injection pressure can lead to underfilling (for example, a PP part experienced a 20% missing edge rate when the pressure was reduced from 120 MPa to 90 MPa). Too high an injection pressure can increase internal stress (for example, a PC lens experienced an 8% higher cracking rate at 180 MPa than at 150 MPa). For thick-walled parts (>5 mm), higher pressures (150-180 MPa) are required to ensure shrinkage compensation. For a PA66 bearing sleeve, shrinkage porosity was reduced from 5% to 0.5% at 160 MPa. For thin-walled parts (<1 mm), controlled pressure (80-120 MPa) is required to prevent flash. For a mobile phone housing, the flash defect rate was only 0.3% at 100 MPa. The stability of injection pressure is equally important. A servo injection molding machine uses closed-loop control to reduce pressure fluctuations to <±2%, and the weight deviation of plastic parts is reduced from ±1% to ±0.3%.
Melt temperature directly affects material fluidity and degradation, and is a key factor in determining the appearance and mechanical properties of plastic parts. Too low a temperature can lead to insufficient fluidity, resulting in noticeable weld marks in the mold cavity (the weld mark strength of an ABS electrical housing at 200°C is 25 MPa, increasing to 28 MPa at 220°C). Too high a temperature can cause material degradation. For example, a POM gear exhibited yellowing and a 10% decrease in impact strength at 220°C. Crystalline plastics must be kept 20-40°C above their melting point (for example, PP has a melting point of 165°C and a processing temperature of 180-200°C) to ensure uniform crystallization. Amorphous plastics must be kept 100-150°C above their glass transition temperature (for example, PC has a glass transition temperature of 150°C and a processing temperature of 260-300°C). The uniformity of melt temperature needs to be achieved through segmented temperature control of the barrel. A certain mold divides the barrel into three sections (rear section 200℃, middle section 210℃, front section 220℃), so that the melt temperature deviation is <±3℃, and the color difference of the plastic part is reduced from ΔE3.0 to ΔE1.2.
The setting of holding pressure and holding time is crucial to the dimensional stability of plastic parts. It replenishes the melt before the gate solidifies and compensates for cooling shrinkage. Holding pressure is typically 60-80% of the injection pressure. For a PE part using a 70% holding pressure, dimensional fluctuations decreased from ±0.2mm to ±0.08mm. Excessively high holding pressures can increase internal stresses. A PS toy cracked at 80% holding pressure, but recovered after reducing it to 60%. The holding time must be long enough to ensure gate solidification. For a PC lampshade, Moldflow simulation determined a gate solidification time of 15 seconds, and the holding time was set to 18 seconds, avoiding sink marks. Too short a holding time can lead to insufficient shrinkage compensation. For a PP bucket, reducing the holding time from 12 seconds to 8 seconds resulted in sink marks as deep as 0.3mm at the bottom. The pressure decay curve during the holding stage also needs to be optimized. A certain car bumper uses a stepped holding pressure (100MPa→80MPa→60MPa), which reduces the shrinkage rate difference between areas with different wall thicknesses from 2% to 0.5%.
Mold temperature significantly impacts the crystallinity, internal stress, and surface quality of plastic parts, requiring precise control based on material type and appearance requirements. Crystalline plastics require higher mold temperatures (50-120°C) to promote crystallization. For example, increasing the mold temperature of a PA66 gear from 60°C to 90°C increased the crystallinity from 65% to 78%, and dimensional stability improved by 20%. Amorphous plastics require lower mold temperatures (30-80°C). For example, the surface gloss of an ABS housing at 50°C reached 90GU, higher than 75GU at 30°C. Mold temperature uniformity is crucial. For example, a washing machine panel experienced warpage of 0.8mm due to uneven water channel layout, resulting in a local temperature difference of 10°C. After optimization, the temperature difference was controlled within 3°C, reducing the warpage to 0.2mm. For exterior parts, a mold temperature controller can be used for precise temperature control (with an accuracy of ±1°C). Using this method, the surface scratch rate of a television front frame was reduced from 5% to 0.5%.
Injection speed determines the time it takes for the melt to fill the mold cavity and its flow pattern. Too fast or too slow can cause quality issues. High-speed injection (50-100 mm/s) can reduce weld marks (a car dashboard’s weld marks disappear at 80 mm/s), but it can easily lead to turbulent flow and bubbles. Low-speed injection (10-30 mm/s) is suitable for precision plastic parts, but excessive filling times can lead to uneven cooling (a PC lens shrinks 1.5% more at the edges than at the center at 20 mm/s). Complex plastic parts require multi-stage speed control. For example, a mobile phone case’s filling is divided into three stages: 30 mm/s (near the gate) → 60 mm/s (main body) → 20 mm/s (corners), reducing the surface defect rate from 8% to 1%. Injection speed and pressure must be adjusted in tandem. One test showed that a 20% increase in speed required a 10% increase in pressure to maintain filling efficiency. This matching improved the production stability of a PP part, achieving a CPK value from 1.0 to 1.6.
