APP – Finetuning (PET)

At this point, the flow of PET should be quite stable and the filament should be quite round and consistent.

The screw speed is probably around 4-7RPM, and the temperatures around 245°C.

At this stage, the filament is still not being spooled.

1. The goal

For the flow to be stable, the material must melt at the right moment:

  • not too soon, otherwise not enough pressure will be generated
  • not too late, otherwise unmelted particles can cause RPM fluctuations or even come out unmelted

The objective of the finetuning step is to narrow down the diameter fluctuation, which can be seen on the Filament Thickness graph.

2. The methodology

In order to control the melting process and achieve a tighter tolerance, you must:

  • Change the settings slightly
  • Wait long enough (usually 15min-20min) to be sure that the output really corresponds to the new settings
  • Look at the tolerance on the filament thickness graph, and see if it has become better or worse
    • If the tolerance becomes tighter, it means you have made the right choice, and can keep adjusting the settings in the same direction
    • If not, or if the changes seem to have a negative impact on the results overall, then you should probably try opposite changes
  • Repeat this loop

Thanks to the previous step in the roadmap (“Fixing obvious extrusion issues”), we know that cooling the filament is tricky : excessive cooling will cause nozzle build-up and/or ‘snakelike’ solidification. Adjusting the temperatures and fancooling must be done carefully.

3. Concretely, in terms of settings ?

Following the experimental loop from section 2:

  • First it is good to focus on the temperatures – all of them at once. You should start by decreasing all the temperatures by 5°C, wait, look at the tolerance, repeat. In most cases, the final temperatures end up being lower than the starting ones. The idea is to quickly explore the entire thermal window, by discovering the extremes.
  • Then, looking at the log and the output, you should be able to tell what seemed to be the best thermal range.
  • Once you have a good baseline, start adjusting each heater individually. In general, H4 and H3 influence the pressure, while H2 and H1 determine the texture of the output. In a lot of cases, the final temperatures are quite close to each other (spread out over a range of 20°C), and H2 is the hottest, followed by H3 or H1. Not always.
  • Once you feel like you have experimented enough with the temperatures, you can choose the best profile and change the speed by steps of 0.5RPM. Usually between 3.0 and 8.0RPM.
  • The fancooling is a special parameter: feel free to change it anytime, but always carefully. Keep in mind that PET was designed to designed to turn solid very suddenly.

4. What we did

As a matter of fact, the settings found during the previous step did not need to be modified.

245°C, 7RPM, 15% fancooling, were optimal settings :

  • the filament was round and smooth
  • looking at the filament thickness graph, the tolerance was tight enough (+/-50microns approximately)

Only the fancooling needed to be adjusted : slightly higher when the room temperature was higher, and vice versa.