Metal founding – Means to shape a forming surface – Pattern
Reexamination Certificate
2001-12-11
2002-10-22
Elve, M. Alexandra (Department: 1725)
Metal founding
Means to shape a forming surface
Pattern
C164S235000, C164S362000, C164S397000, C164S376000
Reexamination Certificate
active
06467530
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to lost wax casting of metals and, more particularly, to the design of a sprue and a base for supporting the sprue that significantly reduces splash and spatter which often occur in the molten metal pouring process, and which lead to cold shorts that result in imperfect castings and loss of precious metals.
BACKGROUND OF THE INVENTION
Industries which produce small castings generally do so using a tree which includes a sprue button, a sprue, feed sprues and patterns which are the objects of the castings. The tree is made of a material, such as wax, that can be removed from the mold by burning or chemical dissolution to leave a cavity which is an accurate reproduction of the original tree. The tree is held upright on a sprue base, which base, together with an outer tube, also held upright by the base, forms an open container in which the tree is located such that the tree can be surrounded (invested) by a material called investment, such as a plaster, which hardens after pouring to make a mold by filling the container with the investment through the open end of the container. The container with the tree and the investment is placed in a vacuum in order to remove any air bubbles before the investment sets or hardens. The sprue base is detached from the container after the investment material hardens and the tree is removed by burning it out in a hot oven or by chemical dissolution, leaving a cavity in the mold that may be filled or cast with liquid metal. The entire process is called lost wax casting.
In presently used apparatus the sprue button or pour hole is formed as part of the sprue base which holds the sprue.
FIG. 1
hereof, labeled “Prior Art”, is a schematic representation of a typical sprue and base apparatus for supporting sprues currently used in the jewelry industry. Shown is horizontally disposed base member,
10
, which supports the cylindrical shaft,
12
, of sprue,
14
, in socket,
16
, formed in sprue button,
18
. Although the sprue is designed to fit snugly into the cavity, occasionally, a sprue is found floating in the investment slurry,
20
, which is poured into container,
22
, formed by base member
10
and vertical flask wall,
24
. A combination of wear in cavity or socket
16
and air trapped beneath the sprue therein, is thought to be the cause of this occurrence. To reduce the tendency of the sprue to be released from cavity
16
, a hot tool is often used to make a seal or junction,
26
, between the sprue button
18
and the sprue
14
. However, this procedure results in a narrowing of the wax sprue causing turbulent flow to occur when liquid metal is ultimately poured into the cavity formed by the sprue button and the sprue. This restricts the movement of the molten metal into the sprue network,
28
, which increases the liquid pool in the sprue button at the end of the pouring process. Additionally, as can be observed from
FIG. 1
, the shape of sprue button
18
by itself introduces an non-uniform transition between the pour hole and the open end of the sprue. Higher mold temperatures must be employed to compensate for the slower metal flow, and casting defects often occur during the pouring of the molten metal into the cavity. However, the high temperatures involved, the need to maintain an appropriate atmosphere surrounding the melting and pouring zone, and the opacity of the mold itself have prevented visual observation of the actual process.
Molten metal which overflows the pour hole in the mold formed by the base of the sprue is called splash, while unfilled or partially filled patterns are known as cold shorts. It is presently believed by practitioners of the lost wax process that splash is a consequence of too rapid filling of the mold cavity; in fact, the present inventor has determined that splash results from a wave that overflows the pour hole when the end of the poured liquid metal stream enters the liquid pool in the sprue button. Cold shorts are the result of metal having been lost from the pour hole causing a reduction in pressure in the sprue itself and increasing the likelihood that patterns near to the pour hole will not fill completely. Such shorts are localized near to the pour hole. Random cold shorts or cold short fills are identified as patterns on a cast tree which have not completely filled that are randomly disposed on the tree. Such short fills are caused by temperatures which are insufficient to permit the metal to flow such that the mold cavity can completely fill before the metal solidifies. This can be the result of too low a metal temperature and/or too low a mold temperature. It is believed by the present inventor that turbulence in the liquid metal flow in the region of the junction of the feed sprues and the main sprue caused by the transition between the pour hole and the main sprue leaves small metal deposits which further restricts the metal flow.
Often voids or inclusions are caused by small pieces of investment material breaking away from the mold and being carried into the pattern cavity by the liquid metal during pouring. It is believed that the sprue and sprue bases currently in use contribute to damage of the investment mold during removal of the sprue base member from the mold, since it is not possible to remove such sprue bases without creating stresses on the sprue and also on the investment material which is weak and easily damaged. In some situations, part of the total volume of molten metal which is introduced into the pour hole splashes out, thereby reducing the pressure head in the cast tree and increasing the likelihood that the patterns nearest the pour hole do not completely fill with molten metal.
Small grains of metal which are separated from the cast tree are called spatter. In the situation where precious metals are employed, these small spherical grains of metal make inventory control difficult, and reuse of the spattered metal is undesirable since the metal may be contaminated, thereby adding to the cost of the castings. It is believed that when the molten metal stream enters the transition between the sprue button and the sprue in existing sprue designs, small metal spheres bounce out of the sprue button. Additionally, spattered metal may interfere with the operation of the casting machines.
Another difficulty with castings produced using existing sprue designs is the formation of bubbles. These are small spherical additions to the castings caused by air bubbles clinging to the wax tree generated when the investment makes a mold of the bubbles which are subsequently reproduced in metal during casting. For sprues currently in use, these bubbles are caused by air trapped in the sprue button or in the sprue itself released toward the end of the vacuum cycle too late to be removed.
Accordingly, it is an object of the present invention is to provide a casting apparatus which reduces molten metal splash that often occurs in the pouring process and which leads to cold shorts or voids and results in imperfect castings.
Another object of the invention is to provide a casting apparatus for reducing molten metal spatter which results in the loss of precious metals.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus for forming a pour hole and main sprue in an investment mold hereof may include: a wax sprue including an elongated upper portion adapted to receive at least one feed sprue, and a flared base; a horizontally disposed base member having a socket formed
Freund Samuel M.
Lin I.-H.
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