Author Topic: ABS lighter then PBT?  (Read 2075 times)

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Offline ajx

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ABS lighter then PBT?
« on: Tue, 27 December 2011, 14:58:25 »
i want to know if there is a small difference of weight between PBDT and ABS
i surprisingly discovered PBT feel stickers then ABS

Offline dorkvader

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ABS lighter then PBT?
« Reply #1 on: Wed, 28 December 2011, 00:21:47 »
Or you can look up the plastics:
http://en.wikipedia.org/wiki/Polybutylene_terephthalate
http://en.wikipedia.org/wiki/Acrylonitrile_butadiene_styrene
(notice: no density info. DV's theory on 'pedia wikis strikes again)

http://www.azom.com/properties.aspx?ArticleID=1998
http://www.substech.com/dokuwiki/doku.php?id=thermoplastic_acrylonitrile-butadiene-styrene_abs
ABS is about 1.05 g*cm^-3
PBT is between 1.30-1.38 g*cm^-3

Despite that ABS is massively more common than PBT (the reason Ripster likes it so much is that Lego uses ABS) information wasn't as readily available on AZOM as for PBT. I highly recommend that PBT page I linked.

In conclusion: since PBT is more dense, it'll weigh more for the same thickness (and profile) keycaps. This is one reason I'm excited about thick PBT.

Cheers!

Offline lenakira

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ABS lighter then PBT?
« Reply #2 on: Wed, 28 December 2011, 17:46:36 »
-Removed-

Offline Findecanor

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ABS lighter then PBT?
« Reply #3 on: Thu, 29 December 2011, 00:41:49 »
Or, you could take keys, ABS and PBT that have been cast in the same mould, and weigh them on a sensitive digital scale ... I think that this would be very unusual, but I have just that:

Key 1: Black 6-unit space bar from a set of double-shot Cherry keys from a Cherry G80-11900 keyboard. Curiously enough.. it has the markings ">PBT<" and "2" in it. I have done the acetone test on it, showing that despite its markings, it is not PBT and probably ABS.

Key 2: Light grey 5-unit space bar from a winkeyfull Cherry G80-1800. It also has the markings ">PBT<" and "2" in it. Acetone test was made, showing that it is PBT.

Except for the colour, both keys look exactly the same, but when I measure them I see that the ABS space bar is 1.1 mm longer than the PBT space bar. I tried it on the G80-1800, and it does not fit.

The weights:
ABS key: 5.62 g
PBT key: 6.65 g

6.65 / 5.62 = 1.18327...

Conclusion: PBT is 18 percent heavier than ABS.

However, it should be said that ABS is a blend of three plastics and that the composition varies a lot between blends. All keys will also contain pigment and a difference in pigments could also affect the weight.
This comparison I made is also a bit inaccurate in that because PBT keys are probably cast in bigger moulds to account for PBT shrinking more after demoulding, making these even heavier.
I did also look at other PBT keys from Cherry, but I could not find any larger keys that were exactly like my double-shot Cherry keys. When I weighed those that were sort-of alike, PBT keys were around 16 percent heavier.
« Last Edit: Thu, 29 December 2011, 01:01:45 by Findecanor »
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Offline dorkvader

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ABS lighter then PBT?
« Reply #4 on: Fri, 30 December 2011, 12:18:20 »
To get the volume, you can put it in a graduated cylinder, and fill it with water, (or alcohol, preferably, as it doesn't wet as much) It'd work best with a key that has no (or fewest) mounts on it, and is otherwise larger in volume (to make it easier to measure) Fill with liquid, measure, put the key in measure, take it out and measure. If it varies, that's because liquid is stuck to the key, so you can estimate it by averaging.

Alternatively, you can make an apparatus, and pulverize some of the plastic, and use Archimedes' method for high precision.