How FDM 3D Printers Work: A Quick Guide for Beginners

Curtis Satterfield, Ph.D.

Curtis Satterfield, Ph.D.

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If you are new to 3D printing you may wonder how exactly 3D printers work. How does a machine take a digital file and turn it into a physical object? In this article we will discuss how 3D printers work in detail.

Fused Deposition Modeling (FDM) or filament printers work by melting thermoplastic and extruding the melted plastic layer upon layer until the object is finished

That is a simple summary of how FDM printers work. In the rest of the article we will cover the printing process in detail and discuss the pros and cons of 3D printing with an FDM printer. If you are new to 3D printing and curious about how FDM printers work, this article is for you.

How an FDM Printer Works

Fused Deposition Modeling (FDM) is a method where thermal plastics are melted and then extruded according to a patter.  In a nutshell, an FDM printer works by melting thermoplastic and extruding the plastic over a build surface called a print bed or build plate. The printer draws all the shapes that make up one layer of the print and then the print head raises and repeats the process. The printer continues extruding layer by layer until it completes the object.

The specifics of how an FDM printer works will differ slightly between printers but the general process is the same for all FDM printers.

 

  1. Thermoplastic filament is fed into the printer and down into the printer’s nozzle.
  2. The print nozzle is heated to a temperature appropriate for the type of filament being used.
  3. The printer is designed to allow movement in the X, Y, and Z dimensions for the print head to deposit the plastic in the print area.
  4. As the material melts it is pushed out or extruded from the print nozzle as the print head moves around the X and Y dimensions tracing out the path designated by the slicing software.
  5. The extruded plastic cools and become bonded to the layer beneath.
  6. The printer will print a layer and then the printer will move the print head up 1 layer to begin printing the next layer.
  7. The printer repeats this process for each layer until the model is fully printed.

FDM Printer Terms

It will be helpful to review different terms related to FDM 3D printing. Below is a list of common terms that you will hear when working with 3D printers.

Bearings – Used to reduce friction of moving parts in the machine. Can consist of both linear and circular bearings. Requires lubrication to prevent seizing over time.

Benchy – The ubiquitous stress testing model for your 3D printer.

Bowden Extruder– This is a style of printer where the extruder motor is attached to the printer’s frame and the filament is pushed through a PTFE tube to the printer head which contains the hot end. Bowden extruders can print faster than direct drive as they do not have as much weight to move around due to the extruder being attached to the frame of the printer.

Build Plate/Print Bed – The surface that the extruded plastic attaches to during the build process. The print bed is usually made of metal and covered with a PEI sheet or a piece of glass.

Cooling Duct – A part that is attached to the cooling fan on the print head that directs air flow at the part being printed.

Cooling Fan – Can refer to several fans on the printer. The following items require fans; the electronic components of the printer, the heat break, and the part being printer.

Direct Drive Extruder – Extruder style where the extruder motor is attached to the print head and feeds filament directly down into the hot end.

Extruder – This is the stepper motor that pushes the filament down into the heater block and nozzle. The drive wheel that is turned by the stepper motor will have teeth that grip the filament allowing it to drive the filament down.

Frame – The structure that makes up the 3D printer, typically made of metal such as aluminum. The term open frame printer means that a printer does not have an enclosure and the build platform is exposed.

Heat Break – The area in the nozzle where heat is dissipated so as not to melt filament before it gets to the hot end.

Heater Block – large chunk of metal that is used to help create a consistent temperature for the nozzle.

Heat Sink – A metallic object designed to pull heat off an area such as a chip on a circuit board or the heat break in a hot end.

Hot End – This consists of the parts necessary to melt the filament including the heater block, heater, thermistor, and nozzle.

Nozzle – The last part of the hot end that the melted plastic passes through as it is extruded. Nozzles are made of brass or steel and come in a variety of sizes from .25mm up to 1.2mm. The size in millimeters determines the maximum width that an extruded line of filament can be.

