Affordable machined parts available for any order size | Contact us now to obtain a quotation!

Flow Marks (Flow Lines) in Injection Molding: What Is It and How To Reduce Flow Marks

Just as any technology has its limitations, injection molding is no exception. For plastic parts with high aesthetic requirements, the presence of flow marks is undoubtedly an unacceptable flaw. Although the generation of flow marks does not affect the tolerance and structural stability of the product at the technical level.

If you frequently encounter flow mark issues in the injection molding process, it’s time to learn how to solve or prevent this problem. This article will take you through what flow marks are, the reasons for their occurrence, and how to prevent flow marks from appearing in plastic injection molding.

What is a Flow Mark?

Flow marks, also known as flow lines, typically manifest as wavy or ripple-like patterns on the surface of plastic parts. These marks remain clearly visible after the plastic has cooled and solidified, representing the uneven flow of the plastic melt within the mold.

Flow marks often appear near the plastic injection nozzle (also referred to as the gate) and may propagate outward along the surface of the part. The generation of this defect is primarily attributed to temperature gradients within the melt, resulting in unevenness in the plastic’s flow during the molding process.

As depicted in the diagram below, flow marks exhibit various forms, including circular, linear, or repetitive patterns. For plastic parts that pursue high quality and visual effects, the existence of flow marks is a problem that needs to be given sufficient attention.

plastic injection molded parts display flow marks of different shapes

What Causes Flow Marks in Injection Molding?

The causes of flow marks during injection molding are multifaceted, usually related to the uneven cooling rate of plastic materials. Here are some common reasons:

Poor Flow of Molten Material

Poor melt flow can result in radial flow marks on the surface of the plastic part with the gate as the center. When low-flow, high-viscosity melt flows in a semi-solidified and oscillatory state into the mold cavity through the gate and runner system, the melt flows along the mold cavity surface and experiences reflux and stagnation due to subsequent melt being continuously injected and compressed. This process leads to the formation of radial flow marks on the surface of the plastic part with the gate as the center.

Improper Mold Design

One of the key reasons for flow mark issues during the injection molding process is mold design defects. When there are flaws in the mold design, it directly affects the flowability and filling effect of the plastic melt in the mold, thereby increasing the occurrence of defects such as flow marks. Examples of mold design defects include poor venting, uneven wall thickness, inadequate lubrication, and undersized or improperly designed gates/runners/sprues.

It’s particularly important to note that if the design of the gate and runner system is too narrow or weak, it can significantly restrict the flowability of the plastic melt, reducing its temperature and pressure. This limitation can lead to various quality issues, including flow marks.

Injection Molding Machine Setup Issue

Injection molding machine parameters play a crucial role in avoiding flow mark issues.

Injection Pressure

If the injection pressure is too low, the plastic melt will struggle to evenly fill the mold cavity, especially in distant or complex areas of the mold. This uneven flow of plastic in the mold leads to the formation of flow marks.

Nozzle and Barrel Temperature

If the temperatures of the nozzle and barrel are too low, the plastic may not reach the ideal melting state during heating, thereby affecting its flowability. This results in uneven flow of plastic in the mold, leading to flow marks.

Dwell Time and Cycle Time

If the dwell time and cycle time of the plastic in the barrel are too short, the plastic may not have fully melted before being injected into the mold, leading to poor flowability and the formation of flow marks. Similarly, if the cycle time is too short, the heating time of the plastic in the barrel may be insufficient, which also affects its flowability.

How to Reduce Flow Marks in Injection Molding?

To minimize flow marks in injection molds, a series of adjustments and repeated experiments are necessary. Here are some suggestions and methods aimed at helping you optimize the injection molding process and reduce the probability of flow mark occurrence.

Adjusting Injection Pressure

Increasing the back pressure, holding pressure, and adjusting the injection pressure are the most direct methods to reduce flow mark defects in injection molding.

Back pressure is the pressure required to be generated and exceeded before the screw retracts. Increasing back pressure helps to push the fluid more evenly through the runner and mold, thereby improving product densification and reducing flow marks.

Holding pressure is the static pressure after injecting plastic, used to maintain a certain pressure in the mold cavity to ensure full filling of the mold and reduce defects caused by cooling shrinkage. Properly increasing holding pressure can improve the surface quality of the product, allowing the plastic to better adhere to the smoothness of the mold cavity.

