With the annoucement of the 10361 Holiday Express Train came a lot of hullabaloo surrounding the new toy train element. Notably because it would be the first commercially sold 3D printed LEGO piece to be available to the market. I know a thing or two about 3D printing so I wanted to talk about it a little bit as well as clear up some misconceptions as well as give my thoughts on it.

I will be talking about:
● Why is LEGO now using 3D printing?
● What we can expect from 3D printed LEGO pieces
● What are the concerns about 3D printing being used in LEGO sets?

The LEGO Group is no stranger to 3D printing. They have been using the technology for prototyping as early as the 2010s. There was an old video of some LEGO employees talking about how they designed new pieces for the Pixar Cars and they detail how they would first 3D print the prototype pieces to test for general fit and shaping (I can’t find the video anymore so if anyone has a link to it feel free to drop it in the comments!).

But the first publicly released official 3D printed LEGO piece wasn’t until 2019 with the LEGO Inside Tour set: The LEGO System House. It featured 66237 Utensil Drafting Table Arm in sand green.

Similary, the next few official 3D printed LEGO pieces were also only given out in rare exclusive contexts such as the Pull-Along Duck and the Pogo Stick in 2022. The duck appears in red as a GWP for an AFOL event and again in green for the 2024 LEGO Inside Tour. The pogo stick was a supporter bonus gift for the “The Secret Life of LEGO Bricks” crowdfunding campaign.

There is one thing that these 3D printed LEGO pieces all have in common: they all having moving parts. The drafting table arm could bend and swivel about that top pivot. The toy duck’s beaks will open and close as the wheels are rolled across a surface. The pogo stick naturally bounces up and down. These are all examples of either compliant mechanisms or finely designed mechanical parts, a hallmark of something that can easily be achieved through 3D printing.

I think LEGO is trying to capitalize on the unique benefits of CNC based additive manufacturing method to create more complex pieces that would normally not be achievable with injection molding. Injection molding is how most LEGO pieces are currently made and it involves injecting hot plastic into a “die” which contains a cavity in the shape of what the final product will look like. Like an ice cube tray forming the ice into cubes. But this places severe limitations on what kind of shapes you can make since the die has to be able to then be removed from the parts once the plastic cools.

For example consider something like a vase. It would be literally impossible to injection mold a vase like the ones shown in the pictures below since the inside of the vase is larger than the opening of the vase.

But 3D printing works differently. There’s many types of 3D printing, the most common of which is called FDM. This is most likely what most people are familiar with, which is essentially a machine that takes plastic filament and melts it and then builds an object layer by layer. Imagine trying to sculpt something with a hot glue gun essentially.

HOWEVER. These LEGO 3D printed pieces are most likely NOT made with traditional FDM printing. It much more likely that they are made with SLS printing (Selective Laser Sintering). How this works is instead you have a chamber with a layer of nylon powder. A laser shoots an outline onto the powder to fuse it into a solid piece. A new layer of the powder is laid down and the process repeats. Eventually you’ll have an entire chamber of powder and within the powder will be the object you printed. Blow away the dust and ta-da!

So a lot of the concerns people will have when they hear that 3D printed LEGO pieces is that they will be ugly or flimsy. But this is mostly due to the association with FDM printing’s layer lines.

In the picture above you’ll see a side by side comparison with a traditional 3D printed part next to an SLA printed part. Note that SLA is not the same as SLS but for the sake of the point I’m trying to make, they’re comparable. SLS provide much smoother detail also stronger layer adhesion. Layer adhesion is the how strongly bonded the plastic is to each other between each layer. With traditional FDM, parts are often weak in the direction along the layer line so parts printed can just break if designed sub-optimally.

SLS printing also offers finer tolerances, which refers to how tightly parts can fit together without accidentally interfering with each other. Take a look at the underside of the previously mentioned 3D printed duck.

With SLS printing, they were able to print this entire thing in one go. Since instead of having to assemble all the moving parts, the duck can be printed as is on the spot. FDM printing doesn’t offer the tolerances needed to print these small parts and still have them move freely. Another benefit of SLS is that you don’t need supports.

Imagine you were trying to print a T shape standing up on an FDM printer. You’d be fine up until you got to the top section of the T. You suddenly wouldn’t be able to print that top straight section because if you were to try and print it, the plastic would just fall straight down since there’s nothing underneath it to support it. Since SLS print happens in a chamber filled with powder, the powder naturally acts as supports and holds everything in place. Take a look at the pogo stick from earlier. Any part of the pogo that doesn’t immediately have anything beneath it would require supports.

But should we be concerned? How will SLS printing hold up to injection molded parts? Is SLS 3D printing the future of LEGO? Do we need to be worried?

Basically, no, fine, no, and no. In that order. Nylon is nontoxic and pretty strong (pure nylon is stronger than ABS actually, which is what LEGO bricks are normally made of. But printed nylon is probably weaker depending on the bond strength). So I don’t foresee it being much of an issue in that regard.

SLS printing is currently still super expensive compared to injection molding and more importantly, not scalable. Injection molding has the benefit of being extremely fast and as long as you have the die, you can just pump out thousands of pieces very quickly. The post processing is also a lot less labor intensive. Whereas with SLS printing you have to wait for the chamber to cool down, clean away all the dust, and then coat it for painting. Right now, SLS polymers are limited to lightly colored nylons due to the nature of how the laser interacts with the powder. So any color would need to be added after the fact. I’m not super familiar with SLS coloring methods but based on the texture shown in the photos it’s probably some form of powder coating, hence the grainy texture.

Long story short, it’s a huge pain to make SLS printed parts and would only be worth it for intricate pieces that have some complex moving parts like the toy train. I’m mainly just confused on how LEGO is planning on making this work from an economics perspective. SLS doesn’t sound like it’s a viable option for large scale manufacturing unless they have tons of SLS printers (which I need to reiterate, are super expensive). Does this mean we will have a sort of “SLS-tax” that just automatically raises the price of any sets containing 3D printed parts? But I am excited for what TLG will do with this new technology now that it is an option. They’ve unlocked a playing field of new types of LEGO pieces. I’m thrilled to see where their creativity will lead them.