What is weld lines defects?

When plastic flows around an obstruction  (for example a hole) in a mold cavity the point where the material comes back together is called a weld line defects. If two more more plastics material flow together after coming together it will become a weld line. Weld lines defects will always have when two or more melt flows meet together. we call knit lines defects as well

Weld lines defects issues and troubleshooting skill

Weld lines defects are formed when two flow fronts are merged together. An obstruction such as a core pin or holes will split the material flow and cause a weld line issue to form on the other side.

Weld lines defects can also form when the flow fronts from two gates merge or when thick and thin sections in a part cause a flow to split and merge. Since plastic cools as it flows, the separate flow fronts are partially solidified when they meet and form the weld line.

This can lead to quality issues. Weld lines issues are usually a visible line on the part surface and can be an appearance issue. Weld lines may also reduce the part strength by 10% – 20% potentially creating a structural issue depending on their location. The strength reduction is worse in glass filled materials because the fibers do not cross the weld line.

The below image is a weld line plot. The black line on the part represent welding line. On this part there are two major weld lines. The weld line in the side of the part is created when the flow fronts from the two separate gates meet. The weld lines at the end of the flow are formed when the flow front travels around the pin that forms the hole and meets up on the other side.Weld lines

To avoid problematic weld lines issues,  it is critical to know where weld lines will form on appearance or strength critical parts. Mold flow analysis provides a weld line plot that details the location of weld lines. Note that the analysis can only be used to predict weld line location, not weld line strength or appearance. However, weld lines can be moved to low stress areas or to higher packing pressure areas of the part by changing the gate location.

If a plastic injection mold is built without considering the impacts of the number and position of weld lines, the mould may not produce an acceptable part,

Welding lines defects troubleshooting skills

Once the mold have been made and the weld lines defects appear, to solve this welding line issue by using below troubleshooting skill will improve this issue.

  • Increase injection pressure
  • Increase melt temperature
  • Increase mold temperature
  • Enlarge vents
  • Change gate location
  • Increase runner diameter

If mold flow analysis is not used to fix  these problems before the mold is built, the plastic mold company will be forced to use one of the above costly solutions!