Recently, I conducted a thematic study on causes and solutions of injection molding defects. I systematically reviewed common quality defects in injection molded parts production, analyzing them from four aspects: problem manifestation, impact, cause, and solution.

Problem Manifestation: Localized depressions and voids appear in injection molded part, often occurring in thicker areas.
Impact: Damages product's appearance and reduces structural strength of thick-walled sections.
Cause Analysis:
Structural Design: Uneven wall thickness or excessively thick areas lead to inconsistent melt shrinkage during cooling.
Mold Factors: Insufficiently narrow runner design, excessively high mold temperature, reduced melt flowability and shrinkage compensation.
Injection Molding Process: Insufficient holding time, too low injection pressure, and too slow injection speed fail to provide sufficient shrinkage compensation pressure for thick-walled areas.
Solutions: Optimize mold thickness structurally to achieve a uniform transition, and remove excess material from unnecessary thick-walled areas.
Improve runner design at mold end and rationally control mold temperature.
In terms of process, increase holding time, appropriately increase injection pressure and speed to ensure sufficient shrinkage compensation in thick-walled areas.

 

Problem Manifestation: Blinders and flash appear on injection molded parts, with overflow at the parting surface and slides.
Impact: Rough appearance, easily scratching operators, and also damaging precision of assembly surfaces, leading to poor component fit.
Cause Analysis:
Raw Material Characteristics: Raw material temperature is too high, or its fluidity is too strong, easily overflowing from mold gaps during injection.
Mold Precision: Poor mold parting surface fit, insufficient mold hardness or strength, unable to effectively lock cavity.
Injection Molding Process: Injection speed is too fast, pressure is too high, and clamping force is insufficient, forcibly squeezing melt into mold gaps.
Solutions: Select a material with moderate fluidity at raw material end, or adjust raw material temperature.
Improve parting surface fit precision at mold end, strengthen mold hardness and strength.
In terms of process, slow down injection speed, reduce injection pressure, and increase clamping force to avoid melt overflow.

 

Problem Manifestation: Warping, twisting, and other deformations occur after demolding of injection molded part.
Impact: It damages product's appearance and causes assembly dimensions to deviate from design requirements, leading to assembly failure.
Cause Analysis:
Structural Design: Insufficient strength of plastic part, excessively thin or overly flat wall thickness, resulting in uneven stress distribution during cooling.
Mold Design: Inappropriate gate placement, improper ejection method, and uneven force during demolding.
Injection Molding Process: Excessive holding time, excessive injection pressure, and excessive residual stress inside plastic part.
Solutions: Structurally, appropriately increase reinforcement of ribs to enhance strength, optimize wall thickness distribution, avoid large and flat designs.
Properly position gate at mold end and optimize ejection mechanism to reduce demolding stress.
Process-wise, reduce injection pressure and shorten holding time to reduce residual stress inside plastic part.

 

Problem Manifestation: Excessive glue (excess glue) or insufficient glue (insufficient glue) appears in localized areas of injection molded part.
Impact: Excessive glue affects appearance and assembly interference; insufficient glue directly leads to functional defects or aesthetic flaws.
Cause Analysis:
Mold Factors: Mold design or manufacturing errors result in excessive/missed glue spots; insufficient ejector pin length leads to abnormal glue spots at ejector pins.
Injection Molding Process: Insufficient injection pressure, poor mold cavity venting, and inability of melt to fill cavity (insufficient glue).
Solutions: Insufficient glue can be resolved by adjusting mold (e.g., modifying ejector pin length, optimizing mold cavity venting).
Excessive glue can be left as is if it does not affect assembly functionality; otherwise, mold adjustment is necessary.

 

Problem Manifestation: White marks appear in the areas of injection molded part ejected by ejector pins.
Impact: Detracts from product's aesthetic appearance, significantly impacting products with high aesthetic requirements.
Cause Analysis:
Mold Factors: Ejector pins are too long or unbalanced, resulting in excessive localized pressure on part during ejection.
Injection Molding Process: Excessive injection pressure, excessive ejection speed, excessive injection speed, and excessive stress/deformation in ejector pin area.
Solutions: Adjust ejector pin length at mold end and correct ejector pin mating surfaces to ensure balanced ejection.
Reduce injection pressure, slow down ejection speed and injection speed to reduce stress in ejector pin area.

 

Problem Manifestation: Scratches and delamination marks appear on the surface of injection molded part.
Impact: Severely damages product's appearance and reduces its perceived quality.
Cause Analysis:
Structural Design: Insufficient draft angle, resulting in excessive friction between part and mold during demolding.
Mold Factors: Poor mold polishing, resulting in a rough surface and scratches on part.
Injection Molding Process: Insufficient mold temperature, excessive use of mold release agent, excessive lubrication or uneven stress on part surface.
Solutions: Increase draft angle of product to reduce demolding friction.
Improve polishing requirements at mold end to ensure a smooth cavity surface. In terms of process, increase injection mold temperature, minimize use of release agents, avoid excessive lubrication.

 

Problem: Due to poor mold venting, melt is abnormally heated and carbonizes, resulting in scorch marks.
Impact: It damages appearance and significantly reduces structural strength of carbonized area.
Cause Analysis:
Mold Factors: Poor mold venting, improper gate placement, melt stagnation and excessive heating.
Injection Molding Process: Plastic temperature too high, injection speed too fast, pressure too high, excessive melt shear heat, or poor venting.
Solutions: Improve venting structure at mold end, properly position gate, and ensure smooth melt flow.
In terms of process, reduce plastic temperature, decrease injection pressure, slow down injection speed, and avoid excessive heating of melt.

 

Problem: At multiple gates, melt merges from two or more directions, forming weld lines.
Impact: It damages integrity of appearance, and structural strength of weld line area is weak. Cause Analysis:
Raw Material Characteristics: Slow raw material flowability, insufficient drying, and moisture content affect melt fusion.
Mold Factors: Insufficiently large cold slug well, improper gate design, poor venting, cold material/gas hindering melt fusion.
Injection Molding Process: Insufficient mold temperature, insufficient material temperature, and insufficient injection pressure result in insufficient melt fusion power.
Solutions: Structurally, add ribs in areas prone to weld lines to strengthen the weld line area.
Improve venting at the mold end and properly design gate and cold slug well.
Process-wise, increase plastic and mold temperatures, increase injection pressure, and promote full melt fusion.

 

Problem Manifestation: Surface bulging/internal air entrapment.
Impact: Damages product appearance, affects structural strength and assembly accuracy.
Cause Analysis: Air entrapment (high speed/low back pressure), resin degradation (high temperature/long residence time), insufficient material drying.
Solutions: Reduce speed/increase back pressure, reduce barrel temperature, dry material, expand venting ports.

 

Problem Manifestation: Small flow marks near the gate.
Impact: Damages product appearance; residual gate or damaged areas affect assembly accuracy.
Cause Analysis: Process (low mold temperature, fast injection), mold (small gate, impact type), material (low fluidity).
Solutions: Increase mold temperature, use multi-stage injection to reduce gate speed, enlarge gate, use a higher fluidity material.

 

Improving quality of injection molded parts requires a multi-dimensional approach, including structural design, mold manufacturing, process parameters, and raw material management.
Only by accurately identifying causes of defects and optimizing design, mold, process accordingly can the quality of injection molded parts be effectively improved, ensuring product's appearance, structure, and assembly performance.