Investment Casting / Additive Manufacturing
The Investment Casting process, also known as ‘Lost Wax Casting’ is a developmental manufacturing process that converts molten metal in a single operation to precision engineered components with minimum wastage of material, energy and subsequent machining.
The process is extremely versatile, cost effective and is excellent for complex components where fine detail and dimensional accuracy are required. It also reduces or totally eliminates the need for costly machining operations and, as several parts can be made as one casting, the time and cost involved for subsequent assembly is removed. This provides numerous advantages and flexibility for engineers and designers and can be a great alternative to other manufacturing processes due to the complex shapes and little surface finishing that is required to the finished parts and the varied metals and alloys that are able to be used.
Investment casting – using the lost wax casting development technique (this reference term is due to the wax that is used in this methodology) – is one of the oldest casting methods known dating back over 5000 years and examples of its uses in producing jewellery and statues can be traced back to use with natural waxes.
Lost Wax casting or Investment Casting can be integrated with a variety of metals and alloys such as carbon steel, stainless steel, aluminium, brass, bronze, copper, zinc and superalloys such as Inconel and Hastelloy.
Investment Casting – The Process
The investment casting process – using the lost wax technique – is one of the oldest casting methods known and examples of its uses in producing jewellery and statues can be traced back several thousand years. With its roots in ancient Mesopotamia and Egypt, its commercial life only really began during World War II when military demands were overtaxing the machine tool industry.
The basic stages of the lost wax process are outlined below:
1. Wax Making – Wax replicas of the desired castings are produced by injection moulding using a metal die. These replicas are called “patterns”.
2. Wax Assembly – These patterns are attached via a ‘gate’ to a central wax stick, referred to as a ‘tree’ or ‘sprue’, to form a casting cluster or assembly, and mounted on a pouring cup.
3. Investing – A shell is built by immersing the assembly in a liquid ceramic slurry and then into a bed of extremely fine sand. Several layers may be applied in this manner.
4. Dewaxing – Once the ceramic is dry, the wax is melted out in an autoclave, creating a negative impression of the assembly within the shell. The shell mould is then fired in a high temperature oven.
5. Casting – The shell is filled with molten metal – metal injection moulding in a variety of materials, using various techniques and, as the metal cools, the parts, gates, tree and pouring cup become a solid casting.
6. Knock out – When the metal has cooled and solidified, the ceramic shell is broken off by vibration or water blasting.
7. Cut Off – The parts are cut away from the central tree using a high speed friction saw.
8. Finishing – Minor finishing operations, such as fettling and grinding, are undertaken to produce a metal casting identical to the original wax pattern.
Additive manufacturing offers several advantages for the Investment casting process. The most common advantage is the fact we are able to do away with moulds and the process can straight away print patterns with extremely fine details. Advantages that of using Additive Manufacturing for the Investment Casting process are that it can reduce the process steps thus speeding up and making it more efficient overall; more complex and intricate jobs can be handled and no mould costs are required.
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