Why are ABS, PBT and POM the only major keycap materials?

[cribbit]

Obviously a lot more plastics exist. Does nothing else give the quality or price that those three can?

[JaccoW]

It mostly has to do with how keycaps are generally manufactured. Extrusion molding is/was the most common. And from the most usable materials ABS was probably the easiest to work with.
Remember, it wasn't until that long ago that Pbt or pom keycaps were really rare. And it wasn't until just 2-3 years ago that we finally saw some real double shot pbt keycaps.

As for some other plastics: Five of the most common plastics.

PET: Probably too susceptible to scratches I guess?
HDPE: It's a polymer so once it is mixed it will harden. That's a slight problem for use in extrusion machines where it might need to stay hot and soft for a longer time.
PVC: Good chemical properties but needs a plasticizer that will leak out over time.
Polystyrene: Pretty food qualities but expands into styrofoam when heated to 200C.

Generally speaking you want a thermoplastics when doing double shot keycaps and most are simply to soft.

[Lepidus]

It mostly has to do with how keycaps are generally manufactured. Extrusion molding is/was the most common. And from the most usable materials ABS was probably the easiest to work with.
Remember, it wasn't until that long ago that Pbt or pom keycaps were really rare. And it wasn't until just 2-3 years ago that we finally saw some real double shot pbt keycaps.

As for some other plastics: Five of the most common plastics.

PET: Probably too susceptible to scratches I guess?
HDPE: It's a polymer so once it is mixed it will harden. That's a slight problem for use in extrusion machines where it might need to stay hot and soft for a longer time.
PVC: Good chemical properties but needs a plasticizer that will leak out over time.
Polystyrene: Pretty food qualities but expands into styrofoam when heated to 200C.

Generally speaking you want a thermoplastics when doing double shot keycaps and most are simply to soft.

What about PC? SP PC caps feel pretty nice.

[keyladding]

POM Keycaps have been around since Cherry early days.

[JaccoW]

It mostly has to do with how keycaps are generally manufactured. Extrusion molding is/was the most common. And from the most usable materials ABS was probably the easiest to work with.
Remember, it wasn't until that long ago that Pbt or pom keycaps were really rare. And it wasn't until just 2-3 years ago that we finally saw some real double shot pbt keycaps.

As for some other plastics: Five of the most common plastics.

PET: Probably too susceptible to scratches I guess?
HDPE: It's a polymer so once it is mixed it will harden. That's a slight problem for use in extrusion machines where it might need to stay hot and soft for a longer time.
PVC: Good chemical properties but needs a plasticizer that will leak out over time.
Polystyrene: Pretty food qualities but expands into styrofoam when heated to 200C.

Generally speaking you want a thermoplastics when doing double shot keycaps and most are simply to soft.

What about PC? SP PC caps feel pretty nice.

Polycarbonate is a durable material. Although it has high impact-resistance, it has low scratch-resistance. Therefore, a hard coating is applied to polycarbonate eyewear lenses and polycarbonate exterior automotive components. The characteristics of polycarbonate compare to those of polymethyl methacrylate (PMMA, acrylic), but polycarbonate is stronger and will hold up longer to extreme temperature. Polycarbonate is highly transparent to visible light, with better light transmission than many kinds of glass.

(source)

Not that great for items that get a lot of wear. And on it's own it is a translucent material. Haven't seen that many opaque colored PC items to be honest.
I used to own their Fallout keycaps and I agree that they did feel nice.

[JaccoW]

POM Keycaps have been around since Cherry early days.

I have only really seen them on spacebars and infilled caps.

[Gajible]

POM Keycaps have been around since Cherry early days.

I have only really seen them on spacebars and infilled caps.

Original DAS boards were POM if I recall correctly, and Model Ms all used Dyesub PBT.

They're not suuuuuper rare, just more expensive.