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# 2.8 Casting our gearwheel

Let's now consider the problem of how best to make the food mixer gearwheel we discussed in Section 1. Could it be made by any of these casting processes? Since it is a simple solid object the answer must be yes, of course, but which processes are feasible? If the gearwheel is to be made from metal then we can consider all of the casting processes we have described:

1. sand casting;

2. gravity-die casting;

3. pressure-die casting.

Let's consider each of these in turn.

Sand casting is unattractive for volume production of this shape and size. The cast wheel would have a rough surface which would need to be machined, and a new sand mould would be needed for every product. Because the wheel is such a small component, there would be a lot of scrap, and so this makes the process rather expensive and time consuming when you need to produce of the order of 100,000 gearwheels. However, sand casting can be used for mass production of parts such as engine blocks for cars, where it is more economical than other processes. We can probably rule it out for production of the gearwheel, though!

Gravity-die casting gives a better surface finish and the die is reusable almost indefinitely. However, even this surface would need some machining in order to achieve the required accuracy of tooth shape of the gear, and to remove the runner and any little sheets of extra material ('flash') at the splits in the mould. In addition, it is a slow process, so gravity-die casting is probably not the best option.

Pressure-die casting looks promising. Here the as-cast surface needs little or no finishing, and provided the casting has the required strength for the application then this would a feasible option.

Next we consider injection moulding. Provided a thermoplastic is acceptable for the gearwheel, this process can be used. It produces an excellent surface finish (better than pressure-die casting) and has a short cycle time. Again, provided the moulding is strong enough, then injection moulding is a feasible option. So at this point in our analysis the first two candidate processes to manufacture our gearwheel are pressure-die casting and injection moulding.