Mold Bases and Plates
Special-Purpose Machines 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.
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. “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 …”
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 clamp slots. The mold maker must select the type of locating ring that will match the platens of the machine in which the mold will
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