Plastic injection molds are divided into two types on the bases of runner system.
Runner system:
Channel system that leads molten plastic from sprue to mold cavity through gate is called runner system. Following are the two types of runner systems:
- Cold runner system
- Hot runner system also known as runner less
Cold Runner System
Cold runner mold can consist of either 2 or 3 plates. The simplest style is the 2 plate system where the runner carrying the molten plastic, is machined into the mold plates. After the part is molded, both the part and the runner are ejected. In mold with three plates, the runner is machined on a separate plate so the parts are ejected alone.
Advantages and disadvantages of cold runner system
| Advantages of cold runner | Disadvantages of hot runner |
| Less expensive to manufacture | Cycle times are slower |
| Color changes can be made quickly | Potential waste from runners especially if they cannot be reground |
| Lower maintenance costs | |
| Accommodate a wide variety of polymers |
Hot Runner System
The hot runner system first came into use in the 60’s, but did not gain popularity until the 80’s and 90’s when technical advances made them more reliable, and the increases in plastics material prices made them more desirable.
Hot runner system Types
- Insulated hot runner
- Internally heated hot runner
- Externally heated hot runner
Heated Hot Runners
In heated hot runner system runners are heated with heater internally in the manifold or externally but the function is the same to keep the plastic resin in a liquid state within the tool except when it passes through the gate into the mold cavity.
Insulated Hot Runners
In this type runner system is not heated externally but it is insulated with thick layer of insulating media all around the runners.
So in hot runner system, the runner is not ejected with the finished part. Instead it stays in the mold ready to fill the cavity to make the next part. In the hot runner system, the amount of resin used is greatly reduced because there is almost no waste of resin to make the parts.
Advantages and disadvantages of hot runner system
| Advantages of hot runner system | Disadvantages of hot runner system |
| Potentially faster cycle times | More expensive to manufacture |
| Eliminates runners and potential waste | Colors cannot be easily changed |
| No need for robotics or someone to remove runners | Higher maintenance costs and potential downtime |
| Better for high volume production | May not be suited to certain types of engineering plastic resins and thermoplastic resins and sensitive materials |
The main key difference is that in cold runner both part and plastic in runner are ejected, while in hot runner only product is ejected.
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