Overview
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
InsulateLess 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.
Hot Runner SystemsFIGURE 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.
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