Flow lines are streaks, bands or ripples on the surface that follow the path the melt took through the cavity. They form when the melt front cools, slows or stalls before it is pressed firmly against the steel. Start with injection speed, then melt and mold temperature. Flow marks are usually a process or gate problem, and the resin is rarely the first cause.
Also described as: flow marks, gate blush, wavy lines, record grooves, tiger stripes, hesitation marks.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| A dull, hazy ring or halo around the gate on an otherwise glossy part (gate blush). | The melt goes through the gate too fast at the start of fill. The first material is sheared hard and skids across the steel instead of wetting it. | Injection speed at the very start of fill; gate size and land length; the cold slug well |
| Fine, closely spaced ripples across the flow direction, like the grooves of a record. Often worst in thin areas or near the end of fill. | The melt front is moving too slowly or is too cold. The skin freezes, the melt rolls over it, and the pattern repeats. | Injection speed; whether the machine reaches its pressure limit during fill; mold and melt temperature |
| A single band or dull line where a thin section, rib or louver joins a thicker wall (hesitation mark). | The melt takes the easy path through the thick wall and stalls at the thin feature. When pressure builds it starts again, but the stalled front has already cooled. | A series of short shots to see where the front stops; injection speed; where the gate sits relative to the thin feature |
| Alternating dull and glossy bands across the flow on a long part. Most often seen in impact-modified or filled polypropylene and in rubber-modified styrenics. | The flow front is unstable and flips from side to side as it advances (often called tiger stripes). | Injection speed (a change in either direction usually moves the pattern); mold temperature; the grade |
| Marks are worse at start-up, after a stoppage, or on one half of the tool only. | The steel is colder than you think, or one half runs colder than the other. | Steel surface temperature on both halves, measured, not the controller setting |
| The streaks are a change in color only. Gloss and texture are the same across them. | Color that is not fully mixed. This is not a flow mark. | A short run in natural; back pressure. See color inconsistency |
| Flow marks started with a new lot or a new grade. | The new material is stiffer in the melt (a lower melt flow), so the front moves slower and cools more. | Melt flow on the COA of the good lot against the bad lot |
Checks in order, cheapest first
- Make a series of short shots. Cut the shot size in steps and lay the parts out in order. They show where the melt front was when the mark formed, and whether it stalled at a thin feature or slowed near the end of fill.
- Check whether fill is pressure-limited. If the actual injection pressure reaches the set limit during fill, the screw slows down and you are not getting the speed you set. Raise the limit before you judge any speed change.
- Change the injection speed. For record grooves and hesitation marks, fill faster. For gate blush, slow the first part of the stroke until the melt is through the gate, then speed up. A speed profile lets you do both.
- Raise mold temperature, then melt temperature. A warmer mold keeps the skin soft long enough to be pressed against the steel. Measure the steel and the melt with a probe. Stay inside the supplier’s range, and watch cycle time and sink.
- Clean the vents and check the nozzle. Blocked vents slow the melt front near the end of fill. A nozzle that runs cold sends a cold slug into the part, which leaves a mark near the gate.
- Compare lots. If the marks arrived with a new lot, compare melt flow on the COAs before you change the process. Also check that the regrind ratio has not changed.
- Look at the tool. A small gate, a long gate land or a missing cold slug well causes blush. A gate placed so the melt reaches a thin feature early causes hesitation. Abrupt changes in wall thickness cause both. These are the expensive fixes, so confirm the cheap ones first.
- Change the resin last. Flow marks are usually not the resin. If speed, temperature and gating are right and the marks still fail the part, an easier-flowing grade is the next place to look.
