Brown Lines or Dark Spots in 3D Prints? How to Fix

You’ve been 3D printing some PLA or other material of a light color and noticed some brown spots or lines in your print, now you’re trying to find a solution. Luckily, you’ve come to the right place, in this post I’m going to detail how to fix brown lines or spots in 3D prints and some other useful information.

How do I fix brown lines or spots in my 3D prints? The best way to fix brown lines or dark spots in your prints is to tweak your speed, travel, temperature and retraction settings in your slicer to get an even extrusion without leaving residue behind. First decrease your nozzle temperature by 5°C. You can also occasionally clean and oil your nozzle or upgrade your hotend.

This is the basic fix, keep reading on to learn why it happens, more ways to fix it, and future prevention methods to stop it from happening again.

Why Do Brown Lines or Dark Spots Happen?

Even though a few of your prints will come out completely fine, some of them may be tainted with brown lines, dark spots or strange residue which ends up ruining your prints. This section will look into the underlying causes as to why this happens.

There is one main reason why you’ll get these kind of imperfections in your prints and a few other possible reasons.

The first and most common reason is your nozzle temperature is too high, especially when printing with a sticky material like PETG.

Some filament just happens to be more sticky and rubbery when melted so when they are being extruded, some of the molten material can get left over inside the nozzle.What can happen is loose fragments get extruded by your nozzle because of some underlying problem which can be parts of your extrusion system, or incorrect settings.

After some time extruding, there can easily be a build up of material residue and burnt flakes left inside the nozzle which burns over time and ends up in your prints.

A combination of your nozzle moving around, getting hot and more material gathering inside the nozzle causes it to soon be extruded out and this is when you’ll see some dark spots.

A less common cause is your teflon-coupler, located near your nozzle can get burnt and deformed if there is some kind of underlying problem. This may have happened when fixing another issue and some kind of loose connection cause heat to be transferred to the coupler.

These things are more likely to happen if you have recently increased the flow-rate, increased the temperature, or have some overextrusion.

Also, when your nozzle presses down on one spot for too long on your print bed, it can cause material to burn. Some people do this purposely to have material stick better but only a little bit of pressing down is required.

If you use the wrong type of oil with your filament it can cause a build up and result in these dark spots. Using the right kind of oil here, rather than adding to the problem can actually help stop filament sticking to the inside of your nozzle.

How Do You Fix Brown Lines or Dark Spots in 3D Prints?

To fix this issue, it’s best to first identify why these brown lines or dark spots are happening. Once you have your root cause, it becomes easier to come up with a solution for it.

In some cases you have some catch-all type fixes which might just work with what ever is causing the problem but sometimes it might come back because you will just be fixing the symptom rather than the cause of it.

It can usually be fixed by tweaking some settings in your slicer, the usual ones being your speed, temperature, retraction or combing.

Sometimes simply making sure your nozzle is tightened down is the solution because plastic can ooze out of the base of the nozzle.

If PLA or some other material has leaked from your extruder or hotend it can burn and turn black from sitting in the heat for so long. Eventually, it will drip down onto your prints.

Go back and check whether your nozzle has a good, sealed connection with your hotend. There should be a little gap of around 0.1-0.2mm so it won’t usually be too wide.

If your nozzle is pressing down too hard on the first layer, re-levelling your print bed can fix this problem. This happens because when the nozzle is pressing down too hard, material isn’t being extruded fast enough, meaning it’s exposed to heat from the nozzle for longer than necessary.

When this happens, one user described using silicon mould release oil by spraying it on tissue, then wiping the nozzle.

What it does is reduces the build-up of melted filament around the interior edges of the nozzle. It works better for some materials more than others such as PLA & ABS. Some other oils could work better but it did work better than PTFE oil.

Some people use silicon socks for the heater block which reduces the chance of residue building up on your heater block and getting burnt.

You can perform a the Atomic Pull Method to thoroughly clean out the extrusion path which will clean out any burnt debris left over from previous printing. If necessary, you can replace your teflon-coupler if it is burnt and deformed.

Sometimes just a good clean of your nozzle will help, because some sticky fragments of material might just be stuck up there, but we want to stop there being these fragments altogether.

Your nozzle will have a big effect on how often this might happen. Some nozzle materials and sizes are better at clearing residue and debris better than others. Stainless steel nozzles are well-known to resist filament sticking down to it, but they don’t retain heat as well as brass. My post on Brass Vs Hardened Steel Vs Stainless Steel Nozzles goes into some pretty good detail.

Preventing Dark Spots & Brown Lines on Your Models

There are a few good ways to increase prevention for dark spots and brown lines in your models:

  • Use the optimal temperature for your filament
  • Balance your printing speed with your travel speed, trial and error
  • Avoid filament that are known to be sticky and rubbery
  • Change your nozzle for a higher quality one
  • Occasionally oil clean out and oil your nozzle (priming) to prevent material sticking
  • Replace your hotend with one that melts filament better

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