Print Head – On a Bowden Style machine this consists of the hot end attached to a gantry. On a direct drive printer, the print head contains the hot end and the extruder motor.

Printer Board – This refers to the electronic circuit board that controls the printer.

PTFE Tube – Polytetrafluoroethylene is used as a low friction tube that filament is fed through on many printers. The PTFE tubing can be used to transport filament from a spool holder to the extruder. Some hot ends contain a PTFE liner.

Spool – This holds the filament. While not technically part of the printer, you will hear this term a lot.

Stepper Motor – Used to drive the movement for each of the three axes and the extruder. A stepper motor is a brushless electric DC motor that divides its rotation into an equal number of steps.

Thermistor – Sensor that is held in the heater block responsible for detecting and reporting the hot end/extruder temperature back to the printer board.

Preparing a 3D model for printing

In order to 3D print there are a couple steps you need to take before actually pressing print. The first step is to find a model that you would like to print. There are many freely available 3D models that you can download from online. Most models will be saved as an STL file. In order to download the models and the software for 3D printing your computer will need high-speed Internet access. To learn more about computers and 3D printing check out my article on the topic.

If you want to create your own 3D models using Blender or a CAD program such as AutoDesk Fusion360 you will need a computer to run the software. After you create the model or part in your CAD program you will need to save it as an STL and import it into your slicer.

Regardless of the method used to get your 3D models you will need to import them into your slicer to process. Ultimaker Cura is a popular free slicing program that you can download and install on your computer. You will need to generate a profile for your printer so Cura can generate G-Code that will work on your printer. Luckily, most software slicers come with a large collection of printer profiles that you can import. As you get more experience with your printer you will want to tweak the settings in your slicer to get the best prints.

After setting up a printer profile you will need to slice your model. Slicing the model generates code that instructs the machine how to move and what paths to follow. In the image below you can see what a layer path looks like after it was sliced.

First Layer Tool Path in 3D Slicer

Slicing the model in your software creates a set of instructions on how the printer should behave for each layer. The model is reduced or “sliced” into a series of layers based on the layer height setting in the slicer. If you have a 3mm cube that is sliced at a .2mm layer height this will result in a print made up of 15 layers.

Below is an image with a small snippet of G-Code.

GCode from a 3D printer file

Once you have sliced the model you will need to transfer the G-Code file from your computer to the printer. The most common method is to copy the G-Code to an SD card and insert it into the printer. The printer will read the G-Code file on the SD card and use the file to print your model. 

3D Models and FDM Printing

FDM printers are great for printing larger models more cost effectively and quickly than a resin printer. The downside is that you will sacrifice some quality and detail when using an FDM printer. You can compensate for the detail loss by lowering the layer height of the print, but you will also increase the overall print time. In addition, when up close to the model you will still see the layer lines of the print. This is not a huge deal unless you are trying to print something highly detailed for painting, such as a tabletop miniature.

FDM printers are more cost effective than resin prints and are a good choice if you need to print lots of large objects. When comparing FDM printers to entry-level resin printers an FDM printer will typically be faster at printing than a resin printer. With good calibration, an FDM printer can certainly produce great looking prints so don’t shy away from an FDM printer because you’ve heard they can’t compete with resin printers.

Here is an image of terrain that I printed using my FDM printers.

Variety of 3D Printed Terrain

Several pieces of FDM printed and painted terrain for tabletop gaming.

FDM printers can also print small objects with a decent amount of details. Below is a picture of a miniature I printed and painted for a tabletop game. I placed a nickel next to the miniature for reference.

Small Wererat miniature printed on an FDM printer and painted

Up close the layer lines are obvious, but from arm’s length they disappear and are not noticeable. If you are not concerned with the visibility of the layer lines an FDM printer is a good choice.

Filament (Thermoplastics) Used in FDM Printing

FDM printers use thermoplastic commonly referred to as filament as the printing medium. There are many kinds of filaments available on the market with different properties and physical characteristics. The easiest filament to get started with is Polylactic Acid or PLA. This material is a favorite among hobbyists as it is low cost and easy to print with. Here is a list of some common filaments.

