Hot runner systems


The ideal injection molding system delivers molded parts of uniform density, and free from all runners, flash, and gate stubs. To achieve this, a hot runner system, in contrast to a cold runner system, is employed. The material in the hot runners is maintained in a molten state and is not ejected with the molded part. Hot runner systems are also referred to as hot-manifold systems, or runnerless molding.

Types of hot runner systems
There are two types of hot runner systems:

Insulated runners  
Insulated runner molds have oversized passages formed in the mold plate. The passages are of sufficient size that, under conditions of operation, the insulated effect of the plastic (frozen on the runner wall) combined with the heat applied with each shot maintains an open, molten flow path.

Heated runners  
For heated runner systems, there are two designs: internally heated and externally heated. The first is characterized by internally heated, annulus flow passages, with the heat being furnished by a probe and torpedo located in the passages. This system takes advantage of the insulating effect of the plastic melt to reduce heat transfer (loss) to the rest of the mold. The second consists of a cartridge-heated manifold with interior flow passages. The manifold is designed with various insulating features to separate it from the rest of the mold, thus reducing heat transfer (loss).

Table 1 lists advantages and disadvantages of the three hot runner systems, which are sketched in Figure 1.

TABLE 1. Advantages and disadvantages of hot-runner systems
Hot Runner Type  Advantages  Disadvantages 
Insulated   Less complicated design.
 Less costly to build.
 Undesired freeze-up at the gate.
 Requires fast cycle to maintain melt state.
 Long start-up periods to stabilize melt temperature.
 Problems in uniform mold filling.
Internally Heated   Improved distribution of heat.
 Higher cost and complicated design.
 Requires careful balancing and sophisticated heat control.
 Should take into account thermal expansion of various mold components.
Externally Heated   Improved distribution of heat.
 Better temperature control.
 Higher cost and complicated design.
 Should take into account thermal expansion of various mold components.

FIGURE 1. Hot runner system types: (a) the insulated hot runner, (b) the internally heated hot-runner system, and (c) the externally heated hot-runner system.