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FDM vs SLA: 3D Printing Explained and Compared

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by Franz Grieser
Oct 8, 2015

FDM vs SLA: All3DP compares these competing 3D printing technologies, explains the advantages and disadvantages of these technologies and which one to use for which purpose.

FDM vs SLA: Explained

The FDM printer Prusa Steel is known for its print quality and reliability (image: Prusa)

FDM is the abbreviation for Fused Deposition Modeling. In FDM, material (in this case: thermoplastics) is deposited in layers to create a 3D printed object. During printing, the plastic filament is fed through a hot extruder where the plastic gets soft enough that it can be precisely placed by the print head. The melted filament is then deposited layer by layer in the print area to build the workpiece.

There is a broad choice of FDM 3D printers for every budget, starting at a few hundred dollars. Filament rolls are relatively inexpensive (from $25 per kilo). These factors made FDM printers so popular among makers and home users.

Formlabs just introduced the Form 2, a SLA printer using an UV laser for curing (image: Formlabs)

SLA is the abbreviation for Stereolithography Apparatus, or simply Stereolithography. Like FDM, SLA is an additive method: Models are built layer by layer. SLA, however, uses a curable photopolymer – typically a liquid resin – that is hardened by applying focussed light or UV light (this process is called curing). SLA printers usually build the models from top to bottom, the build platform lifts the model upwards, out of the resin bath.

The light source is either a laser or a digital projector (the technology is called DLP – Digital Light Processing). Lasers „draw“ the layers; in DLP, an entire slice (a two-dimensional layer) of the model is projected at once into the resin bath.

Laser SLA printers are usually slower than DLP models because of the small surface of the laser beam. In DLP printers, each layer hardens faster as the entire image of one layer is projected onto the resin. Moreover, DLP projectors are more reliable and easier to maintain than customized laser systems as the projectors use the same technology as business and home cinema projectors.

FDM vs SLA: Compared

FDM vs SLA: Materials and colors

FDM printers typically use PLA or ABS filament, some can even handle nylon and a variety of PLA blends (mixed with wood, ceramics, metals, carbon fibre, etc.). Filaments are available in various colors.

Most FDM printers can use standard filament rolls that are available in two standardized sizes (diameter: 1.75 or 2.85mm) from various sources. A few printers (e.g. Zortrax M200 or Tiko) use proprietary filaments or filament boxes – these are typically more expensive than standard rolls but deliver better quality.

Owners of SLA printers have only a limited choice as the resins are proprietary and cannot be exchanged between printers from different makers. The choice of colors is definitely limited: Formlabs for example only offers black, white, grey and clear resins. Customers depend on the supply provided by the manufacturer.

FDM vs SLA: Precision and smoothness

SLA printers such as the Moonray print with high precision – you get details you wouldn't see in a FDM printed object (image: Sprintray, the creators of Moonray)
SLA printers such as the Moonray print with high precision – you get details you wouldn’t see in a FDM printed object (image: Sprintray, the creators of Moonray)

In FDM printers, resolution is a factor of the nozzle size and the precision of the extruder movements (X/Y axis). The precision and smoothness of the printed models is also influenced by other factors: As the bonding force between the layers is lower than in SLA printing and as the weight of upper layers may squeeze the layers below, a number of printing problems may ensue (e.g. warping, misalignment of layers, shifting of layers, shrinking of the lower parts – for more details see this article). These compromise the precision and surface smoothness.

SLA printers consistently produce higher resolution objects and are more accurate than FDM printers. The reason: The resolution is primarily determined by the optical spot size either of the laser or the projector – and that is really small. Moreover, during printing less force is applied to the model. This way, the surface finish is much smoother. SLA prints show details a FDM printer could never produce.

In fact, the fine details an SLA printer produces is the main reason why one would consider getting a SLA printer.

FDM vs SLA: Adhesion/removal after 3D printing

Adhesion to the print bed is normally no problem when using an FDM printer. Printed objects can be easily removed – if the object sticks to the print bed, a palette knife will do.

In SLA printers (such as Form 1+), it can be difficult to remove the printed model from the print platform and often there is a lot of resin left on the platform that you have to remove using a palette knife – and this takes more effort than on a FDM printer.

For this reason, manufacturers such as Carbon3D and WASP came up with new ideas: Carbon3D uses oxygen to create so-called “dead zone” around the printed model (the oxygen keeps the resin at the surface of the model from hardening); WASP changed the position of the projector (from below the resin bath to above it) and claims to have solved the adhesion problem this way. We will see, if these are viable solutions (the printers are not available for testing, yet).

FDM vs SLA: Postprocessing required

After printing on an FDM printer you need to remove supports (if the model has overhangs) and excess plastic either with your fingers or a cutting tool. Sanding helps to get smoother surfaces.

Models printed on an SLA printer such as the Form 1+ are covered in sticky resin that has to be removed in a bath of isopropyl alcohol. This is why you get rubber gloves with the Form 1+ – to protect your fingers from the resin and alcohol. Depending on the model, supports may be required, too – removing them is as easy as with FDM printers.

FDM vs SLA: 3D printing costs

For FDM printers you can get filaments in many colors and even in color gradients (image: 3D-Printshop.biz)

The only consumable in FDM printers are filament rolls. As already mentioned, most FDM printers use the same standardized filament rolls, prices for filament have been declining in the last years. 1 kg of simple filament can be bought for $25, specialized filaments cost more.

In SLA printers, not only resin is consumed: In SLA printers such as the Form series, the resin tank has to be replaced after 2-3 liters of resin have been printed. The reason is that the tank gets smudged inside over time so the light source is no longer able to precisely project the image in the resin. One resin tank costs $59. Formlabs claims to have redesigned the resin system improved in its Form 2 model to enhance the lifetime of the tank. Another component that needs replacing from time to time is the build platform as it gets marred when the user removes the printed model; one platform sells for $99 – we cannot say how often you need to replace the build platform.

The resin is also costly: 1 liter of standard resin for the Form 1+ and 2 costs $149. Form 1+ users say that 1 liter of resin corresponds to 1 kg of FDM filament.

FDM vs SLA: Which One to Use?

In a nutshell: If high precision and smooth finish is your top priority and if cost is of no or of minor importance for a print job, use an SLA printer. If cost does play a role, use an FDM printer.

When to use FDM

  • Rapid prototyping

  • Low-cost models

  • For experimenting

  • When precision and surface finish are not crucial

When to use SLA

  • When intricate details and/or a very smooth surface finish is crucial

  • When strength and durability of the model is not crucial (models made from resin may suffer when exposed to the sun for extended periods)

  • For creating molds for casting to facilitate mass-production (e.g. by jewelry or toy makers)

The lead image is a collage from two images by Stefan Schweihofer and Hans Braxmeier, via Pixabay.

License: The text of "FDM vs SLA: 3D Printing Explained and Compared" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.

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