Troubleshooting · Defect guide
Flow marks, ripples and tiger stripes
Why the surface of a molded part shows patterns, and what to fix first
"Flow marks" covers several defects that look different: concentric ripples around the gate, tiger-stripe bands of alternating gloss, and the snake-like line of jetting. All of them form at the flow front as melt fills the mould, but their causes and fixes differ. This guide helps you identify which one you have, then walks through checks from cheapest to most expensive and is honest about when the material plays a part.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Identify which mark you have
On the shop floor, "flow marks" is used for several different defects. Before touching the machine, look at the shape and position of the mark, because each type has a different fix.
- Ripples around the gate (record grooves): concentric rings like the grooves of a vinyl record, starting near the gate and fading further out. The surface is genuinely wavy when you run a finger over it or look at a low angle.
- Tiger stripes: alternating glossy and dull bands across the flow direction, usually from mid-flow to the end of fill. The bands on the two faces of the part are often offset. Common in PP modified with rubber or talc.
- Jetting: a meandering line starting right at the gate, like a folded rope. It shows even when the part is completely filled.
- Gloss difference: a flat surface with glossy and matte areas but no repeating pattern. Usually uneven mould temperature.
- Not a flow mark: a depression opposite a thick section is a sink mark, and a single line where two melt streams meet is a weld line.
Mechanism: what happens at the flow front
Melt does not advance as a solid plug. At the flow front, material from the core rolls outwards to touch the mould wall (fountain flow) and freezes into the skin immediately. The surface you see is therefore material that touched the wall while it was at the flow front. If the front moves too slowly, is too cold or moves unevenly, that material sets before it is pressed fully against the wall, and the surface ends up rippled or uneven in gloss.
Tiger stripes are a different mechanism. The flow front becomes unstable and the melt slips against the wall alternately on the upper and lower sides, so each band of skin gets a different gloss. Jetting happens before a proper flow front even forms: melt shoots out of the gate as a rope into the empty cavity and is then wrapped by the melt that follows.
Four groups of causes
1. Part design
A long flow path relative to wall thickness, abrupt changes from thick to thin walls that change the speed of the flow front, and glossy dark surfaces, which show surface differences far more than textured ones.
2. Mould
A gate that is too small or faces open space, causing jetting; a mould that is too cold or unevenly cooled; insufficient venting, so trapped air resists the flow front; and no cold slug well, letting cold material from the nozzle tip enter and mark the area near the gate.
3. Process
Injection speed too low, so the front cools on the way, or too high while the melt passes the gate, causing jetting; low melt or mould temperature; and an early V/P switchover or low holding pressure, so the rippled skin is never pressed against the wall. See Injection molding for the background on these settings.
4. Material
Low-MFI grades flow with more resistance and cool quickly at the front. Regrind or a new lot with a different MFI changes the flow front. A masterbatch whose carrier does not flow like the base resin leaves streaks. In PP modified with rubber or talc, tiger stripes depend directly on the compound design. See Polymer basics for more on MFI.
Step-by-step checks, cheapest first
- Pin down the pattern. Rings around the gate, bands across the flow, a snake from the gate, or just a gloss difference: each needs a different fix.
- Run a short-shot series. Fill the part in steps to see where the mark starts, what the flow front looks like, and whether melt is shooting out of the gate as a rope.
- Adjust injection speed. For ripples around the gate, raise speed step by step. For jetting, set a speed profile that is slow while the melt passes the gate and faster afterwards.
- Raise mould temperature step by step and measure the actual surface temperature at several points. If the mark fades clearly, the flow front was too cold.
- Raise melt temperature within the resin supplier's recommended range, measured on purged melt rather than read from the barrel set point.
- Check vents and the cold slug well. Clean any deposits from vents and look for cold material at the gate.
- Mould virgin resin only for 20–30 shots, without masterbatch or regrind. If the mark disappears, the problem is in the blend.
- Change the mould. Enlarge the gate, switch to a design where melt hits a wall or pin first, or retexture the surface. This is the most expensive step.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Low injection speed, cold flow front | Short-shot series shows ripples around the gate | Raise injection speed step by step |
| Speed too high while passing the gate | Short shots show melt shooting from the gate as a rope | Slow first stage of the speed profile, then accelerate |
| Mould too cold or uneven | Compare surface temperature at several points | Raise temperature; check and clean the water circuit |
| Melt temperature too low | Measure the temperature of purged melt | Raise it within the resin supplier's recommended range |
| Low holding pressure or early V/P switchover | Marks fade when holding pressure goes up | Adjust the V/P point and holding pressure |
| Gate too small or facing open space | Jetting persists even with a slow first stage | Enlarge the gate or move it so melt hits a wall or pin first |
| Blocked vents or no cold slug well | Look for vent deposits and marks near the gate | Clean vents; add a cold slug well |
| Inconsistent material or blend | Mould virgin resin only; log lots and regrind ratio | Hold the blend ratio constant; check MFI when the lot changes |
| Tiger stripes in rubber- or talc-modified PP | Stripes stay in the same place after process changes | Review the compound formulation with the resin supplier |
How much can material and additives help?
The honest answer: most flow marks are fixed in the process and the mould. Changing the material makes a clear difference only in some cases. These are the options that actually exist:
- An easier-flowing grade (higher MFI) helps the flow front reach the end of fill while still hot enough. Toughness and other properties may change; confirm the part still meets its specification.
- Compound design in rubber- or talc-modified PP. Tiger stripes depend on the type and dispersion of the rubber and filler. If the stripes remain after the process has been fully optimised, reviewing the compound formulation with the resin supplier is the conversation to have, followed by trials in the actual mould.
- A masterbatch whose carrier matches the base resin reduces streaks caused by components flowing differently. Moulding virgin resin only for comparison tells you whether this is the issue.
- Lubricants or processing aids can improve flow, but overdosing can leave surface deposits, foul vents or weaken weld lines.
What additives cannot fix: jetting from a gate facing open space, a mould that is too cold, the wrong injection speed, and blocked vents.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Flow mark questions we hear often
What causes tiger stripes on talc-filled PP or TPO parts, and can they be fixed?
Tiger stripes are alternating glossy and dull bands running across the flow direction. They come from an unstable flow front: the melt slips against the mould wall alternately on the two sides, so each band of skin freezes under different conditions. They are typical of PP modified with rubber or talc, such as bumper compounds. Adjusting injection speed and mould temperature can reduce them, but in many cases the root cause is the compound itself, such as the type and dispersion of the rubber and filler. Trial changes on both the process side and the material side.
How do flow marks differ from weld lines?
A weld line is a single line where two flow fronts meet, for example behind a hole, behind an insert or between two gates. Flow marks are repeated ripples, waves or bands laid down along the flow path out from the gate. The easiest way to tell them apart is a short-shot series: if the mark sits where fronts converge, it is a weld line; if it forms along the path while the front is moving, it is a flow mark.
Why are flow marks so visible on glossy, dark parts?
Most flow marks are very small differences in gloss and surface texture. A glossy dark surface reflects light sharply, so those differences stand out, while a textured or light-coloured surface hides much more. Once process and mould fixes are exhausted, texturing the mould in areas that do not need gloss is a design option that is genuinely used in practice.
What is jetting and how is it different from ordinary flow marks?
Jetting happens when melt shoots out of the gate as a rope into an empty cavity without touching a wall first. The rope cools and folds back and forth, then the following melt wraps around it, leaving a snake-like line that starts at the gate. Ordinary flow marks come from a flow front that is too slow or too cold. Jetting is fixed by slowing injection while the melt passes the gate, enlarging the gate, or moving it so the melt hits a wall or pin first.
Flow marks still there after tuning the machine?
Send photos of the part taken with light at a low angle, short-shot photos if you have them, and your machine settings on LINE. Our technical team will help you separate mould, process and material causes.