Burn marks come from two different places. A black or brown scorch at the last point to fill is trapped air, squeezed so fast that it heats up and burns the plastic (often called dieseling). Brown streaks or discolored material that move around from shot to shot mean the resin was degraded before it reached the cavity: in the barrel, the nozzle or a hot runner. Tell the two apart first, because the fixes are at opposite ends of the machine.
Also described as: burns, scorch marks, black or brown marks at the end of fill, dieseling.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| A black or brown scorch in the same spot every shot, at the end of fill or in a blind rib, boss or corner | Trapped air. The melt compresses air that cannot escape, and the hot air burns the resin. | Vents at that spot (missing, blocked or too shallow); injection speed at the end of fill |
| A scorch with a small unfilled corner beside it | The trapped air is also holding the melt back. | Vents; whether clamp force is high enough to crush them shut |
| A burnt spot where two flow fronts meet | Air caught between the fronts with no vent at the meeting point. | A vent or overflow at the weld. See weld lines. |
| Burns that build up over a run and go away after the mold is cleaned | Vents are filling with deposit from the resin, additives or mold release. | Vent cleaning interval; how much release is being used |
| Brown or dark streaks along the flow, in a different place each shot | The resin degraded in the barrel: melt too hot, or too long at temperature. | Melt temperature with a probe; shot size against barrel capacity; cycle interruptions |
| Discoloration that starts at the gate and spreads out from it | Shear heating through a small gate or nozzle at high speed, or a hot spot in the nozzle or hot runner. | Injection speed; nozzle and hot runner zone temperatures; gate size |
| Burnt parts for the first shots after a stoppage, clearing as the run goes on | Resin sat hot in the barrel or hot runner while the press was down. | Purging after stoppages; lowering barrel heats when idle |
Checks in order, cheapest first
- Decide which kind of burn it is. Same spot at the end of fill means trapped air. Streaks that move around mean degraded resin. A short shot shows where the cavity fills last.
- Slow the injection speed at the end of fill. Profile the speed so the last part of the cavity fills more slowly. This gives the air time to leave and costs nothing to try.
- Measure the melt temperature and work out residence time. Use a probe in an air shot, not the barrel setting. Back pressure and screw speed add heat that the controller does not show. A small shot in a large barrel keeps resin hot for many cycles.
- Clean the vents and the parting line. Deposit builds up in vents during a run. Check that clamp force is not higher than it needs to be, since excess tonnage can close shallow vents.
- Check the dryer and the decompression setting. Moisture and volatiles add gas that has to escape through the same vents. Too much screw decompression can draw air into the nozzle.
- Compare lots, colors and regrind. Color concentrates, flame retardants and other additives can have less heat stability than the base resin. Regrind has already been through the barrel once. Try a short run of virgin natural resin.
- Look at the tool and the screw. Add or deepen vents where the burn appears, vent through ejector pins, or open up a small gate. Check the nozzle, check ring and hot runner for dead spots where resin hangs up and cooks.
- Change the resin. Burn marks are usually not a resin problem. Consider a different grade only when the process window is too narrow for the part and the press.
Safety. Overheated rigid PVC gives off hydrogen chloride gas and overheated acetal gives off formaldehyde. If either is burning in the barrel, purge, ventilate and keep clear of the nozzle. Never let PVC and acetal meet at processing temperature; purge thoroughly between them.
The numbers
Typical melt temperature range and the longest time the melt should stay in the barrel, for resins that burn or discolor readily.
| Resin | Melt temperature | Max residence in the barrel |
|---|---|---|
| ABS | 425–525°F / 218–274°C (flame-retardant grades never above 475°F / 246°C) | Not published |
| Polycarbonate | 536–653°F / 280–345°C | 4–8 min |
| Nylon 6 | 482–536°F / 250–280°C | 10 min |
| Nylon 66 | 527–580°F / 275–305°C | 10 min |
| PBT, unfilled | 460–536°F / 238–280°C (decomposes above 536°F) | 5–10 min |
| Acetal copolymer | 370–446°F / 188–230°C (never above 460°F / 238°C) | Keep it short. Published limits vary widely, so use the figure for your grade |
| Rigid PVC | 380–410°F / 193–210°C | 15 min absolute |
| Polypropylene homopolymer | 374–525°F / 190–274°C | Purge if idle more than 30 min |
| TPU | 374–455°F / 190–235°C, lower for softer grades | Purge if the cycle stops more than 10 min |
Typical values from published processing guides. A residence limit holds only at the low end of the melt range; the hotter the melt, the shorter the time. Confirm against the data sheet for your grade. The full list is in the melt temp, mold temp and shrinkage chart.
