optimal runner that would fi ll the part adequately. Each run- ner design was simulated with diff erent variables such as: tip diameter, die temperature, slow shot velocity, fast shot velocity, and start of fast shot. After the simulation, the optimal design was chosen to produce the tool so production could start. With such tight tolerances on
fl atness and surface fi nish, cooling lines were placed in specifi c areas of the cavity so a uniform temperature could be achieved. Standard practices at Imperial dictate thermal oil lines are used to pre-heat and stabilize the cavity temperatures, and water lines are used to cool the shot blocks and runner blocks. Initial simulations revealed areas of the cavities that would absorb more heat than others, indicating a need for additional cooling in these areas, such as in the standing steel near the gates. T e use of cooling lines helps
ensure as little shock as possible to the steel during the initial startup of the die and also maintains the tempera- ture during production. Due to tight dimensional tolerances, all the cores in the tool were covered with a chro- mium-based coating to help prevent tool wear and reduce soldering. T is coating also allowed the dimensions throughout the cores to be held within the required tolerance. A vacuum also was applied to reduce any gas porosity that may occur. After the initial startup of the
tool, minor porosity was discovered on the outside of the two snouts after machining. T e amount of machining stock was 0.02 in. (0.5 mm) on each side of the bore cores. T e snouts have a considerable amount of thickness, so shrink porosity would be a com- mon fi nding. To reduce that amount of porosity exposed by machining, stock was added on both the inner and outer diameters of the cores. T is reduced the amount of machin- ing stock and reduced the chances of exposing porosity. T ough the part looks simple, sig-
nifi cant challenges stemmed from the tolerances, low porosity, leak tight- ness and surface fi nish requirements, with limited changes allowed by the
Sept/Oct 2016 | METAL CASTING DESIGN & PURCHASING | 35
Shown is the outside view of the stepper motor housing. The center bore cores have a maximum machining stock on each side to reduce runout.
customer. T is brought on challenges in design and process that required extensive analysis of the part. With a focus on tool design and
quality production processes, Imperial met these obstacles, and has been pro- ducing the part at high volume, serial production rates since 2011.
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