Injection Molding
Injection Molding Defects-Weld Lines, Weld Marks, Meld Lines or Knit Lines
Release time:2019-10-12 17:48:30| Views:

Injection Molding Defects - Weld Lines, Weld Marks, Meld Lines or Knit Lines

Weld Lines and Meld Lines

During the injection molding manufacturing process it is common to see the occurrence of lines where two flow fronts meet due to the inability of those two flow fronts to knit together — these lines are called weld lines or weld mark or knit lines. Weld and meld lines can be caused by holes or inserts in the part, multiple gates, or variable wall thickness where hesitation or race tracking occurs. Weld lines are injection molding defects that cause weak regions in the molded part which could result in breakage when the part is subjected to stresses. If weld or meld lines cannot be avoided, position them at low-stress and low-visibility areas by adjusting the gate position. Improve the strength of weld and meld lines by increasing the local temperature and pressure at their locations.

Difference between Weld lines and Meld Lines

Traditionally, the meeting angle is used to differentiate weld lines and meld lines. As you can see from below picture, a meeting angle, θ, smaller than 135° produces a weld line; greater than 135°, a meld line. Normally, weld lines are considered to be of lower quality than meld lines, since relatively less molecular diffusion occurs across a weld line after it is formed. We call both weld lines and meld lines as weld lines.

Weld lines and meld lines.png

Cause of weld lines defect:

Weld lines could be caused by different causes: material selection, Plastic part design, Injection Mold Design, Injection Molding Process and injection machine

Plastic Material:

Material type and dimension of the filler (fiber or particle)

Part Design:

Holes or Inserts in the part

Thick/Thin Part Section

Mold Design:

Multiple injection gates

Improper Injection Gate Size Location

Injection Molding Process Parameters:

Low Mold Temperature

Low Melt Temperature

Low Injection Pressure

Slow Injection Speed

Improper Flow Rate of injection material

Weld lines in the injection molded plastic parts:

Weld Lines.png

How To Control Weld Lines?

A weld line is often unavoidable, but it is possible to minimize their cosmetic effects to an acceptable degree through careful product design and manufacturing process control.

Plastic Material:

Use materials with high weld line stability.

Product Design:

Masking the presence by integrating fins, lines, raised/recessed lettering or heavy textures into the surface where a weld line would be formed

You can also round edges and corners to prevent wall thickness and resin flow rate changes that can affect the cooling rate of the plastic.

Increase wall thickness

Mold Design:

Improving venting in the weld area

Change the gate location to alter flow pattern

Plastic Injection Molding Process:

Increase injection speeds and injection molding pressure

Increase the mold temperature


In addition a weld line can be moved to an area where it is less visible. Therefore, it is recommended to do a moldflow study prior to the mold being built. molding simulation can predict weld line locations, weld line severity, and the flow front temperature at the weld line. It can predict if flow stops at the weld line or if flow continues in layers beneath the surface, thereby enhancing weld line strength. Simulation can also indicate if the gate is large enough to satisfy the filling speed needs without causing shear-related defects.

Weld lines on the plastic part.png

Want to learn more Injection molding defects such as Short Shot, Sink Marks, Burn Marks, Flow Lines, Warp and discoloration?

A right Mold Manufacturer & Injection Molding Contract Manufacturing Partner will always work with you to discuss practical molding issues in advance and will suggest corrective actions before the plastic mold is built. It’s important for you to know the physical limits to what can be done and how to allow for them in your design.

Contact us today for your injection molds and plastics molding projects.

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