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How to Reduce Flow Mark in Injection Molding


As a leading provider of custom injection molding services, Longsheng understands the challenges that many manufacturers face when it comes to minimizing flow marks in their molded products. Flow marks are unsightly lines or streaks that appear on a plastic surface, caused by uneven flow of plastic material during the injection molding process. These marks not only compromise the aesthetic quality of the finished product, but can also weaken its structural integrity.

Fortunately, there are several effective ways to minimize the occurrence of flow marks in injection molding. In this guide, we’ll walk you through the key factors that contribute to flow marks and offer practical tips on how to reduce their presence.

What are Flow Marks?

Flow marks, also known as flow lines, are visible lines or streaks that appear on the surface of plastic parts. These marks are caused by variations in the flow rate of molten plastic as it moves through the injection molding machine. Flow marks can occur when the plastic cools too quickly or when there are inconsistencies in the molding process.

Why are Flow Marks a Problem?

Flow marks can be a serious problem for injection molding manufacturers, as they can affect the quality and aesthetics of the final product. Flow marks can make parts look unappealing, reduce their functionality, and in some cases, compromise their structural integrity.

Tips for Reducing Flow Marks in Injection Molding

Understanding the Causes of Flow Marks

Flow marks are typically caused by changes in the direction of flow of the molten plastic during injection molding. There are several factors that can contribute to these changes, including:

  • Variation in the wall thickness of the product
  • Changes in the part geometry or design
  • Insufficient injection pressure or flow rate
  • Inadequate temperature and pressure control
  • Poor quality or contaminated raw materials

In order to reduce flow marks, it’s important to identify and address these underlying causes.

Tips for Reducing Flow Marks in Injection Molding

Optimize part design and mold flow analysis: Before starting the injection molding process, it’s important to carefully optimize the part design and analyze the mold flow to ensure consistent material flow and reduce any areas of high shear stress.

Ensure proper temperature and pressure control

Maintaining consistent temperature and pressure levels throughout the injection molding process is essential for preventing flow marks. This includes ensuring that the plastic material is at the correct temperature and pressure before injection.

Optimize injection speed and pressure

Adjusting injection speed and pressure to suit the product design and material properties can help reduce flow marks. Increasing injection speed or pressure can help to ensure consistent material flow and reduce pressure variations.

Use high-quality raw materials

Using high-quality raw materials that are free from contamination or impurities can help to reduce flow marks. This includes ensuring that the materials are properly dried and stored to prevent moisture or other contaminants from affecting the finished product.

Regular maintenance and cleaning

Regular maintenance and cleaning of the injection molding machine can help to prevent flow marks and other issues. This includes ensuring that the mold and barrel are cleaned regularly, and that any worn or damaged parts are promptly replaced.

Injection Molding

Choose the Right Injection Molding Material

The type of plastic you use for injection molding can also impact the likelihood of flow marks occurring. Some plastics are more prone to flow marks than others, so it’s important to choose the right material for your specific application. At Longsheng, we work with a wide range of high-quality plastics that are designed to minimize the chances of flow marks.

Use a High-Quality Mold

The mold used for injection molding can also impact the quality of the final product. A high-quality mold can help to ensure that the plastic flows evenly and reduce the chances of flow marks occurring. At Longsheng, we specialize in custom injection molding, and our team of experts can design and manufacture a mold that is tailored to your specific needs.


Reducing flow marks in injection molding requires a combination of careful design, optimization of the injection molding process, and attention to the quality of raw materials and machinery. At Longsheng, we take a comprehensive approach to injection molding that emphasizes precision, quality, and attention to detail. Contact us today to learn more about our custom injection molding services and how we can help you minimize flow marks in your products.


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