Weld lines and air traps: where they form, how to reduce them

Weld (knit) lines

When the melt separates around a hole, a boss, or multiple gates and then rejoins, a permanent line can remain between the two fronts. This line is both an appearance and a strength problem; especially in glass-fiber-reinforced materials, the fibers end up perpendicular to the joining direction along the line, and strength drops noticeably.

To reduce it

• Increase melt and mold temperature: Fronts that meet hot fuse better.
• Increase injection speed: Leaves less time for the front to cool.
• Add venting: Air trapped between the fronts prevents fusion; opening a vent at the meeting point is often the most effective step.
• Change the gate location: Move the line to a hidden or low-stress region. This is the most permanent solution possible in mold design.
• Choose a more free-flowing grade (one with a higher MFI/MVR value on the data sheet).

Air traps and burning (dieseling)

If the air ahead of the advancing melt can't escape, it gets trapped, heats up rapidly, and burns the plastic: you typically see a black-brown burn mark at the last-filling point of the part. This is called dieseling.

Signs

• Burn mark at the last-filling point, at a corner or at the base of a rib.
• Gets worse as speed increases, improves as speed decreases.
• May appear together with short shots.

Solution order

1. Gradually reduce speed (speed profile) in the last-filling region.
2. Clean the vent channels; they are often dirty or closed at the parting line.
3. Add a vent channel to the mold, or make use of the clearance around ejector pins; solutions using porous steel inserts also exist.
4. Don't excessively increase the mold clamping force: too much tonnage partially closes vent channels.
5. Don't keep the melt temperature unnecessarily high.

Note: These two problems can be confused with each other. If there's a dark mark along the weld line, the problem is an air trap; if the line is colored but clean, the problem is cooling and fusion.

The calculation tool gives you a starting point for speed setting by checking values such as fill speed, shear rate and pressure.