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What Causes Flow Marks in Injection Molding?

Uncover the secrets behind flow marks in injection molding. And you will figure out what is flow mark and the causes of it.

If you've ever held a plastic item and noticed those subtle streaks or wavy lines on its surface, you've encountered flow marks. These marks can be a real headache for manufacturers, affecting the aesthetic appeal and structural integrity of the final product. But what causes them?


What Are Flow Marks?


Flow marks, also known as flow lines, manifest as wavy patterns on a molded object's surface, often appearing as circles or lines after cooling. They indicate irregularities in the plastic flow as it enters the mold due to temperature variations in the melt. These marks exhibit slight color tone differences due to light refraction and pigment redistribution.


Identifying these flaws prompts corrective actions by engineers or plastic injection mold service providers. A well-designed process and mold system are key to preventing flow marks, emphasizing the importance of early recognition to save time and money in the long run.


What Causes Flow Marks?


Flow marks in injection molding stem from various factors within the technology. These issues typically manifest at the gate and often result from machine settings configured prior to gate installation. The key culprits contributing to flow marks include:

1. Slow Melt Injection Speed


The speed at which the melt is injected plays a crucial role. It must reach the mold efficiently, retaining sufficient heat. If the temperature is too low, parts of the melt solidify as it passes through the gate, leading to altered flow patterns and the creation of flow marks.

2. Very Low Melt Injection Pressure


Pressure directly affects the rate of melting. Insufficient pressure hampers compaction, essential for melting homogeneity. Adequate injection pressure evens out flow patterns, preventing the formation of flow marks. Insufficient pressure results in uneven flow and the unwanted appearance of marks.


3. Too Low Mold Temperature


Mold cooling is intrinsic, but if the temperature is excessively low, issues arise. Rapid cooling within the mold can cause premature solidification of the melt around the gate, leading to flow marks. Maintaining an optimal mold temperature is critical to avoid these complications.

4. Too Low Melt Temperature 


The temperature of the melt directly influences its viscosity. Inadequate heat can result in flow markings, indicating operation at a suboptimal temperature. Proper heating is essential to ensure plastic distortion and flow, even if it comes at a cost in terms of fuel consumption.

5. Wrong Gate or Runner Size


Inappropriate dimensions for gates and runners can impede flow. Thin runners restrict the flow, slowing down the melting process and exposing portions of the melt to temperature loss. This issue is particularly pronounced in cold runner systems, where the smaller flow diameter brings the melt's center closer to the runner's walls, contributing to the formation of flow marks.


Tips on Avoid Flow Marks


1. Optimize Melt Injection Speed


Keep it steady and consistent. A slow melt injection speed can lead to uneven flow, causing those pesky marks. Aim for a speed that ensures the melt reaches the mold efficiently, without losing its heat along the way.


2. Ensure Adequate Melt Injection Pressure


Pressure is your friend. Make sure it's high enough to promote even melting. This helps in maintaining a uniform flow pattern, leaving no room for those undesirable flow marks.


3. Maintain the Right Mold Temperature


The mold's temperature matters more than you might think. Avoid setting it too low, as rapid cooling can result in premature solidification and, you guessed it, flow marks. Find the sweet spot for optimal mold temperature.


4. Check Melt Temperature


Don't let it get too cool. The temperature of the melt influences its viscosity. Operating at a lower-than-ideal temperature can lead to flow marks. Ensure you're running at the right temperature for smooth plastic flow.


5. Mind Your Gates and Runners


 Size matters here. Ensure your gates and runners are not too thin, as this can restrict the flow. Optimal dimensions prevent unnecessary slowing of the melting process and reduce the chances of flow marks.


6. Invest in Quality Mold Design


 A well-designed mold is worth its weight in gold. It promotes uniform flow and minimizes the risk of imperfections. Don't cut corners on mold design; it's a crucial factor in achieving a flawless finish.


7. Regular Maintenance is Key 


Machinery needs love too. Keep an eye on your equipment, ensuring it's in top-notch condition. Regular maintenance prevents unexpected hiccups in the injection molding process that could lead to flow marks.


8. Train Your Operators


Knowledge is power. Make sure your operators are well-trained and understand the intricacies of the injection molding process. This includes the importance of maintaining the right settings to avoid flow marks.




Flow marks in injection molding pose an avoidable challenge. Once they mar a product, rejection is likely, leading to potential client loss for your injection molding company. While these marks might not hinder product functionality, they significantly impact its visual appeal.

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