Introduction to investment casting Investment casting is basically a metal shaping technique. It is a foundry practice by which high precision castings are manufactured. This is a specialised foundry technology and is considered a high – tech area. This process has gained popularity on the basis of the superior quality of the castings produced. In addition, more important is the fact that, the cost of a finished component produced by the investment casting process is less than or comparable to that of the conventional production techniques. It is common knowledge that new technologies gain recognition and popularity only if they offer considerable technical advantage along with commercial gain as compared to the existing conventional methods. This is precisely why Investment casting, which has both advantages, has become a much favoured production technology and popular foundry practice of today.
Investment casting caters to a substantial commercial market and competes directly with powder metallurgy, pressing, machining, drop forging, etc …
Advantages of investment casting:
1. This is a high precision casting technique, hence, near net shapes; very near to the finished component is produced.
2. Machining requirements for the finished component is reduced by upto 90%, substantially reducing machining and scrap cost.
3. The process is extremely versatile; almost any intricate shapes, thin walls and tiny delicate components can be cast. This allows for extreme design flexibility.
4. Excellent surface finish is a major advantage of this process.
5. There are no metallurgical limitations for investment casting. Difficult to machine metals are best shaped by this process.
6. The qualities of investment castings are markedly superior to other casting methods. This is an inherent advantage of the process. Applications of investment castings: The applications cover a very wide range. Some of them are listed below.
Investment castings are used in:
-Agricultural equipment.
-Automobile components, on a very large scale.
-Baling and strapping equipment.
-Bicycles and motorcycles.
-Construction hardware and equipment.
-Dentistry and dental tools.
-Prosthetics.
-Computer hardware.
-Electrical equipment, electronic hardware and radar.
-Guns and armament.
-Hand tools.
-Machine tool components.
-Aircraft engines, air frames, fuel systems.
-Aerospace, missiles, ground support systems.
-Cameras.
-Material handling equipment.
-Oil well drilling and auxiliary equipment.
-Components for compressor and pneumatic equipment.
-Components for hydraulic equipment, pump and pipe fittings.
-Pipe fittings for plumbing applications.
-Sports gear and recreational equipment.
-Textile machinery parts.
-Valves and flow control equipment.
-Components for consumer durables.
DESIGN OF INVESTMENT CASTINGS :
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.
DESIGN CONSIDERATIONS:
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.
DIE MAKING FOR PRODUCING WAX PATTERNS.
This is the most important first step in the entire investment casting process. Any error in the die is reproduced in the finished casting. Extreme care is necessary when making a die. The die should be simple in its construction. This will make it easy to use and enhance its production rates.
Certain dimensional changes are intrinsic to the process, namely:
a) Wax shrinkage.
b) Expansion of the shell during firing.
c) Cast metal shrinkage during solidification.
Allowance for each of these dimensional changes is to be incorporated in the die. Dies are usually made of aluminium.
Conventional techniques are used for making dies.
Desirable Properties for Die Materials:
a) The die material should have good thermal conductivity to enable rapid cooling of the injected wax.
b) The material should be sufficiently strong to withstand the injection and clamping pressures.
c) The material should have sufficient wear resistance to provide long tool life.
d) The material should be hard enough so that it can be polished to a high finish, and the finish has a good life.
Pattern Materials: Due to the importance of the patterns, it is very important that they have certain specific properties, such as:
a) Low ash content
b) Good fluidity at injection temperatures to produce detail.
c) Adequate mechanical strength.
d) Very low thermal contraction and expansion ranges.
e) Stability under normal working conditions.
f) Chemical compatibility with dies and shell materials.
g) Ready availability.
h) Reusability.
PRODUCTION OF WAX PATTERNS
This is done using the Hydraulic Wax Injection Machines.
a) Wax is melted and brought to the required temperature in wax melters.<.li>
b) The molten wax is then transferred to the wax injection machines.
c) Wax is injected under pressure into the die at appropriate temperatures. In case the component is large, the dies are cooled using water jackets.
d) The wax patterns are extracted from the dies and stored in a cool place.
e) These patterns are then taken for assembly. Pattern Assembly This is the next important step. Correct design of ingates, and cluster design play a major role in the outcome of the castings. The patterns are assembled to a running and feeding system by welding with a wax-welding gun and with wax glue (stick wax).
The patterns are usually assembled in the form of a fir tree, i.e. the ingate of each pattern is fixed to the central down sprue. Assembly Procedure The assembly procedure is as follows;
1. The patterns are visually inspected before assembly any repairable damages or blemishes are patched using the ‘patch wax’.
