The case is a fairly complex part, as far as injected-molded plastic parts go.
Getting the mold to fill evenly is a bit of a black art. The plastic starts to cool as soon as it starts to flow into the mold. As a result, the leading edge of the injected plastic immediately begins to thicken. There is a limited time in which the mold can be filled, and the part made.
Here is an animation of the mold filling with hot ABS plastic. As the color runs from cool (blue) to hot (red) you can see how the plastic part is formed.
Click on the image to see the animation.