What is weld line in injection molding?

A weld line in injection molding refers to a visible line or ridge that occurs at the junction of two halves of melted plastic that have met but not fully fused together during the molding process. It results from the slow rate of flow of plastic in the mold and can be reduced by proper mold design and control of injection molding parameters such as injection speed and pressure. Weld lines can impact the strength and appearance of the molded part, so they are often considered a defect in high-quality molded products.

weld line injection molding
weld line injection molding

What reason will cause weld line in injection molding parts?

Weld line in injection molding is very normally defects. Weld lines in injection molded parts can occur due to several reasons, including:

  1. Insufficient filling time: If the plastic does not have enough time to fully flow and fuse together, it can result in a weld line.
  2. Improper gate design: A gate that is too small or poorly placed can cause plastic to flow in different directions, resulting in a weld line.
  3. Viscosity variation: If the viscosity of the plastic changes as it is being injected, it can cause the plastic to flow unevenly and result in a weld line.
  4. Non-uniform wall thickness: Parts with non-uniform wall thickness can result in plastic flowing at different speeds, causing a weld line.
  5. Improper injection speed or pressure: Injection speed and pressure that are too high or too low can impact the flow of plastic and result in a weld line.

To minimize the occurrence of weld lines, these factors need to be carefully controlled during the injection molding process.

How to solve the injection molding welding line issue?

Here are some ways to solve the injection molding weld line issue:

  1. Optimize mold design: Proper gate placement, runner design, and part design can help reduce the occurrence of weld lines.
  2. Use balanced pressure molding: This technique helps to ensure that the plastic is filling the mold evenly, reducing the likelihood of a weld line.
  3. Adjust injection speed and pressure: By adjusting the speed and pressure of the plastic as it is being injected, the flow of plastic can be controlled to minimize the formation of a weld line.
  4. Use flow-promoting additives: Certain additives, such as flow improvers, can be added to the plastic to help it flow more evenly and reduce the occurrence of a weld line.
  5. Improve process control: By carefully monitoring and controlling the injection molding process, weld lines can be reduced or eliminated.
  6. Post-mold processing: In some cases, weld lines can be removed or smoothed through secondary processing, such as sanding, grinding, or polishing.

Implementing a combination of these solutions can effectively reduce or eliminate weld lines in injection molded parts.

If you have welding line issues in your injection molding parts and need some help, you are welcome to contact us, we are happy to support.

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