What are your opinions on plate materials?
So far I have only used plastic plates but have a carbon fiber plate on the way.
I was wondering what the differences are and if I made a good enough choice
Tell me what plate you use on your board right now and which plate type you prefer!
I prefer brass and carbon fiber. Aluminum is a safe all-rounder that’s middle of the road (never outstanding but never terrible). I’ve had some FR4 and thin polycarbonate plates that seemed lackluster. Dunno, maybe it was the case or mouting method that caused it. I’ve never had anything but success with brass. But, as you know, that is a personal preference.
aluminum is usually a pretty safe bet if you want something cheap but not terrible.
my personal favorite has to be brass aswell tough. i have no experience with carbon fibre plates, but i like my boards heavy and a hefty, thick brass plate adds some nice weight to the board.
I would be curious to know as well. What about polycarbonate and stainless steel?
For me with my preferences leaning towards flexy builds, I definitely prefer plastics for plate material. POM would be my fav material, but polycarb plates are pretty nice too IME with them so far. POM seems to be a bit more rigid & denser than PC is why I would say I prefer it. Acrylic can actually sound really nice IME, but is a different animal than POM or PC. Acrylic plates need to be thick to prevent cracking. For metals I would have to say brass would be my favorite since I do really like it’s sound profile copper should be pretty similar, I haven’t tried it yet to confirm). Alum is a great all rounder like everyone has said & does allow a tiny bit of flex. Stainless steel I can’t say I really like for MX builds, but goes well with ALPS builds IMO. As for other materials like CF & FR4 they both seem pretty nice. IME fairly rigid typing experiences with them both & they have unique sound profile compared to plastic or metals.
At least that’s all assuming a standard plate width, almost all of those materials have been offered in 4mm variants. I have never tried thick plates myself though since I prefer at least a little flex in my builds. Then there is some innovation going on with CF plates too, I’ve seen a few cast CF plates recently. Then Jaxxstatic is making a CF plate for Rukia that has a special weave. So there is all kinds of crazy stuff out there for plate materials to try. It’s like everything else in this hobby TBH, just gotta try what you think you’ll like best & form your preferences from there.
I’d say that it all depends on what type of switches you use (linear, tactile, clicky) and mounting style, to be simple.
Plate material is about flex/typing feeling as well as sound.
I personally like brass over aluminium and can’t really decide if I like PC or not.
As for other materials - I’ve yet to try them…
Absolutely on both counts, especially mounting method. I was gonna touch on that in my post, but I focused in on just material since OP’s question was specifically about them.
I feel like asking about just plate materials is a bit reductive since the feeling of typing and sound of typing is a combination of 3 crucial factors - case, plate, and switch. Each play their own role in this waltz we call typing.
But FR4, totally FR4 
Same
Carbon fiber has a special place in my heart.
Mainly because I have worked with it building the Swedish naval vessels and there been brainwashed that it’s the superior material except the cost of it.
I’ve always joked with friends that if they want their kids to not get into drugs, get them into cycling or a hobby that takes advantage Carbon fiber so they’ll be too poor to afford drugs 
I have friends that say the same except playing magic cards lol
Hello custom mechanical keyboards.
I 3D printed a thick ~5mm ABS plastic plate and it’s overall quite rigid but has a nice little bit of flex / dampening. Sound is wonderful, solid and rather deep compared to aluminum and presumably other rigid materials.
I definitely prefer it over aluminum and 3DP PLA plastic, which are the only other types I have tried so far.
I’ve used brass, aluminum, carbon fiber, and polycarbonate plates, here are my thoughts on each:
Brass
My favorite, extremely rigid, makes for great sound amplification, and a firm bottom-out. Some might find the bottom-out feel to be too harsh though. It’s also fairly expensive compared to the other offerings.
Aluminum
Goldilocks’ plate of choice, not poor, but also not the greatest (personally), it’s fairly attainable/inexpensive and has the same properties as brass, only I find aluminum to be slightly softer and doesn’t give as large a difference to sound signature. Otherwise, it’s the safest and most vanilla choice of plate.
Carbon Fiber
A fairly malleable plate material, it’s fairly soft, moreso than aluminum, but has the sound signature of polycarbonate. Carbon fiber is also much lighter than metal plates. The bottom-out feel is also comparable to plastics. Carbon fiber is around the same price as aluminum and polycarbonate so it’s fairly inexpensive as well.
Polycarbonate
My least favorite plate material, very very malleable, extremely soft and brittle, and doesn’t give a satisfying sound nor feel. Arguably, the soft characteristics of polycarbonate make a nice typing experience, but building with polycarbonate plates with how bendy they get, and the sound results they convey in a fully-built board, would rather not choose this plate material. It’s cheap for a reason.
I agree with others that aluminum is usually sturdy enough and the price is ok. There can be a difference about how soft the typing feels. I was surprised how much of a difference it was when typing an my Ergodox (after months of not using it) which uses a acrylic plate within a wooden case. vs. my Iris with an aluminum plate. The Iris feels much harder.
I haven’t used a steel plate so far, but it is for sure pretty heavy.
I personally would rather want to try a carbon plate, just did not find a way to get them for an affordable price.