How to choose a FFF/FDM 3D printer – What type of extruder do i need

The extruder is the part of the printer that pushes the plastic into the hot end.
You will find different types/sizes/geometries of extruders.

Each have their pros and cons but the main goal stays the same: push the plastic into the hot end!

They can use different kind of Hobbed bolt to grab the filament and to push it to either the hot end, or through a PTFE tubing.

Greg’s Extruder

This extruder exists for both 3mm and 1.75mm filament diameter.

However you will need to choose the one according to your filament diameter.
Note that what changes is mainly the internal hole size that fits the filament diameter.

They uses what is called Hobbed Bolt. It’s a machined screws with teeth on the flange that allows a good grip on the filament.

Look very closely at this part as the quality of the hobbed bolt is critical to have a proper extrusion!
The original greg’s extruder was purposed to push the plastic directly into the hot end.

• Pros:

The advantage of this is that you limit the nozzle dripping effect. You can also retract the filament slightly and the retraction effect is immediately taken into account.

• Cons:

This setup requires the extruder to be on top of the hot end, and therefore you will have the driving motor on top as well. This leads in having a big weight to move and so it reduces the overall speed of the axis where it belongs.

Bowden Extruder

You also have some variant of this extruder that pushes the plastic into a tube. This setup is called Bowden extruder.
The PTFE tubing has the following characteristics and is adapted for each filament diameter:

• For 3mm filaments:

Inner diameter (ID): 4mm
Outter Diameter (OD): 6mm
Lenght: often 1meter but can vary depending on your setup. The shorter, the better!

• For 1.75mm filament:

Inner diameter (ID): 2mm
Outter Diameter (OD): 4mm
Lenght: often 1meter but can vary depending on your setup. The shorter, the better!

The Pros:

• You can achieve very high speed with this setup because you have a limited amount of weight to move.

The Cons:

• The Hysteresis effect:

As the plastic filament is pushed through the PTFE tube, the tube will get into pressure. When you stop pushing the filament, the residual pressure inside the tube will continue to push the plastic into the tube.

You will therefore have a small dripping/leakage on the nozzle side.
To reduce this effect you will need to retract your filament enough to reduce the pressure inside the PTFE tube to avoid dripping.
This amount of retraction differ a lot from 1 printer to another one, and you might spend some time to figure out the proper amount.
Along with retraction you will have to figure out the retraction speed. The more the better, but your driving motor might be limited to a certain speed/acceleration couple.

Possibles troubles:

You might have different troubles with this kind of setup:

• Your hot end is too cold
• Your PTFE tube is too tight
• Your PTFE tube is too long
• Your PTFE tube is too curled
• Your filament quality is bad, and the diameter exceed the Hot end filament hole tolerance
• Your Hobbed bolt is filled with plastic and cannot push it anymore
• Your nozzle is stuffed

In all those case here what can happen and what you can do:

1. Try to increase slightly your hot end temperature, often a few degrees can make the difference.
2. Try to check your cooling fan, and make sure it doesn’t cool the hot end too much (check your Hot end temperature when this happens)
3. The driving motor will need to push more!
4. If you get motor making a lot of noise, you can try to increase current into the motor
5. If your extruder screws are not tighten enough, you will hear some “click”, in that case try to tighten them a little bit.
6. If your extruder screws are too tight, your motor might struggle to push the filament and thus make some noise.
7. Your filament pneumatic connector holding the PTFE tubing might release the PTFE Tube.
   This often means that the filament is either stuck inside the PTFE tube, or that the nozzle is stuffed with something, or your pneumatic connector is broken (some internal teeth might brake when this happens too often).
8. If your hobbed bolt is filled with plastic, your hobbed bolt will sweep over the plastic, resulting in the motor not pushing the plastic. Clean up your hobbed bolt and try again.

Direct Drive Extruder

Here the goal is to drive directly the filament into the hot end, or the PTFE tube for the bowden version.
The Pros:
Very compact, as you only need a hobbed gear (MK7/MK8/Hobbed shafts, etc…)

The Cons:
You can only use 1.75mm filament, as 3mm filament will require more torque.

Different kind of Direct drive gears

From this nice benchmark :

http://airtripper.com/1676/3d-printer-extruder-filament-drive-gear-review-benchmark/
Here is the result by order of performance:
MK8:

The MK8 Drive Gear provides a good balance of grip and torque to push the filament with force that easily exceeds 4kg.

MK7:

It has excellent grip on the filament and the idler tension is easy to set-up. However, the gears’ large effective diameter may not provide enough torque when nozzle and filament troubles occur. If you’re looking for serious pushing power from a geared stepper motor, the MK7 should be first choice.

Plain Insert drive gear:

Good pushing power. However, the gear can be difficult to set-up without the help of the filament force sensor. If you have good experience with 3d printing and have a well oiled machine, you might get some good performance out of this cheap drive gear.

Raptor drive gear:

As proved with the MK7 Drive Gear, bigger gear teeth don’t mean better grip. However, the Raptor Drive Gear might perform better on a geared extruder where idler tension can be increased, but at the expense of causing more damage to the filament.