Injection Molding Defects: Voids – Understanding the Cause and How to Fix Them

What Are Weld Lines?

Weld lines are visible lines or seams that appear on molded plastic parts where two or more streams of molten plastic meet but do not fully fuse. While they may look like simple surface imperfections, weld lines can create structural weak points in the part. Depending on the application, these weak spots can lead to premature failure, reduced impact resistance, or cosmetic defects that affect the final product’s appearance.

What Causes Weld Lines?

Weld lines form when separate flow fronts of molten plastic converge but do not fully bond at a molecular level. Several factors contribute to this issue, including:

• Cooling Too Quickly – If the plastic starts to solidify before the flow fronts merge, the material won’t properly fuse, leading to a visible weld line.
• Inadequate Pressure – Insufficient injection pressure can prevent the flow from fully merging, creating a weak seam.
• Part Design – Complex geometries, thin walls, and features like holes or inserts can disrupt flow, increasing the chances of weld lines forming.
• Material Properties – Certain plastics are more prone to weld lines due to their viscosity and flow characteristics.

How to Prevent Weld Lines

Minimizing weld lines requires careful adjustments in the molding process and part design. Some key strategies include:

• Increase Mold and Melt Temperature – Keeping the plastic hotter for longer allows the flow fronts to merge more thoroughly, reducing the appearance of weld lines.
• Optimize Injection Speed and Pressure – A higher injection speed and pressure help push the material together with enough force to fuse properly.
• Modify Part Design – Adjusting wall thickness, gate locations, or adding flow leaders can help control how the plastic fills the mold, minimizing weld line formation.
• Use Additives or Different Materials – Some materials or additives improve flow properties and enhance bonding at weld lines.

Strengthen Your Parts with Aprios

If weld lines are causing issues with your parts—whether it's reduced strength, poor aesthetics, or inconsistencies in production—Aprios can help. Our expertise in injection molding allows us to optimize your design, material selection, and process parameters to ensure strong, seamless parts. Reach out to us today and let’s eliminate those weak spots together.