Runner Geometry of Conventional Mold
Runner systems convey the molten material from the sprue to the gate.
The section of the runner should have maximal cross-sectional area and
minimal perimeter. Runners should have a high volume-to-surface area
ratio. Such a section will minimize heat loss, premature solidification
of the molten resin in the runner system, and pressure drop.
The ideal cross-sectional profile for a runner is circular. This is
known as a full-round runner, as shown. While the full-round runner is
the most efficient type, it also is more expensive to provide, because
the runner must be cut into both halves of the mold.
A less expensive yet adequately efficient section is the trapezoid. The
trapezoidal runner should be designed with a taper of 2 to 5° per
side, with the depth of the trapezoid equal to its base width, as shown.
This configuration ensures a good volume-to-surface area ratio.
Half-round runners are not recommended because of their low
volume-to-surface area ratio. The Three Conventional Runner Profile
illustration shows the problem. If the inscribed circles are imagined to
be the flow channels of the polymer through the runners, the poor
perimeter-to-area ratio of the half-round runner design is apparent in
comparison to the trapezoidal design.
Runner Diameter Size
Ideally, the size of the runner diameter will take many factors into
account — part volume, part flow length, runner length, machine
capacity gate size, and cycle time. Generally, runners should have
diameters equal to the maximum part thickness, but within the 4 mm to 10
mm diameter range to avoid early freeze-off or excessive cycle time. The
runner should be large enough to minimize pressure loss, yet small
enough to maintain satisfactory cycle time. Smaller runner diameters
have been successfully used as a result of computer flow analysis where
the smaller runner diameter increases material shear heat, thereby
assisting in maintaining melt temperature and enhancing the polymer
flow. Large runners are not economical because of the amount of energy
that goes into forming, and then regrinding the material that solidifies
within them.
Runner Layout
Similar multicavity part molds should use a balanced "H"
runner system, as shown in the Runner System Layouts illustration.
Balancing the runner system ensures that all mold cavities fill at the
same rate and pressure. Of course, not all molds are multicavity, nor do
they all have similar part geometry. As a service to customers, Dow
Plastics offers computer-aided mold filling analysis to ensure
better-balanced filling of whatever mold your part design requires.
Utilizing mold filling simulation programs enables you to design molds
with:
- Minimum size runners that deliver melt at the proper temperature,
reduce regrind, reduce barrel temperature and pressure, and save
energy while minimizing the possibility of material degradation.
- Artificially balanced runner systems that fill family tool
cavities at the same time and pressure, eliminating overpacking of
more easily filled cavities.
Cold Slug Wells
At all runner intersections, the primary runner should overrun the
secondary runner by a minimum distance equal to one diameter, as shown
in the Recommended Design of a Cold Slug Well illustration. This
produces a feature known as a melt trap or cold slug well.
Cold slug wells improve the flow of the polymer
by atching the colder, higher-viscosity polymer moving at the forefront
of the molten mass and allowing the following, hot, lower-viscosity
polymer to flow more readily into the mold-cavity. The cold slug well
thus prevents a mass of cold material from entering the cavity and
adversely affecting the final properties of the finished part.
Runnerless Molds
Runnerless molds differ from the conventional cold runner mold (see
Conventional Cold Runner Mold illustration) by extending the molding
machine's melt chamber and acting as an extension of the machine nozzle.
A runnerless system maintains all, or a portion, of the polymer melt at
approximately the same temperature and viscosity as the polymer in the
plasticating barrel. There are two general types of runnerless molds:
the insulated system, and the hot (heated) runner system.
Insulated Runners
The insulated runner system (see Insulated Runner Mold illustration)
allows the molten polymer to flow into the runner, and then cool to form
an insulating layer of solid plastic along the walls of the runner. The
insulating layer reduces the diameter of the runner and helps maintain
the temperature of the molten portion of the melt as it awaits the next
shot.
The insulated runner system should be designed so that, while the
runner volume does not exceed the cavity volume, all of the molten
polymer in the runners is injected into the mold during each shot. This
full consumption is necessary to prevent excess build-up of the
insulating skin and to minimize any drop in melt temperature.
The many advantages of insulated runner systems, compared with
conventional runner systems, include:
- Less sensitivity to the requirements for balanced runners.
- Reduction in material shear.
- More consistent volume of polymer per part.
- Faster molding cycles.
- Elimination of runner scrap — less regrind.
- Improved part finish.
- Decreased tool wear.
However, the insulated runner system also has disadvantages. The
increased level of technology required to manufacture and operate the
mold results in:
- Generally more complicated mold design.
- Generally higher mold costs.
- More difficult start-up procedures until running correctly.
- Possible thermal degradation of the polymer melt.
- More difficult color changes.
- Higher maintenance costs.
Hot Runners
The more commonly used runnerless mold design is the hot runner system,
shown in the Hot Runner Mold illustration. This system allows greater
control over melt temperatures and other processing conditions, as well
as a greater freedom in mold design — especially for large,
multicavity molds.
Hot runner molds retain the advantages of the insulated runner over the
conventional cold runner, and eliminate some of the disadvantages. For
example, start-up procedures are not as difficult. The major
disadvantages of a hot runner mold, compared with a cold runner mold,
are:
- More complex mold design, manufacture, and operation.
- Substantially higher costs.
These disadvantages stem from the need to install a heated
manifold, balance the heat provided by the manifold, and minimize
polymer hang-ups.
The heated manifold acts as an extension of the machine nozzle by
maintaining a totally molten polymer from the nozzle to the mold gate.
To accomplish this, the manifold is equipped with heating elements and
controls for keeping the melt at the desired temperature. Installing and
controlling the heating elements is difficult. It is also difficult to
insulate the rest of the mold from the heat of the manifold so the
required cyclic cooling of the cavity is not affected.
Another concern is the thermal expansion of the mold components. This
is a significant detail of mold design, requiring attention to ensure
the maintenance of proper alignment between the manifold and the cavity
gates. (For more information on thermal expansion, see the information
on thermal stress analysis in Thermal Properties.)
Currently there are many suppliers and many available types of
runnerless mold systems. In most cases, selection of such a system is
based primarily on cost and design limitations — be careful in
evaluating and selecting a system for a particular application.