The numbers
Typical melt and mold temperature ranges for the resins where flow marks are most often reported. Flow marks usually improve toward the upper part of the mold temperature range.
| Resin | Melt temperature | Mold temperature |
|---|---|---|
| ABS | 425–525°F / 218–274°C (flame-retardant grades never above 475°F / 246°C) | 80–180°F / 27–82°C |
| ASA | 464–536°F / 240–280°C | 104–176°F / 40–80°C |
| Polycarbonate | 536–653°F / 280–345°C | 158–248°F / 70–120°C |
| Acrylic (PMMA) | 340–520°F / 171–271°C | 100–205°F / 38–96°C |
| General-purpose polystyrene | 356–500°F / 180–260°C | 50–150°F / 10–66°C |
| Polypropylene homopolymer | 374–525°F / 190–274°C | 50–203°F / 10–95°C |
| Polypropylene copolymer | 392–500°F / 200–260°C | 59–149°F / 15–65°C |
| HDPE | 380–525°F / 193–275°C | 50–125°F / 10–50°C |
| Nylon 6 | 482–536°F / 250–280°C | 104–212°F / 40–100°C |
Typical values from published processing guides. The right setting depends on the grade, the wall thickness and the machine, so confirm against the data sheet for your grade. The full list is in the melt temp, mold temp and shrinkage chart.
Why flow marks form
- The melt front rolls outward as it fills. Material from the middle of the stream moves to the front and is laid against the steel, where it freezes into a skin almost at once. Whatever that front looked like when it touched the steel is what you see on the part.
- A slow or cold front freezes before it is pressed flat. The skin stiffens, the melt behind pushes over it, and the cycle repeats. That gives the record-groove pattern. Faster fill, a hotter mold and a hotter melt all keep the front soft longer.
- A stalled front leaves a line. When the melt has a thick path and a thin path, it fills the thick one first. The front waiting at the thin feature cools, and the mark stays where it stopped.
- Gate blush comes from high shear at the gate. A small gate and a fast start put very high stress on the first material into the cavity. It does not copy the steel finish, so it looks dull against the glossy surface around it.
- Finish decides whether it gets rejected. High-gloss, dark and transparent parts show every flow mark. A textured or matte surface hides marks that are still there.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A higher melt flow grade of the same resin | It fills faster at the same pressure, so the front reaches the end of the cavity hotter. | Lower molecular weight, which usually means lower impact strength and stress-crack resistance. More tendency to flash. A new grade for your customer to approve. |
| A grade developed for surface appearance | Some impact-modified polypropylene and styrenic grades are formulated to resist tiger striping on long flow paths. | Usually a higher price and fewer sources. Other properties may differ, so compare the full data sheets. A new grade for your customer to approve. |
| A melt flow range on the specification | Flow marks that come and go by lot often trace to melt flow moving between lots. | Nothing in the part. It takes an incoming check, and it may narrow your choice of sources. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Weld lines. A weld line is a single fine line where two melt fronts meet, downstream of a hole, a core or a second gate. It sits in the same place on every shot. See weld lines.
- Jetting. Jetting is a snake-like squiggle that starts at the gate, where the melt shot across an open cavity instead of spreading out. See jetting.
- Splay. Splay is silvery streaks that fan out along the flow and change from shot to shot. It comes from moisture or gas, and speed changes do not remove it. See splay and silver streaks.
- Color streaks. If the streak is a different shade but the gloss is the same, the color is not mixed. See color inconsistency.
Quick answers
What causes flow lines on molded parts?
Flow lines form when the melt front cools, slows or stalls before it is pressed firmly against the steel. The skin freezes, the melt rolls over it, and the pattern repeats. Start with injection speed, then mold and melt temperature. Flow marks are usually a process or gate problem, and the resin is rarely the first cause.
How do I fix gate blush?
Slow the first part of the injection stroke until the melt is through the gate, then speed up. Gate blush comes from melt going through the gate too fast at the start of fill. If that does not clear it, look at the tool: a small gate, a long gate land or a missing cold slug well causes blush.
Why does my part have wavy lines near the gate?
A dull, hazy ring around the gate is gate blush, caused by melt going through the gate too fast at the start of fill. A nozzle that runs cold can also send a cold slug into the part and leave a mark near the gate. A snake-like squiggle that starts at the gate is jetting, a different defect.
Are flow lines caused by the resin?
Usually not. Flow marks are most often a matter of injection speed, mold and melt temperature, or gating. The resin is worth checking when the marks arrived with a new lot or grade, because a lower melt flow makes the front move slower and cool more. Compare melt flow on the COAs before you change the process.
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