How Would the Weld Line Occur ?

About the weld line

This article has showed the  phenomenon, causes and solutions of the weld line occuring in the prossess of injection molding.

1. Phenomenon

Materials flow in two or more directions. When they join again, the resin temperature drops and they don’t fuse, which results in a weld line.

weld line

2. Cause

  • Large flow resistance in sprue, runner and gate
  • Poor degassing of sprue and runner
  • Poor air removal from the cavity
  • Insufficient degassing of the mold
  • Gate position
  • Insufficient injection pressure
  • Low cylinder temperature (lowresin temperature)
  • Lowmold temperature
  • Slowinjection speed

3. Measures

  • Increase the cross-sectional area of the sprue, runner, and gate
  • Also shorten the length, smooth the surface
  • Provide sufficient gas venting to the sprue and runner
  • Add sufficient air bleeding to the mold
  • Degas the mold
  • Move the gate position so that welds will occur in places with a low load
  • Increase injection pressure
  • Increase cylinder temperature
  • Increase mold temperature
  • Increasethe injection speed
  • (increasing the temperature at which the split resins merge and fuse)

By adding the vent function, the air resistance in the mold is eliminated and the internal pressure is reduced.

Degas is just below the sprue part and at the runner end. Increase the air vent near the final filling part of the product part.

By taking good venting measures with the mold, there is no air resistance in the mold and molding at low pressure is possible.

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