Defect guide

Weld line (knit line)

A line and a weak zone where two melt fronts meet.

Also known as: weld line, knit line, joining line, meld line

Symptoms

  • Line around holes or between gates
  • Break or crack along this line
  • Clear loss of strength in glass-fiber-filled material

Tip: If there is a dark mark along the line, the problem is trapped air; if the line is clean, it is cooling and fusion.

Order of solutions

  1. Increase the melt and mold temperature
  2. Increase the injection speed
  3. Add venting at the meeting point
  4. Change the gate location so that the line moves to a non-critical area
  5. Choose a more flowable grade

Likely causes

Process setting

  • Melt or mold temperature too low
  • Injection speed too low (the front cools down)
  • Holding/pressure insufficient

Raw material

  • Viscous grade; low MFI

Mold

  • No venting at the meeting point
  • Gate location unsuitable

Part design

  • Flow splitting around holes/protrusions
  • Multiple gates

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Other defects

Short shot (incomplete fill)

The cavity does not fill completely; the farthest or thinnest area of the part stays incomplete.

incomplete fill · underfill · non-fill

Flash

Plastic escapes through the parting line or around the ejector pins.

flash · overflow · parting line flash

Sink mark

A dent on the surface opposite a thick section; the surface is drawn inwards.

sink mark · depression · sink

Void (internal vacuum void)

Internal cavity in a thick section of the part; the outer surface may look perfect.

void · bubble · air pocket