< SHELL (CORNING) PROCESS
DIE CASTING
Die casting is a method of creating a cast either by GRAVITY POURING, or PRESSURE INJECTING molten metal into a rigid mould. The defining feature of this process is the use of a permanent mould or DIE from which the process takes it’s name. The die is constructed from a metal alloy capable of withstanding repeated cycles of applied heat and pressure over prolonged periods of use. Dies must be designed in a way that allows the ejection of the formed cast once it has chilled. With this in mind, a complex die is divided into sections that separate to clear any design feature that might LOCK-IN the casting. These restrictions mean that a careful preliminary design process is essential before any object is committed to production by die casting.
The construction of a die can be a potentially complicated engineering undertaking, requiring the skills of a specialist TOOLMAKER to fashion a highly accurate ‘negative impression mould’ of the design out of a metal block. Often a labour intensive procedure, even simple die constructions can cost several hundred dollars, though dies which run to thousands of dollars in development costs are not uncommon. The advantage of this casting process however, is that once the die is complete, a great many casts (frequently ordered and produced in multiples of 100,000+) can be output at a far lower unit cost than would ever be possible using sand or investment casting processes.
Die casting is most suited to the production of large quantities of a simple design – the greater the volume produced, the easier it is to offset the initial development costs. Although die cast designs typically offer an excellent standard of as cast finish and are dimensionally very accurate, some ‘post-cast’ machining of the work will often be necessary. The finishing of die casts normally involves the drilling and tapping of fixing points, plus the application of a surface finish, for instance polishing or patination. These additional procedures take time, and also have to be factored into any ‘per unit’ cost calculation. Artists and designers wishing to work with this process are advised to seek detailed information, and obtain a variety of quotations from both die toolmakers, and die cast founders. The services of a technical draftsman may also need to be engaged, for the drawing up of accurate and notated design information for the toolmaker.
ALLOYS FOR DIE CASTING
The most common casting alloys for die cast production are those based on lighter metals; principally zinc and aluminium, though copper alloys - especially brasses, are also often used for this process (METALS). Lighter alloys used for die casting are usually melted in large electric resistance furnaces (often referred to as BASIN FURNACES). These furnaces are effectively ‘maintained temperature holding tanks’ for the charge. When die casting by gravity assistance alone, small moulds are usually filled from hand held ladles, which are manually recharged from the open topped basin furnace. Automatic and semi-automatic injection casting machines may be fed with metal direct from the furnace.
CONTINUOUS, CENTFRIGUAL & CUTTLEFISH CASTING >
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