What is Flow marks?

Flow marks are a common surface defect in the plastic injection molding process. They appear as lines, streaks, or ripples on the molded part’s surface, typically following the direction of the molten plastic flow. Although they don’t always impact the part’s functionality, they can compromise its aesthetic appeal—especially in visible or visually critical applications.

Flow marks occur when the molten plastic cools unevenly or when there are abrupt changes in speed, pressure, or temperature during mold filling. They may also form around inserts, in narrow sections, or near complex geometries. In these areas, the material may begin to solidify before completing its flow path, leaving visible traces once the cycle finishes.

The main causes of flow marks include low mold temperature, incorrect melt temperature, insufficient injection speed, or poor mold venting. They may also result from an inadequate gate design or the use of low-flow materials. Additionally, if colorants or fillers are not well dispersed, they can worsen the visual impact of flow marks.

Preventing flow marks involves optimizing processing parameters. Adjusting the mold and melt temperatures, increasing injection speed, improving gate and runner design, and ensuring proper venting are some of the most effective corrective actions. It’s also important to select a material with suitable flow characteristics for the part’s geometry and thickness.

Part and mold design also play a key role. Smooth transitions between sections, generous radii in internal corners, and strategically placed injection points can help reduce or eliminate flow marks. In many cases, flow simulations are used during the design phase to identify critical areas and implement design improvements before mold fabrication.

At Plásticos Lezo, we place great emphasis on the visual quality of our plastic parts. When flow marks are detected, our technical team conducts a root cause analysis and applies process adjustments or mold design changes. We work closely with clients to ensure that the final part meets both functional and visual requirements.

We also use advanced monitoring systems to control the injection process in real time and make proactive adjustments. This preventive approach helps us minimize scrap, improve cycle efficiency, and ensure surface quality even in large production runs or complex geometries.

In summary, flow marks are a common but manageable defect in injection molding. Effective control requires deep knowledge of the process, material properties, and mold design. At Plásticos Lezo, we apply technical and engineering solutions to deliver clean, uniform, and aesthetically acceptable surfaces in every product we manufacture.