Comparing my childhood minifigure heads to my post dark age ones, there is now a hollow stud at the top vs the solid ones I grew up with. At what point did this change happen and what was the reason for the change? Up until recently I didn't think it had a functional difference, but I have seen at least one hair piece that takes advantage of the hollow stud to hold it in place (CMF4 punk rocker). Was that a motivating factor for the change?
Part (or mould) changes have numerous reasons, the main ones being:
- decrease mould complexity and thus production cost (for example to )
- decrease plastic quantity used and thus production cost (for example, look at the bottom of several 1 x n bricks — the new ones have hollow tubes)
- increase part sturdiness (for example to — honestly, how many of these did you break as kid?)
- add functionality (I have no example in mind, but considering the cost of new moulds, they probably do that only when it has to be replaced, or they treat it as a new part altogether).
Making sure a part fits better into the system is a huge factor in new part creation nowadays, and designers will get a new part produced easier if the part plays nice with the other parts, mainly in terms of connection possibilities, but also of existing similar parts. In the past, things were way less rational.
I'm not so sure it influences part redesign, but it could. At the time the hollow stud appeared on the minifig heads, however, functionality was certainly not the main interest, so I would bet on mould complexity. Of course, since it added functionality anyway, you can count on designer to use it. So it's a bit of a chicken-and-egg problem: did the designer ask for the new functionality first, or did the mould change first? In this case, I would say the latter.
EDIT: Even if I answered the "full stud" against "hollow stud" part, I was intrigued by the "anti-choking" argument of having not only a hollow stud, but even one which was fully hollow, that is, you could see through it. So I asked Jan Beyer, LEGO community manager, about it and it turns out the "non-choking" reason was indeed correct. Here is his answer:
This is correct, previous the hole was made in the element due to product safety if the element was swallowed. These regulations changes from time to time, and it has been evaluated and decided that the hole is no longer needed due to safety regulations.
One theory I've read online is that this change was made to increase safety in case of choking.
This reason appears in a Gizmodo article:
We added this hole on the top of the head just in case any kids got one of the heads stuck in their throat. That way they would be able to keep breathing.
The article, though formatted as an interview, is a compilation of reader questions asked by the writer on a tour, so it is not clear if these are the exact words of Lego staff.
The same rationale is discussed in a Brickset post:
...there's a book that LEGO came out with in Japan, called "LEGO Book Museum" (volume 1). This guy gave a review of it, which is unfortunately lost to the internet ether. The response indicates that his review indicated an "anti-choking" reference relating to minifigs, but apparently wasn't specific, leading people on LUGNET to speculate that they were referring to the hollow-stud head.
The writer provides a translation and concludes that the book seems to just point out the design change itself:
So, I haven't found anything about choking on the page...
A hollow head stud allows to connect a 4140303 lightsaber blade sticking out the top. (in case you ever wanted to do that)
It's probably used to save plastic. For every 100 hollow-studded heads, Lego probably saves several heads worth of plastic. In the long run, that saves much material.
More a comment as I don't know the real answer, but I wonder if it in part had to do with the small vacuum created when you connect to a body piece. Pulling apart tight heads usually made an audible popping noise, especially if the pieces were wet or dirty.
However, I'm sure the friction from grimy pieces had more to do with the tight fit than the vacuum.