Investment Casting
Introduction
Investment casting uses a piece of ceramic mould. The mould is prepared by surrounding the ceramic material over the wax or plastic pattern. Once the ceramic material solidifies, the wax replica is melted and drained out from the mould and the metal is poured into the mould cavity.
Figure M2.2.1: Steps in investment casting
There are two types of investment casting process namely - solid mould and ceramic shell mould. These two processes differ primarily the way in which the mould is prepared or formed. In the solid mould process the pattern is placed into a container and mould material poured around the pattern and allowed to solidify to make the solid block. On the other hand, in case of ceramic shell mould, the pattern is dipped into ceramic slurry. Pattern is taken out from the slurry and rotated to produce a uniform coating. Further, the coating is allowed to dry and the dipping process repeated over several times. As a result, multiple coat forms a hard ceramic shell mould. The mould is again placed in an oven to drain out the wax so as to create the mould cavity.
Pattern materials
Patterns used for investment casting are injection moulded of either wax or plastic. Paraffin and microcrystalline waxes are the most common base material for patterns.
Typical characteristics and applications
Investment casting is practiced when the following characteristics features are strictly involved:
Intricate shape
Close tolerances
Small size
High strength alloys
Investment casting is preferred in the places with parts involving contoured surfaces, undercuts, other intricate shapes, places where machining is difficult or unfeasible. Typical examples of such mechanical components are: sewing machines, firearms, surgical and dental devices, turbine blades, gear impeller, hand tools, cranks and levers.
General design considerations
Since patterns are injection moulded, a good practice must be followed for injection moulded parts. These include the use of a well located pattern, straight parting line, adequate draft and avoidance of undercuts, generous radii and fillets.
Suitable material for investment casting
A wide variety of materials such as both ferrous and non-ferrous can be used in investment casting. Any metal that can be melted in standard induction furnace or vacuum furnace can be considered for this case. Difficult to machine materials are also good candidate for investment casting. Comprehensive list of materials that can be used in investment casting are given in Table M2.2.1. Table M2.2.1: Suitable material for investment casting. (Source: Design for
Manufacturability Handbook by James G Bralla, 2nd Ed)
Material Fluidity Shrinkage Resistance Castability
to hot tearing rating
Carbon steels
1040 (G10400) BBBB+
1050 (G10500) BBBB+
Alloy steels
2345 (G23450) BBBA-
4130 (G41300) BBBA-
Nickel alloys
Inconel 600 (AMS 5665) (N06600) ABBB+
Cobalt alloys
Monel (QQ-N-288-A) (N04020) ABBB+
Aluminum alloys
Cobalt 21 (R30021) AABA-
Cobalt 31 (R30031) AABA-
A 356 (A13560) AAAA+
C 355 (A33550) AAAA
Tool steels
A-2 (T30102) BBBB+
H-13 (T20813) BBBB+
Copper alloys
Beryllium copper 10C (C82000) AAAA
A -Excellent; B - good; C- poor
Gunmetal (C90500) ACAB+
Detail design recommendations
The following important design guidelines are recommended for investment casting.
Minimum wall thickness: The minimum thickness of casting wall is primarily dependent upon the fluidity of the metal to cast and the length of the section involved.
If the section is long, heavier wall may be required. Suggested wall thickness for various metals is shown in the Table M2.2.2.

Table M2.2.2: Suggested Minimum Wall Thickness for Various Investment-Casting
Metals. (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)
Metal Minimum wall thickness(mm)
Ferrous metals
Low-carbon steel 1.8
High-carbon steel 1.5
Low-alloy steel 1.5
Stainless steel, 300 Series 1.0
Stainless steel, 400 Series 1.5
Cobalt-base alloys 0.75
Nonferrous metals
Aluminum 1.0
Beryllium copper 0.75
Brass 1.0
Bronze 1.5
Flatness and straightness: Deviations from flatness andstraightness can be minimized by incorporatingribs and gussets in the parts. Also, due to occurrence of shrinkage there is a tendency to develop a“dish” (concave) shape instead of flat surface. This can be minimized by designing parts with uniform wall thickness.(Figure M2.2.2)
Figure M2.2.2: Keep walls uniformly thin to avoid “dishing.”
Radii: Even though use of sharp corners is possible by this method, generous radii are preferred. Better quality and more accurate parts can be produced by providing ample fillets and radii. A minimum of 0.75mm fillet radius need to be furnished. However, a radius of 1.5 to 3 mm is preferable.(Figure M2.2.3)

