Plastic Injection Mold is an assembly of parts containing within it an impression into which plastic material is injected and cooled. It's the impression which gives the moulding its form. The impression may, therefore, be defined as that part of the mould which imparts shape to the moulding.
The impression is formed by two mould members:
Cavity and core plates: This is illustrated for a simple container, the basic mould, in this case, consists of two plates. Into one plate is sunk the cavity which shapes the outside form of the moulding and is therefore known as the cavity plate.
Similarly, the core which projects from the core plate forms the inside shape of the moulding. When the mould is closed, the two plates come together forming a space between the cavity and core which is the impression.