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LEGO Minifigures

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LEGO Minifigures

Nov 2021

Engineers know that controlling plastics with precision is a challenge.

LEGO knows a thing or two about plastics. They manufacture hundreds of millions of Minifigures per year. These figures are assemblies with moving parts. Each one must work with every other LEGO ever created. Pulling off perfection on this scale is impressive. That’s why, in the minds of engineers, the LEGO mythos is all about precision.

Well, you might be surprised by what is hiding inside. Scroll down to reveal what is within through CT scans.

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Intro image for the scan.

The Fireman

The claw-like hands are one millimeter thick, but the arms are two millimeters thick. The delta is not a trivial detail. Changes in thickness can lead to defects in plastics. Here, there are 600 micron voids inside of the arms.

On the exterior, the surfaces are clean and square. The torso is the exception, where there are intentionally large angles. Interior surfaces are where the designers hide the draft angles required to release parts from a mold.

Computed tomography picks up both geometric and material information. Seeing the reflective fire vest separate from the body indicates it has thickness. The color change of the vest relative to the body implies a dissimilar material in the coating.

The torso has two planes of symmetry that form an axis in the Minifigure's neck. This makes the neck a lovely place for a gate — the site where plastic is injected. Interesting, then, the placement of a LEGO logo here. Only the L and O are defect-free due to the gate. Can you find another example of this in the scan?

Ignore the voids, and focus on the legs. It is worth driving around this part of the scan to notice how precisely the legs interact with the torso. See the use of flexural walls. These walls ensure the legs are held with pressure even when there are size variations.

Intro image for the scan.

The Alien

It is no surprise that the staff contains voids similar to the arms. It is worth noting the shape of the voids. The circular nature prevents any stress concentration during compression. Under bending, the stress lives at the surface of the part.

The legs of the figure contain R and L designators. These are <200 microns in height yet cleanly separate in the CT scan. These types of features enable automation techniques like OCR to sort and track parts.

Here is a view of precision manufacturing at work. Note where the body meets the legs; the outer edge is nearly collinear front and back. The misalignment is at most 50 microns on this example.

Even with excellent manufacturing tolerances, good designers do not rely on perfection. The arms of the legos have, to our measure, 250-micron tall ribs around them. These ribs force an interference to ensure a snug fit in the sockets while allowing rotation of the arm.

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