If you do not have a specific hotend for every material you are using, you will likely experience some black dots on your prints from time to time. Even when you are using only one material, this can still be a frequent occurrence. This can be from a few culprits, but often it will show itself by having built up material on or in your hotend.

One of the biggest issues with this failure is that you will often not be able to diagnose it until it happens. This means that a black spot might show up on an important section of your print 10+ hours in. This is why it is crucial you maintain the cleanliness of your hotend by frequently purging and cold pulling any residue from your nozzle, along with using a nylon brush and silicone sock.

Ensure your hotend and nozzle is set up properly

Every hotend setup needs to be assembled in a slightly different fashion, but nearly all of them require you to not over-tighten.

When the heater rises in temperature, the metal expands and can cause your once tight nozzle/heater block to have minor gaps. This gap can cause material to ooze out and make its way onto your print. Since this material has been stuck on a hot nozzle before finally being pushed onto your model, it will likely be black and burnt, regardless of the color you are using.

If you notice that your heater block is loose when hot, or that you constantly have to brush off the nozzle or hotend from excess material, you will likely need to tighten these parts.

I always suggest doing the final tightening of your nozzle and heater block when heated to 240°C or higher (if using an all-metal hotend), and using proper gloves and tools. Remember that you have a high chance of burning yourself, so do this with caution.

You also want to make sure to not over-tighten anything. I have broken quite a few heater blocks, nozzles, and heat break barrels due to over-tightening. These parts, especially when hot, can easily snap under pressure. When you are doing this, make sure to only tighten until you know that the nozzle and heat block are not loose, and will not unscrew during printing.

If you are still experiencing material oozing from the gap between your nozzle and heater block you will likely need to upgrade or replace your nozzle, heater block, or entire hotend. Poorly made or worn out parts will not have tight tolerances, and can lead to these gaps in your threads. I have seen images of cheap knockoff products cut in half showing just how poor their tolerances are.

This is why it is important to only purchase name brand parts and not to buy aftermarket knock-offs. For example, if you want an E3D hotend, only purchase from verified dealers such as E3D, Filastruder or Matterhackers, because there are many counterfeit products on the market. Matterhackers is a good source for most 3D printing parts.

Purging material

Every time you switch filaments or after very long prints, you should purge out the material that may have oxidized inside of the hotend. There are a couple of ways that you can do this.

If you are using the same material, you can go ahead and heat the hotend to its printing temperature. Then push down the filament for about an inch, and pull up quickly. Cut off the end and you should be good to go with your next print. Repeat this step as necessary if you are switching to a different color in order to ensure that you do not get any mixed colors during your print.

If you are switching to a material that prints at a higher temperature (such as switching from PLA to ABS), purging is normally simple. Do the same procedure as above, but multiple times in order to confirm that there is no remaining residue within the hotend. If this hotter material is in a different color, then you will follow the same procedure, it will just be more apparent when you haven’t purged enough. Since you are printing at a higher temperature, the majority of the previous material should be removed.

A real issue occurs when you are switching to a material that prints at a lower temperature than your previous filament (such as switching from ABS to PLA). It is likely you will not be able to purge all of the residue material with the above method, because the ABS needs to be purged at a temperature it can properly extrude at.

If you like to live dangerously, you can purge this material by pushing the colder filament through the hotend when it is set to the higher temperature (such as extruding PLA at 245°C when switching from ABS). Push the material through at a steady pace and then pull it out very quickly, making sure to not let it sit. If you attempt this method, you are going to have a higher chance of a nozzle clog, and you may not get 100% of the residue material.

The proper way to get rid of this material would be to do a Cold Pull as described below, or to purge by using a cleaning filament/nylon material.

Purging with cleaning filament or nylon can be done by heating your extruder to a temperature of around 240°C – 250°C (or whatever the material you are using calls for), and then extruding the cleaning filament through as you would with the examples above. Quickly pull the filament out as to not leave any residue, and you will see just how much gunk the cleaning filament was able to pull out.

Though less common when using cleaning filament than with other printing materials, you still run the risk of leaving residual material in the hotend that will come out later as a black, burnt spot. That is why the best method is actually a cold pull.

Cold pull

When switching to a material that prints at a lower temperature than your previous filament you will likely want to do a cold pull. Cold pulls are also very beneficial to do as regular maintenance on your machine regardless.

I personally like to perform cold pulls with either a cleaning filament or Nylon mix, but you can perform them with the material you are trying to clear out. My favorite material to do this with is Nylon 910 by taulman3D. Not only is that material great to print with, it seems to work even better than cleaning filament I have used in the past for removing the oxidized material in the hotend.
What you do is heat the hotend to the temperature of the material you are using to do the cold pull (250°C for Nylon 910). Push the filament through for an inch, or as much is required for you to no longer see the previous material coming out the nozzle.

Then quickly set your hotend to 130°C - 150°C (I normally do 130°C). You don’t want to leave the material sitting in the hotend for a long period of time because it can oxidize itself, or even cause heat creep in your barrel. Once the nozzle cools to this newly set temperature you will want to pull out the filament. This can be difficult if there is a lot of built up residue material, but it normally doesn’t require too much effort.

Once you pull you should see excess burnt or colored material on the filament you just cold pulled. Repeat this process until you no longer see this residue.
This is the best way I know of, other than purchasing a new hotend/heater block to get rid of the excess and oxidized material.

Excess oozing

I recommend reading the “Material Science” page, particularly where oozing is discussed, to help prevent excess material from coming out of the nozzle.
Use a wire or nylon brush to clean nozzle and hotend

You should have a brass or copper brush on hand to clean the nozzle and hotend periodically. Before a print, especially one where you see material has built up on the heater block/nozzle, you will want to clean it with a wire brush (when heated). When the nozzle is hot you can brush off any excess material that has built up.

Many people suggest using a nylon brush instead of a wire brush since it is far less abrasive. I frequently use a nylon brush and only periodically use a wire brush on a very dirty hotend. If using a metal wire brush it is better to use one made of copper or brass as opposed to steel to reduce the amount of nozzle abrasion that occurs. Keep in mind that a normal nylon brush will melt on the heated hotend if left on for any longer than a quick wipe.

This is crucial in maintaining a clean nozzle and reducing the amount of burnt spots experienced on a print.

Don’t leave filament resting in a heated hotend

You shouldn’t heat your hotend until you are ready to extrude. If you leave filament in a heated hotend for long periods of time you will increase your chance of nozzle clogs and oxidization of the material.

Make sure all of your end G-codes have the script M104 S0, which turns off your hotend after completing a print.

Summary of Fixes and Precautions

• Make sure you have a well-made hotend and that everything is tightened when heated to 240°C or higher. Take proper precaution to not over-tighten.
• Purge old material by pushing down new material an inch and pulling out quickly. Cut off any old material that is stuck onto the filament and repeat the process until there is no longer any excess residue.
• Use cleaning filament or Nylon 910 since it works best for this process. If not, you can use the material you are printing with.
• Cold pull by extruding cleaning filament/Nylon through the hotend at 250°C. Allow the nozzle to then cool to 140°C – 160°C, and pull the filament out. Repeat this process until you no longer get discoloring.
• Don’t leave filament in a heated hotend for extended periods of time.
• Use a silicone sock to reduce black spots on your print.