Blog

Weld lines, also known as knit lines, are common defects in injection molding that can impact the appearance, strength, and functionality of molded parts. These lines occur when multiple streams of molten plastic meet in the mold and fail to fully fuse. Understanding how to prevent weld lines is essential for achieving high-quality, defect-free parts. This article provides a comprehensive look into the causes of weld lines, strategies to prevent them, and best practices to ensure flawless injection molding.

Understanding Weld Lines

Weld lines are visible marks or seams on the surface of a molded part where streams of molten plastic have converged. These lines typically exhibit the following characteristics:

• Visual Defects: Weld lines may appear as faint lines or seams on the part's surface, sometimes accompanied by slight discoloration or surface imperfections.

• Reduced Mechanical Strength: Parts with weld lines can have lower mechanical strength and impact resistance due to incomplete fusion of the plastic streams.

• Functional Issues: In critical applications, weld lines can affect part functionality, potentially leading to failure during use.

Causes of Weld Lines

Several factors contribute to weld line formation in injection molding:

• Flow Fronts Meeting: Weld lines form when multiple streams of molten plastic meet but do not fully fuse. This often occurs when flow fronts are too far apart or cool too quickly before merging.

• Mold Design: Poor mold design can exacerbate weld line formation. Factors such as inadequate gate positioning, improper flow channel design, and uneven mold temperatures can affect how plastic flows and solidifies.

• Material Properties: The flow and solidification characteristics of different materials vary. High-viscosity materials or those with a low melt flow index are more prone to weld lines.

• Injection Speed and Pressure: Insufficient injection speed or pressure can lead to incomplete filling of the mold cavity, preventing proper fusion of the flow fronts.

• Cooling Conditions: Uneven or excessive cooling rates can create temperature differences at weld line locations, leading to defects.