Please note, do not adjust too much at once, as excessively high injection pressure can lead to jetting (another defect). Parameters should be adjusted gradually, and changes in the product should be observed to find the optimal pressure parameter settings.

Gate, Sprue, & Runner Design

It’s best to use a circular cross-section for gates and runners, as this shape provides optimal filling. However, if gates need to be placed in weak areas of the part, a square cross-section should be used. Additionally, larger cold slug wells should be placed at the bottom of the sprue and at the end of the runner. Temperature greatly affects the flowability of the melt, so attention should be paid to the size of the cold slug well. The cold slug well should be positioned at the end of the melt flow direction from the sprue.

If the main cause of radial flow mark formation is poor resin performance, low-viscosity resin can be used when conditions permit.

Changes to Temperature Parameters

Variations in mold temperature parameters are crucial for ensuring the flowability, viscosity, and overall quality of the plastic melt.

Most thermoplastic materials have recommended barrel temperature ranges. Operators should ensure that different areas of the barrel (such as the rear, center, front, and nozzle) are set to appropriate temperatures, with temperatures gradually increasing as the plastic progresses through the barrel. Typically, the temperature setting for each heating zone should be about 6°C higher than the previous zone to ensure uniform heating and melting of the plastic as it advances.

Additionally, the nozzle temperature should be increased to maintain melt flowability, usually about 6°C higher than the front barrel temperature. Then, adjust the mold temperature according to material recommendations to ensure it is not too cold, which could affect product quality. Through step-by-step testing and adjustment, find the optimal combination of process temperatures.

Mold Design Optimization

In mold design, incorporating rounded edges and corners can improve the flow of material within the mold, particularly when transitioning into thicker areas, effectively reducing the occurrence of flow marks and ensuring the smooth appearance and overall quality of the product.

Additionally, molds with uniform wall thickness can ensure uniformity in the cooling process, reducing issues such as deformation and cracking caused by temperature differences. For molds with variable wall thickness, particular attention should be paid to the design of rounded corners in thicker areas to further minimize uneven cooling due to non-uniform wall thickness.

To prevent blockages and cooling issues during operation, cold slug wells can be added at the end of the main flow channels. This measure ensures the smooth operation of the mold and the efficient production of the product, further enhancing production efficiency and product quality.

Improving Venting

Insufficient venting can lead to poor venting, causing obstruction to the filling of the melt and preventing the melt front from tightly pressing the condensed skin against the mold surface, leaving flow marks in the direction of flow. Considering venting at the end of each section of the runner can prevent gas from entering the cavity. Venting of the cavity cannot be overlooked. It is best to use venting around the entire perimeter.


In plastic injection molding processes, ensuring products are free from flow line defects is a key step in guaranteeing high-quality finished goods. Flow lines not only impact the aesthetic appeal of the product but may also pose potential threats to structural durability.

To effectively prevent the formation of flow lines, it is essential to gain a deep understanding of the common causes of flow line defects in injection molding and to collaborate closely with experienced manufacturers or suppliers to obtain professional advice and technical support to address these issues.

BOYI is the trusted partner you deserve. We understand the impact of flow lines on product quality. Therefore, throughout the entire production process, from prototype design to final production, we implement preventative measures to ensure your products are flawless.

If you are seeking a reliable injection molding partner to ensure your next production project is defect-free and smoothly brought to market, BOYI is your top choice. With our wealth of industry experience and professional technical team, we provide comprehensive support and injection molding services to ensure your products meet the highest standards from design to production. Feel free to contact us anytime!


How to get rid of flow lines in injection molding?

Repositioning the mold gates further away from the mold’s cooling system can extend the cooling time and mitigate the formation of flow lines. By increasing the nozzle diameter, you can enhance the flow rate of the plastic, which helps prevent premature cooling and subsequently reduces the occurrence of flow lines.

What is the difference between flow lines and knit lines?

The distinction between flow lines and knit lines lies in their visual appearance and formation. Flow lines manifest as wavy patterns of a slightly varied hue on the surface of a plastic part, sometimes forming a ring-shaped pattern near the mold’s entry points. Conversely, knit lines typically emerge as a visible line where two separate plastic flows converge and join.

Tagged: Injection Molding Guide

Scroll to Top