Developed in the early 1900’s, die casting is a permanent mould manufacturing process. The process involves the use of a large amount of pressure in order to force molten metal through a predesigned mould of any industrial purpose metal casing. Since it is a high pressure process, it is employed in casting of metal casings with intricate surface details, pinpoint accuracy in dimensions and very thin walls. Some of the regular products made from die casting are toys, tools, carburettors, machine components and their housings, alloy wheels, engine blocks, engine components and motors of different size and caliber. The castings vary in size and weight and can range from a few grams to almost fifty kilograms.
The process begins with the preparation and clamping of the two halves of the die, each of which comprises of a cavity the exact shape, size and structure of the part to be produced. These cavities are created with the help of hardened tool steel dies which undergo machining to obtain the required shape – quite similar to the injection moulding process. Commonly used materials for the making of these dies are zinc, copper, aluminium, magnesium, lead, pewter and other alloys based out of tin.
Transfer of molten metal
Each die is cleaned of residue from the previous round and re-lubricated to assist in the ejection of the next part. The lubrication time is directly dependent and proportional to the size of the part, and the number of cavities, side cores etc which define the design and structure of the part being produced. Lubrication can be done routinely instead of after each cycle and round, depending on the material being used for the casting. Once the lubrication is done, the two die halves attached inside the die casting machines, are then closed and securely clamped. In order to keep the die securely closed while the metal is being injected, a certain amount of force is required to be applied. The clamping time depends on the size of the machine, since bigger machines with greater clamping forces require more time.
The molten metal is maintained at a preset temperature in the furnace and is then transferred into a chamber from where it is then injected into the die. The transfer method depends upon the type of the die casting machine, whether it is a hot chamber or cold chamber one. Once transferred, the molten metal is then injected at a high pressure into the die. Injection pressures may range from 1000, to 20,000 psi. This pressure holds the molten metal in the die, during its solidification. The quantity of metal to be inserted is fixed and constant and is referred to as the “Shot”. The injection time is what the molten metal takes to fill all the channels and cavities in the die. This time is of a very short duration and helps prevent the possibility of early solidification of any part of the metal. Injection time can also be determined by the thermodynamic properties of the material, as well as the wall thickness of the casing. Greater wall thicknesses require more time for injection.
The molten metal eventually begins to cool and solidify once inside the die cavity. With the solidification of all of the metal inside the die, the final cast shape is formed. The cooling time must elapse and the casting fully solidified, in order to attempt the opening of the die. The cooling time is also estimated from the thermodynamic properties of the metal, the maximum wall thickness of the casting and the complexity of the die.
Once the cooling time has lapsed, the die halves are then pried open and an ejection mechanism pushes the casting out of the die cavity. The ejection mechanism is required to apply force to eject the part since cooling causes the part to shrink and adhere to the die. Once the casting gets ejected, the die is again cleaned and lubricated for the next injection round and clamped shut.
Once the cooled casting has been ejected from the die, the excess material that forms around crevices or patterns or even the main surface of the casting, is trimmed and removed either manually through cutting, sawing or using a trimming press. The scrap material is generally discarded and can also be reused in the casting process. Recycled material may require to be reconditioned to the original chemical composition in order to help combine it with the non recycled metal and reused in the die casting process.