Acrylonitrile Butadiene Styrene (ABS) is a common filament type known for being tough and impact resistant. Typically, ABS is printed at a nozzle temperature of 240°C – 270°C and is good for parts that will be subjected to heat.

Polylactic Acid (PLA) is one of the favorite filaments of the 3D printing community. It is a biodegradable thermoplastic derived from renewable sources such as cornstarch, sugar cane, and potato starch. PLA requires a much lower nozzle temperature than ABS printing between 180°C and 220°C.

Polyethylene Terephthalate (PET) is often used for mechanical parts that require flexibility and impact resistance. PET offers more flexibility than ABS while maintaining flexibility. PETG is printed at temperature like ABS between 220°C and 250°C.

Glycol modified Polyethylene Terephthalate (PETG) is a variant of PET filament that increases the materials durability and impact resistance. Unlike PET this material will not readily absorb water. PETG is also considered food save and can be used for cups, plates, and food containers. It prints at the same temperature range as PET.

PolyEthylene coTrimethylene Terephthalate (PETT) is another modified version of PET. PETT is considered food safe and is approved by the FDA. It prints slightly cooler than PETG between 210°C and 230°C.

Polyamide (Nylon) filament is a synthetic polymer typically used in industrial applications. Nylon is strong, durable, and flexible. Printed between 220°C – 250°C nylon is perfect for industrial parts such as gears, bearings, and mechanical components. The downside to nylon is that it emits toxic fumes when printing. This is important to know as nylon does not produce and odor when it prints but it is toxic and should be used in a well-ventilated area.

Wood filament is made with recycled wood and polymer binding. It is typically used for more decorative applications as it is one of the weaker printing materials. Wood filament is printed in the same nozzle temperature range as PLA at 195°C-220°C.

 

 

3-Dimensional Movement

For a 3D printer to work it must move in 3 dimensions. We refer to theses dimensions as the X, Y and Z axes. The X axis is back and forth relative to the print bed, the Y axis is forward and backwards relative to the print bed, and the Z-axis is up and down.

Creality CR10 with Axes labelled

Creality CR10 – Source: Creality3dofficial.com

Some printers will have the print head move in the X and Y axes. Other models will move the print head in the X and Z axes and will move the print bed in the Y axis. Regardless of the method all FDM printers will move in such a way that the plastic can be distributed to the model in all three dimensions.

A Creality Ender3 is pictured below. The Ender3 is an open frame printer than moves the print head in the X and Z directions. It achieves Y axis movement by moving the print bed forwards and backwards.

Ender3 3D printer with X and Z axes labelled

Creality Ender3 – Source: Creality3Dofficial.com

My Wanhao Duplicator 6, pictured below, moves the print head in the X and Y axes on a gantry and the print bed moves only in the Z axis.

Wanhao Duplicator 6 3D printer with axes labelled

The printer will print an entire slice or layer of the print at one time and then move up one-layer height on the Z-axis. A 3D printer will never go back down to print at lower layers as it always builds up from the previous layer.

It’s (3D Printed) Layers all the Way Down

As stated previously, a 3D printer works by printing one layer at a time on top of the previous layer. The melted plastic is extruded on top of the previous layer and as it cools and solidifies it bonds with the previous layer. One issue this causes in finished prints is that stress along print layers can cause the part to break. If prints are breaking too easily along the layer lines it might be an indication of printing the filament at too low a temperature.

Because a 3D print builds up one layer at a time any problems with previous layers can cause print quality issues or print failure. If an area of the layer is missing filament that was supposed to be there, the next layer will not have the support and structure to print properly. Layer problems at any height can cause issues, but issues with the first layer can cause catastrophic print failures.

The First Layer is the Most Important

Like building a house a 3D print needs a strong foundation and the first layer is the foundation for the rest of your print. The first layer of the print has the all-important job of keeping the print adhered to the bed. For a 3D printer to work properly the print must stick to the build plate, otherwise the model will get pushed around during the printing process. If the model moves around on the print bed filament will not be extruded on the model where it needs to go. If the print breaks free early in the print you can end up with “spaghetti” on your printer.

