Mold action and ejection requirements will often dictate the
use one of three special-purpose mold bases instead of the
simpler A- and B-styles.
One of these is the X-style, or stripper-plate, mold base.
Sandwiched between the “A” and “B” plates, its stripper
(“X”) plate engages the edge of a part and pushes it off the
core. Typically, the X-style sees use with round parts like
cups, caps, and containers. This style of mold base comes
in both five- and six-plate styles- -with the six-plate version
including a support plate.
The AX-style mold base is used for parts requiring core detail
in the cavity side of the mold. When the mold opens, that
core detail is pulled so that the part remains on the ejector
side of the mold. The AX-style is essentially an A-style mold
base with an “X-1” plate, located between the “A” and “B”
plates but attached to the top half of the mold so that it can
pull the part off the core detail.
The “T” style, or three-plate, mold base is used when the
molder would like to separate the part from the runner in
the tool. “T” series mold bases consist of an “A” clamping
plate, “X-1” plate, “X-2” plate, “B” plate, and the ejector
assembly and housing. Unlike the other mold bases, the “T”
series operates with two parting lines. The first parting line,
which occurs between the X-1 and X-2 plates, separates the
part from the gate prior to opening the main parting line. The
main parting line then opens and the X-1 plate is actuated to
pull the runner from the sprue-puller pin, thereby freeing the
runner and allowing it to be ejected separately from the part
being produced.See page 11 for illustrations of standard mold base types.
Choosing A Steel
Steel selection is an important aspect of specifying the right
mold base. Generally there are four standard grades of steel
available. See page 8 for mold and die steel descriptions.
Molding Machine Considerations
After you’ve picked the right style and steel for your mold
base, it’s time to consider variables related to the molding
machine: the locating-ring style, sprue bushing, and
The mold maker must select the type of locating ring that
will match the platens of the machine in which the mold will
be running. Locating rings are available in a wide variety of
configurations to fit most injection machines, but the most
common locating ring has a 3.990 in. outside diameter.
Sprue bushings must also match the machine, so be sure to
determine the proper orifice and radius of the sprue bush-
ing so it will match the machine nozzle. The most common
type of sprue bushing is made from 6145 steel that has been
hardened, ground, and polished for sprue release. In some
applications it is desirable to use a high-conductivity
copper-alloy sprue bushing.
These “high-performance” sprue bushings can cool the sprue
quickly when either the sprue weight is greater than the part
weight, or a rigid target is needed for a robotic sprue picker,
or when scrap would result from a hot sprue coming in
contact with a finished part. High-performance sprue bushings
are fully interchangeable with the standard bushings.
A number of different clamp-slot styles are available.
Whatever the style, make sure it’s compatible with the
thickness of the top clamping plate on your mold base
(ACP, “A” plate, or AX plate).
Finally, the molder needs to determine the correct mold base
height in relation to the maximum space available in the
press. A mold base that won’t run in the appropriate size of
press can turn potential profit into loss. In addition, be aware
of the maximum stroke required to eject the part for the mold.
Mold Bases and Plates
Mold Bases and Plates
Mold Bases: What Every Molder Should Know
“High-performance sprue bushings cool
the sprue quickly when either the sprue
weight is greater than the part weight,
or a rigid target is needed for a robotic
sprue picker …”