Last Edited: September 14, 2021
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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.
Standard Extruder Components
The the hot end (also spelled as HotEnd) is an aluminum block that is heated by a ceramic heater cartridge. The heat produced by this component will lower the viscosity of the filament (i.e., melt it) so that it can be extruded out of a nozzle. Commonly available hot ends can comfortably 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, then the molten plastic will either burn or have too low of a viscosity to be properly deposited onto the build platform.
A temperature probe 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. There are two types of temperature probes that are commonly used in 3D printer extruders: 1) Thermistor and 2) RTD (Resistive Temperature Device). The big difference between these two types is that thermistors decrease in resistance with increasing temperatures while RTD have the opposite trend (increase in resistance with temperature). A more subtle difference between these two probe types is that thermistors have a higher sensitivity and are more responsive to changes in temperature than RTDs. However, RTDs have a larger temperature range.
Standard extruders typically come with a 100k thermistor that can measure temperatures up to about 300C. If you have the hardware to print at higher temperatures then you will need a PT1000 RTD that can measure up to 500C. At a later date, I will create a guide on the necessary upgrades for high temperature printing.
The blue silicone “sock” fits snugly over the hot end. All 3D printers move the extruder in at least one dimension, and by moving the extruder you are passing air across the hot end, which will induce thermal fluctuations. The silicone sock acts as an insulator maintaining the thermal stability of the hot end. The sock not only insulates the outside world from the hot end, but it also insulates nearby components, such as a Z probe, from the radiant heat of the hot end.
The cold end is directly above 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 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 minimizes the amount of molten filament.
An important and often underrated component is the heat break, which divides the hot end from the cold end. This part has a low cross-sectional area that restricts heat flow from these two components. Heat creep is when too much thermal energy makes it past the heat break allowing filament to melt in the cold end. Heat creep is typically an inherent issue with an extruder design or the use of poorly manufactured parts, such as those often used in clones, but reducing the printing temperatures can help.
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 small pin hole of a defined diameter, ranging from 0.25mm to 1.00mm (for standard flow extruders). 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, but this softness results in low wear resistance. Composite filaments 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.
To change a nozzle you have to unscrew it from the hot end. Fortunately, almost all nozzles follow the “RepRap” style and use an male M6 x 1 thread. Nearly all modern extruders have hot ends with the female version of this thread. This makes it easy to shop nozzles from different vendors and manufacturers.
Filament Diameter and Voltage
There are two specifications that you have to be mindful of when purchasing an extruder: 1. Filament Diameter and 2. Voltage. Extruder product listings typically look as follows: E3D V6 HotEnd - 1.75 mm and 24 V. Here the 1.75 mm refers to filament diameter and the 24 V is supply voltage needed to power the hot end.
There are two standard sizes for filament diameters: 1.75 mm and 2.85 mm. The filament section of this guide has a writeup comparing the benefits of these two diameters. If you are building a printer from scratch, then going with an extruder that accepts 1.75 mm filament is advisable because this is the more common filament size. Please note: an extruder that supports 3 mm filament cannot be fed 1.75 mm filament.
There are two common voltages that 3D printers run on: 12 V or 24 V. All new printers should be built to run on 24 V, and this includes picking a 24 V extruder. 24 V is far superior to 12 V and you can read all about it in the power supply section.
Which Extruder Should You Buy?
There are a lot of different extruders on the market, but three designs stand out: V6, Dragon, and Mosquito. Read about the pros and cons of each model below.
Slice Engineering Mosquito
The V6 is the oldest extruder design of the three and has really stood the test of time. The V6 has the most accessories and almost every printer on the market is compatible out-of-the-box with this extruder. If you cannot natively install the V6 on your printer, then there is definitely an aftermarket bracket to help you.
The one notable issue with the extruder’s design is that it relies on the heat break as a structural member. It holds the hot and cold ends together. To prevent heat from spreading to the cold end you want the heat break to be as thin as possible, but this is not possible with V6’s design. If the heat break is too thin, then the V6 would snap if the extruder experienced any force during installation or when changing nozzles. Therefore, the V6’s heat break is quite thick and can suffer from heat creep (as discussed above).
While the “groovemount” of the V6 is widely accepted in the industry, it is difficult for the DIYer to design a mount that properly grabs onto the top of the extruder. Also, if not tightened down properly, the V6 can spin, which can stress the heater and thermistor wires. A rigid mount that uses threaded holes would make it much easier to build a custom printer around a V6.
Even with those drawbacks, the price of V6 is the most competitive and is still the most commonly used.
The Dragon solves the heat break problem of the V6 by having four insulated rods connect the cold end to the hot end, relieving any kind of mechanical pressure on the heat break. This allows for the heat break to have an ultra-thin wall thickness (~0.1mm), which virtually eliminates heat creep. Also, it is much easier to change nozzles because the rods keep the hot end from spinning, so the nozzle can be unscrewed without having to clamp the hot end. The dragon accepts the common V6 nozzles (reprap style), so there is plenty of diameters and hardened materials to choose from. However, the Dragon is a little bit more expensive than the V6. For mounting, the Dragon has the same groovemount as the V6 and can in almost all cases be swapped in for any V6.
