Flow Lines and Flow Marks in Injection Molding

Flow lines are a visual record of how a part filled. They show as wavy patterns, concentric halos near the gate, or streaky bands across a surface. They are usually gloss or shade variation, not raised or sunken geometry. On an internal bracket they are often harmless. On glossy consumer faces they are a common cosmetic reject.

The Mechanism Behind Flow Marks

As melt advances through the cavity, material at the flow front cools against steel. A smooth, hot front freezes uniformly and leaves no trace.

If the front hesitates, pulses, or cools too much during travel, the surface freezes in stages. Each stage leaves a faint witness. The visible result is a ripple or ring from the gate, or a path along the flow.

That separates flow lines from related defects. Jetting is a snake-like strand of cold material from the gate. A weld line is the seam where two flow fronts collide. A flow line is the texture of a single front’s journey.

Diagnostic Hierarchy: Process, Tooling, Design

Flow marks are mainly thermal and speed-related. Work them in order so you know whether the fix is a routine adjustment or a tooling change.

1. Process Adjustments

Most flow lines respond to hotter melt, hotter mold, and better injection speed profiling.

• Low melt or mold temperature: A cold, viscous front freezes in ripples. Higher temperatures let material flow smoothly and heal its own history.
• Slow or unsteady injection speed: The front hesitates and stamps each pause into the surface.

A capable molder usually clears process-driven flow marks during routine T1 sampling. A hotter mold can lengthen cycle time, which may reopen piece-price talks on high-volume parts.

2. Tooling Limitations

If flow lines survive thermal and speed changes, look at the tool.

• Undersized gate or runner: Pressure loss starves the front so it slows and cools early.
• Poor gate location: A gate that forces a long, awkward fill path gives material too much time to cool before end of cavity.

These are gate design issues that need steel work. When a supplier says “we need to enlarge the gate,” they have left process tuning for tool correction.

3. Part Design

Abrupt wall-thickness changes make the front speed up or slow down. Those velocity shifts stamp the surface at every transition. If flow marks align with internal ribs or thickness steps, smooth those transitions. That is wall thickness discipline, not only cosmetics.

Sourcing and RFQ Strategy

Define cosmetic surfaces before tooling starts. Flow lines only matter where they are seen. Suppliers need to know which faces require high gloss and which will be textured (texture hides what gloss shows).

Expect flow marks on early T1 samples. Require resolution before final mold approval. Fix them through process or tooling. Do not negotiate them into the acceptance standard.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.