INVESTMENT CASTING - IMMENSE COST SAVINGS POTENTIAL
Investment casting, or the lostwax process (a term still applied to the production of jewelry and works of art), has the distinction of being not only the oldest metal process known to man, but also the most advanced technology in the sphere of metal forming techniques employed by manufacturing industry today.
The industrial use of investment casting started during World War II when the demand for critical aircraft parts outstripped the ability of the country's machine shops to provide them. A turbine blade about 3 inches tall, weighing less than one-quarter pound, in heat resistant stainless steel, is considered to be one of the first production applications. Investment casting was able to fill the bill and the process grew rapidly during and after the war years as many new applications were found.
This year aerospace and ordnance related parts will represent about half of the $1.6 billion worth of investment castings produced in the U.S.A. The remaining $800 million is comprised of an amazingly wide range of complex shapes in virtually any alloy imaginable, appealing to just about every industry.
The following partial list will provide insight to the broad acceptance of the investment casting manufacturing method.
- Aircraft Airframe
- Aircraft Engine
- Aircraft Fuel Controls
- Agricultural Machinery
- Automotive Parts
- Business Machine Components
- Chemical Equipment and Controls
- Electrical Switchgear
- Food Processing Equipment
- Farm Implements
- Gas Turbines
- Hand Tools
- Industrial Valves
- Marine Hardware and Instruments
- Medical Instruments and Equipment
- Mining Machinery
- Navigational Equipment
- Orthopedic Implants
- Oil Field Equipment Components
- Packaging Equipment
- Precision Machining Equipment
- Pneumatic Nailers and Staplers
- Permanent Magnets
- Pumps and Air Compressors
- Small Arms-Rifles
- Security Systems
- Sports Equipment
- Surgical Instruments
- Textile Machinery
- Wood Cutting Tools
Designers like to specify investment castings because of the design flexibility it gives them and the close dimensional tolerances which they make available. In addition, they can utilize metals which are difficult or impossible to machine.
While parts are still in the design stage, understanding of the basic production techniques used to make investment castings will enable engineers and designers to take full advantage of this method of producing complex precision parts quickly and economically.
The investment casting process begins with the production of a wax pattern with the exact geometry of the finished casting. This pattern is usually made by injecting liquid wax or plastic into a metal die cavity. During construction, the die cavity is made oversize, as required, in order to compensate first for volumetric shrinkage of the hot wax as it cools and later for contraction of the molten metal as it solidifies in the mold. An investment casting wax pattern die may be a simple hand-filled split mold or may be a fully-automated tool with one or more cavities.
An investment casting "gate" is a projection added to the pattern. It serves three functions: it holds the patterns to the runner system or sprue to form a cluster; provides a passage for the wax to be drained out after the mold is formed; and guides molten metal into the mold cavity.
Patterns fastened to the gate by one or more runners are attached to a sprue and pouring cup, all of which are made of wax. Patterns, runners, sprue and pouring cup comprise the mold cluster.
The ceramic-shell, mold-making process starts by dipping the entire wax cluster into a fine ceramic slurry and then coating it with a fine ceramic sand. After the initial coating dries sufficiently, the cluster is dipped into progressively coarser grades of ceramic material. When the shell is between 3/16" and 5/8" thick, it is placed in a heated environment that melts the wax pattern gates, runners, and sprue. The remaining ceramic shell contains dimensionally precise cavities of the casting shape desired.
Shell molds must be "fired" to burn out the last traces of pattern wax and to assure permeability as well as fired strength. They are generally ready for pouring after a few hours in the furnace.
After the molten metal has been poured and cooled, the ceramic mold material is broken away from the cast cluster. The individual castings are cut away from the runners by abrasive cut-off wheels. Finally, the gate protrusions are removed and the castings are cleaned and ready for inspection and any specified secondary operations.
Advantages of the investment casting process include:
- Reproduction of intricate shapes with fine detail
- Elimination or reduction of machining requirements
- Minimal generation of scrap materials (runners, gates, etc., can be remelted)
- Combining the features of an assembly into one casting
- Use of alloys which are difficult to machine
- Manufacturing parts with superior density, uniformity, and strength
- As-cast surface finishes of 70 to 90 microinches on non-ferrous parts and 100 to 125 microinches on ferrous parts
In addition to the unfavorable cost of the six-part machined fabrication method, inventory control and coordination of several vendors was a constant problem.
While the investment casting process is a more exacting process than conventional foundry processes, it should be kept in mind that dimensional limitations do exist due to variables that can only be controlled to a reasonable extent.
Tight tolerances can be held as part of the casting process. As in most manufacturing methods, costs increase as tolerances become tighter. According to the Investment Casting Institute, normal linear tolerances are +/-.010" for dimensions up to 1", with +/-.005" for each additional inch thereafter. These tolerances apply to all casting dimensions and will result in the lowest possible production cost.
Tighter premium tolerances can be supplied (at a cost), usually through secondary operations. Premium tolerances normally apply only to specified dimensions. Typical premium tolerances might be +/-.003" on dimensions up to 1/2", +/-.005" on 1", +/-.008" on 2", with an additional +/-.002"/in. up to 5", and an additional +/-.001 "/in. thereafter.
The individual design characteristics of your particular part, such as the relationship of holes, slots, bosses, and sectional thickness variations, play an important role in establishing attainable production tolerances. It would be wise to review the critical aspects of your part with your investment casting foundryman before finalizing the dimensions of the casting.
With the aid of robots and other process equipment improvements, investment castings that weigh in excess of 200 pounds each are in production today. It would be reasonable to estimate that 90% of all investment castings produced have a weight of less than 40 pounds.
In the face of sky-rocketing production costs, present users especially value the immense cost savings potential of the investment casting process. Your best place to start is with a part that requires considerable machining. Review the critical dimensions, relax tolerances in noncritical areas, and send the drawing out for quotation. Why delay investigating whether or not the benefits of the investment casting process can be employed in your cost reduction programs?
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