Introduction to FDM 3D Printing
What is FDM 3D printing?
Fused Deposition Modeling (FDM) - modeling by depositing molten material, or Fused Filament Fabrication (FFF) - manufacturing with a meltable filament, represents an additive manufacturing process that belongs to the family of material extrusion. In FDM 3D printing the object is built by selectively applying molten material in a predetermined path layer by layer. The materials used are thermoplastic polymers in the form of filament.
FDM is the most widely used technology for 3D printing. Globally, it has the largest installed base of 3D printers, which often makes it the first such technology that people encounter.
This article presents the main principles and key aspects of FDM technology.
The designer must take into account the capabilities and limitations of the technology when creating a part with FDM, as this will help them achieve the best result.
How does FDM 3D printing work?
This is how the FDM production process works:
Initially, a spool of thermoplastic filament is loaded into the 3D printer. Once the nozzle reaches the desired temperature, the filament is fed to the extrusion head and into the nozzle, where it melts.
The extrusion head is attached to a 3-axis system, allowing it to move in X, Y, and Z directions. The melted material is extruded into thin strands and deposited layer by layer at predetermined locations, where it cools and hardens. Sometimes the cooling of the material is accelerated by using cooling fans attached to the extrusion head.
"To fill a given area, multiple passes are required (similar to coloring a rectangle with a marker). When one layer is completed, the building platform moves down (or in other machine settings, the extrusion head moves up) and a new layer is applied. This process is repeated until the part is fully completed."
Characteristics of FDM 3D printing
3D printer parameters
Most FDM systems allow for the adjustment of several technological parameters. These include the nozzle and build platform temperature, build speed, layer height, and cooling fan speed. These parameters are typically set by the operator, so they should not affect the designer.
For the designer, the size of the fabrication and the height of the layer are significantly important:
The available build size of a desktop 3D printer is usually 200 x 200 x 200 mm. For industrial 3D printers this size can be up to 1000 x 1000 x 1000 mm. In case the work requires a desktop machine (to reduce costs for example), a large model can be broken down into smaller parts and then assembled.
The layer height used in FDM printers typically ranges from 50 to 400 microns. It can be specified when ordering. A smaller layer height provides smoother parts and captures distorted shapes more accurately, while a larger layer height produces parts faster and at a lower cost. The most commonly used layer height is 200 microns.
Distortion of 3D printed parts
Warping is one of the most common defects in FDM. This occurs while the extruded material cools during solidification, causing a change in its dimensions. During printing, different parts of the model cool at different rates. Accordingly, the rate at which their dimensions change also varies.
Differential cooling leads to the accumulation of internal stresses that pull the base layer upwards, causing it to warp. From a technological perspective, warping can be prevented by more careful monitoring of the temperature of FDM system (for example, on the build platform and the chamber) and by increasing the adhesion between the part and the build platform.
Design solutions can also reduce the likelihood of distortion:
- Large flat areas (for example, when constructing a rectangular box) are more susceptible to distortion and should be avoided whenever possible.
- Thin protruding elements are also susceptible to distortion. In these cases, this can be avoided by adding a small amount of additional material at the end of the element to increase the area that is in contact with the building platform.
- Sharp corners tend to distort more often than rounded shapes. In such cases, rounding the corners of the model is considered good design practice.
- The different materials are susceptible to distortion in different ways: ABS is usually more sensitive to distortion compared to PLA or PETG, due to its higher glass transition temperature and relatively high coefficient of thermal expansion.
Layer adhesion
In FDM, good layer adhesion is of utmost importance. When the molten thermoplastic is extruded through the nozzle, it is pressed against the previous layer. The high temperature and pressure melt the surface of the previous layer and allow the new layer to bond with the previously printed part.
"The strength of the bonds between different layers is always lower than the basic strength of the material."
Therefore, FDM parts are anisotropic - their strength along the Z axis is always less than their strength in the plane of X, Y. That is why, when designing FDM parts, it is important to consider their orientation.
This is clearly seen when comparing tensile test parts printed horizontally in ABS with 50% infill and test parts that are printed vertically. The horizontally printed parts have about 4 times greater tensile strength in the X, Y printing direction compared to the Z direction (17.0 MPa vs 4.4 MPa) and stretch almost 10 times more before breaking (4.8% compared to 0.5%).