Figure M2.2.3:Use generous fillets and radii.
Curved surfaces: Both concave and convex surfaces are possible to cast in this category. Generally concave surfaces can be cast with greater accuracy.
Parallel sections: Parallel sections can be classified into two geometric categories namely: forks or yokes and clamp or pinch collars. In case of fork, as the thickness t increases, the width of the opening w also must increase (Refer Figure M2.2.4).
Figure M2.2.4: Forks and yokes have minimum practical slot widths.
The recommended minimum slot width w for clamps with ferrous and nonferrous metals are 1.5, and 1 mm respectively. ( See Figure M2.2.5)

Figure M2.2.5: Recommended minimum slot widths W for clamps and pinch collars are 1.5 mm for ferrous metals and 1.0 mm for non-ferrous metals.
Keys and key ways: The ratio of width (W) to depth (D) for ease of casting of keys and keyways are kept 1 or more. Recommended minimum key width which can be cast is 2.3mm and 1.5mm for ferrous and non-ferrous metals respectively.(Refer
Figure 2.2.6)
Figure M2.2.6: Keys and keyways should have a widthto-depth ratio of 1:1 or more.
Holes: Whenever holes are to be incorporated a minimum size (diameter) must be greater than or equal to 1.5mm and 2.2 mm for nonferrous and ferrous alloys respectively are to be used.
Blindholes: In general, blind holes are not recommended for casting. If it is inevitable, the depth of the hole should not exceed the diameter.
Through holes: Tolerances for blind holes and through holes are same. L/D ratio should not exceed 4:1 and 5:1 for ferrous and copper aluminum respectively.
Ceramic cores: In the design of pattern mould, if any intricate internal configuration
(example internal thread) has to be made with a ceramic core then provision must be provided in the mould to support the ceramic core the time of wax injection.

Draft:To remove wax pattern from the mould a small amount of draft is provided to the pattern. The recommended values lie in the range between 1/4° to 1/8° .(Refer
Figure M2.2.7)
Figure M2.2.7: A small amount of draft is necessary for the removal of patterns from the moulding die.
Screw threads: Both internal and external threads can be investment casted. But it is not a usual recommended practice.
Undercuts: Even though undercuts doesn’t create any problem to the casting operation but has significant effect in pattern moulding operation and hence this has to be avoided.
Dimensional factors:The most significant factor influencing the dimensional accuracy of investment casting is the shrinkage of the materials used. Some degrees of shrinkage are there in all levels: wax pattern, investment material and cast material.
The normal shrinkage allowance provided for shell mould and flask mould method are 1.6 to 1.7 % and 1.1% respectively. Other factors adversely affecting dimensional control are
 Temperature variations in pouring
 Fluidity of the metal used
 Pattern and pattern-mould dimensional variations
 Pattern distortion during handling
 Investment-mould cracks and other variations
 Wax shrinkage and sag over a period of time Recommended tolerances for variousangles, general dimensions, roundness and flatness are provided in Figure M2.2.8.
Figure M2.2.8: Recommended tolerances for angles: (a) angular openings as A (±1.5°); (b) angular shapes as B (±0.5°); (c) parallelism (±2.0°).
Table M2.2.3: Recommended tolerances for angles (Source: Design for Manufacturability
Handbook by James G Bralla, 2nd Ed)
Dimension(mm) Recommended tolerances(mm)
Normal Tight
Up to 6 0.4 0.08
6–13 0.10 0.4
13–25 0.13 0.4
25–50 0.18 0.4
50–100 0.40 0.8
100–150 0.60 1.1
Over 150 0.80 1.5