In previous videos we always showed the extruder assembled. This time we take it apart. The subject is the screw, the heating zones and the question of why a pellet extruder behaves so differently in operation from a filament extruder – and when each is the right choice.
The basis
As a basis we use an extruder from our supplier and partner Princore, which we have adapted for our field – technical ceramics. Most of these extruders are in circulation with a brass body. Our version has a hardened barrel. Added to that are hardened nozzles from 0.8 mm upwards; in some cases we fit diamond nozzles from Gühring. The plastic sleeve around it is our own design.
The feed zone is the heart of it
A stepper motor with a 5:1 gearbox drives the screw. The aluminium body above it doubles as the feed: this is where the pellets drop in and initially sit loose.
The grooves in the feed zone are decisive. The rotation of the screw swirls the pellets and draws them in actively – without them there would be no reliable material feed. A blade also rotates with the screw, freeing larger fragments if something jams.
Flight depth creates the pressure
Look at the screw and you notice that the flight depth changes along its length: deeper at the back, shallower towards the nozzle. That is no accident. The shallower the flight depth becomes, the more pressure we build. At the nozzle we measure around 5 bar on our extruders; on considerably larger industrial extruders we have seen up to 30 bar – but that is a different league.
How strongly this flight depth varies differs considerably between manufacturers. For us one thing matters above all: a clean, thorough mixing of the melt.
Hot at the bottom, cold at the top
Temperature management is a subject in its own right on a pellet extruder. The heater sits at the bottom – that is where the pellets are melted, compressed and pressed through the nozzle opening. At the top, in the feed zone, exactly that must not happen: if the pellets start to melt there, the feed is disrupted.
We therefore cool the feed zone actively and blow cold air through the cooling fins. In our version we changed the geometry for this: in the original layout the warm air blows towards the operator standing in front of the printer – in ours it is directed away. A large part cooler with a curved fan duct handles part cooling below.
Some extruders work with several heating zones and are correspondingly longer. That brings considerably more heating capacity – and therefore higher throughput.
Why every material needs its own parameters
Because we genuinely create a melt, every material behaves differently. And every nozzle with a different outlet creates a different back pressure. Until now that value has been set from experience.
This is exactly where we want to come in: in future the machine should work out that parameter for itself, rather than someone searching for it from experience. We are well aware that this does not make matters simpler – more on it in our article on machine learning in pellet-based printing.
The toolhead board on the head
Our circuit board sits in the same place. It brings all the wiring together over a short distance and creates a serial interface: only one six-core cable goes into the control cabinet. The hopper on top is interchangeable – for a spout taking the hose of the pneumatic conveying system.
For comparison: the filament extruder
In a filament extruder everything happens in the solid state. A geared drive creates the feed and pushes the wire into a hot end, where it is melted for the first time.
That is simpler to begin with: the feed rate is always the same because the material is solid and has a constant diameter. In return you are limited at two points – how much can be melted per second, and how much force the extruder can apply to push the material through. Because nothing flows there on its own.
Which one, when?
For very fine parts the filament extruder is the more suitable choice. Where high throughput and heating capacity matter, you are better served by the pellet extruder – along with gradient printing, material blends and the lower material price.
Both work well. It is simply a question of the application. You will find a detailed comparison of the two processes in our article on FFF and FGF.
Chapters in the video
The video is in German; the chapter titles below link straight to the relevant passage.
- 0:00 · Intro – pellet and filament extruders compared
- 0:23 · The sleeve and the Princore base
- 1:08 · Motor, 5:1 gearbox and feed
- 1:28 · The feed zone: how the pellets are drawn in
- 1:57 · The hardened version and nozzle options
- 2:16 · The blade against jammed pellets
- 2:52 · Screw flight: mixing and pressure build-up
- 3:43 · 5 to 30 bar at the nozzle
- 3:58 · Heating zone, melt and feed cooling
- 4:25 · Several heating zones and back pressure per material
- 4:46 · Parameters by machine learning in future
- 5:24 · Our sleeve: hopper, hose and toolhead board
- 6:05 · Airflow: feed cooling and part cooling
- 7:08 · The filament extruder by comparison
- 8:10 · When filament makes sense, and when pellets do
- 8:34 · Components used and conclusion