Types of mould in plastic injection moulding

Plastic Injection moulds can be classified into 2 types – A cold runner mould and a hot runner mould. They can further be single cavity moulds or multi-cavity moulds. Types of Injection Mould Based on Mould Opening and cold runner system –

A Two plate plastic injection mould consists of one parting plane (parting line). At this plane, the mould splits into two halves (1 cavity side and 1 core side) In this type of plastic injection mould the runner system must be located on this parting plane.
Usually, in a two-plate injection mould, the gates are on or around the edge of the parts. 

The illustration below shows a 2 cavity mould with the gates at the edges of the part. 

2 Plate Injection moulds are usually constructed with a cold runner design and these runners need to be reground for use. The construction of a Three Plate Injection mould consists of two parting planes (parting lines) and the mould splits into three sections when opening and before the part is ejected. The runner system can be located on one parting plane and the part on the other parting plane.
The advantage of a Three plate injection mould is that we have the flexibility in the gating location. The part can be gated anywhere along its surface.

A Typical 3 plate mould construction with cold runner
3 Plate Injection moulds are also constructed with a cold runner design and these runners need to be reground for use.

Hot Runner Systems mould ( Runner less moulding)
Moulds that are designed with a Hot Runner System (HRS) eliminate the entire runner system. The main task of the hot runner system is to maintain the flow of the material with a constant temperature and constant pressure from the injection moulding machine barrel into a closed mould to form the part. This eliminates the runner and saves the cost of material that would have otherwise been collected and reground for reuse. Most HRS are used in multi-cavity moulds or in moulds when the part is a large size and the runner flow length is more.

What are the advantages & disadvantages of the Hot Runner System technology…

Reduced wastage of raw material in the form of runners and sprue.
Faster cycle times can be achieved.
Prevents the overfilling or short fill of the mould cavity
Prevents sink marks and flashes on the component
A wide range of components can be produced with high quality

Initial tooling costs are high
Maintenance costs are high
Colour changes are slow
Thermal-sensitive materials are not suitable for Hot runner systems.

Main components in a Hot runner system

The Manifold is the key part of any Hot runner system. The manifold distributes the molten material from the Injection moulding machine to the various nozzles. The manifold also holds the nozzle in the correct position in the mould assembly.
The Nozzle is another key component in the Hot Runner System which is designed to inject the plastic material into the cavity of the mould.

The Nozzles are classified into 3 types –
The free flow nozzle – here the plastic material flows freely in the hot runner system into the mould cavity.
The Torpedo Nozzle – Here the material flows through a specially designed torpedo tip into the cavity of the mould.
The Valve Gate nozzle- as the name suggests, the valve gate controls the entry of the plastic material into the cavity of the mould. The valve gate pin is operated sequentially rather than simultaneously for the flow of the material into the mould cavity.

Why are Injection Moulds so Expensive?

A well-designed plastic injection mould is subject to very high pressure during the moulding operation on the injection moulding machine for every cycle. The mould construction is done by selecting the right tool steels and machining with tight tolerances and assembling. Generally, the life of the mould is estimated at 1 million shots without deformation. The cost of the mould is expensive due to the various material costs and machining costs involved in the process of manufacturing a mould. An additional cost is incurred when moulds have to be designed with a good hot runner system.