Hi Alan
Most molds are in two halves that clamp together with the plastic injected into the space between. As they have to pull apart, clearly with that method there are limitations to what shapes you can achieve. Slide moulding adds another dimension, literally - a section of the mold that slides in from the side as well, enabling, for instance, hollow gun barrels to be produced. Dragon were one of the first to do this, Trumpeter and
Tamiya more recently. Revel has started using this process as well. As yet I am not aware of
Airfix adopting it.