Burn marks and discolouration in 3D printing — 5 causes of dark streaks

Symptom: the finished model shows dark streaks, brown patches or matte, scorched discolouration. They usually appear on small parts, on spires and anywhere the hotend circles for a long time over the same fragment. The colour differs from the rest of the print, and the surface in those places can be rough and slightly tacky to the touch.

Burn marks are the result of thermal degradation. Polymer that sits too long in a hot nozzle, or that is taken above its range, begins to break down: it changes colour, releases volatile decomposition products and deposits on the walls of the melt channel. That deposit later returns to the model as dark streaks. One thing is worth remembering — what decides the outcome is not temperature alone, but the product of temperature and the time the material spends in the hot zone. That is why identical settings can be perfectly safe on a large model and catastrophic on a ring printed over three hours.

Most common causes and fixes

1. The nozzle temperature exceeds the material's range

The simplest and most common cause. Profiles downloaded from the internet are often written for another manufacturer's filament, and a difference of 20–30 °C between materials in the same family is normal. PLA printed at the temperature intended for PETG will turn brown within a quarter of an hour.

How to check: compare the temperature in your profile with the manufacturer's range in the table below. If you are at the upper limit or above it, you already have your answer.

What to do: come down to the middle of the range and print a temperature tower, dropping in 5 °C steps. You are looking for the lowest temperature at which the layers still bond well. With PLA and its variants, burn marks usually disappear after two or three steps down.

Porima material	Nozzle range	Bed	Recommended cooling
PLA	200–230 °C	0–60 °C	100%
PLA Pastel	200–230 °C	0–60 °C	80–100%
Tough PLA	210–240 °C	0–60 °C	100%
PLA Premium	210–240 °C	0–60 °C	80–100%
PLA Wood	210–240 °C	0–60 °C	80–100%
Hyper PLA+	220–250 °C	0–60 °C	100%
PLA/CF	220–250 °C	50–70 °C	60–100%
Silk PLA	230–260 °C	0–60 °C	80–100%
PETG	240–260 °C	60–80 °C	100%
ABS	250–280 °C	80–110 °C	0–40%
ASA	250–280 °C	90–120 °C	0–30%
PA (nylon)	260–290 °C	90–120 °C	30–100%
HT PETG	270–300 °C	100–120 °C	0–10%

Source: the official FDM technical data table from the manufacturer, Porima. Full listing of all 25 materials: print temperature table.

2. The print is too slow for the temperature you have set

Speed and temperature are one system, not two independent sliders. When printing slowly, less material passes through the nozzle per unit of time, so every portion of polymer spends longer in the heater block. The same temperature that works beautifully at 80 mm/s starts to burn at 20 mm/s.

How to check: look at where the discolouration sits on the model. If it is only on small details, on lettering and in places slowed down by the minimum layer time, speed is to blame rather than the heater itself.

What to do: on slow, small prints lower the temperature by 5–10 °C relative to the profile you use for large models. Alternatively raise the minimum layer speed instead of letting the slicer slow down to a few millimetres per second, and set part cooling to the maximum permitted for the material.

3. The material degrades in the hotend during a long print

On prints of a dozen or more hours with a small cross-section, the material effectively stands still in the heated channel. The polymer slowly breaks down, darkens and sticks to the walls. This affects in particular materials printed hot: HT PETG in the 270–300 °C range, PA at 260–290 °C, or PC/ABS. The symptom builds hour by hour — the first layers are clean, the last ones dirty.

How to check: extrude by hand after the print has finished. If a clean nozzle first delivers a dark strand and then a lighter one, burnt material was sitting in the hotend.

What to do: do not leave the printer hot and idle before the start or after the end of a print. On very long jobs with a small cross-section, consider printing several copies at once so that something is always flowing through the nozzle. After every print like that, clean it with a cold pull.

4. Charred deposits in the nozzle and on its face

Carbon deposits rarely build up in a single day. They accumulate in layers after every material change and after every print where something stuck to the outer edge of the nozzle. At some point a piece breaks away and is dragged into the model wall as a black dot or a streak running across several layers.

How to check: heat the nozzle, push out 10 cm of material and look at the strand against the light. Dark specks or uneven colour mean a dirty channel. Look at the nozzle face too — a baked-on ring of deposit is visible with the naked eye.

What to do: clean the nozzle hot with a brass brush, then do two or three cold pulls. With stubborn carbon build-up it is simpler and cheaper to replace the nozzle than to fight it. Remember that PLA/CF requires a hardened nozzle — brass wears out within a few dozen hours on carbon-fibre-filled material, and an enlarged, ragged bore in itself encourages deposits to stick.

5. Retraction pulls molten polymer into the cold zone

Retraction that is too long pulls molten material above the melt zone, into the heat break. There the polymer solidifies, on the next move it is pushed back into the heat, and eventually it lands on the model as a dark lump. In the jargon this is called coking. The side effect can be more serious than the discolouration: a partial clog and a filament jam.

How to check: count the retractions. If the model has many separate islands and the discolouration appears exactly at line starts, the cause is retraction, not temperature.

What to do: shorten the retraction distance. For a direct drive extruder stay within 0.5–1.5 mm, for a Bowden setup 3–6 mm, and do not exceed those values without a clear reason. Also limit the number of retractions per layer and enable Avoid crossing perimeters, so the hotend has to retract the filament less often.

When discolouration is not a burn mark

Not every colour change means burnt polymer. Silk PLA changes its degree of gloss with temperature and speed, so a difference in shade between the walls and the top surface is normal. PLA Wood naturally darkens at the upper end of the range, because it is the wood filler reacting, not the polymer — if you want a lighter result, print closer to 210 °C and remember that the manufacturer recommends a 0.6 mm nozzle for this material. Streaks after a spool change are usually the remains of the previous colour in the channel, which clears after a few dozen centimetres of extrusion. And finally: matte, rough bands on PETG more often come from moisture than from overheating. Wet filament crackles in the nozzle and leaves a greyish bloom that is easy to mistake for a burn mark.

Related problems

• Clogged nozzle — charred deposits usually end in exactly that, a blocked channel.
• Blobs & zits — lumps on the surface, often from the same deposit on the nozzle face.
• Stringing — the other classic symptom of too high a temperature.
• Wet filament — gives grey, matte bands mistaken for burn marks.
• Smell and fumes while printing — thermal degradation always releases something, worth knowing what.
• The printer stops extruding mid-print — the endgame of carbon building up in the hotend.
• Filament jam — the result of material solidifying in the cold zone after too long a retraction.