Investment casting

A brief process description

The origin of investment casting or lost wax processing is southeast Asia, and reaches back to the time between 4500 and 4000 BC. Archeological findings from the bronze period, approximately 2500 BC, confirm the use of lost wax processing. At that time, models were made of beeswax, wrapped in clay, and dried. The wax was then melted out and the clay form was filled with molten metal. To date, there have been no changes to this basic principle. Just that other materials are used, which allow for better surfaces, temperatures and precision.

Investment casting is used today, when the workpieces require thin walls,
the highest dimensional precision, top surface qualities, or complex contours. Investment casting is also done when alloys are used that are either difficult or impossible to process mechanically.

The process begins with the construction of the injection molding tools which will be used later to produce the wax molds (ill. 1). ZOLLERN
produces these tools themselves, made of either aluminum or steel, depending on the quantity. But before this phase, ZOLLERN technicians advise the customers on issues pertaining to economical production, feasibility, and last but not least, the proper alloy selection. After the tools have been produced, a special wax mixture is injected into the aluminum or steel tool on an injection molding machine, to create a one-to-one model of the intended workpiece. This wax model is adhered to a wax tree, which is then attached to a model cluster (ill. 2).

This is  mechanically immersed into the slurry, a ceramic mass made of fire-proof powder and a binding agent, adhered with fire-proof sand. This process is repeated until the mold has the desired stability and ceramic strength (ill. 3).

The wax is subsequently melted away from the model in a melt-out oven, and the ceramic form is fired to between 900 and 1200 degrees. Simultaneously, the selected alloys are combined from the various elements and put into the smelter on the oven. While the smelter is heating the alloy to the correct casting temperature, the ceramic molds are also pre-heated prior to casting, to avoid any danger of cracking due to the extreme temperature variation (ill. 4).

There are three options for the subsequent casting (ill. 5). One option is "open casting." In this case, the molten alloys are poured into the ceramic mold with a crucible. The second option is "vacuum casting." This is used when oxygen-stabilized alloys are being cast. In a closed casting plant, a negative pressure prevents oxygen from coming into contact with the alloy atmosphere which would promote oxidation. The third option is the "low-pressure casting technique." In this case, the molten alloy is slowly and evenly cast or pressed into the ceramic mold by means of a riser. After the ceramic mold is cooled, a water stream or pneumatic hammer destroys the ceramic, and the finished casted pieces appear on the model cluster. The individual components are separated from the model cluster in the separation unit, so that the finishing process can proceed (ill. 6).