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Common Defects in Injection Molding and Ways to Prevent Them

Learn simple tricks to avoid defects in injection molding. Follow our easy guide for flawless results in your projects.

Injection molding stands out as an excellent plastic molding process, yet like any method, it has its downsides. Quality issues persist, affecting product safety and performance. These issues arise from mold manufacturing, material nature, and tooling design.


Common Problems and Practical Solutions in Injection Molding


Flow Lines

Wavy patterns on flat-based mold products, like buckets or plastic boxes, result from cooling speed variations. To remedy, increase injection speed, material temperature, and pressure for uniform filling. Opt for an even molding surface and enlarge the nozzle diameter.


Burn Marks

Rust-colored or black discoloration on plastic edges, known as burn marks, stems from trapped air or resin overheating. Lower mold and melting temperatures, decrease injection speed, and enhance gas vents and gates to release trapped air. Also, reduce molding cycle times.



Uneven mold shrinkage causes warping, impacting product performance. Slow down the cooling process, lower material temperature, switch to materials with less shrinkage, and redesign the mold for symmetry.


Air Pockets and Vacuum Voids

Air bubbles inside products from inadequate pressure and fast cooling can compromise structural integrity. Increase injection pressure, choose lower viscosity materials, and position mold gates strategically for proper air release.


Sink Marks

Small depressions on flat plastic surfaces occur due to slow cooling. Raise holding pressure, increase cooling time, and thin component walls for faster surface cooling.


Weld Lines

Visible bonding lines occur when identical parts have varying temperatures. Increase material temperature, injection speed, and pressure to prevent premature cooling. Use a matrix without partitions and materials with lower melting temperatures.



Squiggly lines on the surface, called jetting, result from premature solidification. Lower injection pressure, increase temperature, and redesign the gate for optimal filling.



Poorly cleaned melting crucibles lead to discoloration from overheated plastic pellet mixing. Clean molds regularly, use purging compounds, and select colors with good thermal stability for consistency.



An extra layer on the surface, resembling a faded coating, occurs due to contamination. Pre-dry plastic, avoid contamination during pellet mixing, and invest in automated ejection mechanisms.



Escaping molten plastic creates extended thin patches on the product extremities. Increase clamp pressure, maintain mold cleanliness, and balance injection speed, mold temperature, and ventilation.

injection molding

Specifical Ways to Prevent Defects in Injecion Molding


Mind the Temperature


Just like Goldilocks and her porridge, the mold temperature has to be just right. Too hot or too cold, and you might encounter some unwanted surprises. Find that sweet spot, and you're golden.


Pressure Control 


Balancing the pressure in injection molding is crucial. It's like a seesaw; find the equilibrium, and you'll have a smooth ride.


Quality Materials 


Using top-notch materials is like having the Avengers on your side. They ensure strength, durability, and a flawless finish. Invest wisely, and your products will thank you.


Regular Maintenance 


Your mold needs some love too! Regular maintenance checks are the TLC injection molds crave. A well-maintained mold is a happy mold, producing flawless products every time.



Injection molding, while prolific in plastic manufacturing, presents challenges. These defects, though common, are manageable with proper measures. For professional plastic injection molding services and comprehensive defect solutions, visit our website. Your manufacturing journey deserves expertise and precision.

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