Metal founding – Process – With measuring – testing – inspecting – or condition determination
Patent
1996-06-07
1999-06-22
Batten, Jr., J. Reed
Metal founding
Process
With measuring, testing, inspecting, or condition determination
164119, 164133, 1641553, 164306, 164312, 164337, 266239, B22D 1804
Patent
active
059133589
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a casting apparatus and method, for use in low pressure casting in particular, and to provide metered quantities of metal for other casting processes, in general.
The majority of castings are now made by pouring liquid metal vertically under gravity in a turbulent fashion into moulds. The turbulence is often responsible for the majority of defects found in castings. The advantages of filling moulds in an upward direction in a non-turbulent manner have been known for a number of years and several systems have been devised which utilize this filling mode. However, such systems have not been popular because of their cost and general inflexibility for normal foundry use.
A typical casting apparatus which incorporates upward filling will usually comprise a large holding furnace from which the liquid metal is pumped either by an electromagnetic or pneumatic system.
For pneumatic systems, the large volume of gas which has to be pressurised renders such an apparatus slow in operation. Furthermore, the furnace has to be opened up frequently to recharge it with liquid metal, which causes an interruption to the casting cycle. The most usual form or this type or low pressure machine has three pressure regimes; first, a rapid pressure application to fill the riser tube or feeder tube; a second and lower rate of pressurisation to fill tile casting more slowly; and a final intensification pressure, commonly in the region of 4 bar, which is applied during the casting solidification stage. Although predominantly used with metal moulds, this pneumatic form of low pressure casting has also been used with plaster and sand mounds. Other types of pneumatic furnaces have been adapted to dispense a metered volume of liquid from the riser tube onto a launder for use in either gravity casting or high pressure die casting.
The alternative type of low pressure casting system using electromagnetic pumping units are expensive to install and costly to maintain since the electromagnetic pumps operate in an aggressive environment of liquid metal. Such low pressure pumping units do have one advantage over the conventional pneumatic furnaces in that the flow of metal through the electromagnetic pump is, in principle, easily controlled and can be electronically monitored in a fully computerised system. Electromagnetic pumping systems currently operate only in large holding furnaces for large runs of identical castings of constant alloy composition. Although it is sometimes claimed that they are intended for use in small foundries, such types of casting apparatus, being costly and inflexible in operation, are often unsuitable for use by jobbing foundries which constitute a large sector of the casting industry.
In order to overcome some of the special difficulties of both types of low pressure furnace, a casting apparatus was devised in which a small ceramic pressurising vessel, having a closable orifice in its lower region, was partially immersed in a reservoir of molten metal to effect replenishment of the pressurising chamber with metal after each casting operation. In such a way, only a small chamber is required to be pressurised to move metal upwardly into the mould cavity. However, the pneumatic apparatus described relied entirely on pressurisation by direct control of gas pressure through pressure gauges and/or pressure switches. Such a pneumatic control system is obviously limited in its range of operation by the number of independent pressure gauges and switches constituting the control assembly. Clearly, for complex metal flow profiles into complex casting cavities the hardware associated with such control assemblies becomes burdensome. Furthermore, the questionable mechanical stability of the ceramic crucible at the operating temperature and its possible limited lifespan due to thermal cracking, coupled with replacement costs of the crucibles used as the pressure vessel, create problems in selecting an economic crucible material. That is, whereas the refractory lined steel pressure vessel of a conven
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Batten, Jr. J. Reed
Hi-Tec Metals Ltd.
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