Investment Casting Overview
Investment casting is an industrial process which employs in-process control at every point. Highly refined on-line process control methods are backed up with laboratory skill. Every casting shipped--and this applies equally to orders for a dozen castings or half a million--can be relied on to meet the designer's performance specification.
The principles of investment casting are the same for both the solid mold and shell processes. It is only in the method of forming the ceramic mold that they differ. Both require a pattern, gating to a central sprue, a ceramic mold (either solid or shell), removal of pattern by melting, pouring metal into the cavity left by the melted pattern, removal of mold material from the cast cluster and cutting of castings from the sprue.
The process begins with the production of a one-piece heat-disposable pattern. This pattern is usually made by injecting wax or plastic into a metal die. These dies may range from a simple, hand-operated single cavity tool to a fully automated multicavity tool, depending on production quantities and complexity of the part.
A heat-disposable pattern is required for each casting. These disposable patterns have the exact geometry of the required finished part, but they are made slightly larger, to compensate for volumetric shrinkage (a) in the pattern production stage and (b) during solidification of metal in the ceramic mold.
The pattern carries one or more gates which are usually located at the heaviest casting section. The gate has three functions:
Patterns are fastened by the gate to one or more runners and the runners are attached to a pouring cup. Patterns, runners and pouring cup comprise the cluster or tree, which is needed to produce the ceramic mold. The number of runners per section and their arrangement on the pouring cup can vary considerably, depending on alloy type, size and configuration of the casting.
The Ceramic Shell Mold Process
This process has largely replaced the original block or solid mold process except for some smaller high volume parts and for some aluminum casting where the mold is plaster bonded. The ceramic shell mold technique involves dipping the entire cluster into a ceramic slurry, draining it, and then coating it with fine ceramic sand. After drying, this process is repeated again and again, using progressively coarser grades of ceramic material until a self-supporting shell has been formed.
The shell may run from 3/16 to 5/8 in. thick. The coated cluster is then placed in a high temperature furnace or steam autoclave where the pattern melts and runs out through the gates, runners and pouring cup. This leaves a ceramic shell containing cavities of the casting shape desired with passages leading to them. The cluster is dipped into a ceramic slurry and then, after draining and whilst the slurry coating is still wet, the cluster is coated with dry sand i.e., the stuccoing operation. This is repeated until the shell reaches the correct thickness.
The ceramic shell molds must be fired to burn out the last traces of pattern material and to preheat the mold in preparation for casting usually in the range 1600-2000 F. Because shell molds have relatively thin walls they can be fired and ready to pour after a few hours in the furnace. The hot molds may be poured utilizing static pressure of the molten metal head, as is common in sand casting, or with assistance of vacuum, pressure and/or centrifugal force. This enables the investment casting foundry to reproduce the most intricate details and extremely thin walls of an original wax or plastic pattern. Melting equipment employed depends on the alloy. For non-ferrous alloys, gas fired or electric crucible furnaces are usually used. For melting of, ferrous alloys, high frequency induction furnaces are most commonly used.
After the poured molds have cooled, the mold material is removed from the casting cluster. This is done by mechanical vibration and chemical cleaning. Individual castings are then removed from the cluster by means of cut-off wheels and any remaining protrusions left by gates or runners are removed by belt grinding. The casting is then ready for secondary operations: heat treating, straightening, machining and whatever inspection is specified. Then, following any secondary operations, the castings are ready to ship.