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How Does a 3D Printer Extruder Work? Facts Explained

Extruders are the heart of your 3D printer, and they manipulate your filament and turn it into a viscous liquid suitable for creating a 3D print. However, these blocks — full of various parts — can be confusing if you don’t know what’s under the hood.

A 3D printer extruder works by pushing filament through a cooling area, then a heated area before it escapes the nozzle. Extruders come in several varieties, but they all contain a cold end and hot end that feed in more filament and maintain the ideal temperatures for printing.

In this article, I’ll lead you through every step your filament goes through — from when it enters your extruder to when it hits the print bed. I’ll also tell you about the differences between each extruder style, listing their pros and cons to give you a comprehensive idea of what’s going on in your 3D printer’s extrusion system. Let’s get into it!

What Is a 3D Printer Extruder?

You likely already know what an extruder is. However, you might not know where the extruder stops and ends. For example, some people have different ideas of what an extruder is. So, let’s get to the bottom of this misconception and talk about what an extruder is and what it’s not.

A 3D printer extruder is the mechanism on your 3D printer that feeds filament in, heats it, and pushes it out as you print. The extruder contains many components (including the hot and cold ends) consisting of parts like the temperature sensor, drive gear, nozzle, and fans.

There are two standard 3D printing extruder styles: the Bowden tube and direct drive. These extruders contain the same parts, except the Bowden-style extruder has a long Bowden tube. Otherwise, these extruders include the same hardware (but more on that later).

Essentially, an extruder contains a channel that processes filament. However, various parts are along this channel, and each element interacts with the filament differently.

So, the extruder uses several components to turn your rigid spool of filament into an even, molten string of material.

You can divide the processes inside an extruder into two primary functions.

The upper half of the extruder (called the cold end) contains the parts that push the filament through your extruder’s channel, maintaining an even pressure level to keep your layers straight and consistent.

However, the lower half of the extruder (the hot end) heats your filament and maintains the print temperature using fans, sensors, and heat elements.

The Filament Extrusion Process: From Spool To Print

If you look inside an extruder, it’s like a mini car wash with multiple mechanisms and steps that change as the filament creeps through it.

So, with all these steps, let’s break it down and go through the process, following your filament through the extruder.

1. The Filament Goes Into the Extruder

With all FDM or FFM 3D prints, you start with a spool of filament and a 3D printer. It’s usually best to preheat your 3D printer before feeding the filament in so the hot end is warm from the get-go. To attach your filament to the extruder, you’ll have to release the tension in the feeding mechanism, which you can usually do by pressing a lever or button on your extruder.

Then, you’ll feed the filament into a small hole on the top or side of the extruder. That’s where the fun begins.

2. The Drive Gear and Bearing Feed the Filament Into the Extruder

Once you start printing, the extruder engages the drive gear and bearing. The drive gear — a thin, flat, toothed gear — is the component that pushes your filament down through the extruder. It’s attached to a stepper motor to provide the ideal pressure for your extrusion rate.

You’ll find the bearing on the other side of the channel where the filament runs. This bearing uses a spring to press against the filament, ensuring that you don’t lose tension.

3. Fans Blow Cool Air on the Filament

As the drive gear and bearing turn, feeding your filament down through the extruder, the filament makes its way to the fans. These fans serve several purposes. Firstly, they help create division between your hotend and the cold end. They keep the filament cool as it nears the hotend, ensuring your material doesn’t get runny or warm before reaching the heating element. 

So, without the fans, your filament might melt before it should, creating a solidified plastic mess inside the extruder.

Secondly, your extruder fans also ensure that the hot end doesn’t get too hot since the heat could melt your extruder’s housing and any other plastic printer parts.

4. The Filament Enters the Hot End

You’ve made it to the hot end, where the exciting stuff happens. Inside the hot end, you’ll find the heat sink, heating block, temperature sensor, and heating element. Below these parts is the nozzle. The filament works from the fan section to the heat sink, a metal tube that screws into the heat block. 

The heat sink keeps the filament cool as it gets closer to the heat block. This cooling area ensures that the filament doesn’t soften before it reaches the heat block, protecting it from getting too hot and melting or burning inside your extruder.

Then, the filament’s tip enters the hot end block or heater block, the warm component that’ll liquefy your filament.

The hot end block maintains consistent heat using a temperature sensor and heat element. The heating element will warm up the metal (and potentially PTFE) inside the hot end until it reaches the print temperature you set in your Slicer.

Then, if the hot end gets one degree warmer than the set print temp, it’ll disable the heating element. When the temperature drops by a fraction of a degree, the sensor will turn the heating element. So, using this sensor, the temperature stays almost entirely constant, ensuring that you never over-or under-heat your filament.

5. The Molten Filament Escapes From the Nozzle

As the drive gear and bearing continue pushing more filament through the extruder, the pressure causes the viscous, molten filament to exit the hot end block through the nozzle. There’s usually a tiny heat break between the nozzle and hot end block to ensure that the filament cools as it hits the build surface and that it adheres but doesn’t spurt or run down previous layers.

As this occurs, the rest of your 3D printer is hard at work determining where that filament will go.

