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Proof-Casting Cavities & Gun Chambers
Cerrolow 117® , Cerrosafe® and Cerrobase®
The alloy should be melted in a clean iron ladle. The source of heat should be removed as soon as the alloy is completely melted, at which time it is ready to pour. The solidified casting should be removed from the cavity, mold or chamber before or when it cools to room temperature. If it is allowed to remain in the mold too long it may be gripped to the mold.
The cavity to be tested must be free from undercuts to permit removal of the casting. Usually, no parting medium is necessary, but in some cases a thin film of a light weight oil (non-detergent) or graphite may be used for coating the inside of the cavity before casting the alloy.
It is important that the molten alloy be just hot enough to assure filling of the cavity so that solidification does not occur before filling is complete. This is usually about 10-25 degrees above the melting temperature of the alloy. If the alloy is overheated when pouring, danger of distortion of the part is likely, due to the transfer of heat from the alloy to the part causing undue expansion.
Deep, narrow cavities should be filled by pouring the alloy through a tube of sufficient length to reach the bottom, withdrawing the tube gradually as the liquid level aproaches the top. Sometimes better results are obtained when pressure can be exerted on the molten metal alloy. A head of metal in an extended pouring gate usually surfaces. The height may vary, but a gate of one to two times the depth of the cavity may be sufficient. Internal threads may be proof cast with low-melting alloys as the slugs are easily removed by "backing-out". A parting medium is necessary for proof casting threads.
Small cavities are proof cast with solid castings with these Cerro low melting alloys. The weight of medium and large castings may be reduced materially by using a wood core on the surface of which small head nails or staples may be driven and left protruding to serve as chaplets before centering the core in the cavity. The space between the inside of the cavity and the outside of the core should be 1/4" or more. The core need not follow the exact contour of the cavity. Dry hardwood is recommended for the core. The intervening space can then be filled with the molten alloy.
Proof-casting requires factors of minimum pouring temperature and medium volume change to assure accurate measurements. All the proof-casting media must initially shrink sufficiently to facilitate removal from the cavity.
CERROLOW 117® is preferred for measurements of light,
thin sectioned materials, because of its very low pouring temperature. This alloy is molten at 117 deg. F. and can be poured at just a few degrees above its melting temperature.
CERROSAFE® is the alloy most generally used for proof casting since its pouring temperature is 190 deg. F. It shrinks slightly for a few minutes after it solidifies due to the thermal contraction. This shrinkage is gradually off-set during the first hour after the casting reaches room temperature by the growth which takes place witin the alloy. Measurements made in approximately one hour after cooling to room temperature may be relied upon to be highly accurate.
CERROBASE®, with a melting temperature of 225 deg. F., is generally used for larger cavities such as forging dies.
The advantages of using any of the above listed Cerro Alloys for making proof-castings are: greater accuracy of proof casts through less expansion or distortion of the part being proved; assurance of reproduction of detail do to the expansion of the alloy on solidification; and the reduction in calculations to allow for shrinkage of the proof-casting medium itself, which is necessary in other less accurate proof-casting materials. It is economical to use Cerro Alloys for proof-casting because they do not deteriorate and can be re-used repeatedly.
Alloy being cast from an iron ladle into a closed mold.
3 minutes later, showing the fidelity of detail and rapid solidification.
A varitety of proofcastings of dies and molds
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