Technical document

Quality defects and solutions of precision injection molding products

2025-07-04 10:01:15 precision injection molding
In the field of precision injection molding, product quality is the core factor for enterprises to gain a foothold in the market. Although intelligent manufacturing technology has significantly improved production stability, various quality problems still occur in actual production. The following analyzes common problems and proposes targeted solutions combined with the application of new-era technologies.
I. Loss of Dimensional Accuracy
  1. Problem Performance: Critical dimensions exceed the preset tolerance (such as ±0.01mm), directly affecting the assembly accuracy of components and possibly leading to the failure of the entire machine.

  1. Cause Analysis: Insufficient stability of the injection molding machine's metering system, with barrel temperature fluctuations exceeding ±1℃; uneven distribution of the mold's temperature field, resulting in significant differences in shrinkage during the cooling stage; unreasonable setting of pressure-holding parameters, leading to poor replenishment effect.

  1. Countermeasures: Use a closed-loop control injection molding machine equipped with a 0.01mm-level displacement sensor to calibrate the injection volume deviation in real-time; upgrade the temperature control system to PID + fuzzy control to compress barrel temperature fluctuations within ±0.5℃; configure a multi-zone mold temperature controller to ensure that the temperature difference between each area of the cavity is ≤0.5℃; optimize the cooling curve according to material characteristics (for example, PC materials need 30% longer cooling time) and achieve precise temperature control with infrared temperature measurement.

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II. Surface Quality Defects
  1. Sinks and Depressions

  • Problem Performance: Local indentations appear at the transition of wall thickness, which not only affects the appearance but may also become stress concentration points.

  • Cause Analysis: Insufficient pressure-holding pressure or time, resulting in untimely melt replenishment; improper design of gate position, leading to excessive pressure loss due to too long melt flow path.

  • Countermeasures: Implement a stepwise pressure-holding process (initial 80% of rated pressure, reduced to 60% after 3 seconds) and dynamically adjust through pressure feedback; use CAE mold flow analysis to optimize gate layout, controlling the maximum flow distance within 50mm; adopt conformal cooling water channels in thick-walled areas (>3mm) to accelerate local solidification speed.

  1. Silver Streaks and Bubbles

  • Problem Performance: Silver-white streaks on the surface or bubbles inside, which are particularly common in hygroscopic materials such as PA and PC.

  • Cause Analysis: Excessive moisture content in raw materials (>0.02%), generating water vapor at high temperatures; too fast injection speed leading to incomplete exhaust of the cavity and entrainment of air to form bubbles.

  • Countermeasures: Use a dehumidifying and drying system with a dew point ≤-40℃, and set drying parameters according to material characteristics (for example, PA66 requires 4-6 hours); adopt three-stage injection speed control (low speed in the gate section, medium speed in the main runner section, and high speed at the end of the cavity), and set 0.01-0.02mm deep micro 排气槽 on the parting surface.

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III. Structural Integrity Defects
  1. Flash and Burrs

  • Problem Performance: Excess material edges appear on the mold mating surface, requiring additional trimming processes, increasing costs and possibly affecting assembly.

  • Cause Analysis: Insufficient or uneven clamping force, leading to mold sealing failure; long-term use resulting in mold mating gap exceeding 0.005mm.

  • Countermeasures: Regularly detect the clamping force fluctuation value to ensure it is ≤±2%, and replace the high-precision servo valve if necessary; use a laser diameter gauge to monitor the mold gap, and repair worn parts to ≤0.003mm by electrical discharge machining; appropriately reduce the injection pressure by 10-15% for high-viscosity materials such as POM.

  1. Weld Lines

  • Problem Performance: Obvious seams form at the confluence of melt splits, with strength 20-30% lower than the main body.

  • Cause Analysis: The temperature of the melt drops by more than 15℃ when converging, resulting in insufficient diffusion and fusion of molecular chains; poor exhaust of the cavity, with accumulated gas hindering melt combination.

  • Countermeasures: Add heating rods (temperature 5-10℃ higher than the barrel) at the weld line position to increase the local melt temperature; adopt sequential valve gating technology to control the opening time difference of each gate to ≤0.5 seconds; set 1 exhaust slot per 100mm circumference, with the depth of the exhaust slot for ABS materials ≤0.015mm.

IV. Insufficient Mechanical Properties
  1. Warpage and Distortion

  • Problem Performance: The product bends or twists after demolding, with straightness deviation exceeding 0.1mm/m, affecting assembly accuracy.

  • Cause Analysis: Excessive difference in molecular orientation (shrinkage difference between flow and vertical directions >2%); uneven mold cooling causing internal stress concentration.

  • Countermeasures: Use a rotating screw (rotational speed fluctuation ≤5rpm) to reduce melt shear stress; identify high-stress areas through mold flow analysis and set stress relief slots; adopt a slow cooling process for crystalline materials such as POM to control the cooling rate difference ≤5℃/s.

  1. Strength Reduction

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  • Problem Performance: The mechanical properties of the product decrease, with tensile strength 15% lower than the standard value, making it prone to fracture.

  • Cause Analysis: Uneven plasticization (unmelted particles accounting for >3%); too fast injection speed leading to molecular chain breakage; excessive proportion of recycled materials (>20%) or too many impurities.

  • Countermeasures: Replace with a barrier screw to improve plasticization effect, ensuring that the melt temperature distribution difference is ≤3℃; use servo drive to control the injection acceleration ≤50mm/s²; monitor the purity of raw materials through an online near-infrared detection system, and filter recycled materials through a 100-mesh filter before adding.

In the new era, full inspection can be realized through machine vision (0.005mm accuracy), ultrasonic flaw detection can identify bubbles ≥0.1mm, and combined with digital twin models to simulate and optimize process parameters, the qualified rate of products can be increased, significantly reducing quality loss.

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