Breaking Down the Design: LEGO – Perfecting the Plastic Brick

There is far more than you’d think that goes into manufacturing rectangular bricks of plastic. While certainly not as complex as many of the plastic components made for car interiors or high-end consumer products, no one would argue the point that LEGO bricks are extremely well engineered. The closer you look, the more details you start to notice about how much has gone into making these foot-destroying little blocks one of the highest quality products manufactured across any industry.

Let’s break down the design a bit and see just what they’ve done to perfect the plastic brick.

Perfecting the Basics

Plastic part design starts by setting the thickness of the part. It’s hugely important and often difficult decision as the wall thickness is a balance between several factors, including strength, dimensional stability, manufacturability, and cost.

Strength: The thicker the wall, the stronger each brick will be. They’re good on this front – the BBC reports that an average LEGO brick can support 950 pounds, equating to a tower more than 2 miles high before the bottom brick collapses.

Dimensional stability (i.e. will it warp?): Injection molded products are somewhat counter-intuitive as thinner walls are typically less likely to warp out of shape. This is incredibly important as warped bricks would not snap together in trademark LEGO fashion.

Manufacturability: Make the walls too thin and you need, quite literally, tons more pressure to fill the mold while also creating high-levels of stress as the melted plastic fills the mold that can lead to degradation.

Cost: LEGO makes 19 billion elements each year, so shaving off even a fraction of the wall thickness would result in massive cost savings.

For the DUPLO brick we’re looking at, the sides of the brick are 1.5mm thick—very close to the minimum recommended wall thickness for the ABS (acrylonitrile butadiene styrene) plastic used. What’s interesting is that the top face is only 1mm thick which is well below the standard guideline for ABS, but is done for a very good reason that we’ll explain next…

Perfectly Straight Walls


One of the biggest challenges when injection molding non-circular shapes is getting plastic to corners of the part at the same time as everything else. If this doesn’t happen the part will not shrink uniformly and each side of the part will bow in or out causing problems snapping bricks together.


The left image shows a model using flow leaders and the right is a uniform 1.5mm thickness. Notice how the flow accelerates through the flow leaders helping the plastic reach the corners at the same time as the rest of the wall.

LEGO solved this issue by using a combination of reduced thickness (as explained above) and flow leaders on the underside of the horizontal wall. The raised paths start from the center of the part where the plastic is injected and travel first to the bosses and then to the corners of the part. As the plastic flows through the mold it follows the path of least resistance, traveling through these paths faster so the corners fill at exactly the same time as the shorter walls. This results in a uniform rate of shrinkage as the plastic cools so the walls remain flat.

You’ll notice this approach does not work for the long walls of the 2×4 DUPLO brick as the middle fills with plastic well before the corners. So it is supported by a rib that prevents them from bowing inwards both due to shrinkage as well as in use by kids with abnormally strong hands (and possibly 35 year old children).

A Perfect Stack

A box with straight sides is just the first step in making sure every brick stacks together just right. Across the part, the surface is completely free from any marks or defects except one location – the spot right in the middle of the top surface where the plastic is injected into the mold. That’s what’s called ‘gate vestige’. It’s a necessary evil that has to be dealt with and LEGO does so in a wonderfully simple way. They dented it. That is, they created a small circular impression so that they don’t have to deal with the little nib of plastic after it comes out of the mold. It just sits below the top surface of the part so blocks can be stacked evenly and there’s no post-molding operation required to clean it up.


Another necessary element of injection molding is the need to add draft to the side walls so that the part can be easily ejected from the mold. However, this would result in a slight overhang when stacking blocks on top of each other as the bottom would be slightly larger than the top of the brick below. For most manufacturers this would not even register as a problem…but we’re talking about the perfect brick. LEGO moved the parting line up about 1mm above the bottom edge of the part, so the wall tapers back in and a rounded edge is added to give the appearance that it is the same dimension, creating a seamless transition from one brick to the next.


One last detail that ensures a perfect stack can be found on the bosses underneath. The bosses and the ribs are arranged to accept the posts on the top of the brick below. If you look closely at the bosses, you’ll notice that the wall is reinforced at each spot where the post will contact it. This is just another subtle detail that, according to LEGO enthusiast Phillipe Cantin and his custom built LEGO test contraption, allow the bricks to be assembled 37,111 times before they wear out.

Perfectly Manufactured

To support the volume of bricks made each year the manufacturing process has to be dialed in so manual intervention is rarely, if ever, necessary.

So they needed to find a way to deal with the features of LEGO bricks that make getting them out of the mold a bit of a trick. When the posts on the top of the part cool down and shrink, they are likely going to ‘stick’ to the pins coming out of the mold that form them. The result is a part that can attach itself to the wrong side of the mold when it opens so it can’t be automatically removed.

Another simple fix. Between the small ribs on the inside of the part there are small extrusions that come out about a tenth of a millimeter from the wall. These are there solely to grip the part when the mold opens so it stays on the right side of the tool.

But with a very minimal draft angle and the ‘grippers’ on the part, more force is now required to eject the part while still making sure it isn’t damaged. Rather than using ejector pins that would apply force to the thin horizontal wall, LEGO uses three ejector sleeves on the base of each of the bosses that apply the ejection force to the strongest structural feature of the part.

The Perfect Brick

There’s clearly a lot of engineering that has gone into these…and we’re still only examining what is visible on the part itself. You can be certain that just as much effort has gone into engineering the material to get the exact mechanical and manufacturing properties they require, designing the mold to achieve the highest quality part with the shortest cycle time possible, and setting up a robust manufacturing process that can be replicated at manufacturing sites across multiple continents.

The question of how easy it is to design and manufacture something is not always based on the obvious simplicity or complexity of the object. You also have to account for the standard of quality and how close one gets to the absolute best possible design.

Brian Sather is a product marketing manager at Autodesk. An engineer by trade, he likes taking things apart and putting them back together. Half the things in his home work twice as well as they should, and the other half don’t work at all.

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