APP – Fixing obvious extrusion issues (PET)

Once your material starts coming out of the nozzle for the first time, the settings adjustment process begins.

The first step is to fix the “obvious” extrusion issues.

At this stage, it is important to not spool the filament, just pull it.

1. What are “obvious” extrusion issues ?

The obvious issues are the ones you can see with the naked eye. In general, they are easy to notice, understand, and fix.

The most common extrusion issue (thickness deviation) is not in this list. It has to do with the flow stability, which is a lot more difficult to manage and time-consuming. This will be the focus of the next finetuning step.

Fixing the first extrusion issues will result in a homogeneous, smooth output that comes out continuously and allows you to focus on the tolerance.

2. Troubleshooting each issue

Most extrusion issues can be fixed by tweaking the experimental process. But sometimes, when the material is really incompatible with the filament making process, only changing the material can solve the issues.

Important note : When you adjust the settings during this step, we recommend you to apply the same changes to all heaters, whenever possible. Because this step is not the final finetuning step, it is easier to work with rough baselines rather than specifics.

Solid bumps

  • The main cause is the presence of impurities (dust) in the batch of regrind. It is difficult to remove 100% of the foreign particles from the batch.
  • The regrind might not be homogeneous in size. All the particles should be smaller than 4mm.
  • The material might only be melting partially. This does not mean that the temperatures should be risen. Decreasing the temperatures by small steps should increase the viscosity, the friction, and therefore enhance the melting.

Weak flow (almost no material comes out)

This can happen when:

  • Ratholing or bridging is happening in the hopper. Then you can use the Feeder.
  • The material is not viscous enough. This can happen with injection grades for example, or when the temperatures are too high, or when the material degrades (hydrolysis or another form of degradation)
  • There is not enough pressure to push the material. You can try to decrease H4 and H3 while increasing H1.
  • The barrel is partially or totally clogged. Purging the machine with Devoclean MidTemp might unclog it.


  • Lower the temperatures (small steps of 5°C)
  • Make sure the material has been properly dried

Solidification/build-up at the tip of the nozzle

PET can solidify very fas upon exiting the nozzle, and stick to its tip. This issue becomes more likely when the temperature in the room is cold.

  • Make sure the magnetic plate is well-positioned on the hole around the nozzle
  • Angle the fans so that they blow as downward as possible (not horizontally nor straight toward the nozzle)
  • Increase the temperatures (especially H1)
  • Lower the fancooling percentage

When this issue happens, you can carefully use tweezers to pull the residue off the nozzle.

Streaks on the surface

This refers to continuous streaks, or lines, carved into the surface of the filament.

Note: fibers can make the surface rough with lots of lines, so can bubbles, but those lines are not continuous.

  • A build-up of solid material at the tip of nozzle may cause this issue.
  • If the nozzle is damaged, then it should be changed.

Filament falling “like a snake”, not vertically down through the sensor

This can cause zeros in the reading, because the filament leave the reading angle of the sensor.

  • Decrease the fancooling
  • When this happens, it is often easier to decrease the fancooling by a lot, or completely, and manually re-position the filament in the puller again, nice and straight

Filament sticking to the puller wheel

  • Increase the fancooling
  • Decrease the screw speed
  • Decrease the temperatures (especially H1)

Filament ovality

The filament is round when going down through the sensor, but can get flattened/squeezed between the wheels. There are several solutions:

    • Increase the fancooling
    • Decrease the temperatures (especially H1)
    • Decrease the screw speed

Sensor reading zeros

If the filament sometimes moves out of the sensor’s reach, then this is normal (see “Filament falling “like a snake” ” above).

If not, it means the sensor reads zeros when it should not:

  • The sensor must be cleaned.
  • Light from the room is interefering with the optical sensor. Try to cover the opening of the sensor with a piece of tape. This issue is extremely rare.

3. What we did

We mainly faced the following issues :

  • Difficult cooling (causing nozzle build-up, “snakelike’ solidification, bad control of the temperature in the room) : the solution was to make very small and accurate adjustments around 15% (depending on the temperature in the room).
  • Impurities : we had to clean the bottles and our Shr3d It more thoroughly. Protecting every step of the process from dust, is key.
  • Temperatures : PET being very liquid but also very sensitive to temperature variations, we cautiously decreased all the temperatures down to 245°C. That seemed to both stabilize the flow and give the filament a good solid behavior for spooling.
  • Weak flow : ratholing was disturbing feeding and regrinds have a low bulk density. We mounted the Feeder onto the Filament Maker and increased the screw speed to 7RPM.