Injection molding defects in plastic products involve a complex process that requires precise coordination of materials, molds, machinery, and process parameters. Any issue in these areas can lead to defects.
Short Shot
Phenomenon: The mold cavity is not completely filled, resulting in an incomplete product.
Main Causes:
Insufficient injection pressure/speed: The melt cannot overcome flow resistance.
Low material/mold temperature: Poor fluidity of the melt.
Unbalanced or undersized gates: Improper design of runners or gates.
Poor venting: Trapped gas at the end of the cavity creates resistance.
Solutions: Increase injection pressure, speed, and temperature; improve mold venting; optimize gate design.
Flash
Phenomenon: Excess thin layers of plastic appear at the mold parting line or insert gaps.
Main Causes:
Insufficient clamping force: Cannot withstand injection pressure, causing the mold to separate.
Excessive injection pressure: Exceeds the clamping force limit.
Mold wear or misalignment: Contamination or wear on the parting surface prevents proper sealing.
High material temperature: Low melt viscosity allows flow into gaps.
Solutions: Increase clamping force; reduce injection pressure; inspect and clean the mold parting surface.
Sink Marks
Phenomenon: Localized depressions or voids on the product surface, typically in thick-walled areas.
Main Causes:
Insufficient holding pressure/time: Unable to compensate for volumetric shrinkage during cooling.
Uneven or insufficient cooling: Thick areas cool slower, leading to greater shrinkage.
Product design issues: Non-uniform wall thickness with significant variations.
Solutions: Increase holding pressure and time; optimize cooling system design; modify product design for uniform wall thickness.
Weld Lines
Phenomenon: Lines formed where two or more melt fronts meet and fuse.
Main Causes:
Low mold temperature: The melt fronts cool too quickly, preventing proper fusion.
Poor venting: Trapped gas at the weld line.
Improper runner/gate design: Long flow paths or unfavorable convergence angles.
Solutions: Increase mold and material temperature; improve venting; add overflow wells or adjust gate positions.
Splay Marks / Silver Streaks
Phenomenon: Silver or white streaks resembling hairline cracks on the product surface.
Main Causes:
Moisture or volatiles: Insufficient drying of the material generates steam during heating.
Degradation: Overheating or excessive screw speed causes material breakdown and gas formation.
Entrapped air: Excessive injection speed traps air in the melt.
Solutions: Thoroughly dry the material; reduce barrel temperature; decrease injection speed; improve venting.
Warping
Phenomenon: Bending or twisting of the product after cooling, deviating from the intended shape.
Main Causes:
Internal stress imbalance: The fundamental cause.
Uneven cooling: Different cooling rates on opposing sides cause inconsistent shrinkage.
Molecular/fiber orientation: Higher shrinkage in the flow direction than perpendicular to it.
Unbalanced ejection: Uneven ejection force or improperly placed ejector pins.
Solutions: Optimize cooling channel layout; adjust holding pressure curves; lower mold temperature; balance the ejection system.
Black Specks / Streaks
Phenomenon: Black or brown spots or streaks on or inside the product.
Main Causes:
Material degradation: Residual material in dead zones of the barrel carbonizes over time.
Contamination: Mixing of different materials, colors, or dust.
Screw or barrel wear: Metal shavings mix into the melt.
Solutions: Clean the barrel and screw; inspect and clean hot runners; prevent material contamination; reduce barrel temperature.
Poor Surface Gloss / Flow Marks
Phenomenon: Loss of desired surface gloss, appearing as a foggy or orange-peel-like rough area.
Main Causes:
Low mold temperature: The melt cools too quickly upon contact, failing to replicate the mold’s smoothness.
Material issues: Contamination or insufficient lubricant in the material.
Slow injection speed: The melt front advances too slowly and cools prematurely.
Solutions: Increase mold temperature; raise injection speed; ensure material purity.
Addressing different injection molding defects requires comprehensive consideration. Solutions may involve adjusting process parameters or modifying mold features such as cooling channels and venting systems. This necessitates evaluation and planning by an experienced engineering team to develop an effective solution.