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What is FDM 3D printing?

Get to know the basics of Fused Deposition Modeling, also known as FDM 3D printing. Learn why this 3D printing technique is so affordable, and why it is an excellent choice for quick, low-cost prototyping. Delve into FDM materials and the advantages and disadvantages for designers and engineers alike. 

Written by Alkaios Bournias Varotsis

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Fused deposition modeling (FDM), also known as fused filament fabrication (FFF), is an additive manufacturing process that falls within the category of material extrusion. In FDM, an object is built by selectively depositing melted material in a predetermined path, layer by layer. The materials used are thermoplastic polymers, which come in a filament form.

3D printing technology tree, highlighting FDM.

FDM is the most widely used 3D printing technology. It comprises the largest installed base of 3D printers globally and is often the first 3D printing technology that people encounter. This article presents the basic principles and the key characteristics of the technology, with the aim of helping engineers get the best possible results from FDM printing.

Video: How to prototype like a pro with FDM 3D printing

This 10-minute video teaches you everything you need to know to get started with FDM 3D printing.

How does FDM work?

The FDM fabrication process works by first loading a spool of thermoplastic filament into the printer. Once the nozzle has reached the desired temperature, the filament is fed into the extrusion head and nozzle, where it melts. 

The extrusion head is attached to a three-axis system that allows it to move in the x-, y- and z- directions. Melted material is extruded in thin strands and deposited layer by layer in predetermined locations, where it cools and solidifies. Fans can be attached to the extrusion head to accelerate the cooling. 

To fill an area, multiple passes are required, similar to coloring in a rectangle with a marker. When a layer is finished, the build platform moves down (or in some machine setups, the extrusion head moves up) and a new layer is deposited. This process is repeated until the part is complete.

Schematic of a FDM printer
Schematic of a typical FDM printer

FDM printer parameters

Most FDM systems allow for the adjustment of several process parameters. These include the temperatures of both nozzle and build platform, build speed, layer height and cooling fan speed. An operator usually sets these adjustments, so they are rarely a designer’s concern.

Build size and layer height are, however, of concern.

The build size of a desktop 3D printer is commonly 200 x 200 x 200mm, while industrial machines can be as big as 1,000 x 1,000 x 1,000mm. If a desktop machine is preferred (for example, for cost reduction), a big model can be broken down into smaller parts and then reassembled .

Typical layer height used in FDM varies between 50 and 400 microns, and can be determined upon order placement. A layer of shorter height produces smoother parts and captures curved geometries more accurately, while a greater height produces parts more quickly and cost effectively. 

Design tip: A nice compromise is to use layers that are 200 microns thick. Read this article discussing the impact of layer height in 3D-printed parts.

What are the characteristics of FDM 3D printing?

While there are variations in the extrusion systems of fused deposition modeling 3D printers and parts may have different qualities, there are also some common characteristics (some good, some bad) that you can expect when you FDM print.

Warping

Warping is one of the most common defects in FDM. When extruded material cools during solidification, its dimensions decrease. Since different sections of the print cool at different rates, their dimensions also change at different speeds. Differential cooling causes the buildup of internal stresses that pull the underlying layer upward, causing it to warp, as shown in figure 3. Warping can be prevented through closer temperature monitoring of the FDM system (namely, of build platform and chamber) and increasing adhesion between the part and the build platform. 

Design choices can also reduce the likelihood of warping: 

  • Large flat areas (such as those characteristic of a rectangular box) are more prone to warping and should be avoided when possible. 

  • Thin protruding features (such as those characteristic of fork prongs) are also prone to warping. Warping can be avoided by adding sacrificial material at the edge of the thin feature (e.g. a 200-micron thick rectangle) to increase the area that touches the build platform.

  • Sharp corners warp more often than rounded shapes, so adding fillets to the design is good practice. 

  • Different materials are more susceptible to warping. ABS is generally more sensitive to warping than PLA or PETG.

Schematic showing edge warping of a FDM part
As newly deposited layers cool they shrink, pulling the underlying layer upward resulting in warping.

Layer adhesion

Secure adhesion between the deposited layers is critical for an FDM part. When the molten thermoplastic is extruded through the nozzle, it is pressed against the previously printed layer. High temperature and pressure cause this layer to remelt and enable the new layer to bond with the previously printed part. 

Bond strength between the different layers is always lower than the base strength of the material. This means that FDM parts are inherently anisotropic: their strength in the z-axis is always smaller than their strength in the XY plane. For this reason, it is important to consider part orientation when designing parts for FDM .