Why plastic burns in the mold or the barrel
- Trapped air heats when it is compressed. The cavity is full of air before each shot. If the melt arrives faster than the air can leave through the vents, the air is squeezed into the last corner, gets hot enough to scorch the resin, and leaves a black mark. This is the same effect that ignites fuel in a diesel engine.
- Degradation is time and temperature together. Every resin tolerates a certain time at a certain melt temperature. Run hotter and the safe time gets shorter.
- Shear adds heat you did not set. High screw speed, high back pressure, fast injection and small gates all heat the melt beyond the barrel setting.
- Hang-up spots cook resin slowly. A worn check ring, a mismatched nozzle or a dead corner in a hot runner holds resin for many cycles and then releases it as dark streaks.
- Some grades have a narrower window. Flame-retardant grades, and some colors and additives, start to break down at lower temperatures than the base resin. That is a property of the grade, not a defect in the lot.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A higher-flow grade of the same resin | Fills at a lower melt temperature, speed and pressure, so there is less shear heat and the air is pushed out more gently. Melt flow index explained | Higher flow usually means lower toughness, and the part may flash more easily. |
| A heat-stabilized grade | Tolerates more time at melt temperature. | Usually a higher price, and a new grade for your customer to approve. It does nothing for trapped air. |
| Less regrind, or none | Less heat history going back into the barrel. | More scrap to sell or store. Nothing in the part. |
| A different color concentrate or carrier | A colorant with better heat stability, in a carrier matched to your resin, stops streaks that come from the color and not the base resin. | A new color match and a new approval. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks more or less makes a different size part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Black specks. Separate dark particles scattered through the part are bits of carbonized resin or contamination breaking loose, not a scorch. See black specks and contamination.
- Splay. Silver or whitish streaks along the flow come from moisture or gas in the melt. Burn streaks are brown or black. See splay and silver streaks.
- Color streaks. Streaks in the part’s own color, lighter or darker, come from poor mixing of the colorant. See color inconsistency.
- Short shots. Trapped air can leave an unfilled corner with no scorch at all. The cause and the venting fix are the same. See short shots.
Quick answers
What causes burn marks on plastic parts?
Burn marks come from two different places. A black or brown scorch in the same spot at the end of fill is trapped air, squeezed so fast that it heats up and burns the plastic. Brown streaks that move around from shot to shot mean the resin degraded in the barrel, the nozzle or a hot runner.
Why is there a black mark at the end of fill?
A black mark at the end of fill is trapped air. If the melt arrives faster than the air can leave through the vents, the air is squeezed into the last corner and gets hot enough to scorch the resin. Check for vents that are missing, blocked or too shallow at that spot, and check injection speed.
How do I stop gas burns in the mold?
Start by slowing the injection speed at the end of fill, which gives the air time to leave. Then clean the vents and the parting line, and check that clamp force is not higher than it needs to be, since excess tonnage can close shallow vents. Check the dryer and the decompression setting too, because moisture and volatiles add gas.
Are burn marks caused by the resin or the process?
Burn marks are usually not a resin problem. Most come from trapped air or from melt that was too hot or held too long. Color concentrates, flame retardants and other additives can have less heat stability than the base resin, so try a run of virgin natural resin. Consider a different grade only when the process window is too narrow.
Why are the first parts after a stoppage burnt?
Resin sat hot in the barrel or hot runner while the press was down. The burnt parts usually clear as the run goes on. Purge after stoppages and lower the barrel heats when idle. Overheated rigid PVC gives off hydrogen chloride gas and overheated acetal gives off formaldehyde, so purge, ventilate and keep clear of the nozzle.
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