Last updated: 2026-09-02 · 5 min read

What Is Gate Blush — and How Is It Different From Flow Marks?

Gate blush is a hazy or glossy halo right at the gate where hot, sheared melt first slams into the cavity surface; flow marks run as rings or lines along the fill path where the advancing front stalls, cools and re-advances. Same family of surface defects, different mechanisms — and the difference matters, because blush is fixed at the gate and the process, while flow marks are fixed along the path and the front speed.

Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. See also related guide.

Key Facts

  • Blush mechanism: the first melt through the gate is fast, hot and highly sheared; it sprays against the cold steel, leaves an over-packed or burnt-looking halo, and the stress pattern freezes into the surface. It is always gate-adjacent and roughly concentric.
  • Flow-mark mechanism: the melt front slows as it fills, a thin cooled layer forms, and the front then pushes that skin forward, printing rings or rivers downstream. They follow the fill pattern — around bosses, across thin sections — wherever the front hesitates.
  • The fix location differs: blush yields to gate geometry, gate size, injection speed profile and shear management; flow marks yield to fill-speed increases, wall balance, gate placement and front-stall elimination. Fixing the wrong mechanism wastes trial weeks.

    • Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. The related guide covers adjacent topics.

    Side by Side

    Attribute Gate Blush Flow Marks
    Appearance Halo or glossy haze at the gate; sometimes dull ring Rings, arcs or rivers downstream along the fill path
    Location Within millimeters of the gate Wherever the front stalls: thin walls, around features
    Mechanism Shear and over-stress at the jetting first contact Front hesitation; cooled skin pushed ahead
    Process levers Slow first stage, gate size up, shear-friendly gate design Faster fill through the stall zone, uniform wall, hotter mold
    Tool levers Gate land and type, cold-slug well, runner corner polish Gate relocation, wall thickness balancing, flow leaders

    Fixing Blush, in Order of Cost

    1

    Soften the first contact

    Slow the injection profile in the first stage so melt eases onto the steel instead of spraying, then accelerate once the front is established. On the process sheet this is a speed-profile change — no steel touched, often the whole fix.

    2

    Manage the gate shear

    Check gate size and land: an undersized gate turns the runner into a shear heater. Sizing up the gate, opening the land, or moving to a shear-friendlier gate type changes the melt the cavity receives.

    3

    Check the cold slug

    The cooled slug at the runner tip can be what lands at the gate. A proper cold-slug well and polished runner corners keep the coldest material out of the first contact zone.

    4

    Confirm melt state

    Under-dried resin, low melt temperature, or degraded regrind all amplify blush. Verify the melt is dry, at temperature, and single-grade before blaming geometry.

    Fixing Flow Marks

  • Speed through the stall: raise fill velocity in the zone where marks appear so the front never hesitates — profiled fill, not blanket speed.
  • Balanced walls: marks cluster where thin sections feed thick ones; evening the wall or adding flow leaders removes the hesitation itself.
  • Hotter mold surface at the problem region: a warm steel face keeps the leading skin from freezing early.
  • Gate placement: moving the gate so the longest, most uniform front crosses the visible face eliminates the ring geometry entirely.
  • Check venting: trapped air at the front slows it locally; a vent at the stall point is sometimes the whole fix.

    • Frequently Asked Questions

    Can polishing the mold remove blush?

    It can help the appearance temporarily, but blush is stress and shear history in the part surface, not dirt on the steel.
    If the melt condition that created it continues, the halo returns with the next shots. Fix the gate and the profile first; polish last.

    Why do marks get worse on textured parts?

    Texture multiplies the visual contrast of both defects: the rough surface scatters light, so any gloss change reads strongly. Textured parts also need higher fill speeds to replicate texture uniformly — which is exactly the lever that removes flow marks, and the one that can worsen blush if the gate entry is abrupt. Texture plus gate design must be engineered together.

    Who fixes it — the molder or the mold maker?

    Process levers first — both sides can try profile, speed and temperature changes in an afternoon. When those cap out, the cause decides: blush that survives process tuning is gate geometry (mold maker); flow marks that survive are fill balance and gate placement (mold maker with the designer). The trial report’s cause statement is what assigns the work.

    Key Takeaways

    1. Blush is gate-adjacent and shear-driven; flow marks are path-hesitation and skin-driven — diagnose by location and shape before touching anything.
    2. Blush yields to first-stage speed, gate shear management and clean melt; flow marks yield to fill speed through stalls, wall balance and gate placement.
    3. Texture and glossy visible faces demand both defects be engineered out together — gate entry and fill profile are two ends of the same decision.

    Bring photos of the defect with the part oriented to its gate map, and aCatMould will call the mechanism by name — blush or hesitation — and attach the fix to its owner: profile changes on the process sheet, or gate and path changes in the tool, with the correction verified against the same photos at re-trial.

    Sources
    Plastics Technology — surface defect troubleshooting
    MoldMaking Technology — gate design practice
    BASF — injection molding defect guide

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet