Flow Marks and Record Grooves: Reading the Surface of a Filling Problem

What a flow mark is

A flow mark, sometimes called a record groove, is a series of faint concentric lines or a wavy pattern on the surface of a moulded part, usually radiating from the gate. It is the shape of the advancing flow front, frozen into the surface as the polymer cooled against the mould.

The mechanism is a hesitation effect. The flow front advances in a series of surges: the centre moves ahead, the material near the cold mould wall hesitates, then the centre slows and the wall catches up. Each surge leaves a slight ridge, and if the melt is viscous enough and the wall cold enough, those ridges do not relax before the part solidifies.

That is why flow marks are a filling problem and not a surface problem. They are not caused by a rough tool, and polishing the tool will not remove them. They are caused by the flow front advancing unevenly and the material freezing before it can smooth itself out.

Speed is the first lever

The depth of the mark falls steeply with injection speed. At 10 mm per second the marks are about 18 microns deep, at 30 they are 11, at 60 they are 6, at 100 they are 3 and at 200 they are 1.5. Below about 3 microns they are invisible on a textured surface and barely detectable on a polished one.

The reason is that a faster front spends less time hesitating. The material reaches the wall and is dragged along by the shear rather than stopping and restarting, and the front advances as a steady wave rather than a series of surges. Faster filling also generates more shear heat, which raises the local temperature and lets the surface relax.

The limit is jetting. Above a certain speed, and below a certain gate size, the melt enters the cavity as a rope rather than a wave, and that produces a much worse defect than a flow mark. The practical window is fast enough to avoid record grooves and slow enough to avoid jetting, and it is usually wider than people think.

Temperature is the second

A cold melt is viscous, and a viscous melt does not relax the ridges before it freezes. The severity index runs 9 at a 200 °C melt, 7 at 220, 5 at 240, 3 at 260 and 1.5 at 280. The relationship is close to linear over that range, which makes it easy to predict: a 20 °C rise removes about two points of severity.

Mould temperature matters as much and is often neglected. A cold mould chills the surface instantly, which freezes the ridges in place. Raising the mould temperature by 20 °C is often more effective than raising the melt temperature by 20 °C, because it keeps the surface mobile for longer rather than making the bulk hotter.

Table 1 — The five levers and what each one buys.

The gate and the runner

A small gate is a flow mark generator. It forces the melt through a restriction at high shear, which produces a cold, oriented, uneven front that enters the cavity and leaves a record of itself. A larger gate, or a fan gate that spreads the flow, gives a much smoother entry.

The runner matters too. A long, thin runner loses heat and pressure, so the material that arrives at the gate is cooler and slower than the material that left the nozzle. A short, generously sized runner delivers a better front. This is one of the cases where a slightly larger runner costs almost nothing and fixes a surface defect.

Gate position is the third factor. A gate that directs the flow straight at a wall produces a hesitation on the far side and a strong flow mark there. A gate that directs the flow along the wall, or into an open area, produces a much more even front.

Materials that mark easily

Flow marks are worst in materials with a high viscosity and a sharp melting point, because both prevent the surface from relaxing. Semi-crystalline polymers with a narrow processing window, and filled grades with a high filler loading, are the usual offenders.

Table 2 — How prone each material is to flow marks, and why.

The practical consequence is that a tool that runs a clean part in ABS may produce visible marks in a talc-filled PP with the same settings, and the settings have to change with the material rather than being carried over. That is obvious in theory and frequently ignored in a shop that runs both.

Telling it from the neighbours

Three surface defects are routinely confused with flow marks, and they have different causes.

• A weld line is a single line where two fronts meet, not a series of ripples. It is a strength problem as well as a cosmetic one.
• A colour streak is a line of different colour, not a ridge in the surface. It comes from dispersion, not from flow.
• A drag mark or a scratch is a single line with a direction, and it is caused by ejection or handling, not by filling.
• A gate blush is a dull patch right at the gate, caused by shear, and it does not extend along the flow.

The distinction is worth making with a fingertip as well as an eye. A flow mark is a ridge you can feel with a fingernail across the flow direction; a colour streak is not. That test takes a second and it settles the diagnosis before anyone touches a machine setting.

A sequence that works

1. Raise the injection speed in steps and watch the marks. Most flow marks disappear between 60 and 120 mm/s.
2. If speed alone does not do it, raise the mould temperature by 10 to 20 °C. This is often the single most effective change.
3. Raise the melt temperature within the material's window, in 10 °C steps, and stop at the first sign of degradation.
4. Check the gate size and the runner. If the marks start sharply at the gate, this is where the answer is.
5. Consider a higher melt index grade only after the process and the gate have been exhausted.
6. If the part is textured, check whether the marks are actually visible in the texture before spending money on a fix.

The last item is the one that saves the most money. A 6 micron flow mark on a matte texture is invisible, and a part that is cosmetically acceptable should not be re-engineered because a polished sample showed a defect that the production texture hides.