Lego with hollow posts underneath - dates?

Question

When did TLG change the small posts on the underside of the modern bricks and plates , eg 1x6 and 1x4, to hollow ones?

Answer 1

I can confirm that some bricks have hollow posts. I ran across this in attempting to answer a "how much does a brick weigh" question. What I thought was a homogenous batch of bricks turned out to have a few with hollow posts. The bricks were all purchased new in the last three years.

From a "material cost productivity" standpoint, it will be in TLG's best interest to migrate to the hollow posts since parts from these molds will require less material to serve the same function (but not immediately). An expedient time to do this would be when a mold is at end of life requires replacement. So, I would expect to see significant variation continue for years to come. The balance point is the cost of replacing a single mold with the reduction in processed material costs over the expected remaining life of the mold. So the older the mold is the less cost effective it is to replace prematurely, but the newer one represents less time in service to amortize the mold over fewer piece parts... both of these things mean that the replacement will likely be when the mold normally reaches end of life. Since the useful life is very long, it is more likely that we are seeing parts from line or factory expansions.

Answer 2

Answering the question tl:dr style: Around the 90's.

Long story, quote from another user:

I recall having read somewhere that LEGO didn't replace all the molds in one go during the 90's. LEGO kept using the old molds until they were worn out. This meant that both solid and hollow tube bricks and plates were present in sets during that time, sometimes even in the same set.

This is an example of the main difference between plates:

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Answer 3

I’m going through a Lego lot from the '90s (around '91 to '95) and there are some pieces that have a hollow bottom and some that have a solid bottom. On the other hand, I think the solid bottom was around in the early 2000s too. It just really comes down to the fact that there are many different Lego factories and they needed to make another mould of the exact same piece at different times because it’s being used more frequently. After they update the piece, in this case to having a hollow bottom, but sometimes they keep the old mould because it still works. I have gotten these sets and some colours had solid bottom and some didn’t in different colours so I imagine that in that factory one machine with the newer mould was making that piece in that one colour and the older mould was making it in the different colour. Even more annoying in that sealed set that I mentioned, there was even one colour with some pieces with hollow bottom and solid bottom but the set that I’m talking about is from 2003 and it’s not plates that I’m talking about but bricks. But if the solid bottom did not make it to the early 2000s then I would say the hollow bottom came out around maybe '94-'95 and probably did not retire the old mould until they needed to replace it with the updated version. But also one other thing too is if the piece was around early 2000s they might’ve kept the old mould in storage instead of destroying it in case they needed it if, I don’t know, it was busier that year, and they needed to take it out of storage or the other mould was down but it’s best to check if you see anyone opening a sealed set of the set that you’re getting pieces to it and see what mould variation is shown in the video on the set.