Investment Casting (19.04.2011-15.00)
Investment casting is actually a very old process-used in ancient China and Egypt are more recently performed by dentists and jewelers for a number of years. It was not the end of World War 11, however, that it attained a significant degree of industrial importance. Products such as rocket components and jet engine turbine blades required the fabrication of high-precision complex shapes from high-melting-point metals that are easily machined. Investment casting offers almost unlimited freedom in both the complexity of shapes and the types of materials that can be cast, and millions of investment castings are now produced each year.
Investment casting uses the same type of molding aggregate as the ceramic molding process and typically involves the following sequential steps:
1. Produce a master pattern-a modified replica of the desired product made from metal wood, plastic, or some other easily worked material.
2. From the master pattern, produce a master die. This can he made from low-melting point metal, steel, or possibly even wood. If a low-melting-point metal is used, the die may be cast directly from the master pattern. Rubber molds can also be made directly from the master pattern. Steel dies are often machined directly. eliminating need for step 1.
3. Produce wax patterns. Patterns are made by pouring molten wax into the master or injecting it under pressure (injection molding), and allowing it to harden. Release agents, such as silicone sprays, are used to assist in pattern removal. Plastic and frozen mercury are alternate pattern materials. The polystyrene plastic may be preferred for producing thin and complex surfaces, where its higher strength and greater durability are desired. Frozen mercury is seldom used because of its cost. handling problems, and toxicity. If cores are required, they can generally be made from soluble wax or ceramic. The soluble wax cores are dissolved out of the patterns prior to further processing, while the ceramic cores remain and are not removed until after solidification of the metal casting.
4. Assemble the wax patterns onto a common wax sprue. Using heated tools and melted wax, a number of wax patterns can be attached to a central sprue and runner system to create a pattern cluster, or a tree. If the product is sufficiently complex that its pattern could not be withdrawn from a single master die, the pattern may be made in pieces and assembled prior to attachment.
5. Coal the cluster or tree with a thin layer of investment material. This step is usually accomplished
by dipping into a watery slurry of finely ground refractory material. A thin but very smooth layer of investment material is deposited onto the wax pattern, ensuring a smooth surface and good detail in the final product.
6. Form additional investment around the coated cluster. After the initial layer has dried, the cluster can be redipped, but this time the wet ceramic is coated with a layer of sand or coarse refractory, a process called stuccoing. After drying, the process is repeated until the investment coating has the desired thickness (typically 5 to 15 mm or 4 to 5 . g inch with up to eight layers). As an alternative, the single-dipped cluster can be placed upside down in a flask and liquid investment material poured around it. The flask is then vibrated to remove entrapped air and ensure that the investment material now surrounds all surfaces of the cluster.
7. Allow the investment to fully harden.
8. Remove the wax pattern from the mold by melting or dissolving. Molds or trees are generally placed upside down in an oven where the wax can melt and run out, and any residue subsequently vaporizes. This step is the most distinctive feature of the process because it enables a complex pattern to be removed from a single-piece mold. Extremely complex shapes can be readily cast. (Note: In the early years of the process, only small parts were cast, and when the molds were placed in the oven, the molten wax was absorbed into the porous investment. Because the wax "disappeared," the process was called the lost-wax process, and the name is still used.)
9. Heat the mold in preparation for pouring. Heating to 550" to 1100°C (1000" to 2000°F) ensures complete removal of the mold wax, cures the mold to give added strength, and allows the molten metal to retain its heat and flow more readily into all of the thin sections and details. Mold heating also gives better dimensional control because the mold and the metal can shrink together during cooling.
10. Pour the molten metal. While gravity pouring is the simplest, other methods may be used to ensure complete filling of the mold. When complex, thin sections are involved, mold filling may be assisted by positive air pressure, evacuation of the air from the mold, or some form of centrifugal process.
11. Remove the solidified casting from the mold. After solidification, techniques such as mechanical chipping or vibration, high-pressure water jet, or sand blasting are used to break the mold and remove the mold material from the metal casting.
(MATERIALS AND PROCESSES IN MANUFACTURING 10th edition, J. Temple Black, Ernest Paul DeGarmo, Ronald A. Kohser, p.304)
Vinyl (19.04.2011-15.05)
Wide range of types, from thin, rubbery films to rigid forms; tear resistant good aging properties; good dimensional stability and water resistance in rigid forms; used for floor and wall covering, upholstery fabrics, and Iightweight water hose; common trade names include Saran and Tygon
(MATERIALS AND PROCESSES IN MANUFACTURING 10th edition, J. Temple Black, Ernest Paul DeGarmo, Ronald A. Kohser, p.169)
The Niche Enterprise (19.04.2011-15.45)
When a niche strategy is applied, a concentration on selected target groups, market segments or parts of a product range takes place. Moreover the demarcation of a geographical market is possible. Niche strategies offer the chance to generate profits, even with focussing, which is achieved either by a cost lead related to the pursued target or through a high degree of differentiation. In a niche, both variants can be realized simultaneously, thus, this strategy type represents combination of the two generic strategies. A possible negative aspect, however, might be the conflict between profitability and maximizing turnover, as the niche market share represents a limitation as compared with the total market.
(Entrepreneurship in a European Perspective: Concepts for the Creation and Growth of New Ventures, Christine K. Volkmann, Kim Oliver Tokarski, Marc Grünhagen, p.208)
Fatigue Retardation (19.04.2011-16.15)
The fracture of machinery is mostly due to fatigue fracture mode. The retardation phenomenon for the crack growth due to the overload has an important effect on fatigue life of the material. Therefore, many studies had been conducted in order to predict fatigue life. Up to now, the mechanisms for the retardation have been supposed the compressive residual stress on crack tip and the related crack tip plasticity, crack closure, crack tip blunting, crack tip strain hardening, and crack tip branching etc. This retardation mechanism due to the overload had been much studied as the basis for the crack growth behavior and the life prediction. Mostly, however, those studies are investigated for identical specimen thickness. The influence of specimen thickness is most important among the factors affected retardation.
(Structural integrity and fracture: proceedings of the International Conference on Structural Integrity and Fracture, Arcady V. Dyskin, Emad Sahouryeh, p.167)
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