3D Printing: Comparing FDM, SLA and SLS

By Mitch

December 2024

Whether you’re interested in getting into 3D printing or already an expert, then you’re probably aware that there are a few methods currently in use. Each one has different characteristics and in fact, completely different methods of printing but they all end with the same result, a 3D printed object.

In this article, I'm going to run through the basics of what each method is and how you use it. I’ll also look at the pros and cons of each method and compare and contrast between them.

So, let’s get started by looking at probably the most commonly used method of 3D printing available today.

FDM

FDM (Fused Deposition Modelling) prints in 3D  using a continuous filament made from a thermoplastic material. Various different materials can be used in FDM printing but the most popular and commonly used is PLA (Polylactic Acid) due to its ease of use and relatively low working temperatures. PLA becomes workable at around 200ºC but also cools quickly making it ideal for FDM printing.

A typical FDM printer works on the motion of three axis, X,Y and Z. The X and Y axis move in the horizontal plane so X from left to right and Y from front to back. The Z axis allows for the vertical movement of the “hot end” from where the heated filament is extruded.

The most common diameter of filament in use is 1.75mm which is then pushed or extruded through the heated nozzle of the aforementioned hot end which can be anything from 0.1mm to 1.0mm. The majority of users use a 0.4mm diameter nozzle which delivers the right amount of filament for most uses. The filament is then deposited on to a heated print bed (moving in the Y axis direction) in layers and builds the model or object in that manner from the base upwards.

The design of the object to be printed is determined by data produced by the designer and saved as an STL format file which can then be transferred to the printer. The design and data formatting process is pretty much the same for each of the printing methods we’re looking at but we won’t go into the details of that in this article.

Pros

Easy to use. Once you’ve mastered the process it becomes second nature

Economical. Machines start at around $200 USD and materials are relatively cheap.

Ideal for home use. If you have a space to put the printer then that’s all you need

Larger build volume. Basic FDM printers can produce relatively large objects

Good quality prints. Get the settings just right and your prints will be perfect

Cons

Steep learning curve. Getting to know everything can be a challenge for some

Additional costs. Failed prints cost money and replacement parts add to the expense

Noise/fumes. Some print filaments can give off fumes and older printers can be noisy

Long print times. It can take days to print a large, high-quality item in some cases

Consistency and Reliability. Prints can fail for many reasons such as nozzle clogging and under extrusion

SLA

Stereolithography (SLA) 3D printing is a different process altogether from FDM but ultimately produces very similar results. Often just referred to as resin printing, SLA uses UV light to form polymers from the liquid resin and thereby form the body of a 3D solid.

That’s the process described in it’s basic form but the machinery and technology used is far more delicate and precise than an FDM printer. The level of fine detail produced using SLA is more defined making it the ideal medium in which to 3D print intricate miniatures or small prototypes.

I’ve used the word “small” there as the average home use SLA printer only has a build plate of 80x130mm and a Z axis height of 160mm. Compare that to a basic FDM printer with a build plate of 220x220mm and a Z axis height of 250mm and you can see the difference. The level of detail and quality of the print though are much more impressive than an FDM printer but there are some added considerations.

As I’ve said, SLA printing uses a liquid resin which is poured into a vat at the base of the printer. This vat is situated above the light source screen and the Z axis lowers into the vat as the UV light gradually reacts with the resin. The 3D object is still built up in layers but the process is inverted so your model will be upside down as you look at it.

The added considerations are that the resin is toxic and can give off fumes so a well-ventilated workspace is key. Also, there is a fair amount of post processing to perform on the printed object. The most important aspect of this is that it needs to be washed and cured. Washing involves cleaning off any resin residue using an IPA (Isopropyl Alcohol) solution and curing requires the object to be exposed to a further UV light source to ensure it hardens properly. Both of these actions can be performed naturally by just using an appropriate container to clean the object and then leaving it in the sunlight for a while. Ultimately though this isn’t a precise method so a washing and curing machine would need to be bought separately.

Again, the design and delivery process of the digital file is pretty much the same as with FDM printing.

Pros

Fine detail. Models and miniatures will have more defined intricacies

High Quality. Resin prints will usually be solid rather than have infill so will be more durable

Quicker printing. Objects ordinarily will take less time to print than FDM

Compact and quiet. SLA printers are all self-contained and virtually silent

Cost. Average home machines are still fairly cheap and start at around $200 USD

Cons

More involved design process. Things like supports and drainage holes are essential

No variety. The material pretty much all the same apart from colour

Errors. If you make a mistake with the design or the print fails then that’s it

Can be toxic. Fumes and ventilation are a big consideration with SLA printers

Additional costs. Resin is expensive and doesn’t last long. Peripherals also add cost

SLS

SLS (Selective Laser Sintering) is a wholly different process than FDM and SLA but does compare to both in the use of a layering system to produce the 3D printed object.

It uses a high-power laser as the power and heat source to sinter a powdered material which is usually a nylon or polyamide but could also be glass or metal. By sintering we mean that the powder is subjected to the heat of the laser which then forms it into a solid without heating it to the point where it will liquify.

The build area is known as the powder bed and the laser scans each layer of the digital design before it fuses the appropriate powder into each layer of the object. The powder is gradually delivered from a separate container and then rolled onto the powder bed surface so that the laser can perform its actions.

The process is a newer technology than FDM and SLA printing and not for home use as the machinery required is on a much larger industrial scale. The main use of SLS printing is in the filed of rapid prototyping which is ideal as the 3D printed objects are likely to be stronger and can be produced fairly quickly.

Pros

Precise modelling. The technology allows for high quality objects to be printed

Cheap materials. The use of powder means that recycled materials can be used

Rapid prototyping. Industrial sized machines can turn out 3D objects quickly

Colouring. The powder can be easily dyed to any colour

No supports needed. The laser targets only the deign areas and loose powder fills gaps

Cons

Rough surfaces. As its powder being used you won’t get a smooth finish

More waste. Unused powder particles can fuse to the bed making then unusable

Machine costs. These are industrial type machines that are expensive to buy and run

Material choice. The most commonly used is nylon but other materials have less strength

Higher shrinkage. The high heats of the sintering mean that cooling cause shrinkage

And Finally...

Hopefully that will have given you a basic overview of the three different 3D printing processes but there’s a lot more you can find out by actually trying the processes for yourself. Whichever method you use will of course be dependent on what you need to 3D print so I hope that this brief guide will help.

As always, thank you for reading this article and remember to look for my other pieces which you'll find under my profile.

3D Printing: Comparing Methods