FDM vs SLA: Which 3D Printing Technology is best for you?

One of the first questions many people ask when they’re new to 3D printing is “What type of printer should I get?” If you’ve been looking into printing plastics, you may have already heard of the two main types. These are Fused Deposition Modelling (FDM) and Stereolithography (SLA).

Both 3D printer technologies share the benefits of additive manufacturing but vary in their execution. There are many factors that might influence which one is most suitable for a given task. Here, we break down the main differences between FDM and SLA so you can decide which type best suits your needs!

FDM vs SLA: How does the 3D printing technology work?

FDM

In its simplest terms, FDM printing (also known as “Fused Filament Fabrication”, or FFF) is a method that builds objects layer by layer. It does this by laying down lines of melted material. This is called extrusion.

A thermoplastic printer usually consists of at least:

• A extruder, which melts a thin filament of solid thermoplastic with a hot nozzle and deposits it in layers.
• A build plate to provide a surface for the extruder to deposit material on.

The build plate and extruder both move around in 3D space. The motion of these parts depends on the model. In the Ultimaker S3 for example, the extruder moves horizontally in the XY-plane while the build plate moves vertically in the Z-axis.

SLA

SLA printing also builds up parts layer-by-layer, but differs in the mechanism used to make the layers. This difference begins with the material used; a liquid resin instead of a solid filament. Instead of an extruder depositing plastic in layers, SLA printing uses guided UV lasers to draw patterns onto the surface of a filled vat of resin.

The laser ‘cures’ the parts of the resin it touches in a 3D printing process called “vat polymerization”. This means that wherever the laser hits, bonds will be created to connect adjacent molecules and form a solid.

SLA 3D printing has fewer moving parts, because only the build plate moves, translating vertically in the Z-axis to expose new layers of resin. The build plate also doesn’t necessarily have to hold the part on its top face!

The Formlabs 3 printers contain a build plate which suspends the manufactured part upside down inside the vat of resin, slowly pulling it out to allow the laser at the bottom to cure additional layers. This removes the size of the vat of resin as a constraint on part size (Learn more here about Stereolithography and how it works in our blog).

SLA does require more steps in the printing process. Certain materials require a final curing stage after the print is complete for example.

Note: While SLA is the focus of this comparison, there are other vat polymerisation techniques. These include DLP and LCD printing, which use light projection and digital screens respectively to cure the resin layers instead of a laser. You can read more about how DLP compares to SLA printing.

FDM vs SLA: Print Quality

FDM

The resolution of an printed part is dependent on the size of the extruder nozzle and how precisely the extruder/build plate moves in space. A popular nozzle size, used on printers such as the Stratasys F900, is 0.01 inch. It is usually easy to replace the nozzle with a smaller one if required.

Smaller nozzles can create finer lines and increase surface detail, at the cost of print speed. However, it is important to note that the extruder movement resolution needs to be high enough to take advantage of a smaller nozzle size.

Due to the number of moving parts, there are a number of ways print quality can be adversely affected:

• Lack of layer adhesion – The extruded thermoplastic is usually deposited in a rounded cuboid shape. This means that gaps can easily form between layers. The layers may also not adhere to each other when printed. These issues mean that parts can have directional weaknesses in their vertical axis, known as anisotropy, which can reduce strength by up to 90%.
• Layer weight – Layers higher up in a print can weigh down on lower layers, causing them to shift around or sag. This can result in warped, inaccurate parts.
• Thermal stress – The process of heating thermoplastic material to a near molten state can cause residual stresses in printed components.

Many aspects such as machine calibration and careful management of the printing environment can ameliorate some of these issues. Luckily, these aspects have been researched and optimized for many printer models by a large community of users online, so there is a wealth of knowledge to pull from.

SLA

SLA print resolution is very high as it purely relies on the size of the laser spot. With a resolution as small as 25 microns, this means that much more precise parts can be made and this is one of the main draws of the process. SLA printing is very well suited to any application where highly detailed parts with a high-quality surface finish are required.

The SLA printing process itself also results in lower directional weaknesses. As mentioned earlier, the process of curing creates chemical bonds in the resin on each layer (also known as “cross-linking”). These bonds constitute each of the solid layers.

However, the chemistry of the resin means that there is a semi-reactive state between each layer that’s printed. This means that when the whole part is cured, the same strong bonds making up each layer also form between the layers.

This means that SLA parts have no gaps between layers and negligible directional weaknesses. SLA printed parts can therefore be waterproof and airtight.

