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10 Most Common Plastic Part Defects in Injection Molding

Injection molding is a widely used manufacturing process for producing plastic parts. However, it is not without its challenges, and understanding the most common plastic part defects is essential for ensuring high-quality production. In this article, we will explore the 10 most common plastic part defects in injection molding, categorized into surface defects, dimensional defects, and structural defects.

Key Takeaways

  • Understanding the most common plastic part defects is crucial for maintaining high-quality production standards.

  • Proper mold design and maintenance are essential for minimizing plastic part defects in injection molding.

  • Effective process control and monitoring can help identify and address plastic part defects early in the production cycle.

  • Material selection plays a critical role in minimizing dimensional defects such as warping and shrinkage in injection-molded plastic parts.

  • Structural defects such as short shot and splay can be minimized through proper injection molding parameters and tooling design.

Surface Defects

Sink Marks

Sink marks are a common surface defect in injection molding. They appear as dimples or shallow depressions on the surface of the molded part. Sink marks are caused by thicker than normal cross sections, non-uniform cooling, or insufficient packing pressure. These defects can affect the aesthetics and structural integrity of the part. It's important to address the root causes of sink marks to ensure high-quality molded parts.

Weld Lines

Weld lines are formed when two flow fronts meet and result in a visible line on the surface of the part. These lines can weaken the structural integrity of the part and affect its appearance. Proper mold design and optimized processing parameters are crucial for minimizing the occurrence of weld lines.

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Flow Lines

Flow lines are visual patterns that can appear on the surface of plastic parts due to the varying speed at which the molten plastic flows into the injection mold. These lines often look like wavy patterns or ring-shaped bands on the part where the flow of the plastic has slowed down or changed direction.

To minimize the appearance of flow lines, consider the following tips:

  • Adjust the injection speed to ensure a consistent flow of plastic.

  • Optimize the temperature of the molten plastic and the mold to prevent premature cooling.

  • Design the mold with smooth transitions and adequate venting to facilitate even flow.

Remember that flow lines may not affect the structural integrity of the part, but they can be aesthetically displeasing and indicative of potential issues with the injection molding process.

Dimensional Defects


Warping is a common dimensional defect in injection molding that results in the part taking on a twisted or distorted shape. This can occur due to uneven cooling or improper mold design. Addressing the root causes of warping is crucial for ensuring the dimensional accuracy and integrity of the final product.

To prevent warping, consider optimizing the cooling process, adjusting the mold design, and using materials with lower shrinkage rates. Additionally, maintaining uniform wall thickness and minimizing internal stresses can contribute to reducing the occurrence of warping in plastic parts.

Warping Prevention Strategies

  1. Optimize cooling process

  2. Adjust mold design

  3. Use materials with lower shrinkage rates

  4. Maintain uniform wall thickness

  5. Minimize internal stresses


Shrinkage is a common issue in injection molding that affects the quality of produced parts. It occurs when the material contracts as it cools, leading to dimensional inaccuracies and surface imperfections. To mitigate shrinkage, careful monitoring of molding conditions is essential. This includes controlling factors such as temperature, pressure, and cooling time to minimize the impact of shrinkage on the final part dimensions. Additionally, adjusting the material composition and gate design can also help reduce the effects of shrinkage.


Flash is a common dimensional defect in injection molding, occurring when excess material seeps out of the mold cavity, usually along the parting line or inserts. This defect typically results from excessive pressure or poor mold design. It can lead to increased production time and material waste. Addressing flash issues early in the manufacturing process is crucial to prevent further complications and maintain product quality. Implementing regular mold maintenance and optimizing process parameters can help minimize the occurrence of flash defects.

Structural Defects

Short Shot

Short Shot is a common defect in injection molding that occurs when the mold cavity is not completely filled with the molten plastic. This can result from factors such as low injection pressure, material flow restrictions, and inadequate venting. To prevent short shots, it is important to optimize the injection pressure, ensure proper material flow, and improve venting in the mold design. Additionally, using the right mold temperature and gate size can also help prevent this defect. It is crucial to address these factors to achieve high-quality plastic parts without short shots.


Splay is a cosmetic defect that manifests as silver or white streaks on the surfaces of molded parts. It occurs when moisture or other contaminants are present in the resin. The presence of these contaminants causes the resin to decompose, resulting in the formation of gas bubbles during the injection process. This leads to the streaking effect on the surface of the molded parts. To prevent splay, it is important to ensure that the resin is properly dried and free from contaminants before the injection process. Additionally, maintaining consistent processing conditions can help minimize the occurrence of splay.


Brittleness is a critical structural defect in plastic parts that can lead to catastrophic failure under stress. It is characterized by a lack of flexibility and a tendency to fracture or break easily. This defect is often caused by improper material selection or processing parameters. Addressing brittleness requires careful consideration of material properties and processing conditions to ensure the final product meets the required mechanical specifications. Manufacturers must prioritize material testing and analysis to identify potential brittleness issues early in the production process. Additionally, implementing quality control measures and regular inspections can help mitigate the risk of brittleness in plastic parts.


In conclusion, addressing these common plastic part defects in injection molding is crucial for ensuring product quality and reducing manufacturing costs. By implementing effective quality control measures and leveraging advanced technologies, manufacturers can minimize the occurrence of defects and improve overall production efficiency. It is imperative for industry professionals to stay updated on the latest advancements in injection molding processes and materials to mitigate the impact of defects and enhance the performance of plastic parts.

Frequently Asked Questions

What causes sink marks in plastic parts?

Sink marks are caused by variations in wall thickness, cooling rates, and part design, leading to uneven shrinkage during the cooling process.

How can weld lines be prevented in injection molding?

Weld lines can be prevented by optimizing the mold design, using proper gate locations, and controlling the injection speed and pressure to minimize flow front collisions.

What is the main cause of flow lines in plastic parts?

The main cause of flow lines is inadequate material flow and pressure, resulting in visible lines or streaks on the surface of the part.

What factors contribute to warping in injection-molded parts?

Warping is often caused by uneven cooling rates, residual stresses, and inadequate part design, including insufficient draft angles and wall thickness variations.

How can shrinkage in plastic parts be minimized during injection molding?

Shrinkage can be minimized by adjusting processing parameters, using proper gate locations, and optimizing the part and mold design to ensure uniform cooling and material flow.

What measures can be taken to prevent flash in injection-molded parts?

To prevent flash, proper mold maintenance, adequate clamping force, and precise control of injection pressure and temperature are essential to ensure that the mold remains closed during the entire injection cycle.

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