For example, tensile test pieces printed horizontally in ABS at 50% infill were compared to test pieces printed vertically and found to have almost four times greater tensile strength in the XY print direction than the Z direction (17.0 megapascals compared to 4.4 megapascals) and elongated almost 10 times more before they broke (4.8% compared to 0.5%).

Since the molten material is pressed against the previously printed layer, its shape is deformed to an oval. This means that FDM parts always have a wavy surface, no matter what layer height is used , and that small features, such as small holes or threads , may need to be post-processed after printing.

Schematic of the FDM material extrusion profile
The FDM material extrusion profile.

Support structure

Because molten thermoplastic cannot be deposited on thin air, some geometries require support structures. Read this detailed article explaining the use of support structure.

Support material might be difficult to remove, so it is often easier to design parts to minimize the need for supports. 

Support is usually printed in the same material as the part. Support materials that dissolve in liquid also exist, but they are used mainly in high-end desktop or industrial FDM 3D printers . Using dissolvable supports increases the overall cost of a print.

Infill and shell thickness

To reduce print time and save on material, FDM parts are usually not printed solid. Instead, the outer perimeter—called the shell—is traced using several passes, and the interior—called the infill—is filled with an internal low-density structure. 

Infill and shell thickness greatly affect the part’s strength. For desktop FDM printers, the default setting is 20% infill density and 1mm in shell thickness, which provides a nice compromise of strength and speed for quick prints.

FDM prints with different infill density
The internal geometry of FDM prints with different infill density

The main characteristics of FDM are summarized in this table:

FDM
Materials Thermoplastics (PLA, ABS, PETG, PC, PEI etc)
Dimensional Accuracy ± 0.5% (lower limit ± 0.5 mm) - desktop
± 0.15% (lower limit ± 0.2 mm) - industrial
Typical Build Size 200 x 200 x 200 mm - desktop
900 x 600 x 900 mm - industrial
Common layer thickness 50 to 400 microns
Support Not always required (dissolvable available)

Common FDM materials

One of the key strengths of FDM is the wide range of materials available for use, which includes commodity thermoplastics (such as PLA and ABS ), engineering materials (such as PA, TPU and PETG ) and high-performance thermoplastics (such as PEEK and PEI ).

The material used affects the printed part’s mechanical properties and accuracy as well as its cost. The most common FDM materials are summarized in the table below. For more details, see this review of the main differences between PLA and ABS —the two most common FDM materials—and an extensive comparison of all common FDM materials , below.

Material Characteristics
ABS + Good strength

+ Good temperature resistance

- More susceptible to warping
PLA + Excellent visual quality

+ Easy to print with

- Low impact strength
Nylon (PA) + High strength

+ Excellent wear and chemical resistance

- Low humidity resistance
PETG + Food Safe*

+ Good strength

+ Easy to print with
TPU + Very flexible

- Difficult to print accurately
PEI + Excellent strength to weight

+ Excellent fire and chemical resistance

- High cost

Post-processing

FDM 3D printed parts can be finished to a very high standard using various post processing methods, such as sanding and polishing, priming and painting, cold welding, vapor smoothing, epoxy coating and metal plating. There is a wide variety of post-processing options for FDM parts.

What are the advantages of FDM 3D printing?

  • FDM is the most cost-effective way to produce custom thermoplastic parts and prototypes.

  • Lead times are short (typically a few days) due to the ubiquity of the technology.

  • A wide range of materials is available, suitable for both prototyping and some functional applications.

What are the disadvantages of FDM 3D printing?

  • FDM has the lowest resolution compared to other 3D printing technologies, so it is not suitable for parts with very small details.

  • Parts are likely to have visible layer lines, so post-processing is required for a smooth finish.

  • The layer adhesion mechanism makes parts inherently anisotropic.

Dive into the hands-on details with this guide to designing parts for FDM.

FDM best practices

  • FDM can produce prototypes and functional parts quickly and cost effectively.

  • There is a wide range of materials available for FDM.

  • Typical build size of a desktop FDM 3D printer is 200 x 200 x 200mm. Industrial machines have a larger build size.

  • To prevent warping, avoid large flat areas and add fillets to sharp corners.

  • FDM is inherently anisotropic, so it is unsuitable for mechanically critical components.

Want to learn more about 3D printing? Read our full guide: What is 3D printing?

 

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