Designing Investment Castings Overcoming Challenges
Designing Investment Castings Overcoming Challenges involves challenges such as achieving precise dimensional tolerances, managing surface finishes, and ensuring flatness and straightness. 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.
Designing Investment Castings Overcoming Challenges, Investment castings offer high precision, with typical dimensional tolerances of 0.05 mm/cm and the possibility of 0.03 mm/cm, though tighter tolerances can increase costs. Dimensional accuracy depends on geometry, requiring close collaboration between designers and founders. Surface finishes are significantly smoother than sand castings, with RMS values for low-alloy steel ranging from 63 to 125, compared to 500 to 1000 for sand castings. Flatness is controlled within 0.05 mm/cm, and straightness within 0.1 mm/cm. Radii tolerances are ±0.075 mm/cm. For holes, through-hole length should not exceed five times the diameter, while blind holes should be no more than 2.5 times the diameter. Threads can be cast in hard-to-machine alloys but are costly. These guidelines reflect the design flexibility of investment casting, offering more freedom compared to other manufacturing processes.
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.