Flow lines and injection molding defects: definition, how to intervene during molding, causes and remedies

Flow lines and injection molding defects

By flow lines and injection molding defects we mean all the imperfections that can occur on molded parts during the molding of plastic.

There are numerous defects that can occur during the production of plastic parts.

In this article we will focus on flow lines.

Before going into the details of this defect, it is important to provide some information on the injection molding production process.

Injection molding is a very popular industrial production technique for the production of plastic objects.

The molten plastic inside the mold is cooled and solidified, shortly after the mold opens and the molded piece is extracted manually or thanks to robotic systems.

Defining Flow Lines, From Theory to Practice

Let’s now move on to defining the flow lines.

These are imperfections that occur on the surface of the printed product.

The appearance of flow lines is easily recognizable: the printed component appears with stripes of a different color than the rest of the material.

Flow marks in injection molding do not affect the functionality of the molded part but do compromise its aesthetics.

For this reason, it is important to understand how to intervene in the production process to definitively resolve the problem and obtain high-quality injection-molded parts.

What causes flow lines?

The causes of this problem are largely attributable to variations in the cooling rate of the plastic.

Another element that influences the formation of flow lines are changes in direction, i.e. changes in the orientation of the flow of the molten plastic inside the mold.

Furthermore, the sliding of plastic on variable thicknesses can also cause this defect.

In addition to the elements mentioned there are other parameters that can cause the creation of flow lines.

For example, too low melting temperature, too low injection speed, too small dimensions of the guides

Remedies for injection molding defects

From what has been said above, it is evident that to repair the defect in the flow lines it is necessary to intervene on the mold design or on the molding parameters.

Some interventions to overcome this problem could be:

• increase the nozzle diameter to delay cooling;
• intervene on the injection speed, pressure or temperature of the material;
• consider changing the material to the same characteristics required by the project.

If what is indicated is not possible, it is possible to intervene on the piece with:

• Painting or other finishes on the printed piece;
• Inserting texture into mold to cover flow line marks;

In short, the solutions can be numerous and of different nature.

Obviously, to choose the most suitable solution, it is definitely necessary to entrust your project to a supplier who has a very solid experience in design, production and injection molding.

To have a summary diagram of the main defects that can occur, download our guide “Solutions for molding problems”, you will have a very useful summary diagram to use as a vademecum for your projects.

Our experience, concrete examples and case studies

In 20 years of activity and experience in the design and production of plastic parts we have hundreds of production equipment that we have on contract for injection molding.

The following images are all projects that we have created on behalf of our customers respecting deadlines, quality of the printed pieces, and complete customer assistance for all problems during production and printing.