In addition, since the melted material is pressed against the previous layer, its shape is deformed into an oval. This means that FDM parts will always have a wavy surface, even at low layer heights. Therefore, small elements like small holes or threads may need to be further processed after printing.
Support structure
The support structure is essential for creating figures with protruding elements in FDM. The molten thermoplastic material cannot be deposited on thin air, which is why some figures require such a support structure.
"Often, the surfaces printed on the support are of lower quality than the other elements of the part. That is why it is recommended that the part be designed in such a way that the need for support is minimized."
The support is usually printed from the same material as the part itself. There are also support materials that dissolve in liquid, but they are mainly used in high-end desktop or industrial FDM 3D printers. Printing on soluble supports significantly improves the surface quality of the detail. However, the overall costs increase significantly, as a specialized dual-extrusion machine is required. Also, the price of the soluble material is relatively high.
Density of filler and casing
FDM parts are not printed solidly to reduce printing time and save materials. Instead, the outer perimeter is traced using a shell (shell), while the interior is filled with a low-density internal structure called infill (infill).
The density of the fill and the shell significantly affects the strength of a given part. For desktop FDM printers the default setting is 25% fill density and a shell thickness of 1 mm, which is a good compromise between strength and speed for fast printing.
Frequently used materials for FDM 3D printing
One of the key strengths of FDM is the wide range of available materials. They can be both widely used thermoplastics (PLA, ABS) as well as engineering materials (PA, TPU, PETG) and high-performance thermoplastics (PEEK, PEI).
The material used affects both the mechanical properties and the precision of the printed part, as well as its cost.
The most commonly used FDM materials:
ABS
✔️ Good health
✔️ Good temperature stability
❌ More susceptible to distortion
PLA
✔️ Excellent visual quality
✔️ Easy to process
❌ Low impact force
Nylon(PA)
✔️ High strength
✔️ Excellent resistance to wear and chemicals
❌ Low moisture resistance
PETG
✔️ Safe for food *
✔️ Good strength
✔️ Easy to process
TPU
✔️ Good flexibility
❌ Difficult to process accurately
LIKE
✔️ Excellent strength relative to weight
✔️ Excellent fire and chemical resistance
❌ High price
* The interface cuts of the layer are critical points for bacterial growth, so thorough cleaning is necessary after each use.
Subsequent processing
FDM parts can be finished to a very high standard using various post-processing methods. These can include sanding and polishing, priming and painting, cold welding, vapor smoothing, epoxy or metal coating.
Advantages and limitations of FDM 3D printing
The main advantages and disadvantages of the technology are summarized below:
✔️ FDM is the most cost-effective way to produce thermoplastic parts and custom prototypes.
✔️ The lead time for FDM is short due to the high availability of the technology.
✔️ A wide range of thermoplastic materials is available, suitable for both prototyping and some non-commercial functional applications.
❌ FDM has the lowest dimensional accuracy and resolution compared to other 3D printing technologies, making it unsuitable for parts with complex details.
❌ FDM parts often have visible layer lines, which requires post-processing to smooth the surface.
❌ The layer adhesion mechanism makes FDM parts anisotropic.
Main characteristics of FDM:
| Materials | Thermoplastics (PLA, ABS, PETG, PC, PEI, etc.) |
|---|---|
| Measurement Accuracy |
Table: ± 0.5% (lower limit ± 0.5 mm) Industrial: ± 0.15% (lower limit ± 0.2 mm) |
| Construction dimensions |
Table: 200 x 200 x 200 mm Industrial: 1000 x 1000 x 1000 mm |
| Layer height | From 50 to 400 microns |
| Support | Not always required (soluble are available) |
Measurement accuracy:
Table: ± 0.5% (lower limit ± 0.5 mm)
Industrial: ± 0.15% (lower limit ± 0.2 mm)
Construction dimensions:
Table: 200 x 200 x 200 mm
Industrial: 1000 x 1000 x 1000 mm
Layer height:
From 50 to 400 microns
Support:
Not always required (soluble are available)
FDM 3D printing: Summary
FDM 3D printing can produce prototypes and functional parts quickly and at a low cost from a wide range of thermoplastic materials.
- The typical size of construction of desktop FDM 3D printer 200 x 200 x 200 mm. Industrial machines have a larger manufacturing size.
- "To prevent distortion, it is important to avoid large flat areas and to add rounding at sharp corners. For optimal results, consult with professionals." 3D printer services.
FDM is anisotropic, so it is not recommended for mechanically critical components.
