Investment Casting Process | Lost Wax Investment Casting | Investment Casting Foundry

Investment Casting (or) Lost Wax Method

A wax duplicate of the desired casting is created to be invested into a "Ceramic Slurry".

The slurry covered investment can be dipped into alternating coatings of sand and slurry until a suitable thickness of shell is achieved that can hold the molten metal after the investment is burnt out.

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The "Burn-Out" process requires that the investment and coating are inverted in an oven that is fired to 1800F so that the investment can flow out and be recovered. The refractory coating is also cured in this procedure.

This process is beneficial for casting metals with high melting temperatures that cannot be moulded in plaster or metal.

Parts that are typically made by investment casting include those with complex geometry such as turbine blades or fire arm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.

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Principle

Method also called as precision investment casting. The method involves the use of expendable Pattern with a shell of refractory material surrounded to form a casting mould. Since the pattern made up of wax is melted out and gets destroyed. That is why the name-"Lost wax method".

Process parameters of Investment casting

Process principle

Refractory slurry is formed around a wax or plastic pattern and allowed to harden. The pattern is then melted cut under mould is baked. The molten metal into the mould and solidifies.

Size limits

As small as (1/10) inch but usually less than 10 lb.

Thickness limits

As thickness as 0.025 inch but less than 3 inch.

Typical tolerance

Approximately 0.005 inch.

For the first inch and 0.002 inch for each additional inch.

Draft allowance

Not required.

Surface finish

50 to 125 micron.

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Procedure

1. Produce a master pattern

The pattern is 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 be made from low-melting-point metal, steel, or possibly even wood. If low-melting-point metal is used.

3. Produce wax patterns

Patterns are made by pouring molten wax into the master die, or injecting it under pressure, and allowing it to harden. Plastic and frozen mercury have also been used as pattern material.

4. Assemble the wax patterns onto common wax sprues

The individual wax patterns are attached to a central sprues and runner system by means of heated tools and melted wax. In some cases, several pattern pieces may first be united to form a complex.

5. Coat the cluster with a thin layer of investment material

This step is usually accomplished by dipping the cluster into a watery slurry of finely ground refractory material.

6. Produce the final investment around the coated cluster

After the initial layer is formed, the cluster can be redipped, but this time the wet ceramic is coated with a layer of sand and allowed to dry. This process can be repeated until the investment coating is the desired thickness (typically 5 to 15 mm).

7. Allow the investment to fully harden

8. Melt or dissolve the wax pattern to remove it from the mould

This is generally accomplished by placing the moulds upside down in an oven, where the wax melts and runs out, and any residue subsequently vaporizes.

9. Preheat the mould in preparation for pouring

Heating to 550 to 1100°C (1000 to 2000°F) ensures complete removal of the mould wax, curves the mould to give added strength, and allows the molten metal to retain its heat and flow more readily into all of the thin sections.

10. Pour the molten metal

Various methods, beyond simple pouring, can be used to ensure complete filling of the mould, especially when complex, thin sections are involved.

11. Remove the casting from the mould

This is accomplished by breaking the mould away from the casting. Techniques include mechanical vibration and high-pressure water.

Handgun graphic, Black and White
Graphic by Sgt. Jason Luber

Advantages

i) Smoother surfaces (1500 to 2250 micro-mm root mean-square).Close tolerance (of +0.003 mm/mm)

ii) High dimensional accuracy

iii) Intricate shape can be cast

iv) Castings do not contain any disfiguring parting line

v) Machining operations can by eliminated

 

Disadvantages:

i) Process is relatively slow.

ii) Use of cores makes the process more difficult.

iii) The process is relatively expensive than other process.

iv) Pattern is expandable.

v) Size limitation of the component part to be cast. Majority of the castings produce weight less than 0.5 kg.

 

Applications

The products made by this process are vanes and blades for gas turbines, shuttle eyes for weaving, pawls and claws of movie cameras, wave guides for radars, bolts and triggers for fire arms, stainless steel valve bodies and impellers for turbo chargers.

While investment casting is actually a very old process and has been performed by dentists and jewellers for a number of years, it was not until the end of World War II that it attained any degree of industrial importance.

Developments and demands in the aerospace industry, such as rocket components and jet engine turbine blades, required high-precision complex shapes from high-melting-point metals that are not readily machinable.

Investment casting offers almost unlimited freedom in both the complexity of shapes and types of materials that can be cast.

Shell Mould Casting | Shell Casting Process | Shell Casting Materials

Shell Moulding (or) Croning Shell Process

Introduction

Shell moulding is a process for producing simple or complex near net shape castings maintaining tight tolerances and a high degree of dimensional stability. Shell moulding is method for making high quality castings.

Principle

The process is based on the principle of capability of a thermosetting resin and sand mixture to assume the shape of a preheated metal pattern to form a dense, quickly hardened shell mould.

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Process parameters of shell moulding Process

Sand coated with a thermosetting plastic resin is dropped onto a heated metal pattern, which cures the resin.

The shell segments are stripped from the pattern and assembled. When the poured metal solidifies, the shell is broken away from the finished casting.

Advantages: Faster production rate than sand moulding high dimensional accuracy with smooth surfaces.

Limitations: Requires expensive metal patterns. Plastic resin adds to cost; part size is limited.

Common metals: Cast irons and casting alloys of aluminium and copper.

Size limits: 30 g minimum usually less than 10kg; mould area usually less than 0.3 m2

Typical tolerances: Approximately 0.005 cm

Draft allowance: 1/4 to 1/2 degree

Surface finish: 1/3 – 4.0 microns

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Steps involved

There are different stages in shell mould processing that include:

1. Initially preparing a metal-matched plate

2. Mixing resin and sand

3. Heating pattern.

4. Inverting the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill).

5. Curing the shell and baking it

6. Removing investment

7. Inserting cores

8. Repeating for the other half

9. Assembling the mould

10. Pouring the mould

11. Removing casting

12. Cleaning and Trimming.

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The shell mould casting process consists of the following steps.

a) Pattern creation:

A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials.

b) Mould creation:

First, each pattern half is heated to 175-370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.

c) Mould assembly:

The two shell halves are joined together and securely clamped to form the complete shell mould. If any cores are required, they are inserted prior to closing the mould. The shell mould is then placed into a flask and supported by a backing material.

d) Pouring:

The mould is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mould cavity.

e) Cooling:

After the mould has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.

f) Casting removal:

After the molten metal has cooled, the mould can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mould.

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Advantages of Shell Moulding Casting

1. Good casting detail and dimensional accuracy are possible.

2. Moulds are lightweight and may be stored for extended periods of time.

3. Has better flexibility in design than die-casting.

4. Is less expensive than investment casting.

5. Capital plant costs are lower than for mechanized green sand moulding.

6. Metal yields are relatively high.

7. Sand: metal ratios are relatively low.

8. Gives superior surface finish and higher dimensional accuracy, and incurs lower fettling costs than conventional sand castings.

Disadvantages:

i) Higher cost of match plate

ii) Size of casting is limited

iii) Serious dust and fume problems

iv) Carbon pickup in case of steels.

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Applications

Cylinders and cylinder heads for air cooled IC engines, automobile transmission parts, cast tooth bevel gears, brake beam, hubs, and track rollers for crawler tractors, steel eyes, gear blanks, chain seat brackets, refrigerator valve plate, and small crank shafts.