Some defects are a record. Flow marks, jetting, gate blush, and cold slug scars are all frozen evidence of how the melt front behaved during the fraction of a second it took to fill the cavity — and each pattern has a distinct signature that points at a distinct cause. Learning to read them turns a vague “the surface looks bad” into a specific diagnosis.
They share a common thread: the leading edge of the melt was too cold, or moving wrong. Whether the result is a ripple, a snake, a halo, or an embedded scar depends on where and how that went wrong.
Wavy flow marks: the front stalled and skinned over
Wavy or rippled flow marks — the ones that look like ripples radiating from the gate — come from a melt front that’s moving too slowly while cooling too fast. Here’s the mechanism, and it explains the fix: the melt closest to the mold wall chills and forms a skin. That skin is stiff. Instead of the front advancing smoothly and laying fresh melt against the steel, the chilled leading edge stalls, then gets pushed forward in surges, so the material can’t stay in contact with the mold wall properly. Each surge freezes as a ripple.
Because the cause is “too cold and too slow at the front,” the fixes all push the same direction:
- Raise injection speed so the front arrives before it skins over — this is usually the strongest lever.
- Raise mold temperature so the skin doesn’t form as aggressively at the wall.
- Raise melt temperature to improve flowability.
- Enlarge the gate or nozzle orifice if the restriction is what’s slowing the front.
Note this is the opposite prescription from burn marks, which want less speed. That’s why identifying the pattern matters before you turn anything.
Jetting: the melt shot in as a stream
Jetting looks completely different — a snake-like or worm-track mark running from the gate into the part. It happens when melt fires through a small gate into a large open cavity as a free stream instead of spreading as a proper front. The stream cools on the outside as it travels, then the rest of the shot fills around it, and the snake stays visible as a permanent scar with different surface and structure.
The fixes are about breaking up that free stream:
| Approach | Why it works |
|---|---|
| Enlarge the gate | Lower velocity through the gate; less stream-like entry |
| Direct flow against a wall or core immediately after the gate (impingement) | The stream hits something and spreads into a proper front |
| Reduce injection speed at the start of fill | Slower entry through the gate, then speed up once the front is established |
| Change gate type or location | A fan gate spreads flow; a different location may avoid firing into open space |
| Raise mold temperature | Reduces how much the stream skins over as it travels |
Note the pattern: jetting wants slower at the gate, while wavy flow marks want faster through the cavity. Multi-stage injection resolves that apparent contradiction — slow through the gate to establish the front, then accelerate to keep it hot.
Gate blush: instability right at the entry
Gate blush is a small halo of flow disturbance right around the gate — a dull or streaky ring in the immediate gate area. The cause is unstable flow as the melt passes through the restriction: too much velocity and shear at the gate itself.
The contributing factors are consistent:
- Process: mold temperature too low, injection speed too high through the gate
- Mold: gate size too small (concentrating shear), or a gate geometry that disturbs entry
- Material: low-flow resin struggling through the restriction
The most effective single measure is a multi-stage injection profile that slows the melt specifically while it’s passing through the gate, then increases speed once it’s into the cavity. Raising mold temperature helps too. Note that “just slow the whole shot down” costs you cycle time and can create the wavy flow marks described above — the point of staging is to be slow only where it matters.
Cold slugs and the well that catches them
Between shots, the melt at the very tip of the nozzle sits against relatively cool steel and forms a chilled plug — a cold slug. On the next shot, that plug is the first material into the tool. If it makes it into the cavity, it doesn’t melt back in; it embeds as a visible blemish, a flow disturbance, or a weak spot, and it can also block or disturb the gate.
The countermeasure is a cold slug well — a small trap at the end of the sprue and at runner junctions where flow changes direction, sized to catch that first chilled material before it reaches the gate. If a tool has no cold slug well, or one that’s too small, cold-material defects will recur no matter how well the process is tuned. Supporting factors:
- Nozzle temperature too low increases how much slug forms between shots.
- Excessive decompression can pull cold material back into the flow path.
- Long open times (slow cycle, interruptions) give the slug more time to form.
Telling them apart
| Pattern | What you see | Root cause | Direction of fix |
|---|---|---|---|
| Wavy flow marks | Ripples radiating from the gate | Front too slow, skinning over at the wall | Faster fill, hotter mold and melt |
| Jetting | Snake or worm track from the gate | Free stream fired into open cavity | Slower at the gate, impingement, bigger gate |
| Gate blush | Dull halo right around the gate | Unstable, high-shear flow through the gate | Stage the profile slow through the gate; hotter mold |
| Cold slug mark | Discrete embedded blemish or scar | Chilled nozzle plug entered the cavity | Cold slug well; nozzle temperature |
A practical sequence
- Classify the pattern — ripple, snake, halo, or discrete scar. They have different, sometimes opposite, fixes.
- For ripples: raise fill speed and mold temperature first.
- For jetting or blush: stage injection to slow through the gate, and look at gate size, type, and impingement.
- For cold slug marks: check for a cold slug well and adequate nozzle temperature — this is usually a tooling answer.
- Change one variable at a time, and remember that speed helps one of these and hurts another.
FAQs
What causes wavy flow marks radiating from the gate?
A melt front that’s advancing too slowly while cooling too quickly. The material touching the mold wall chills into a stiff skin, so instead of flowing smoothly the front stalls and then surges forward, and each surge freezes as a ripple — the cooled outer layer effectively obstructs the melt behind it from laying against the wall properly. The fixes all add heat or speed at the front: raise injection speed (usually most effective), raise mold temperature, raise melt temperature, and enlarge the gate or nozzle orifice if the restriction is limiting flow.
What’s the difference between jetting and a flow mark?
They look and behave differently. A flow mark is a wavy ripple pattern from a front that stalled and surged. Jetting is a distinct snake-like or worm track running from the gate, caused by melt firing through a small gate into a large open cavity as a free stream rather than spreading into a proper front — the stream skins over and stays visible after the rest fills around it. Crucially, their fixes are opposite at the gate: flow marks want faster filling, while jetting wants slower entry through the gate, plus impingement against a wall or core, or a larger gate.
What is a cold slug well and why does my tool need one?
Between shots, melt at the nozzle tip chills against cooler steel and forms a solid plug — a cold slug. On the next shot that plug is the first material in, and since it won’t remelt properly, it embeds in the part as a blemish or weak spot and can disturb the gate. A cold slug well is a small trap at the end of the sprue (and at runner direction changes) that catches this chilled material before it reaches the cavity. Without one, or with one that’s undersized, cold-material defects recur regardless of process tuning.
How do I fix gate blush without slowing the whole cycle?
Use a multi-stage injection profile. Gate blush comes from unstable, high-shear flow as the melt passes through the gate restriction, so the effective fix is to slow the melt specifically while it’s crossing the gate, then increase speed once it’s into the cavity. Slowing the entire shot would also reduce blush but costs cycle time and can create wavy flow marks from a front that then chills en route. Raising mold temperature helps as well, and if blush persists, the gate may simply be too small for the material’s flow.