The Mosquito is the most expensive of the three desgins. Similar to the dragon, the Mosquito also solves the heat break problem with four insulated rods, which allows for a thin heat break and also, quick changing of the nozzles. The Mosquito is 30% (~20mm) smaller than the V6 and dragon. These height savings give you additional Z clearance for taller prints. But possibly the best feature, is the fixed mounting of this extruder. The Mosquito opted for mounting holes instead of the goovemount, so it can be simply bolted down.
Extrusion Drive Anatomy
It takes quite a bit of force to push molten filament out of a small nozzle that is a fraction of the size of the filament’s diameter. It is the extrusion drive’s job to convince the filament to go into the extruder out the other side.
The extrusion drive is powered by a stepper motor that either directly or indirectly drives a gear with ridges or teeth. These sharp features bite into the smooth but hard surface of the filament providing sufficient traction. A second hobbed gear or smooth bearing sits on the opposite side of the filament and confines the filament’s movement to up or down.
A stepper motor requires about 0.7 N-m of torque to properly drive the filament gear(s). Very few Nema17 motors have this kind of torque. A larger motor could be used, but heavier motors are not ideal because they can slow down the tool head if the extrusion drive is attached directly to the extruder (more on extrusion drive placement later). Therefore, most extrusion drives utilize gears to increase the torque output of small “pancake” Nema17 motors. 3:1 and 5:1 are typical gear ratio used in commercially available extrusion drives. Keep in mind that higher gear ratios will result in higher torque but also proportionally lower speeds. At some point your printer’s speed could be limited by the gearing of the stepper motor if you choose too high of a gear ratio.
Extrusion Drive Placement
In some 3D printer designs the extrusion drive sits on top of the extruder and moves along with it (i.e., direct drive setup). 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 (i.e., bowden setup). 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 increased distance between the hobbed 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 drive 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.
Which Extrusion Drive Should You Buy?
The designs of all extrusion drives are converging on the Dual Drive technology that was first introduced by BondTech. Gripping the filament on both sides allows for consistent extrusion and retraction. All three extrusion drives that I recommend are dual drive: BMG, LGX, and LDO Orbiter. Each of these extrusion drives will come in a couple different specifications to direct drive different types of extruders. For example, a stanard BMG has a groove mount for the V6 and dragon, while the BMG-M has the bolt on mount for the Mosquito. Below the product listings are descriptions for each drive and differentiating factors.
It should be pointed out that there are 3D printable extrusion drive designs that are significantly cheaper than commercial offerings. The Voron Clockwork is a repackaged BMG dual-gear drive, which cost a fraction of a genuine BMG, but you have to source your own gears and bearings. Clones also exist for many of the designs, which could save you money or could cause a headache if these knockoffs are manufactured poorly.
The Bondtech QR extruder was the answer to the unreliable extrusion that plagued 3D printers in the 2010s. After a couple of iterations, the BMG superseded the QR extruder as a lighter alternative (~210g including motor). The BMG would go on to be the most popular 3rd party extrusion drive. Today, aftermarket kits allow you to install a BMG extruder on nearly all 3D printers, including those manufactured by the likes of Prusa, MakerBot, and Ultimaker.
The appeal of the QR extruder and now the BMG is the dual-drive hardened steel hobbed gears. These gears have the profile of the filament cut into them and with lots of sharp little teeth have a high surface contact with the filament. Both drive gears are actively driven by a 3:1 gear ratio for triple the torque output from the motor.
While ultra-reliable, the BMG does command a high price tag for its gears and SLS printed housing. This cost does not even include a pancake NEMA 17 motor that is required for operation.
The Large Gears eXtruder or LGX for short is a separate product line from Bondtech but could be viewed as the next iteration of the BMG. The LGX uses larger drive gears to push the filament. In theory, larger gearing means more accurate extrusion and retraction as well as a larger contact area for more pushing force. The larger gears add about 10g of extra weight (220g total including the motor) compared to the BMG. The new ratcheting latch system lets you quickly adjust tension across filament types.
Even though the LGX appears to be 25% more expensive than the BMG it does comes with a pancake stepper motor and is ready to run out of the box. So the difference in price is much smaller than you may think. The larger gears, tensioning system, and sleek looks are worth the small premium.
The LDO Orbiter is a rare example of a Thingiverse design that went on to be successfully commercialized. Even better, the original creator, Dr. Lorincz Rober, was included in the process of bringing the Orbiter to market with the help of LDO motors. It’s too common for companies to plagiarize designs on Thingiverse or other open-source platforms and not provide any compensation to the creator.
The LDO is a strong extruder that is capable of 9.4 kg of pushing force, but at the same time is 25% lighter than the BMG (150g including motor). These impressive specifications are a result of a 7.5:1 planetary gear train. Planetary gearboxes are compact ways to produce massive amounts of force in a small for factor. The round LDO pancake motor only weights 75g, which is nearly half the weight of a normal pancake stepper motor (134g)
The icing on the top is that the Orbiter uses the same large Bondtech gears, but costs half of the price.
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.