Last updated: 2026-09-04 · 5 min read

Flow Mark Prevention — How Do You Eliminate the Wavy Lines Near the Gate on Your Injection Molded Parts, and What Causes Them?

Flow marks (tiger stripes) are wavy patterns near the gate caused by jetting — the melt enters the cavity at high velocity through a small gate and pulses rather than flows smoothly. Each pulse leaves a mark on the surface. Flow marks are cosmetic only and do not affect structural integrity. But on glossy or dark surfaces they are unacceptable. Fix hierarchy: enlarge the gate, raise mold temperature, slow injection at the gate.

Key Facts

Four root causes: gate too small (melt velocity too high → jetting), mold temperature too low (melt freezes on contact), injection speed too high (same jetting mechanism), material too cold (viscosity too high to flow smoothly).

Most visible on glossy surfaces, dark-colored parts and large flat areas near the gate. Marks from a small gate disappear 50–100mm from the gate; marks from low mold temperature appear across the entire part.

Fix hierarchy: enlarge the gate (most effective) → increase mold temperature → reduce injection speed at the gate → increase melt temperature. Design prevention: fan gate, flow leader, gate relocation, surface texture.

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.

What Causes Flow Marks and Why Do They Appear Near the Gate?

Four root causes, each with a distinct mechanism:

Root Cause	Mechanism	How to Identify
Gate too small	Melt velocity through the gate is too high — causes jetting	Marks concentrated at the gate; disappear 50–100mm away
Mold temperature too low	Melt freezes on contact with the cold mold surface	Marks appear across the entire part, not just near the gate
Injection speed too high	Melt enters the cavity at high velocity — causes jetting	Marks near the gate; more pronounced on thin walls
Material too cold	Melt viscosity too high — does not flow smoothly	Marks accompanied by short shots or high injection pressure

Flow marks appear near the gate because that is where the melt first enters the cavity at high velocity. The melt passes through a small gate (e.g., 1mm deep) into a larger cavity (e.g., 3mm wall) — the velocity change causes the melt to jet rather than flow smoothly. Jetting creates alternating high-pressure and low-pressure zones on the cavity surface, which appear as wavy lines parallel to the flow direction.

The identification pattern matters for diagnosis. If marks are concentrated at the gate and fade with distance, the gate is too small or the injection speed is too high. If marks appear across the entire part, the mold temperature is too low. If marks come with short shots, the material temperature is too low. The pattern tells you which fix to apply first.

What Are the Fixes in Order of Effectiveness?

Four fixes, ranked by how directly they address the root cause:

Enlarge the gate. The most effective fix. Increase the gate depth (e.g., from 1.0mm to 1.5mm) — this reduces melt velocity through the gate and eliminates jetting. The gate should be sized so melt velocity is below 1 m/s. For a fan gate, increase width as well as depth. For a pin-point gate, increase the pin diameter. The trade-off: larger gate vestige — but on cosmetic parts, flow marks are usually the bigger problem.
Increase the mold temperature. A warmer mold surface keeps the melt fluid longer — it flows smoothly instead of freezing on contact. Increase from 40°C to 80–100°C for ABS, PC, PC/ABS. Higher mold temperature also reduces melt viscosity at the cavity wall, suppressing the jetting pattern. Trade-off: 5–10% longer cycle time — acceptable for cosmetic parts.
Reduce injection speed at the gate. Use a two-stage injection profile: slow injection (10–20 mm/s) as melt passes through the gate, then fast injection (80–120 mm/s) to fill the rest. The slow injection eliminates jetting; the fast injection ensures the cavity fills before the melt freezes. The easiest fix (no tooling changes) — but may not fully eliminate marks if the gate is too small.
Increase the melt temperature. A hotter melt has lower viscosity and flows more smoothly. Increase by 10–20°C within the material supplier’s range. Trade-off: higher risk of material degradation (burn marks, gas traps) — monitor the part after increasing temperature.

For design-phase prevention, four changes are available before the mold is built: use a fan gate or tab gate instead of a pin-point gate (spreads melt across a wider area, reducing velocity); add a flow leader past the gate (a 2–3mm deep channel that gives the melt a controlled entry path); move the gate to a non-cosmetic area where flow marks are hidden; or add a light texture (SPI C2–D1, 3–8 μm Ra) that hides marks by breaking up reflected light.

Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. The related guide covers adjacent topics.

Flow marks are a surface phenomenon — the melt freezes in a jetting pattern on the surface while the bulk of the part is unaffected. The fix is to eliminate the jetting, not to compensate for it.

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

Frequently Asked Questions

Are flow marks a structural defect or only cosmetic?

Cosmetic only. The wavy pattern is a surface phenomenon — the bulk of the part is unaffected. Tensile strength, impact strength and dimensional accuracy are not reduced by flow marks. However, on parts that are both cosmetic and structural, flow marks indicate that the melt flow is not optimal. The same jetting that causes flow marks may also cause weld lines, air traps or voids elsewhere. If flow marks are present, check the rest of the part for structural defects.

The practical test: if the flow marks disappear after enlarging the gate or raising the mold temperature, the root cause was jetting and the part structure is fine. If they persist after all primary fixes, investigate other flow issues.

Which materials are most susceptible to flow marks?

High-viscosity materials are most susceptible: ABS, PC, PC/ABS and POM show flow marks more easily than PP, PE or PA (which have lower melt viscosity and flow more smoothly). Dark-colored materials (black, dark gray, dark blue) show flow marks more than light-colored ones because the wavy pattern creates a gloss difference more visible on dark surfaces.

Glass-filled materials show flow marks less than unfilled ones — the glass fibers disrupt the jetting pattern. If you are designing a cosmetic part in ABS or PC, plan for flow mark prevention during the DFM phase: specify a fan gate, high mold temperature and consider a surface texture. Fixing flow marks after the mold is built is expensive.

Can back pressure help eliminate flow marks?

Back pressure (resistance applied to the screw during plasticizing) improves melt quality by compressing the melt, removing trapped air and creating uniform temperature. Higher back pressure (50–100 bar) can reduce flow marks by eliminating cold spots in the melt — cold spots have higher viscosity and jet more easily.

But back pressure is a secondary fix. It does not address the root cause of flow marks: jetting at the gate. The primary fixes are gate enlargement, mold temperature increase and injection speed reduction. If flow marks persist after the primary fixes, increase back pressure by 20–30 bar as a supplementary measure.

Key Takeaways

Flow marks are caused by jetting at the gate — melt enters at high velocity.

Fix hierarchy: enlarge gate → raise mold temp → slow injection at gate → raise melt temp.

Design prevention: fan gate, flow leader, gate relocation, surface texture.

Cosmetic only — structural properties are unaffected.

Flow marks are preventable with proper gate design, mold temperature control and injection speed profiling.
If your part has a cosmetic surface, specify the gate type and location during the DFM review — and request a short-shot study to confirm that the melt flows smoothly into the cavity without jetting. The fix hierarchy is clear: start with the gate, then the mold temperature, then the injection profile. Each fix addresses the jetting that causes the marks.