Metal founding – Process – Shaping a forming surface
Patent
1993-06-15
1996-11-12
Bradley, P. Austin
Metal founding
Process
Shaping a forming surface
B22C 900
Patent
active
055730558
DESCRIPTION:
BRIEF SUMMARY
This invention relates to water dispersible moulds for use in making foundry castings or injection mouldings.
The term "mould" as used in this specification includes both a mould for producing castings with or without cavities, and a core for producing a cavity in a cavity-containing casting, and combinations of such moulds and cores. The term "casting" used in the specification encompasses foundry casting and other moulding processes such as injection moulding.
Cores and moulds are made from sand or other refractory particulate materials and it is customary to add binders in order to give the necessary properties of flowability (to enable the core/mould to be formed), stripping strength (to enable cores/mould to be handled soon after forming) and the ultimate strength to withstand the conditions occurring during Casting.
The refractory particulate materials and binder are formed into a core or mould by various processes which include ramming, pressing, blowing and extruding the mix into a suitable forming means such as a core box, a moulding flask, or a moulding or mould box. A mould is generally left in the forming means or alternatively it may be removed therefrom; a core is removed from the forming means, optionally after a curing step in which the core is cured to a higher strength than the green strength. If the Curing step is omitted the core requires sufficient green strength so that on removal from the forming means the mixture does not collapse. The core or mould is then allowed to cure, artificially cured or baked to further increase its strength so that it will resist the pressure and erosion effects of the molten metal and retain its shape without breakage or distortion until the metal has solidified. Some binders for the refractory particulate materials result in cores which are difficult to remove from the cavity after casting. Some cores, particularly those employing a sodium silicate binder, increase in strength when exposed to high casting temperatures. The result is that the core is not water dispersible and is difficult to break up mechanically in order to remove it from the casting.
It is well known to employ, for the production of castings, cores or inserts made from a ceramic composition around which the metal or alloy is cast. The cores or inserts are removed after casting by mechanical means, for example by percussion drilling, or in the case of complex shapes or fragile castings by dissolution in a solvent which does not react with the metal of the casting. Alternatively, if an organic binder is used the casting and core may be heated to a temperature approaching the melting point of the casting to break down the organic binder.
A suitable core must satisfy a range of requirements. shape throughout the casting process; it must withstand elevated temperatures; it must be removable from the casting without damaging the casting; and it must be made of a material or materials that do not damage or weaken the casting. The core must also be stable and provide a high quality surface finish.
U.S Pat. Nos. 3,764,575, 3,963,818 and 4,629,708 each disclose methods for using dispersible cores in a casting process, For instance U.S Pat. No. 4,629,708 uses a mixture of a water soluble salt, a calcium silicate and a binder. Examples of suitable materials of the water-soluble salt include potassium chloride, sodium metasilicate or preferably sodium chloride. The binder may be a paraffin wax, a synthetic organic resin, a silicone resin or preferably polyethylene glycol. The mixture is injection moulded and then fired to drive off organics and to sinter particles of the water soluble salt. After casting the core is removed by dissolution in water. The nature of the core material means that time needed for removal of the core can be commercially unacceptable. The solution being in Contact for a relatively long period with the casting can cause corrosion.
U.S Pat. No. 3,764,575 discloses a core comprising a water soluble salt, such as alkali or alkali earth metal chlorides, sulphates or borates, wate
REFERENCES:
patent: 1751482 (1930-03-01), Leasman
patent: 3764575 (1973-10-01), Anderko
patent: 3968828 (1976-07-01), Toeniskoetter
patent: 4070195 (1978-01-01), Toeniskoetter
patent: 4078599 (1978-03-01), Makiguchi
patent: 4602667 (1986-07-01), Moore
patent: 4768567 (1988-09-01), Travillian
Challand Nigel
Melling Richard
Borden (UK) Limited
Bradley P. Austin
Miner James
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