Processes - Casting

COMPARISON OF CASTING METHODS

INVESTMENT CASTING (LOST WAX PROCESS)

This method of precision casting is one of the oldest known and was certainly used for making the brass and bronze statues of Buddha and other religious images throughout the East. Because of the high degree of accuracy obtainable, it is used nowadays for precision casting of high melting-point alloys.

An accurate permanent metal mould for the required component is made, in which a replica of the component is cast in wax, complete with runner sections. This expendable wax model is now dipped in a slurry, or suspension in water, of a ceramic material which dries to form a coating adhering to the wax. Further coats are given until a sufficient body of ceramic is built up and dried. The whole is fired or baked to harden and strengthen the ceramic mould, during which the wax replica melts and burns out so that it is ‘lost’.

By this means a very precise mould with good surface finish is made, in which it is possible to cast high melting point alloys and produce a casting which requires a minimum of machining. As many of these alloys are extremely difficult to machine the expense of the casting process is more than justified.

DIE CASTINGS

Where castings are made from low melting-point alloys, the metal can be cast directly in a metal mould, which is then opened to remove the component. The metal may be poured in the normal way to produce a gravity die casting, or may be forced into the die under pressure to give a pressure die casting. The materials used are limited to low melting-temperature alloys and a zinc-base alloy is generally used for parts which carry little load. Aluminium is also die-cast and, in isolated cases, brass, but its melting point is the limit of temperature at which the process is used.

It is important to note that the dies which form the mould must be designed so that the casting can be removed in one piece without damaging the mould. The split line must be chosen with great care and in some cases multi-part ides with more than one split line are used. Ejector pins are provided to remove the solidified casting from the dies as they open. The dies are usually designed to produce more than one component per cycle of the die-casting machine, according to the component size, and up to twelve-impression dies may be used for small components.

After the component has been cast, the sprue is broken off and any flash is removed, usually by a clipping operation in a press.

The accuracy available is such that machining of the components can be kept to a minimum, being carried out only where a particular fit is required. An indication of the quantity of work produced by die casting can be gained by listing some of the components in a modern motor car which are made by this process. It should be noted that all these components carry very little load. They include:

Pressure die-castings can be produced as tiny parts such as small gears for instruments or as large as 35kg in mass.

Specialist Casting

Irvine Case Study

The vast majority of engine crankcases are pressure die cast, mostly in the Far East because it is not economic to pressure die cast in the UK, it costs twice as much both for the tooling and having it done. Some of the lower volume engines are investment cast, using the lost wax method. The problem with it is that you get an inconsistency in the castings caused by movement in the wax during production, so they are difficult to make fixtures for them to go into for machining operations.

The 20 car engine is investment cast. It is likely that in a year or so this will be changed to a pressure die cast. It is much more expensive. The tooling for an investment casting is probably 25% of the tooling for a pressure die casting, but the cost of the individual crank case is 6 times higher. If you can do the volume, pressure die casting is by far the best way, it is also far more accurate. You get a better surface finish as well because you don’t get the flow lines from when the wax leaves the mould. You should select the tooling method that is most suitable for the volume you are going to be producing. Some of the castings of back plates use gravity die casting. The back plate of the 20 engine is machined from solid using CNC machines. The advantage of CNC machining compared to casting is that you are not committed to high of tooling costs that will change if you want to change the design. The design change for CNC are parameters in the machining programme. This can be easily edited your design is changed. You can run relatively short batches and change things rapidly without having to commit to a tooling change, which is good. The down side is that things that you CNC manufacture take a lot longer to manufacture, but you do get a lovely finish, depending on the tooling that you’re using. If a product looks ugly, people are reluctant to buy it, it doesn’t really matter how good the specification is. There are exceptions to that, in the world of racing engines, what it looks like doesn’t matter at all, performance is the main criteria for success.

Specialist Coring

Irvine make a very special racing engine which uses an advanced version of investment casting, with double wax coring. Normally with any kind of coring system for the internals of the engine have mechanical sliders in them, (metal sliders that make the internal shapes) and they has to mechanically retract when removing the casting from the mould. You have to have a certain angle of taper to make sure the tools come out; any reverse angles or any really shallow tapers and the tools won’t come out. Because of that, it is limiting on the engine design and therefore performance of the engine. To overcome this the 15 engine has a ‘soluble core’ for the internal parts which is a solid core rather than a removable mechanical core. It is dissolved out with water, after casting as opposed to melting with the heat of the molten metal. It is a very expensive, very specialist method. When we originally started using it we were one of the first engineering companies in the UK in any shape or form to use this particular system. It’s relatively common now but only in specialist manufacture.

Casting Picture Gallery