Learn the differences between the 3D printing technologies FDM and SLA, and which one to use for which application.

FDM and SLA Simply Explained

FDM is the abbreviation for Fused Deposition Modeling. In FDM, a strand of 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 spools are relatively inexpensive, starting from $25 per kilo. These factors made FDM printers so popular among makers and home users.

You can find the best FDM 3D printers here: 2020 Best 3D Printers (Winter Update)

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 focused 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 often 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. The printed models have to undergo a post-processing process, though.

Overall, there are less budget-friendy SLA machines than FDM 3D printers. Resin printers can often be found in a professional context, although the prices came down in the last years.

You can find the best SLA printers here: 2020 Best Resin 3D Printers (Winter Update)

FDM vs SLA: The Differences

FDM vs SLA: Materials and colors

FDM printers typically use PLA, PETG, or ABS filament. Most FDM printers can handle nylon, PVA, TPU and a variety of PLA blends (mixed with wood, ceramics, metals, carbon fiber, etc.) Filaments are available in various colors. Some manufacturers even offer a service to manufacture RAL colors by demand.

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 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 more limited pallet of resin materials. Quite often these are proprietary and cannot be exchanged between printers from different makers. The choice of colors is also limited. Formlabs, for example, only offers black, white, grey and clear resins. On the other hand, they offer more durable or highly specialized materials (i.e. dental, heat-resistant, or flexible resins) for industrial uses.

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)

In FDM printers, the printer’s 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 an FDM printer could never produce.

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

FDM vs SLA: Adhesion/removal after 3D printing

Adhesion to the print bed is a topic when using an FDM printer. Printed objects can be relatively easily removed – if the object sticks to the print bed, a palette knife will do.

In SLA printers, 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 an FDM printer. Industrial printer manufacturer Carbon3D even came up with a new idea: They use oxygen to create so-called “dead zone” around the printed model (the oxygen keeps the resin at the surface of the model from hardening).

FDM vs SLA: Postprocessing 

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. More on supports here: 3D Printing Supports: 3 Easy Steps to Success

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 most SLA printers – 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

Consumable in FDM printers are nozzles and 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 PLA filament can be bought for $25, specialized filaments cost more.

In SLA printers, not only resin is consumed: In SLA printers, 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. Depending on the manufacturer and model, resin tanks will set you back around $40 to $80.

Another component that needs replacing from time to time is the build platform as it gets marred when the user removes the printed model; a platform can cost up to $100.

The resin is also costly: 1 liter of standard resin will set you back $ 80 to $150.

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

Building low-cost models

Great for hobbyists and makers

When precision and surface finish isn’t 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)

When to use a 3D printing service

There’s a third option that can save you a lot of money. You don’t have to buy a 3D printer to get something printed, you can use a 3D printing service. You can get more information on 3D printing services in this article: 2020 Best Online 3D Printing Services

All3DP also offers a service that lets you compare the costs of popular 3D printing services.

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

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