FDM vs SLA: Which one is best for you?

Intro

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 types 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 the two so you can decide which type best suits your needs!

How they work?

FDM

In its simplest terms, FDM printing (also known as “Fused Filament Fabrication”, or FFF) involves laying down lines of melted material layer-by-layer to build up a component. 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 process called “vat polymerization”. This means that wherever the laser hits, bonds will be created to connect adjacent molecules and form a solid.

SLA 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).

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 the former compares to SLA printing here.

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 Ultimaker 3, is 0.4mm. However, 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 optimised 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.

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 colours 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.

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 colour.

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.

Cost

FDM

In the hobbyist bracket, low-end machines are incredibly inexpensive. Entry-level machines can be bought from just over £100 (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 £1000.

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 budget-friendly 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 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.

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.

Final Thoughts

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

For more information on which type of printer you may require, contact us on 01926 333 777 or check out our contact us section.