3D Printer "Spaghetti"

3D printer “Spaghetti” – Source: iFunny

Because 3D prints can take many hours or even days, it can be infuriating when a print breaks loose many hours into the print. If a print breaks free you will need to start the print over. A primary reason for prints breaking free is lack of bed adhesion and that is what we need to avoid on the first layer. There are many reasons why a print may not properly adhere to the bed. If you want to learn more about bed adhesion, read my article here.

FDM Printer Calibration is Important

3D Printed Benchy Calibration Test

A picture of the ubiquitous Benchy that I printed for calibration

At the least a printer needs all the nuts, bolts, and belts tight to reduce slop. The more slop or play your machine has the lower the quality of your prints will be. The printer also needs to have its Z-offset calibrated so that the first layer adheres to the bed properly. When setting up a 3D printer for the first time it is important to go over the machine and make sure everything is installed correctly and properly tightened. The image below represents the difference in print quality between a tight and loose belt.

Loose belt results in bad print quality

Even between machines of the same make and manufacturer there will be slight differences that cause them to need calibration. Calibration is the act of testing various aspects of your printer to ensure it is working to the best of its ability. An example is calibrating your extruder motor to ensure that it is extruding the exact length of filament the G-Code requires. Other forms of calibration include printing speed, PID tuning, and movement settings such as jerk and acceleration. Calibration will take time to learn and complete but are well worth the effort.

Learn about how to calibrate your FDM printer by reading my article here.

How Fast do 3D Printers Print?

3D printers are not as fast as inkjets and laser printers. Prints can take anywhere from a few minutes to many hours.

Printing speed is the speed in millimeters per second that the print head will move while printing. Increasing the print speed will decrease the time it takes to print the model. But there is a trade-off of quality and time. The faster you run the printer the lower the quality of the final print will be.

The range of printing speeds for FDM printers range from 50mm/s up to 150mm/s. Some printers can run faster than 150mm/s but on average 150mm/s is an upper limit for budget machines. If you are considering purchasing a 3D printer pay attention to the print speed as it will greatly impact your total print times. You should also check out user communities for the printer you are considering and see what print speeds they have achieved with the printer. The advertised print speed of a printer may not be the actual speed you can print at reliably.

Layer height has a large impact of the total print time of an object. As the layer height decreases the number of layers and the print time increases. Most users attempt to balance quality and speed by finding a layer height that offers good detail while not drastically increasing the print time. For FDM printers printing at .2mm layer height offers decent quality and speed.

The need for FDM printers to fill in each layer with thin lines of plastic adds time to a print job. In order to reduce the amount of print time needed slicers allow us to choose the percentage of infill when printing the model. Lowering the infill density of the model allows us to speed up the print time while still retaining strength in the model.

The more infill you use for a model the longer the print will take to complete. The less infill you use for your model the less overall strength the part will have. You will need to determine what kind of use your part will have and how much strength the part will need.

If you would like more information about printing times for both FDM and resin printers read my article here.

Should I buy an FDM Printer?

At this point you may be wondering if an FDM printer is right for you. There are many considerations when it comes to purchasing a 3D printer, but the most important is how you plan to use your printer. A 3D printer is simply a tool that we use to accomplish tasks. Purchasing a tool without a purpose results in the tool sitting and collecting dust after the newness has worn off.

If you know what you would like to use your 3D printer for then the decision becomes much easier. The main points about FDM printers are that they can print larger objects, are faster, and overall, less expensive than a resin printer. All that does come with a trade off in detail. FDM printers cannot print at the level of detail that even a budget resin printer can. If you need a highly detailed model, then resin printing is the way to go. If you want to print large models and minute details are not that important then FDM should be your choice.

If you are interested in purchasing an FMD printer check out my recommended FDM printers here.