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Here's the part in question (2955):

enter image description here

While Technic parts typically have a recess to allow the ends of pins to be flush with the surface, this part (and perhaps others) carry this recess all the way to the edge of the part in a U shape.

I looked through instructions from a few sets to see if I could figure out if there was any utility to this, but I wasn't able to find anything. Here's an example of the part used in the Mobile Crane (8460):

enter image description here

Is the some purpose to this design, or is this just a quirk of this part?

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4  
Seeing how LEGO parts are injection molded, I'd say it's because the mold has to be moved away from the part without damaging it. So if the recess weren't there, the mold would just plow through the part in question. –  LarissaGodzilla Apr 2 '13 at 8:57

1 Answer 1

up vote 10 down vote accepted

As suggested by bumbumfish, it is indeed required by the molding process to be able to remove the part from the mold. This does exist in many other parts, such as Technic frames:

Technic, Liftarm 5×7 Open Center ThickTechnic, Brick 4×4 Open Center

The alternative is a mold with moving parts, but this is more expensive, and can't be used anyway where there is not enough room for the mechanism (e.g. inside frames).

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Another good (if maybe more subtle) example of this is the pin joiner which now has a slot in the middle. That slot is similar to the recess seen in the other parts, in that it simplifies the moulding process greatly. (The previous version required two sub-parts to be assembled together, which is even worse than a mould with moving parts) –  Joubarc Apr 2 '13 at 11:46

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