The extruder is the element responsible for pulling the filament into the hotend so that enough pressure is generated inside it for the melted material to flow constantly and homogeneously through the nozzle.

Depending on the position of the extruder, two different types of extrusion systems can be distinguished: Direct and Bowden.

Direct Extrusion Systems

In direct systems, the extruder is located in the print head directly attached to the hotend, moving in solidarity with it. Because they have a short and straight path to the hotend, they can more precisely control the pressure at the nozzle using shorter retraction distances. Additionally, since they compress less during extrusion and there are no gaps in the path to the hotend, it is much easier to use flexible and elastic filaments.

Within direct extruders, two types can be distinguished: standard and compact. In standard direct extruders, the heatsink serves as a separation element between the hotend and the extruder. These types of extruders minimize heat transfer between the hotend and the extruder, allowing the use of high-temperature materials without experiencing overheating problems in the extruder. They are also usually less prone to "Heat Creep" with PLA.

Compact direct extruders usually incorporate the heatsink into their own housing, allowing the hotend to be attached directly to the extruder, minimizing the filament path. These types of extruders are recommended for printing flexible or elastic filaments with low hardness.

Image 1: Standard (left) and compact (right) direct extruders. Source: Bondtech

The main disadvantage of direct extruders is that, being integrated into the print head, they increase its weight causing greater inertia during movement. Compared to Bowden systems, it is advisable to use lower acceleration and direction change speeds. In general, virtually all direct extruders use filament in 1.75 mm format.

Bowden Extrusion Systems

In Bowden systems, the extruder remains fixed and guides the filament to the hotend through a tube, usually made of PTFE. This type of system significantly reduces the weight of the print head, allowing for higher accelerations and direction change speeds compared to direct systems. However, they present a significant drawback: the long distance between the extruder and the hotend, coupled with the larger internal diameter of the Bowden tube, leads to the appearance of gaps and greater filament compressions, making retraction control more complicated. Material optimization, especially for flexible and elastic materials, in this type of system is much more complex than in the case of direct systems. Additionally, certain flexible and elastic materials with low hardness, as well as fragile filaments like metallic or ceramic ones, may not be printable on some printers with this type of extruder.

Image 2: Bowden extruder setup. Source: Bondtech.se

There are Bowden extruders compatible with both 1.75 mm and 2.85 mm filament. Because the 2.85 mm format requires higher traction forces to achieve the same pressure as in the 1.75 mm format, it is advisable in these cases to use high-power motors or those incorporating reducers. When using flexible filaments, it is always recommended to use systems compatible with 2.85 mm filament.

Calibration and Maintenance

To avoid problems, it is important to adjust and maintain the extrusion system correctly. There are several common points in both direct and Bowden extruders that should be checked regularly.

1. Extruder Tension

Most extruders on the market include a system that allows adjusting the filament traction tension. It is very important to correctly adjust this parameter, as too low tension will cause filament slipping, leading to problems with lack of extrusion or inconsistent extrusion, while excessive tension will cause bite marks due to filament wear, reducing its cross-section and potentially causing extrusion failure.

Image 3: Bite marks on filament caused by excessive extruder tension.

Not all bite marks on the filament may be caused by a problem with the extruder tension adjustment. Sometimes it may be caused by a clog in the hotend. To differentiate the cause, the simplest thing to do is to push the filament until the extruder grabs it again and check if the plastic flows normally through the nozzle.

Image 4: Extruder tension adjustment screw. Source: Bondtech.se

It is recommended to readjust the tension every time a material with different properties is used. ABS, nylon, TPU, or TPE may require very different tension adjustments. To adjust the tension, the following steps should be followed:

1. Loosen the tension to the maximum

2. Start the 3D printer extrusion and load the filament.

3. Increase the tension slowly until the wheels start to pull the filament and wait until plastic starts to come out of the nozzle.

4. Increase the tension further until the plastic flows continuously and evenly, if necessary.

The appropriate tension is the minimum necessary to pull the filament consistently.

2. Tractor Wheel Maintenance

Extruders pull the filament using two opposing wheels, at least one of which has a grooved or rough surface. It is not common for wear to appear on these wheels, except when using abrasive materials (filaments reinforced with fibers, metallic or ceramic). If these types of filaments are regularly used, it is advisable to periodically check the condition of the wheels and replace them when signs of wear appear.

Image 5: Filament traction wheels. Source: Bondtech.se

In addition to checking for wear on the wheels, it is also necessary to check their cleanliness. It is common for plastic residues released from the filament due to friction to accumulate over time, which must be removed to prevent them from reaching the hotend and causing a clog. Cleaning can be done using a small brush and in many cases does not require disassembling the extruder, as many of them have an access area to facilitate this task.

Image 6: Access port for cleaning wheels. Source: Bondtech.se

The last maintenance task required by the wheels consists of lubricating the geared areas, either between the motor and the drive wheel or also between the two wheels in the case of DDG (dual drive geared) systems. It is very important to apply very small amounts of lubricant, and only in the geared areas, to prevent contamination of the filament or lubricant from reaching the hotend.

3. PTFE Tube Maintenance

Most extrusion systems include a section of PTFE tube at some point.

In Bowden systems, a long PTFE tube is used to guide the filament from the extruder to the hotend. It is very important to use high-quality PTFE tubes with adjusted and constant internal diameters. These PTFE tubes should be replaced when they show signs of wear or deterioration, as well as when bends or notches appear.

In many direct systems, small segments of PTFE tube are also used. In systems with all-metal hotends, it is common for there to be a PTFE tube that guides the filament from the traction wheels to the heatbreak inlet, while in systems with non-all-metal hotends this PTFE tube extends to the nozzle. This small tube is usually a consumable item that should be replaced frequently, especially in non-all-metal hotends. It is advisable to always use those provided by the hotend manufacturer, as they usually have special countersinking or machining at both ends. In these cases, using standard PTFE tube sections cut manually often causes clogging problems, either because the tube does not have the correct length or inlet shape or because the internal diameter is not correct.

Image 7: PTFE insert design of an extruder for Prusa Mk3s+. Source: Prusa Research.

The maintenance and adjustment of the extruder must always be accompanied by a correct maintenance of the hotend, as when working together, problems in the hotend can lead to problems in the extruder and vice versa.