What is Weld Line Defects in injection molding

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 line defects in injection molding is a line or lines on the part surface that caused the part looking ugly or cuased part lifetime, or even broken easily. These weld lines occur usually around holes or at the material flow joint area in the injeciton molded part, some welding lins is very small, specially if the molded part made in firber glass material, must be very carfully in the massive production.

Welding line

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.

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 in injection molding 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!

Case study weldline in injection moulding

Several years ago a customer transferred an old single cavity mold to our facility. The part was a very simple rectangular ring. It had two sub-gates, one at each end of the part on the short side of the rectangle. The material was glass-filled and made a very strong part.

We made our first sample submission and were approved for production immediately. The next day we delivered the first parts for their order. A couple of weeks after the plastic parts were delivered I received an email from our customers General Manager thanking us for the very fast service, then he went on to say none of the parts were usable because they did not pass “the hammer test”.

The first words out of my mouth were “what is the hammer test? And why wasn’t this test requirement talked about before this”. after communication I observed the hammer test. It was quite simple; the part was set on a metal table so it stood up resting on its long axis. A technician then proceeded to beat the daylights out of the part with the pointy end of a steel masonry hammer.

The part broke in the very middle of the long side. I examined the part and asked if they had this problem before. The general manager said this happens all the time and it is a major problem. As it turned out the hammer was used to seat a metal piece between two of the plastic parts to make the assembly. The metal pieces were too stiff to assemble by hand so the hammer was used.

My further examination showed that the failure had occurred at one of the weld lines in the part. Weld lines occur when plastic flows around a core and then joins back together, maybe sometimes you can not see this by eyes, but if the part has a function requirement, like this case the part need load some power, so even the weld lines does not existing still will be broken, because there is still joint line that that filling joint area.

The solution was very simple, the problem was that no one looked at the way the part was gated and how the part would be assembled. I told the Custom that we will slightly change the mold a little and send you the new samples that would not break.

I had one of our mold that have weld line issue on the molded part and we simply blocked off one of the two gates. This meant that all the material would flow through one gate and the single weld line that resulted from the new flow pattern would be on the short axis. The old flow pattern produced a weld line in the middle of both of the long sides. We tested the parts with our own hammer and could not produce a failure. I immediately sent the newly re-gated parts to the customer and asked them to test them with their hammer.

They could not break the parts either, we took the first parts back and scrapped them and we delivered the new parts the next two days.

Sincere Tech is one of top 10 plastic injection molding companies in China that offer custom injection molding and delvier to the worldwide, we have rich experience to solve all of molding defects, all of our molding parts will be 100% meet your requirement, if you have any project that needs plastic injection molding service from China, welcome to contact us.