Subject:
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Re: A question of humidity...
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Newsgroups:
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lugnet.general
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Date:
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Fri, 16 Jul 2004 17:39:35 GMT
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Viewed:
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1303 times
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In lugnet.general, David Koudys wrote:
> A few brief stories--
>
> I've had LEGO displays in fish tanks for the little fishes to swim through.
And I'm sure they appreciate it each time they turn around and "discover" it for
the first time in their lives. BTW, mixing a 12" pleco and a LEGO pirate ship
is a recipe for frustration. If you have any skeletons, make sure they're only
holding things that float, so they'll be easy to find when they get knocked
loose. Also, I wouldn't advise putting any metal or rubber parts into a fish
tank, as they might not do as well in the water as the hard plastic parts.
> After years of staying in the fish tank, I found that the LEGO pieces were
> stuck together quite firmly, and some pieces took much effort to pry apart.
That's probably caused by minute mineral deposits in the gaps between the
pieces, or maybe swelling of the parts. There's not any sort of direct bond
between the individual pieces, if that's what you're thinking.
> I filed that away for no real reason.
>
> I bought a house last August and promptly set up a LEGO train layout in the
> basement. Then spring and summer arrived with much rain and humidity, and I
> noticed that many of my large blue baseplates sitting on the wooden train
> modules were getting all warped, and that my 9 volt train wheels were really
> starting to bind. Dragging out the dehumidifier last week and leaving it on
> for a few days (and subsequently dumping lots of collected water down the
> drain), I've noticed that the blue baseplates are gradually flattening out,
> and the 9 volt wheels are not binding as bad as they once were.
>
> So a few things--
>
> There must be different 'grades' of plastic (ABS, whatever) that LEGO bricks
> are made from
Eh, sort of. Different materials, to be specific. Basic opaque bricks are made
of ABS, which sinks. More flexible pieces (like sabers) are made from a
different plastic (still don't know what) which floats. Clear ABS is
prohibitively expensive (and probably not "optically clear" either), so clear
parts are almost assuredly made from a third plastic, possibly acryllic or a
material directly derived from it (which would explain why they're more brittle
than their opaque counterparts, especially in cold weather)
> --Every blue baseplate warped--none of the green or grey ones warped.
> I thought that this fact alone was rather odd.
Indeed it is.
> The tolerances on 9v wheelsets must be quite small for the humidity to affect
> them that much.
>
> I didn't think that LEGO bricks could 'absorb' water moisture. Always had
> it in my mind that ABS was 'waterproof'. Eh, live and learn.
ABS is quite hygroscopic (meaning it absorbs water), but it doesn't suffer any
direct damage as a result (which is a good thing, because it'll absorb moisture
directly out of the air, unless you have zero humidity). The one instance where
this is a critical problem is when you're making parts with it, because the
forming temperature (even for sheet-forming applications, like vacuum-forming,
pressure-forming, or match-mold forming) is higher than the boiling point of
water, so all those water molecules turn into steam, and occupy 1000x the space
of liquid water, so the instant the plastic structure begins to relax, the steam
causes it to honeycomb (or "popcorn" if you prefer). Instead of a seemingly
solid plastic structure, it'll have all these little air pockets scattered
throughout, which greatly weakens the structure of the material. Now, some
plastics are more hygroscopic (meaning they'd need to be dried immediately prior
to processing because even a few minutes of exposure to moderate humidity will
be cause them to absorb enough to honeycomb) and others are less so (meaning you
can dry a batch for a full 8-hour shift and have it sitting out in high humidity
with no noticable problems), while other plastics aren't hygroscopic at all
(meaning you can soak it in water for a year immediately prior to running it,
and as long as you dry off the outer surface it'll be perfectly fine).
If ABS swells at all due to moisture absorption (I'm not positive that it does,
but I'd be surprised if there's not a little swelling, if only on the surface),
it might be causing your train wheels to bind. Also, if there are any metal
parts visible, you could check them for surface rust.
A few likely explanations for your green baseplates (which are either
vacuum-formed or pressure-formed, btw, not injection-molded) might be the
plastic bias when they ran them (the direction in which the raw sheet was
extruded in relation to the layout of the thermo-forming mold and oven), the
quality of the green sheet (many problems introduced during extrusion will not
go away during the subsequent thermo-forming process), how well the
thermo-forming machine was tuned in while running them (different colors of the
same material will behave in slightly different ways during the forming process)
the percentage of regrind material used in the green sheet, or even maybe
something to do with the coloring agent and how it reacts with the ABS plastic
and/or moisture.
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Message is in Reply To:
 | | A question of humidity...
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| A few brief stories-- I've had LEGO displays in fish tanks for the little fishes to swim through. After years of staying in the fish tank, I found that the LEGO pieces were stuck together quite firmly, and some pieces took much effort to pry apart. (...) (20 years ago, 16-Jul-04, to lugnet.general)
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