The process of investment casting is specified over the other manufacturing methods due to unlimited flexibility of design. This flexibility permits one to take full advantage of the functional choices without making concessions necessary to fabricate the component by other means. Designers recommend the use of investment casting in any of the following situations:
(a) When the design is so complicated that a one-piece casting could replace a part comprised of several components that are fabricated by other methods or assembled.
(b) When mechanical properties required are attainable from alloys that are difficult to machine.
(c) When costly machining is to be avoided.
(d) When tolerance of +/- 0.1 is satisfactory on most dimensions.
(e) When fine surface finishes are required.
The choice of investment casting process for a certain component will not be economical if the part can be made as automat machined part, die casting, stamping or forging without secondary operations.
If a component is designed in terms of the more conventional manufacturing processes, it is quite possible that the design may be changed or modified radically by the design freedom offered by the accurate and versatile investment casting process. This may call for redesigning in some cases. During design of an investment casting, attention is to be paid to the ease of production and keeping production cost low. The following are some of the very basic design rules:
a) Keep section thickness constant, where a change cannot be avoided, keep the change gradual.
b) Make use of adequate fillets and chamfers to avoid sharp corners / edges.
c) Do not cause a number of sections to meet at one point. Try to stagger the junction.
Dimensional tolerances: Most investment castings can be economically cast at tolerances of 0.05 mm/cm. tighter tolerances of 0.03 mm/cm can be cast if required. It is not practical to specify needlessly close tolerances, as it would unnecessarily increase cost. The dimensional accuracy obtainable is conditioned by the geometry of the casting. Close collaboration between the designer and the founder will help produce most accurate castings within a set price range. Surface finish: Investment castings are on an average 5 to 15 times smoother than sand castings. For low alloy steel investment castings the RMS values range from 63 to 125. Where as for sand castings the RMS values range from 500 to 1000. Flatness: The dimensions of the castings will significantly affect the flatness obtained. The judicious use of ribs on the casting will minimise bowing and distortion. The flatness can be controlled within 0.05 mm/cm of the as cast state. Straightness: The as-cast straightness can be easily controlled within a tolerance of 0.1 mm/cm of cast length. Radii: The general tolerances obtainable for all radii are around +/-0.075 mm/cm of cast radius. Cast holes: For through holes, the maximum length of the hole should not exceed 5 times the diameter. For blind holes, the maximum length should not exceed 2.5 times the diameter. These are only thumb rules. An infinite variety of holes of different shapes and sizes can still be cast. Threads: Threads can be cast in alloys that are difficult to machine.
Otherwise, casting threads becomes expensive and should be avoided. The above points are only thumb rules, and not essential limitations of the process. The investment casting process offers a design freedom not met by any other manufacturing process.
THE INVESTMENT CASTING PROCESS:
The process can be described as follows:
1. A metal negative of the desired component is first made. This is called a die.
2. Wax is injected into this die to get a low melting point positive. This is called a pattern.
3. A ceramic negative is built around this wax positive. This is the mould or shell.
4. The wax positive is melted out leaving behind a cavity in the ceramic shell. This negative is a monolithic ceramic mould with a cavity having the exact shape and dimensions of the desired component.
5. Metal is cast into this cavity.
6. The ceramic shell is then broken away to reveal the metal component.
This process uses disposable patterns and moulds. Investment casting is a mass production technique. It is also a high precision casting method. Castings with tolerances as close as +/- 0.05mm are cast regularly. From the concept of the investment casting process described above, it is possible to divide the entire investment casting process into different sections or divisions, as indicated below. Die making for producing wax patterns.
I. Production of wax patterns.
II. Mould or Shell building.
III. Shell de – waxing.
IV. Shell firing or sintering.
V. Metal melting and pouring (casting).
VI. Shell knockout, fettling and casting finishing.