Nice to be appreciated! Maybe I should be doing some YouTube vids.
Ahh, yes, that grainy finish... That's been done by "spark erosion", more correctly termed electro-discharge machining (EDM). The moulds I make are "machine-finish", so you get swirl patterns from the cutter. That can be polished out, however. (I did a memorial coaster last year and the mould for that was mirror finished. I might have a photo somewhere, though it's done a bit of work since I first made it...)
With machining, you cut the negative of the part, with EDM you first cut an exact replica (copper or graphite) and then burn it into the metal with tiny lightning bolts. It's an added step in production, so adds to the cost. Normally only used for parts that require a cosmetic finish or have geometries too hard to machine. You can tell how much time someone spent making the mould by looking at the size of the speckles. The finer they are, the longer it takes. (You'll also only see the speckles on the outside. The inside surface is normally machined.)
You can do this in a home workshop as well if you want. I think the Canadian chap who does AvE (Chris?) on YouTube is building one, and "The Post Apocalyptic Inventor" (Gerolf?) is assisting with the electrics in Germany.
Some of the parts in my photo above may also look a bit "bubbly" - that's normally due to too much water content in the material. (My homebrew CNC will actually hit 10um, so the moulds are pretty good.) Choice of material makes a big difference, but for prototypes I often just use what I've got rather than sourcing specific grades. Colourants can also have a big effect, even though they're only added at 1%. (Had a big problem with the penguins due to needing different colours. Only two worked without significant bubbling.)
And yes, most of my work is substantially flat. I can go deep if needed, possibly up to 50mm at a push, but it gets harder to cut the moulds, and harder remove the part. (Flat items just lift out nicely.) The other limit is the "shot weight" of the machine - i.e., how much material you can inject in one go. I'm limited to about 40ccm, and I'd guess these table top machines are considerably less. (The penguins used about 38ccm to give you an idea. They're solid polyurethane, and about 100x 60x 10mm in size.)
I did the inside housings for these as well:
http://www.kielowatts.co.uk/product/contact-colour-optionsThey're the black square in the mouldings photo with the 8 slots through them for the cap-touch contacts. Nice example of how plastics and electronics come together. (I ended up doing the circuitry and PCB layout for the first version as well. All old school 4000 series chips as he didn't want to use a microcontroller, but that's another story!)