Yes, calculators are made from plastic ^^

[Stefan]

Here is a PCB shot of the Casio fx-991DE-Plus I took a while ago.
(I am actually not sure whether this is the DE-Plus or the ES-Plus, because the PCBs are the same.) I guess they only differ in the chip under the black resin. Gosh, I would love to know whether there is a hidden JTAG port or similar under the resin and whether they used some rewritable program memory :-D
The PCB in the Casio is also fixed with these molten plastic domes to the front. I removed them with a knife, but there is nothing interesting on the front side of the PCB.

One thing which would interest me: Does the HP-300s+ also gets rather unresponsive with big expression? For example hit 10 times the sqrt key and then type 111111 (or any other digit) rather fast, then the casio only catches 111 or so. I find that extremely annoying, and can be uncomfortable in exams when time is short.

The 300s+ PCB looks much more clean than the one in the Casio. It's funny, because the 300s+ is actually ~5$ (or ~25%) cheaper than the Casio. Also the HP has large ground (or VCC)planes everywhere, whereas the Casio has just empty space. Also look at the vias on the casio: On the casio they are made from the same black resistive material as the button sensing areas on the front of its PCB
The HP uses metal vias and metal sensing areas for the buttons (as one can see at the reset button which is on the back of the HP)