What is an Extruder?
You might be surprised to learn that this is a matter of some debate. Some maintain that the extruder is only that part that feeds the filament through the nozzle (the cold end). Others contend that only those components used for melting the filament constitute the extruder (the hot end).
The way we see it at i3D is that you can’t have one without the other. Consequently, we will define the extruder here as having two components: the hot end and the cold end.
The cold end refers to upper portion of the extruder where filament is fed, and then passed along into the hot end for melting and extrusion onto the print bed. The filament is not heated at all here.
The most basic cold end consists of a stepper motor, toothed gearing, a hobbed bolt or gear, a spring loaded idler to hold onto the filament and a section of PTFE (Polytetrafluoroethylene is a compound like Kevlar) tubing to guide the filament into the hot end.
Stepper Motor – the stepper motor drives the motion and extrusion of filament in almost all desktop 3D printers. Stepper motors are brushless DC motors that achieve a high level of precision in small movements and impart full torque at low speeds.
You may hear the stepper motor referred to as a NEMA 17 motor. NEMA stands for National Electrical Manufacturer Association, and is the organization that sets the standard for electrical devices. “17” is a reference to the face plate size – in this case 1.7” x 1.7”
Parts are required to be attached to the stepper motor’s driveshaft in order to grab and physically grab the filament and push it along on its path to the hot end.
These parts are a combination of gears and hobbed bolts – or shafts – serving as a pinch wheel along with a bearing or other stiff frictionless material. The bearing is often spring loaded to maintain pressure on the filament and allows for the free movement of the filament (as dictated by the rotating of the hobbed bolt/gear).
The exact configuration of these parts varies per manufacturer. Some manufacturers use fully metal components, others reduce cost by implementing polymer components. Some use an idler to push the filament against the feeder gear, others use to gears to reduce the chance of slippage.
There are two popular variations of extruder beyond the basic componential makeup of the cold end and they are the direct drive and the Bowden drive, and each have their pros and cons, as discussed above.
The cold end of the extruder feeds the filament to the hot end.
Inside the hot end, the filament passes into a heated chamber, where it transitions from solid to fluid before it is pushed out and deposited on the build plate. It sounds simple, and for the most part, it is.
But there is still a lot going on – and a few parts – to keep in mind.
First, there is the filament feed tube. In both the Bowden and direct drive extruder this will simply be the PTFE tube running from the cold end to the hot end. Note that not all direct drive extruders feature such a tube, and when they do, it is typically no longer than 1”. More often though, direct drive extruders run the filament directly into the print head.
Note that since high temperatures are required to melt the filament, a good degree of accuracy is required to determine when a material becomes molten and the management of the temperature is crucial.
The feed tube inserts the filament directly into the heat break through the heat sink. The heat break, which is threaded into the heat sink, is often simply a threaded stainless steel (or other non heat conducting metal, such as titanium) tube.
Divided into two parts and sometimes featuring a treated interior surface, the heat break allows filament to pass freely into the nozzle for extrusion. The heat break, in combination with the heat sink, creates a specific boundary at which the filament is hit with high temperatures.
The upper portion, which is actively cooled by the heat sink and a dedicated fan prevents heat from escaping the hot end and weakening the filament before it’s where it needs to be for extrusion.
This phenomenon is known as ‘heat creep’.
The lower portion of the heat break sits within a heater block, along with a heater cartridge, temperature relaying thermistor and nozzle. Usually constructed from aluminum, the heater block ensures a seamless transition for the filament from the open end of the heat break tube, into the nozzle.
The temperature to melt the filament is provided by the heater cartridge (or coil). Under an electric current, the heater cartridge gets hot, transferring heat to the nozzle via the heater block they are both encased in.
The thermistor – which is essentially a small probe – relays the temperature of the block to the control board which allows for corrections to be made as needed.
Lastly, the molten plastic is deposited using a nozzle.
The nozzle is on the one hand the simplest of the components that make up the extruder, and yet at the same time one of the more important ones since it has a direct impact on your print.
The nozzle is a machined piece of metal that tapers to a desired diameter. The smaller the diameter, the finer the stream of extruded plastic. Nozzles periodically need replacement. Since the preferred default for nozzles is brass, they are soft and over time wear especially when used to extrude composites like blended or carbon-fibre materials.
Nozzles are available in different materials – like stainless steel and tungsten carbide – and in different diameters.