2. Any flashes along the die parting line are carefully removed.
3. The ends of the ingates are shaped to match the shape of the sprue and welded to it.
Important notes:
1. The sprue and runners are gravity cast using recycled wax.
2. The gating and runners should be positioned to ensure easy cut – off of the finished casting.
3. The size of the cluster / tree should be properly chosen to maximise yield. The assembly should be light enough to be easily handled by the worker.
4. Enough space should be provided between the individual patterns to allow a shell of uniform thickness to form on all the surfaces of the pattern.
MOULD OR SHELL BUILDING
This is the process where the ceramic shell is built around the wax trees / assemblies. The most economical and popular process is the dip process. This process is extremely versatile and offers very accurate control of the shell properties. The dip process; The wax tree is invested with ceramic coatings, layer by layer, until a sufficiently thick mould is formed. The wax tree is first dipped in ceramic slurry and dry sand is sprinkled (‘ stuccoed ‘) on it. A number of such coats (slurry plus sand) are applied until the desired thickness is achieved. The dipping procedure;
The step by step shell building procedure is as follows
1. Dip the wax tree into the primary ceramic slurry.
2. Stucco with fine sand.
3. Dry this coat in controlled conditions of temperature and humidity.
4. When dry, wet this tree in the primary backup ceramic slurry.
5. Stucco with slightly coarser sand and dry under same conditions.
6. Repeat this procedure using progressively coarser sands until the required thickness is reached.
7. When a particular coarseness of stucco sand is reached, secondary backup slurry is used.
8. The number of dips varies from 8 to 12 depending on the size and shape of the casting.
Properties of Shell / Mould materials:
(a) The material should have refractory properties.
(b) It should be thermally stable i.e. dimension changes on heating should be minimal.
(c) Chemical compatibility with other mould materials and metal to be cast is important.
(d) It should resist hot deformation, i.e. the material should not become soft, at molten metal temperatures, and deform.
(e) Low cost and easy availability of consistent quality material is also important.
Some of the ceramic materials used are
(a) Silica (SiO2)
(b) Zircon (ZrSiO2)
(c) Zirconia (ZrO2) etc….
SHELL DEWAXING:
Dewaxing is the process where the wax patterns are melted out. This leaves behind the monolithic ceramic shell having cavities of the exact shape and dimensions of the wax patterns. This is also the most delicate step in the entire investment casting process.
The dewaxing methods:
1. Flash heating: the shells are placed in a furnace that is preheated to a high temperature. This sudden exposure to heat enables proper dewaxing of the shells.
2. Steam autoclave dewaxing: here the shells are placed in a steam autoclave. The vessel is closed and super heated steam, at high pressure, is let into it. This causes very efficient dewaxing. Steam dewaxing is the most efficient method of dewaxing as almost no shell cracking is noticed here.
3. Boiling water/wax dewaxing: the shells are simply immersed into a bath of boiling water or wax.
The wax inside the shells is thus melted out. There are a number of disadvantages associated with this method. The only advantage being the low capital investment involved.
SHELL PRE-HEATING
The shell after dewaxing is preheated. This is done to develop the high temperature bond of the shell. The preheating is usually done in a box type electrical furnace or in a directly fired oil / gas furnace. It may be noted that the shells are poured when still hot. This is done to prevent thermal shock and minimise casting defects.
METAL CASTING
There is no limitation to what metal can be cast in the investment casting process. Components in non-ferrous metals are cast regularly along with regular steel, stainless steel, cast iron, etc…. Components in titanium alloys are also investment cast. Gold ornaments are also investment cast forming an independent industry on its own. Hence, metal or metallurgy is no limitation here. Conventional casting methods like, ladle pouring or furnace pouring, are used for casting metal into the investment casting shells. Vacuum melting-casting techniques are used only where metallurgy specifications require such care. The gravity pouring method is found to be adequate for all other casting needs.
FETTLING AND CUT OFF: After the cast has solidified and the shell cooled, it is passed to the knockout stage. The shell is broken away using the pneumatic knock out machine. Additional cleaning methods like blasting and chemical leaching are used to remove shell fragments from hard to reach areas, bores and undercuts. After the cleaning operation, the castings are separated from the sprue and the runner system. Abrasive cut-off wheels, saws, oxyacetylene torch, gouging rods, etc… are used for this purpose.
The cut off witness is kept as small as possible. This is ground off in fettling, along with other casting blemishes, if any. Endless belt grinders, pedestal grinders and pneumatic die grinding tools are used for the light and delicate finishing operation. Care has to be taken to prevent damage to the casting during the fettling operation. In case there is a need the castings are further finished using glass bead/ sand/ refractory grit/ ceramic/ aluminium/ SS/ steel shot blasting techniques. Electro polishing and buffing are sometimes required on stainless steel castings. The castings are inspected at appropriate stages to ensure quality of metal, dimensions and physical properties. Standard foundry inspection methods are used here. The castings are then packed and shipped.