In precision injection molding production, product quality directly impacts a company's market competitiveness. While the development of intelligent manufacturing technology in the new era has significantly improved production precision, some common quality defects still exist. The following analyzes the causes of specific defects and provides solutions.
I. Dimensional Accuracy Deviation
1. Defect Manifestation: Critical dimensions of the product exceed tolerance ranges (e.g., ±0.01mm), affecting assembly performance.
2. Cause Analysis: Unstable injection volume of the injection molding machine, excessive fluctuations in barrel temperature (exceeding ±1℃), uneven mold temperature distribution, or insufficient cooling time leading to inconsistent shrinkage.
3. Solutions: Employ intelligent injection molding machines equipped with high-precision displacement sensors (0.01mm accuracy) and adjust injection volume in real time through a closed-loop control system; upgrade the PID temperature control system to keep barrel temperature fluctuations within ±0.5℃; use multi-zone mold temperature controllers to ensure mold cavity temperature differences are ≤0.5℃; optimize the cooling cycle based on material characteristics (e.g., PC material requires a 30% longer cooling time) and monitor the product cooling curve using infrared thermography.
II. Surface Quality Defects
Sink Marks and Depressions
1. Defect Manifestations: Localized depressions appear where the product wall thickness is uneven, affecting appearance and structural strength.
2. Cause Analysis: Insufficient holding pressure or too short holding time, resulting in inadequate melt shrinkage; unreasonable mold gate design, leading to an excessively long melt flow path.
3. Solution: Employ a stepped pressure holding process (initial pressure 80% of rated pressure, reduced to 60% after 3 seconds), with real-time feedback and adjustment via pressure sensors; optimize gate location using CAE mold flow analysis software to shorten melt flow distance (maximum 50mm); use conformal cooling water channels for thick-walled areas (>3mm) to accelerate local cooling.
Silver Stripes and Bubbles
1. Defect Manifestation: Silver-white stripes or internal bubbles appear on the product surface, commonly seen in hygroscopic materials such as PA and PC.
2. Cause Analysis: Insufficient drying of raw materials (moisture content >0.02%), generating moisture during melting; excessively fast injection speed, trapping air that is not promptly expelled.
3. Solution: Use a dehumidifying dryer (dew point ≤ -40℃), adjusting the drying time according to the material (e.g., PA66 requires 4-6 hours); adopt a three-stage injection speed (low speed filling the gate, medium speed flowing through the main runner, high speed sprinting to the cavity end), and add micro-venting grooves (0.01-0.02mm deep) to the mold parting surface.
III. Structural Integrity Defects
Flash and Burrs
1. Defect Manifestation: Excess material edges appear at the mold parting surface or insert gaps, requiring secondary trimming.
2. Cause Analysis: Insufficient clamping force; poor mold cavity sealing; mold wear leading to increased clearance (exceeding 0.005mm).
3. Solutions: Detect mold clamping force fluctuation (should be ≤±2%), and replace the servo valve to improve control accuracy if necessary; periodically check mold clearance using a laser diameter gauge, and perform EDM repair on worn areas to ensure clearance ≤0.003mm; for high-viscosity materials (such as POM), appropriately reduce injection pressure by 10-15%.
Weld Lines
1. Defect Manifestation: A noticeable seam forms at the point where the two melt streams merge, with strength reduced by 20-30% compared to the bulk material.
2. Cause Analysis: Excessive temperature drop (>15℃) during melt fusion, resulting in poor molecular chain diffusion and fusion; poor mold venting, leading to air accumulation and hindering melt flow fusion.
3. Solutions: Install heating rods (5-10℃ higher than the barrel temperature) at the weld line location to locally increase the melt temperature; employ sequential valve gating technology to control the opening time difference of each gate (≤0.5 seconds) to prevent multiple material flows from merging simultaneously; increase the number of venting grooves to ensure at least one venting point per 100mm circumference, with a depth ≤0.015mm (ABS material).
IV. Mechanical Property Defects
Warping Deformation
1. Defect Manifestation: The product bends or twists after demolding, with dimensional deviations exceeding 0.1mm/m.
2. Cause Analysis: Uneven molecular orientation (shrinkage difference between the flow direction and the perpendicular direction >2%); uneven mold cooling leading to stress concentration.
3. Solutions: Use a rotary screw (speed fluctuation ≤5 rpm) to reduce melt shear stress; simulate stress distribution through mold flow analysis and install stress relief grooves in high-stress areas (such as corners); use a slow cooling process for crystalline materials (such as POM) to reduce the cooling rate difference (≤5℃/s).
Insufficient Strength
1. Defect Manifestations: The product is prone to breakage under stress, and the tensile strength is more than 15% lower than the standard value.
2. Cause Analysis: Poor melt plasticization (unmelted particle ratio >3%), or excessively fast injection speed leading to molecular chain breakage; impurities in the raw material (such as recycled material ratio exceeding 20%).
3. Solutions: Use a barrier screw to improve plasticization uniformity (melt temperature distribution difference ≤3℃); use a servo motor to control the injection speed, ensuring acceleration in the acceleration phase ≤50mm/s²; monitor raw material purity in real time using an online melt quality analyzer (such as a near-infrared spectrometer), and filter recycled material through a 100-mesh filter before addition.
V. New Era Intelligent Inspection and Prevention System
1. Online Inspection Technology: Deploy a machine vision system (0.005mm accuracy) on the production line for full dimensional and appearance inspection of each molded product; use an ultrasonic flaw detector to detect internal bubbles (identifying bubbles ≥0.1mm), and upload data to the MES system in real time to create a quality traceability archive.
2. Predictive Maintenance: Monitor the operating status of the injection molding machine screw using vibration sensors, record the aging trend of the barrel heating coil using temperature sensors, and combine AI algorithms to provide early warnings of potential faults (e.g., automatic alarm when heating coil power decay exceeds 10%).
3. Digital Twin Simulation: Construct a digital twin model of the entire production process, input raw material characteristics and equipment parameters to simulate the probability of defect occurrence, and optimize process parameters in advance (e.g., if the simulation shows a defect rate of 3% for a certain parameter combination, automatically adjust to a defect rate <0.5%).
By addressing the above defects in a targeted manner and combining the application of new-era intelligent technologies, the pass rate of precision injection molded products can be improved, while reducing the cost of defective products, creating significant benefits for enterprises.
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