Edited by Leslie Gordon
Until only a few years ago, investment casting was not a viable way for die casters to produce prototype parts to replicate die-casting magnesium without the use of expensive tooling. That’s because molten magnesium reacts with the fused-silica mold shell. To solve this problem, investment-casting foundry Aristo-Cast Inc., Almont, Mich., has developed a proprietary inhibitor treatment it uses with lost wax investment casting to create prototype magnesium parts that come close to die-cast quality and dimensional integrity, and with similar physical attributes. Better yet, the technique eliminates the need for expensive tooling and slashes the time to build prototypes from weeks or months to days.
For a brief refresher on lost wax investment casting, recall that the technique makes near-net parts that have complex shapes and thin walls. First, a wax pattern is created using various rapid-prototyping methods or an inexpensive injection mold. The replica of the part is given to technicians who assemble the patterns onto a post or a sprue, which is fastened to a pouring cup. Workers dip completed assemblies into a cleaning bath and then into a slurry. From there, wet assemblies are either inserted into a fluidized sand bed or a “rainfall sander,” where sand particles stick to the wet surface. After the assemblies dry, robots are used to add layers of progressively coarser material until the needed shell thickness is built up. The wax is melted, leaving a precise shell or cavity, which workers fill with molten metal from a transfer ladle. When the metal cools, a vibratory process or waterjets crack-off the shell. Workers remove the individual castings with high-speed cut-off wheels and clean the parts in a salt bath. They remove gates by grinding and perform a final cleaning using a variety of finishing machines.
Aristo-Cast provides prototype magnesium castings to a variety of industries including automotive, aerospace, military, and medical. For example, the process was used to build a prototype of a windshield-wiper motor for Trico Products, Rochester Hills, Mich. The first step entailed evaluating whether the parts that made up the motor could be manufactured to Trico’s satisfaction using the magnesium-prototype investment-casting process. At this stage, meetings took place between the two companies to make sure that the investment casting could be made using die casting for high-volume production.
Next the design team selected a method to manufacture the pattern for the investment casting. Aristo-Cast used its 3D wax printers to produce the first five sets of motor components. Once the initial prototype run was approved, Aristo- Cast constructed low-cost injection molds to supply the limited production run of 50 motors. The project was completed in less than six weeks.