FDM vs SLA: Material Choice

FDM

FDM machines have a great selection of material to choose from, mainly due to their prevalence. There are a large number of suppliers and an enormous range of materials and colors to choose from. Furthermore most printers can use any suppliers materials, meaning owners are not exclusively locked in to using their manufacturers filaments.

This choice of materials means that parts manufactured can vary greatly in their physical properties. Polycarbonate (PC) or ABS can be used to make strong, mechanically performant parts. Thermoplastic Polyurethane, or TPU, is flexible and has high impact resistance. Although there are SLA resins that can match some of these properties, the sheer quantity and range available for printers is currently unmatched.

Stratasys F900 material canister

SLA

Due to lower popularity at this point in time, resins are less ubiquitous than filaments. While there is some choice, printer manufacturers often make their own proprietary resins, preventing the use of other brands or types on their machines. This limits choice in material and color.

Formlabs Resin

Another issue is that standard resins often produce brittle parts, and are therefore unsuitable for any high load applications or situations involving significant mechanical force. While industrial resins have better properties, and post-curing can improve the qualities of a newly printed part, both incur additional costs.

FDM vs SLA: Cost

FDM

In the hobbyist bracket, low-end machines are incredibly inexpensive. Entry-level machines can be bought from just over $110 (as of June 2020), although the cheapest machines often suffer from reliability and precision issues due to the low-quality materials used. Despite this, decent starter printers can still be purchased for under $1,100.

Material costs are also very modest due to the quantity of suppliers and manufacturers, although price and quality are often tightly correlated when purchasing filaments.

Other than buying replacement parts if any component of the printer fails, there are negligible other sources of expenditure, and overall printing is one of the most cost-effective ways to enter the 3D printing ecosystem.

SLA

Stereolithography printers are more expensive starting from a thousands of pounds. Prices are continuing to come down as SLA 3D printers become more widespread. The same also applies to resins, which currently cost more than their filament counterparts.

However, compared to FDM printing, the SLA printing process requires more secondary equipment. Protective gloves to handle the resin with, solvents to wash off excess resin and masks/ventilators to avoid the fumes are all recommended items.

There is also the matter of post-curing stations, which are required for certain types of resin and can be relatively expensive. All in all, SLA printing can require a significant investment to get started.

FDM vs SLA: Ease of Use

FDM

Much of the appeal of FDM printing comes from it’s ease of use. Furthermore, if you encounter any difficulties, there is an enormous online community, and experts like ourselves who are happy to help.

Any sources of difficulty associated usually comes from improving print quality. Printing new objects or with new materials often requires significant calibration. The relatively high number of moving parts can mean that sometimes troubleshooting a printing issue can be difficult for a beginner.

SLA

SLA printing has a few more steps to the process. The resins used are sometimes toxic. The process could be described as “messier” than FDM, as it requires an extra cleaning stage.

Preform deals with the automatic generation of support material. Therefore its easy for anyone to start printing with a Formlabs machine.

One benefit SLA has over mechanical simplicity of the printer, with fewer sections requiring checking on the printer itself. However, as mentioned there are more steps to the full process.

SLA Post Processing

FAQs

Q1 – FDM vs SLA: which is faster?

If both of these processes used the same layer height, SLA would be faster because a laser moves faster than an FDM nozzle. However, SLA usually prints in a much smaller layer height (finer resolution) so more often than not FDM prints faster.
Most Industrial and hobbyist SLA prints target the 100 micron resolution (~.004”) as the go to, and drift up to 150-200 as-needed for “Faster print modes”. Most FDM 3D printers, alternatively, are capable of a minimum of 200 Micron slice height, often getting into the 100 to 150 micron range, but typically targets thicker layers like 250 micron height (~.010”) as standard slice height to ensure a balance of speedy build and great part strength.

Q2 – What are the disadvantages to FDM?

• FDM parts with more layers are weaker than those with fewer layers because there are more locations for cracking/failure to occur, as well as having more surface area per bead of deposited filament to bond between layers (thicker layers = wider toolpaths of material).
• Often, depending on the material, FDM parts are considered to be uglier than SLA parts because of the visible ‘layer lines’ from this process which will need to be sanded down for a smooth surface finish, whereas off the printer SLA layer lines tend to be far less visible to the eye.
• Nozzle clogging can be common and very annoying to clean out/fix.
• Calibrations are required fairly often, either to ensure that the printer knows its distance from the fixed tip location to the print bed, or to know the difference in height between a model tip and a support tip.

Q3 – What are the main problems with SLA?

