What are the Ways to Avoid Injection Molding Flow Lines?

by Bobby Coyle

Injection molding, flow lines, and flow markings are frequent aesthetic flaws. Flow lines often appear as wavy lines or streaks on the surface of the molding and are caused by uneven molten material flow and inconsistent cooling in various regions of the molding.

Even though flow lines shouldn’t compromise the accuracy or the structural integrity of a molded item, they can be unattractive and may be unacceptably disruptive for components that must meet strict visual requirements.

The basic principles of injection molding flow lines are covered in this article, along with how you can avoid them through outstanding mold design and molding methods and reduce their visibility through post-processing techniques. In this article, we will discuss the method to avoid injection molding flow lines and minimize their existence.

What do Flow Lines in Injection Moulding Mean?

In circular patterns, wavy flow lines, and plastic injection molding or ripples on a molded part’s surface that are often prominent close to the molding’s gates, where material joins the mold cavity. Occasionally they resemble the grooves on a vinyl record.

Maintaining a uniform material flow is one of the most challenging aspects of injection molding in order to adequately fill the entire cavity and have the molding cool at a consistent pace throughout to avoid warping. When the molten plastic has cooled unevenly inside the mold, flow lines frequently result. When molten material encounters a hollow wall, it cools and solidifies while the inner portion of the stream continues to flow.

Where the circulating material attains the portion against the wall that has prematurely consolidated, a ripple occurs. Uneven cooling can have a number of variables, such as mold design, injection pressure, and mold temperature.

How to Avoid Flow Lines?

Notwithstanding the fact that flow lines are not amongst the more critical injection molding flaws, the majority of manufacturers and clients are opposed to seeing them. Thankfully, flow markings can be eliminated by altering the injection molding process and the mold itself.

With injection molding simulation tools like mold flow analysis, mold design, and significant parameters can normally be adjusted to improve efficiency.

Reducing the Presence of Flow Lines

Even though it’s usually ideal to address the problem of flow lines at their root, that is, by changing the circumstances for material flow, it is sometimes possible to merely minimize their appearance by covering them up. Several post-processing and polishing techniques can be used to accomplish this.


Adding texture to the mold itself is one method for reducing the visibility of flow markings. By providing the surface of the mold with a mottled or uneven texture, you can prevent the ugly flow lines that are more noticeable on flat surfaces. Nevertheless, depending on the part’s planned application, this may have a detrimental influence on how well it works. Mold texturing has the added advantage of not requiring special treatment for each and every molding.


Painting and pad printing are two post-processing methods that can conceal flow lines. The fact that only one color pigment can be established in the injection molding pellets before they undergo molding, depending on the part’s intended operation, painting one or more surfaces may be advantageous. Pad printing may be employed in order to add pictures or logos on a molding’s surface and, by the way, to hide flow lines.


Metallization, in which a thin metal coating is put to the surface of the part, is another technique for hiding flow lines. All of these techniques can reduce the visibility of flow lines, but they also raise the price per unit.

Last Remarks

Inadequate injection pressure and speed are some of the main causes of flow lines. Low pressures enable the material to enter the mold cavity slowly. As a result, flow lines are formed as certain areas of the material cool before others. Increasing the nozzle diameter and back and hold pressure will be helpful in facilitating the passage of material through the opening.

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