"What I don't fully understand is the concept of "slides" in a mold."
Using an Athearn 40 foot box car kit as an example, the following might
help to understand the concept. Keep in mind that I'm a modeler, not an
injection molding engineer, so my terminology might not be correct, but you
should be able to get the general idea.
The molds for the kit could consist of several movable pieces:
1) A mold for the interior of the car
2) Molds for each side of the car
3) Molds for each end of the car
4) A mold for the roof of the car.
Most, if not all of these components of the entire mold would be considered
"slides". Each of them slides into the proper position so the shell of the
car can be cast, and the hot styrene is injected into the mold. As soon as
the casting (molding?) has cooled, the slides move away from the casting
allowing the casing to be ejected from the machine. The slides move back
into position and the process is repeated for the next casting.
If it's in your library, there's a picture of a multipart slide mold at the
bottom of page 4 of Kalmbach's "Building Plastic Railroad Models" (ISBN
0-89024-540-1), printed in 1979.
Hope this helps,