Investment Casting - Cutting Edge of Modern Production Metallurgy
The investment casting process is the most ancient of the metalcasting arts and at the same time, also the most modern. Investment casting was a viable technology before the pyramids where built by the Egyptians. Its earliest known applications were in China during the Shang Dynasty (1766 to 1122 BC). These artisans were skilled enough to create art works of bronze with delicate filigree able to rival the investment castings of today. The next surviving use of investment casting was during the Aztec Civilization. The few castings that have survived show a complete mastery of jewelry crafting technique. The earliest known foundry text was written by a monk, Thophilus Presbyter, in 1100 A.D. and was the guide used by the Italian sculptor, Benevenuto Cellini. In 1540, Cellini was able to cast a 3 1/2-ton bronze statue of Perseus and Medusa's Head which is still viewed today. Investment casting was rediscovered during the early part of the Industrial Revolution. It was used to make dental fillings and inlays. It was World War II, however, that jump-started the investment casting industry. A sudden and urgent demand came about for finished and precise parts for aircraft and arms. Information from the dental and jewelry industries was employed to produce investment castings in inconceivable quantities. After the war, the industry was able to expand into many other areas where complex metal parts were needed. By 1953, the investment casting industry began to standardize and the Investment Casting Institute was formed.
Although the process is an extremely old art, investment casting is the cutting edge of modern production metallurgy. This can be attributed to its great versatility. A casted part can be varied to create the most delicate, or the most massive, product imaginable. Investment castings are at work in the subzero vacuum of space and in the fiery flame chamber of jet aircraft. No other metallurgical method combines the unique properties of investment casting. The advantages to industrial applications of investment casting are too numerous to mention, however, it is worth reviewing several of the benefits for comparisons sake.
Of course, two factors of importance are cost and increased manufacturing capacity. It is easy to compare the expense of a component using an investment casting as the raw material against the cost of another method. In all business, optimum total cost is a goal. This leads us to increased manufacturing capacity. Many times this is a hidden advantage of investment castings. Casting a product may leave 90% or more of the final shape available immediately. Machining is minimal with investment castings.
Investment casting offers almost unlimited design flexibility. Even complex assemblies can be cast as a unit. Holes, slots, bevels, serrations, thin sections, knife-edges and other configurations can be produced. Examples of the diversity offered by investment casting will be addressed later.
The close dimensional tolerances that can be obtained by this method of production are superior to all other production methods. Many parts can be used "as cast" with no additional machining. Investment casting also can be so adaptable as to offer better tolerances than those shown as standard or premium.
Due to the vast alloy selection possible with investment casting, engineers and designers can build parts the way they should be built. This may also allow an upgrade of the part to include properties of a stronger, less machinable alloy. There are five basic areas that should be examined before an alloy choice is made:
- Investment castings are used in such diverse environments as air, fresh and seawater, hot gases, corrosive chemicals and space. Temperature variation can be from cryogenic to 2200F. The parts may be exposed to a particular environment for years or moments. This exposure may be continuous or occasional.
- Mechanical properties are another consideration in alloy selection. Evaluation of hardness, short time tensile properties, stress rupture and creep properties, impact strength, wear and corrosion resistance against the environments mentioned above will narrow the field of usable alloys.
- Each alloy had its own unique foundry characteristics. This uniqueness becomes increasingly important when considering the size and/or complexity of the casting. Castings with thin sections require an alloy with good fluidity and intricate castings will require an alloy with high resistance to hot tearing as well as good fluidity. Castability is another important factor to be considered in the selection of an alloy. How castability factors are handled will determine the degree of production success attained.
- Fabricating characteristics consist of heat treatment, machining and grinding, welding and straightening. Most investment casting alloys require some type of heat treatment before use. The type of heat treating required varies by alloy. Welding is another fabricating characteristic that varies by alloy. Some alloys are designed to be more readily welded than others.
- The final consideration in the choice of an alloy is cost. Cost is a factor in all four of the other categories. Material cost is of minor importance compared with the costs of the environmental and manufacturing considerations. There are times when selecting an expensive alloy on a pound-for-pound basis will result in a less costly finished part: the more costly alloy may be easier to cast, hold more consistent dimensions, resist hot tearing and have a better as-cast finish. The more expensive alloy may have a longer part service life or higher strength-to-weight ratios. It is wise to consider all the options when choosing an alloy.
The diversity offered by the investment casting process is almost limitless. In the Investment Casting Handbook (1980) there are twenty-one (21) different applications listed for investment casting. Some of these are as follows:
Aircraft: Airframes This application utilizes many of investment casting's strengths: dimensional tolerance, good-as-cast surfaces, structural integrity to meet x-ray requirements, and alloys that are unmachinable and can operate at high temperatures. Engines Utilizes resistance to corrosion, high temperature tolerance, weight control issues and more strength per pound. Fuel Systems Utilizes weight saving design with functionality. Instruments Utilizes weight reduction by the alloy selected. Aerospace Utilizes the one-piece casting capabilities and allows freedom of design. Computers Utilizes the wide design flexibility. Electrical Motors and Switch Gears Utilizes the good electrical conductivity that alloy selection can offer. Electronics Utilizes the one-piece investment casting with attention paid to the quality of appearance. Navigational Systems Utilizes simple "wax welding" to allow patterns to be assembled into complicated structures. Food Machinery Utilizes stainless steel to solve machining difficulties and high waste. Gas Turbine Utilizes the capabilities of high temperature and stress levels. Machine Tools Utilizes single unit construction and superior wearing qualities. Medical and Dental Utilizes corrosion resistance and extreme detail to finished dimensions. Pumps and Air Compressors Utilizes higher pressure ratings which encourages resistance to shock and deformation. Small Arms and Rifles Utilizes savings in manufacturing costs. Textile Machinery Utilizes a wide alloy selection and design freedom. Typewriters and Office Equipment Utilizes superior mechanical quality. Weapon Systems Utilizes superior density and integrity to allow thin sections with exceptionally sharp detail.
Since 1970 Delvest has honed the delicate blend of ancient skill and modern technology to produce state-of-the-art precision investment castings. The combined work history of its management staff exceeds 150 years of experience with the "Lost Wax" method. This process allows maximum freedom in gating and cluster assembly, both of which are critical to casting integrity and cost effectiveness. Conveniently located in West Chester, PA. at a 35,000 square foot plant, Delvest is committed to maintaining the highest quality standard in the investment casting industry. While, at the same time, providing customers with quality products, on-time delivery and competitive pricing.
A team of experienced-management is on hand to provide you with the ultimate in craftsmanship from design to complete part. Master pattern makers, design staff, quality control inspectors and dedicated employees balance their efforts. This commitment to excellence has earned Delvest the reputation of a "full-service foundry".
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