Injection-molded parts can be affected by various defects that compromise both appearance and functional performance. These imperfections may arise from challenges in the molding process, such as material selection, mold design, or processing parameters. Addressing a particular defect often requires coordinated adjustments by the injection molding service provider, mold designer, or part engineer.

In this article, we explore the 9 most common injection molding defects and provide insight into their causes, along with practical solutions to ensure high-quality, reliable, and defect-free parts.

Sink Marks

Sink marks appear as depressions or dimples on the surface of injection-molded parts, most commonly in thicker areas where the outer layer solidifies faster than the inner material. These surface imperfections can affect both the appearance and structural integrity of the part if not properly addressed.

Common Causes of Sink Marks

Sink marks are typically caused by uneven cooling or insufficient packing pressure during the molding process. Parts with varying wall thicknesses are especially prone to this defect, and materials with higher shrinkage rates can exacerbate it.

How to Prevent

Design parts with uniform wall thickness wherever possible. Adjusting packing pressure and holding time can compensate for material shrinkage in thicker sections. Optimizing the mold’s cooling system to achieve even temperature distribution also helps reduce the incidence of sink marks. In some cases, using low-shrinkage materials or employing gas-assisted molding can further minimize surface depressions.

Warping

Warping is a deformation that occurs when parts bend, twist, or distort after ejection from the mold. This defect typically arises from uneven cooling or internal stresses that develop during solidification, leading to dimensional inaccuracies or assembly issues.

How to Prevent

Ensure uniform cooling throughout the part by optimizing mold design and cooling channel layout. Maintaining consistent mold and melt temperatures and adjusting cooling times can further reduce internal stress. Additionally, selecting materials with lower shrinkage rates and optimizing gate location and size can help parts retain their intended shape.

Short Shots

Short shots occur when molten plastic fails to fill the mold cavity, resulting in incomplete parts. This defect can lead to wasted material, increased production time, and reduced manufacturing efficiency.

Common Causes of Short Shots

Insufficient injection pressure or speed, low melt temperature, or poor venting that traps air inside the mold. High-viscosity materials or complex mold designs with thin walls or distant cavities can also make it difficult for the molten plastic to fill the cavity.

How to Prevent

Adjust the injection speed and pressure to ensure proper flow of the molten plastic. Maintaining optimal melt temperature and properly calculating the shot size for each mold design also helps achieve complete cavity fill. Additionally, improving venting systems and optimizing gate design can prevent trapped air and facilitate smoother flow into all sections of the mold.

Flash

Flash occurs when thin layers of molten plastic escape from the mold cavity, commonly along parting lines, ejector pin locations, or gate areas. This excess material typically happens when the injection pressure is too high, the clamping force is insufficient, or the mold components are worn or misaligned. Overpacking the mold can also contribute to flash formation.

How to Prevent

Preventing flash requires ensuring that the mold is properly aligned and the clamping force is adequate to counter the injection pressure. Regular inspection and maintenance of mold components to identify and repair wear or damage can further reduce the likelihood of flash.

Additionally, optimizing packing parameters and injection settings can minimize this defect while maintaining the quality and dimensional accuracy of the molded part.

Weld Lines (Knit Lines)

Weld lines, also known as knit lines, are visible lines that appear where two melt flows meet but fail to bond properly. These imperfections can affect the surface appearance and may reduce the structural integrity of injection-molded parts.

Common Causes of Weld Lines

Weld lines often form due to multiple flow paths, such as around holes or cores. They can also result from low melt temperature, slow injection speed, or poor material compatibility, which prevent the molten plastic fronts from fusing completely.

How to Prevent

Increase the melt temperature and injection speed to promote better fusion. Relocating gates or modifying part design can help avoid interrupted flow paths. Choosing materials with better fusion properties also reduces the likelihood of weld lines, ensuring stronger, more visually consistent parts.

Burn Marks

Burn marks in injection molding appear as black or brown discolorations on the part surface. These defects often occur at the ends of the flow path or near gate locations. They can indicate underlying issues such as trapped air burning, overheating of the material, or residual moisture.

How to Prevent

Improve venting to allow trapped air to escape and reduce injection speed or melt temperature to avoid overheating. For moisture-sensitive materials, pre-drying the resin before molding can further minimize the risk of burn marks. Adjusting processing parameters and ensuring proper mold design can help produce parts with smooth, defect-free surfaces.

Bubbles and Voids

Bubbles and voids are internal hollow spaces or air pockets that can form within injection-molded parts. Bubbles are often near the surface, while voids are more common in thicker areas where rapid cooling or trapped air prevents proper filling. These defects can compromise both the structural integrity and appearance of the part.

How to Prevent

Pre-dry materials thoroughly to remove moisture or volatile gases. Increasing packing pressure and holding time helps the molten plastic fill cavities. Additionally, optimizing cooling rates and gate design ensures uniform solidification and reduces the likelihood of trapped air forming voids.

Ejector Pin Marks

Ejector pin marks are protrusions or impressions left on the surface of injection-molded parts where the ejector pins push the part out of the mold. These marks can affect the cosmetic appearance and, in some cases, the functional fit of the part.

How to Prevent

Adjust the ejector force and speed to prevent excessive pressure on the part. Proper alignment and placement of ejector pins can also reduce surface impressions. Adding draft angles, improving mold surface finish, and, if needed, using more pins with smaller diameters can further prevent this defect and produce smoother, defect-free surfaces.

Jetting

Jetting occurs when wavy flow patterns or streaks form near the gate due to turbulent or uncontrolled molten plastic flow. This defect is commonly caused by high injection speed, poor gate design (such as small gates facing open cavities), low melt viscosity, or moisture in the material. Jetting affects the surface appearance and quality of the molded part.

How to Prevent

Reduce injection speed or use multi-stage injection for smoother flow. Optimizing gate design, such as using submarine or fan gates, helps evenly guide the plastic into the mold cavity. Adjusting the melt temperature to achieve proper viscosity and pre-drying moisture-sensitive materials further minimizes the risk of jetting.

Final Thoughts on Injection Molding Quality

While the defects listed above are common in injection molding production, many other challenges can arise in real-world manufacturing. Avoiding these issues requires DFM expertise and proactive guidance that goes beyond simply knowing what can or cannot be produced. With Livepoint, a trusted injection molding supplier in China, you gain practical part design recommendations and process insights to prevent defects before they occur, ensuring higher quality and greater efficiency in your production.