PA6 Marking & Surface Defects in Injection Molding: Flow Marks, Splay, Burn Marks & Streaks

What can 'PA6 marking' mean on a molded part?

In plastics production, the word marking can describe several different things. It may mean an intentional material-identification mark such as >PA6<, a laser-marking operation, or an unwanted visible mark that appears after molding. This guide focuses on the third case: unintended surface marks on injection-molded PA6 and PA6 compounds.

The first troubleshooting step is therefore to name the appearance as precisely as possible. A silver streak, a wave following the melt front, a dark burn near the end of fill, a weld line, a dull patch or visible glass-fiber streaking should not be treated as the same defect simply because all of them can be described informally as 'marking'.

Silver streaks and splay: check moisture, volatiles and thermal history

PA6 is hygroscopic, so material moisture is a frequent starting point when silver streaks or splay appear. Moisture exposed to melt-processing temperatures can contribute to visible streaking and can also affect polymer stability. The material history, package condition, dryer performance and time between drying and molding should be reviewed together.

Splay is not proof of excessive moisture by itself. Contamination, volatile components, excessive thermal history, residence time or other gas-generating conditions can create similar appearances. Confirm the grade and lot, compare the drying record with the supplier's guidance and check whether masterbatch, regrind or another material addition changed when the defect began.

• Grade, lot and packaging condition
• Drying time, temperature, dryer condition and material exposure after drying
• Measured moisture when the process or qualification plan requires it
• Regrind, color masterbatch or other additions
• Melt temperature, residence time and signs of degradation

Flow marks, flow lines and gloss variation

Some PA6 surface patterns follow the motion of the melt front rather than appearing as random streaking. Flow marks, record-groove-like lines and local gloss changes can be influenced by the way the melt fills and cools against the cavity surface.

Useful checks include melt and mold temperature history, injection profile, gate design, wall-thickness transitions, hesitation, cooling balance and the point in the fill sequence where the mark forms. A universal instruction such as simply increasing temperature or speed is not appropriate for every grade or mold; the corrective direction depends on the actual mechanism and should remain inside the validated processing window.

• Does the mark follow the flow direction or repeat at a consistent distance from the gate?
• Did the defect begin after a mold-temperature, cycle-time or injection-profile change?
• Is there a thin-to-thick transition, hesitation area or gate restriction near the mark?
• Does the appearance change by cavity, gate or part location?

Burn marks, gas marks and weld-line appearance

Dark or brown marks near the end of fill, around ribs or in poorly vented regions can be associated with trapped gas, local shear or thermal degradation. Venting condition, injection profile, local restrictions and residence history should be checked before assigning the problem to the resin alone.

A weld line forms where separate melt fronts meet. Its appearance and strength can be affected by flow-front temperature, venting, gate layout, part geometry and, in reinforced compounds, fiber orientation. A visible weld line is therefore both a cosmetic observation and, in some parts, a possible mechanical-design consideration that should be validated at part level.

• Mark location relative to vents and end-of-fill areas
• Evidence of trapped air or local burning
• Gate, runner and flow-front convergence pattern
• Melt residence time and thermal history
• Part-level strength requirement at any weld line

Glass-filled PA6: reinforcement can change the surface pattern

Glass-fiber-reinforced PA6 adds fiber orientation to the surface-quality discussion. Streaks with reinforcement, visible fiber read-through, gloss differences and directional appearance can be influenced by flow pattern, mold surface, gate position, wall thickness, mold temperature and the complete compound formulation.

A PA6 GF30 grade should not be judged only by its nominal glass percentage. Matrix resin, fiber system, additives, flow behavior and molding conditions can change both appearance and dimensional response. When a surface issue begins after changing suppliers or moving between GF15 and GF30, compare the complete grades and the actual molding record rather than assuming the reinforcement percentage is the only difference.

A practical diagnostic sequence for PA6 surface marks

Photograph the defect, record the cavity and exact location, and note when it first appeared. Then confirm the material identity and lot, review storage and drying, compare machine settings with the last known-good run, inspect gates and vents, and check whether mold maintenance or cooling conditions changed.

Change one major variable at a time and record the result. If the issue follows a resin or grade change, retain samples from both materials and provide the supplier with defect photos, drying history, machine data, mold location and the previous reference grade. This gives a technical review enough context to separate material, machine, mold and part-design factors.

• 1. Describe and photograph the mark
• 2. Record gate, cavity and end-of-fill location
• 3. Verify grade, lot, drying and material additions
• 4. Compare process settings with the last known-good run
• 5. Check mold venting, gate condition, cooling and recent maintenance
• 6. Change one major variable and document repeatability