The injection mould is an assembly of part containing within it an impression into which plastic material is injected and cooled. It is the impression which give the moulding its form. The impression may,therefore,be defined as that part of the mould which impart shape to the moulding.
The impression is formed by two mould members:
1. The cavity, which is the female portion of the mould, gives the moulding its external form.
2. The core, which is the male portion of the mould, form the internal shape of the moulding.
Cavity and Core Plate
The basic mould in this case consists of two plate. Into one plate is sunk the cavity which shape the outside form of the moulding and is therefore known as the cavity plate. Similarly, the core which project from the core plate forms the inside shape of the moulding. When the mould is closed, the two plate come together forming a space between the cavity and core which is the compreesion.
During the injection process plastic material is delivered to the nozzle of the machine as melt; it is then transferred to the impression through a passage. The material in this passage is thermed the sprue, and the bush is called a sprue bush.
Runner and Gate
The materials may be directly injected into the impression through the sprue bush or for mould containing several impression ( Multi-Impression mould ) it may pass from the sprue bush hole through a runner and gate system before entering the impression.
If the material is to pass without hindrance into the mould the nozzle and sprue must be central to the machine and this can be achieved by including a locating ring.
Guide Pin and Bushing
To mould an even-walled article it is necessery to ensure that cavity and core are kept in alignment. This is done by incorporating guide pillars on one mould plate which then enter corresponding guide bushes in the other mould plate as the mould closes. The size of guide pillar should be such that they maintain alignment irrespective of the applied moulding force; this they are normally able to do.