Once your printer completes a layer, it’ll stop the drive gear, retract the filament, and start again when your carriage reaches the proper coordinates. This process continues until you have a fully finished 3D print on your bed.

Are There Different Types of Extruders?

There are two different types of extruders. These are Bowden tubes and direct extruders. These extruders work in the same way, but their configurations are different. Bowden-style extruders are significantly more common than direct extruders.

So, not all extruders are the same, but they ultimately perform the same processes.

What’s the Difference Between a Bowden and a Direct Extruder? 

The difference between a Bowden and a direct drive extruder is the extrusion system’s cold end location. In a Bowden system, the cold end is not on the X-axis carriage but on the side of your printer, where a long Bowden tube connects it to the hot end on the X carriage.

So, let’s take a deeper look into these differences and discuss the pros and cons of each extruder type.

Bowden Extruders

Bowden-style extruders are the most common style, and you’ll find them in most 3D printers for personal use.

In these extrusion systems, your cold end and hotend are in entirely different places. On most 3D printers, you’ll find the cold end bracketed to the side of the Z-axis carriage. The hot end will always be on the X-axis carriage.

A Bowden tube (or hollow plastic tube) connects these two components. As the extruder extrudes your filament, it must move the filament through this tube to get to the hot end.

Pros

  • Since the extrusion parts on the X-carriage are smaller and fewer, Bowden extruders can print faster and are less likely to skip.
  • Bowden extruders are easier to repair and replace since the cold and hot ends aren’t directly connected. So, you can upgrade the Bowden tube, hot end, or cold end individually without replacing the entire system.

Cons

  • Finding the ideal retraction settings can be challenging since the printer has to feed your filament through the Bowen tube from the cold end to the hot end.
  • It’s challenging to print with flexible filaments since they’re more likely to jam or bend inside the Bowden tube.
  • Budget 3D printers with Bowden-style extrusion systems are more likely to come apart than direct-drive systems. Specifically, the Bowden tube is much more likely to pop off while you’re printing.
  • The tube may interfere with your extrusion rate, creating minor inconsistencies.

Direct Drive Extruders

Direct drive extruders don’t separate the cold end from the hot end. They’re usually block-shaped, and inside, you’ll find the cold end directly stacked on top of the hot end.

Pros

  • Setting retraction is simple since the mechanism that feeds in your filament is so close to the hotend.
  • They’re ideal for printing flexible filaments.
  • Direct drive extrusions systems are reliable and rarely come apart.
  • Direct drives give you more control over your extrusion rate.

Cons

  • Direct drives put more weight on your X-axis carriage, slowing down your print speeds.
  • It’s difficult to replace individual parts since they’re all connected into one sealed block.

For a visual representation of the difference between Bowden and direct drive extruders, check out this fantastic YouTube video from ModBot:

What’s the Difference Between an All-Metal and a PTFE-Lined Hotend?

The difference between an all-metal and PTFE-lined hotend is the material used to guide the filament through the nozzle. All-metal hotends are usually stainless steel, while PTFE-lined hotends consist of polytetrafluoroethylene, a synthetic polymer.

All-Metal Hotends

All-metal hotends consist of stainless steel in most cases, although you may find brass varieties. These hotends are much more reliable than their PTFE counterparts, but they aren’t as smooth. Thus, all-metal hot ends experience clogging much more frequently.

However, since these hot ends are metal, they’ll degrade very slowly and may never need replacement. In addition, they’re essential for printing food-grade 3D prints since PTFE is toxic when you heat it.

Pros

  • Stainless steel hotends aren’t toxic, so you can use them for food-grade 3D printing.
  • They’ll withstand higher printing temperatures without degrading.
  • They’re slightly more expensive than PTFE-lined hot ends.

Cons

  • Filaments may stick to the metal, creating clogs in your extruder.
  • Retraction doesn’t perform as well and often results in clogging.

PTFE-Lined Hot Ends

Your 3D printer likely came with a PTFE-lined hot end. PTFE, or polytetrafluoroethylene, is a synthetic polymer known for its non-stick properties. Because it’s so smooth and non-adhesive, using it in your hot end reduces clogging and makes retraction easy on your 3D printer.

Bowden tubes usually consist of PTFE to reduce bending and sticking as your printer extrudes. However, you may find a PTFE liner in direct drive and Bowden tube hot ends.

Pros

  • Retraction is easier and faster since the PTFE is so smooth.
  • PTFE significantly reduces the chances of clogging.
  • They’re slightly less expensive than all-metal hot ends.

Cons

  • PTFE degrades at higher print temperatures (above 240° C or 464° F), making it unsuitable for 3D printing filaments such as nylon, PETG, and polycarbonate.
  • PTFE may be toxic.
  • You need to replace PTFE liners regularly.

Final Thoughts

A 3D printer extruder feeds filament into a chamber that first secures and cools the filament, then heats it and pushes it through the nozzle. Extruders can either be Bowden tube-style or direct. Although each of these styles has its advantages and disadvantages, they work very much the same way to extrude your filament and make it a workable, moldable material.

Likewise, your hotends can be all-metal or PTFE-lined, and each of these will affect performance, but they both perform the same processes to heat and liquefy your filaments.