Manufacturing Processes - General Investment Casting

Lost Wax or Investment Casting Handbook

Investment Casting

Investment casting comes under the general category of casting as a manufacturing process and uses expendable moulds and patterns.This process is sometimes referred to as the lost wax process and is one of the oldest metal forming processes around dating back nearly 5000 years.

There are approximately 5 general stages to the component casting program.

1. A wax pattern is produced by injecting molten wax into a metal or rubber die that can be split to remove the pattern.

2. The wax patterns are then assembled onto a tree with feeding and gating systems built in. Once assembled the tree is dipped into a refractory slurry and is then coated or stuccoed with refractory powder and set using either an investment drying process or chemicals to chemically set it. The dipping, coating and setting stage is repeated several times to build up a ceramic layer that is about 5 to 10mm thick.

3. The wax is removed from the ceramic shell that has been formed by heating it in a furnace or steam autoclave.

4. For the casting stage the ceramic mould that has been created is fired in a mould and pre-heated to accept the molten metal. The metal is then poured into the feeder until the mould is full and then allowed to cool. To access the castings the ceramic shell has to be chiseled off revealing the metal version of the tree that has replaced the original wax version.

5. The components are now fettled from the tree and put through any further required operations to prepare them for final purpose. this could include machining,grinding and or heat treatment.

The process is best suited for small, or relatively small, complex 3D components. It is also suitable for most metals including reactive metals that require the casting to be done under vacuum. Larger components can be produced using this process and have been, but it is normally because the complexity of the shape dictates it.

Pros

  1. Very accurate and repeatable with good surface finish
  2. Wax can be recycled
  3. Less need for secondary operations i.e. minimal post casting machining

Cons

  1. Equipment costs are relatively high, particularly with respect to vacuum systems required for reactive metals
  2. Production rates slow due to multiple process stages
  3. Refractory materials are expensive and cannot be recycled
  4. Micro-structures are relatively coarse

Investment casting is used extensively in the aerospace and power generation industries for the production of turbine blades, being a process that lends itself perfectly to the requirements for production of complex 3D shapes.

It also lends itself very well to the production of items of jewelry, one of the original uses of the process from its conception when beeswax was used to create the pattern and clay was used as the investment.

Investment Casting or Lost Wax Casting Process

Photographs taken by Mark Bolton, April 2007 and used here under the Creative Commons license ref: http://creativecommons.org/licenses/by-sa/2.0/deed.en   The Investment Casting Shell
Photographs taken by Mark Bolton, April 2007 and used here under the Creative Commons license ref: http://creativecommons.org/licenses/by-sa/2.0/deed.en The Investment Casting Shell
The Finished Component from the Investment Casting Process
The Finished Component from the Investment Casting Process

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