The electrical/electronic industry uses small components, like connectors and bobbins that are molded in multi-cavity molds. On the other hand, large multi-gated parts are used in the automotive industry, e.g. bumpers and dashboards. Yet both can benefit from the cost and technical advantages of hot runners.
Cycle time reduction is possible when cooling of a cold runner would determine the cycle time. Following are typical advantages and disadvantages of hot runner systems:
Production increase (cycle)
Flexible choice gate location
Critical molding conditions
Critical temperature control
Start-up problems (tailing)
Color change problems
Abrasion (reinforced plastics)
Critical mold design (no dead spots)
In selecting a hot runner system, the following factors have to be taken into account:
Number of parts
Total flow path
Residence time (esp tailing)
Taking all these factors into consideration, there is still a choice between many types and variations of hot runner manifolds and nozzles. General recommendations cannot be given. The best option depends on the thermoplastic and the requirements of the specific application.
The following guidelines should be respected:
- Natural runner balancing
- Minimal pressure-losses
- Sufficient heating capacity for manifold and each single nozzle
- Accurate, separate temperature controls for manifold and nozzle
- Effective insulation between manifold and mold
- Optimal mold temperature control
- No dead spots and flow restrictions in manifold and nozzles
- Limited residence time of melt in the hot runner
- Adequate sealing of runners
The figure below shows various basic types of nozzle configurations with their typical advantages and disadvantages.
With respect to externally and internally heated manifolds the same conclusions are applicable as for nozzles.
A relatively cheap and robust alternative for hot runners is the hot runner/ cold sprue. The hot runner manifold is followed by a short cold sprue that eliminates the use of expensive nozzles.
Advantages and disadvantages of basic hot runner configurations.