Difference between PBT and POM?

[Davox]

Which one is better and why? And would it justify the cost on a Cherry MX Clear keyboard?

[davkol]

POM is usually very smooth, while most PBT keycaps have rough texture (I consider it more pleasant when my fingers get sweaty). Neither is better or worse, they're just different. Definitely much better than ABS for most people.

[mauri]

Some people would argue they are _the_ key cap material, I don't mind so much as long as it's not ABS, slimy buggers.

[Findecanor]

POM is more slipper to the touch than PBT, yes. However, keys can also have more or less texture moulded into them. For instance, Signature Plastics' ABS keys have more texture than Cherry's PBT keys but that texture will wear down faster than Cherry's.
Of all keys, I like Topre's keys the best. They are both PBT and they have quite a rough texture moulded into them.

[jabar]

It is all about the mold. Ducky PBTs have awesome texture whereas SP-made Deck keys are much smoother. From what I've read POM material is more rubbery perhaps? Haven't tried any myself.

[daerid]

If you've ever picked up a standard guitar pick, there you go: POM. Not sure what everyday objects would be that are made of PBT.

[Findecanor]

I would not be surprised if parts of the MX and/or MY switches were made from POM.
BTW, a common brand name for POM is "Delrin".

[dadgh]

geekhack.org/index.php?topic=43272.msg882548#msg882548

First of all, the idea that PBT is "textured" and ABS can be "textured" equivalently is false unless the fundamental laws of physics and chemistry have suddenly become null and void. A large part of PBT's texture comes from the way the plastic cures, not from the mold itself. If you use an identical mold for ABS and PBT (which by the way, I don't believe you can due to various differences) the ABS will cure more smoothly than the PBT every single time.

Secondly, the fact is that ABS has a much lower abrasive resistance than PBT and POM. The lower the cost of the ABS, the lower the resistance to abrasion. This is why I can shine ABS keycaps in a matter of weeks if not less, and actually wear through them in less than a year, but get 2-3 years out of PBT and POM.

The key differences between PBT and POM are mostly to do with the manufacturing process and chemical structure.
Polybutylene terephthalate (how's that for a mouthful) has a less uniform finish, giving PBT keycaps their signature texture. However, with specific techniques, PBT can in fact be made as smooth as ABS - the IBM two piece keycaps are PBT lowers and uppers, and the lowers are some of the smoothest plastic you will ever see. PBT is also harder to work with and more chemically complex.
Polyoxymethylene (try and pronounce it, I dare you) offers superior rigidity to PBT with lower typical friction. But you all already knew that. See, POM's primary US "trade name" is Delrin. Yes, that Delrin. Now you begin to understand the structural rigidity difference.
Acrylonitrile butadiene styrene by comparison is chemically simple, easy to work with in (most commonly sold and handled as pellets.) It also has a very low glass transition point of 105C, but no melting point, which reduces the operating costs relative to PBT and POM.

Part of the reason PBT is so expensive is that it has extreme soluble sensitivity, and a low soluble temperature of 60C (what makes it possible to dye PBT, in fact.) This is offset by a very high 260C+ temperature required to mold it. Heat requires power and power costs money - PBT can literally cost more than twice as much as ABS in this regard.
POM slots in nicely between ABS and PBT in that it offers durability comparable to PBT, lower friction, and higher rigidity. Since I used BASF Ultradur above, I'll use BASF Ultraform - 166C melting point. Meaning the energy costs for POM in fact, slot roughly between ABS and PBT. As you can guess, the 'lower friction' does in fact mean that POM has a smoother texture compared to PBT.

There's a whole bunch of other stuff that has to do with the molding method, extrusion versus pelletized, and so on that I honestly know nothing at all about. The key differences between all of them as far as keycaps go come down to durability, rigidity, and coefficient of friction.

As to which is best? Sure, ABS double-shots are definitely superior in terms of quality to ABS pad-printed. That's just chemical and mechanical fact. POM and PBT will both outlast ABS - again, that's chemistry and engineering talking.
Whether or not ABS is superior to PBT or PBT is superior to POM for your use is a strictly personal decision though. Anyone who says otherwise is trying to sell you something.

This message brought to you by "RootWyrm used to work in a building that also housed a plastics supply company and got curious about why ABS didn't melt." 

[dante]

I prefer POM over all material types because I feel it has both the best attributes of ABS (slick) and PBT (texture).

[elttaboi]

Wouldn't those be opposites?