• The SLA post processing workflow can be lengthy, tedious, and messy due to the liquid resin.
  Liquid resin is a skin irritant, so nitrile gloves are required, and some people dislike the overall smell from the liquid resin.
• SLA Printing utilizes a single material, forcing parts to be printed with rigid support structures that use pin points to attach to the model to ensure that they do not fail while printing.
• This supports can often be difficult to remove, and in almost all scenarios leave behind visible marks on the part wherever they were required, and typically require a moderate amount of operator time to sand off of the part.

Q4 – Are SLA prints stronger than FDM?

SLA parts have isotropic properties, meaning their material properties are the same regardless of what direction you measure them. FDM parts are weaker in the perpendicular direction due to the reliance on layer adhesion. That being said, SLA resins are often chosen for applications requiring strong, durable parts with excellent chemical resistance, whereas FDM materials are preferred for parts needing impact resistance and flexibility.

Q5 – Are printing materials for FDM printers more durable than on SLA printers?

Most SLA resins tend to be less durable than the average FDM material, with some standout resins having critical exceptions for rigidity, heat resistance, and chemical resistance, as well as some with more “Polypropylene like” properties that make them suitable to parts with snap fits.
FDM filaments, alternatively, tend to be on average stronger, boasting the ability to print in both durable base materials like ABS, Polycarbonate, and Nylon (actual thermoplastics), and ranging up to extremely durable and in some cases Aerospace grade Carbon Filled Nylons and Polyetherimides (Ultems, PEEK’s, PEKK’s), many of which are considered replacements for materials like aluminum.

Q6 – What’s the key difference to consider when comparing SLA and FDM printers?

• One of the key differences between SLA and FDM when considering what technology is right for your organization is determining what you will be doing with your printed parts.
• Most Industrial SLA systems tend to have a single large vat, which makes changing resins difficult to do, so organizations with these machines typically find a single resin that checks most of the boxes in what they need.
• Small format SLA printing can be more versatile, enabling more easily the changing of material types, though this typically requires some additional components and extra cleaning steps to take place during the process.
• ALL levels of FDM printing, however, have the capability of printing in multiple filaments, and changing between one another is quite easy, by comparison, enabling FDM to also have a more flexible range of capabilities when it comes to strong/weak materials, depending on the needs of your project.

Q7 – Is SLA 3D printing expensive?

Resins are usually more expensive than filaments for FDM printing, though there are certainly some exceptions to this rule on both sides. Typically, for both technologies, there are some materials that are on the low end for cost per unit (cubic inch, cubic cm, liter, etc), as well as some that are significantly higher.
Printers themselves from both technologies also cover a large range of prices, on the hobbyist end getting as low as sub-$10k USD, and on the top end Industrial systems in botch technologies can reach $500kUSD, with a variety of options in between.

Q8 – Is SLA the most accurate 3D printing technology?

No, there are other more advanced forms of additive manufacturing that are more accurate such as LPBF of metals and Material Jetting, aka PolyJet printing, that more closely align to the original dimensions of a modeled part. With that said, many Industrial SLA systems can achieve respectably high levels of dimensional accuracy, which put high end SLA printing near the top of the list of technologies capable of achieving high levels of dimensional accuracy.

Q9 – What do I need to do after SLA printing?

There are several steps for post-processing of SLA printing:

1. Once the part is printed, while wearing nitrile gloves you will remove the build surface from the machine and wash the excess liquid resin off your parts. This is traditionally done in IPA but TPM is also acceptable and some companies create their own cleaning solutions. This can be done with the parts still attached to the build plate or not, which is often a recommendation from either the OEM of the printer itself, the resin manufacturer, or sometimes operator preference.
2. Then, you will remove the parts from the build plate, remove the support structures (these snap off, and typically require additional sanding to smooth down “nubs” to the surface), after which the parts are set to dry for a period of time.
3. Following the dry time, most resins require the parts to post-cure in a UV chamber. Be sure that your UV light’s wavelength matches the resin type – Formlabs, for example, uses 405nm wavelength resins and Stratasys’ Origin One uses 385nm, though most Industrial SLA resins cure at 355nm.
4. Some resins may benefit from a thermal bake after UV curing, but otherwise this whole process should take 10-30 minutes of hands on time, with UV cure times (typically automated) taking 10-60 minutes depending on the resin type and part geometry.
5. Once post-cured (UV, thermal, or both), it is safe to touch the parts without nitrile gloves.

Final Thoughts

Both FDM and SLA are mature, well-understood 3D printing methods with a variety of use cases. While FDM technology can be cheaper and easier to get started with, SLA printers have higher print quality and are the only choice for certain projects.

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