The extruder is the defining characteristic of FFF 3D printing. There are other types of additive manufacturing (e.g. stereolithography, inkjet printing, etc.), but the way in which the extruder heats and liquifies a thermoplastic before depositing it on the build platform is what makes this printer a FFF printer. There are two functional parts of an FFF extruder: the hot end, which softens and liquifies the plastic filament, and the extrusion drive, which forces filament into the hot end. At the risk of overcomplicating this simple mechanism, I would like to point out that the solid filament that enters the hot end acts as a piston forcing the filament that has been melted out of the nozzle. The plastic that exits the nozzle will be called the fiber. The fiber is deposited in a layer-by-layer process by the system of linear rails and carriages to form 3D geometries. We will take a closer look at the components that make up the extruder, but if you start to get confused just remember the extruder is nothing more than a glorified hot glue gun.
The nozzle is the last part of the extruder the plastic touches before being deposited on the build platform. The molten plastic exits the nozzle through a a small pin hole of a defined diameter, ranging from 0.25mm to 1.00mm. The extruded fiber takes on the diameter of the pin hole, and higher resolution prints can be achieved by using nozzles with smaller pin hole diameters. When selecting a nozzle, the other important consideration besides the pin hole diameter is the material the nozzle is made of. The cheapest nozzles are made out of brass because brass is soft and easy machine. However, soft materials like brass have a low wear resistance. Composite plastics containing hard and sharp particles like carbon fiber and sand will eat away at the inside of a brass nozzle, increasing the size and irregularities of the pin hole. To print these erosive plastic composites its best to use a hardened steel or even a gem-stone tipped nozzle.
The Hot End
The nozzle screws into the hot end, which is an aluminum block that is made hot by a ceramic heater cartridge. Commonly available hot ends can comfortable reach temperatures north of 250C, so it is always best practice to never touch the hot end even if the printer is turned off. Each printable thermoplastic has a narrow temperature range where it exhibits the best properties for printing. If the hot end is below the melting temperature of the plastic then it can’t be extruded. On the opposite end of the spectrum, if the hot end is too hot, the molten plastic will either burn or have too low of a viscosity to be properly deposited onto the build platform. A thermistor located next to the heater cartridge measures the temperature of the hot end, providing regulatory feedback to make sure the hot end stays within a set temperature range.
The Cold End
The cold end immediately follows the hot end and typically has cooling fins and a fan. The presence of a cold end can seem counterproductive to the main function of the extruder, which is heating the filament. For people new to 3D printing, the extruder not only has to eject filament on demand but it also has to retract filament in order to move to unconnected features or parts on the build platform. The longer the distance of preheated filament the less responsive the extruder will be and the more likely the extrusion drive is to jam. The cold end restricts molten filament the aluminum block of the hot end.
The Extrusion Drive
The extrusion drive is what pinches the filament and pushes it through the hot end. Intuitively, the filament has to be cold, so that the gears in the extrusion drive can grip the filament. This is why a properly functioning cold end is so important for the printing process. The gears that have the splines cut into them and are responsible for grabbing a hold of the filament are known as hobbed gears. The extrusion drive is powered by a stepper motor which turns the hobbed gears. The process of jamming filament down through the cold end into the hot end requires a surprising amount of force. For this reason, a geared stepper motor is typically used for the extrusion drive.
Extrusion Drive placement
In some 3D printer designs the extrusion drive sits on top of the extruder and moves along with it. In other designs, the extrusion drive is fastened to the frame and is connected to the extruder by a long tube known as a bowden tube. There are pros and cons for both placements of the extrusion drive. With the bowden tube extrusion drive system the print head can accelerate and deaccelerate faster because the print carriage does not have the added weight of the extrusion drive. This can significantly speed up print time. The drawback for this setup is the distance between the hobbled gear that is pinching the filament and the hot end. The longer this distance the less responsive the filament extrusion will be. This hysteresis or lag is amplified when printing flexible filaments, so it’s best to avoid the bowden setup when you expect to print flexible parts. For the direct extrusion system the pros and cons are swapped. You can’t accelerate and deaccelerate as quickly because the carriage has the added mass of the stepper motor but the extrusion and retraction of the filament is more responsive. I tend to always employ direct extrusion as the maintenance is minimal. Further, I tend to print at lower speeds anyways because part quality is always my highest priority.
There are a lot of different ways to successfully build a 3D printer, many of which are not covered on this website. If you want to learn more about FFF extruders, then click through some of the links below to external websites and forums.
The table below contains parts specific to this section that Dr. D-Flo uses and recommends. Depending on your printer build these parts may or may not be compatible. It is best to use the products below as a launching point to explore similar products on the linked websites. Affiliate links may be present below (depending on the vendor).