What Causes Jetting Marks in Injection Molds?

Formation Mechanism of Jetting Marks

Jetting marks are common surface defects in injection molding processes, typically manifesting as serpentine flow patterns or wavy traces. Essentially, they are flow marks formed when molten plastic generates turbulence under high-speed and high-pressure conditions, then rapidly cools and solidifies upon contact with the mold cavity wall.

What Causes Jetting Marks in Injection Molds?

Undersized Gate:

Oversized gates cause increased pressure and accelerated flow velocity when molten plastic passes through, resulting in jetting on product surfaces.

Excessive Injection Speed:

Overly fast injection creates strong impact forces as material passes through the gate, leading to jetting mark formation.

High Melt Viscosity:

Higher viscosity materials exhibit poorer flowability, making them prone to jetting mark formation on mold surfaces.

Insufficient Holding Pressure:

Inadequate holding pressure prevents complete mold cavity filling, leaving jetting marks on products.

Detailed Causes of Jetting Marks

When molten plastic passes through constricted areas (nozzle, runners, gates) and suddenly enters an open, wider space, flow direction changes create serpentine patterns.
If melt fails to directly contact mold walls or encounter obstacles during filling, the flow front cannot properly expand in the cavity, ultimately leaving jetting marks.

Methods to Reduce or Eliminate Jetting

Enlarge Gate Dimensions

Adopting short-and-wide gate runner designs minimizes jetting marks.

Optimize Gate Position

Relocating gates to provide flow barriers significantly reduces jetting.

Reduce Injection Speed

Implementing multi-stage injection, especially controlling speed through gates.

Lower Melt Viscosity:

Improve flowability by increasing melt temperature, raising mold temperature, or selecting higher-flow material grades.

Increase Holding Pressure:

Ensures complete cavity filling to minimize jetting marks.

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