GB1597565A - Gravity casting - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO GRAVITY CASTING (71) We, MITSUBISHI JIDOSHA KOGYO KABUSHIKI KAISHA, a Japanese company, of 33-8, Shiba 5-chome, Minato-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a gravity casting process and apparatus suitable for die casting of a metal such as an aluminum alloy utlizing gravity.

A so-called gravity casting process utizing the head difference of a melt has been widely employed for casting various metals and alloys on account of the fact that such a process is quite simple in principle and can easily be put into practice.

However, the yield rate (the proportion of the weight of products to the weight of metal melt cast to form the products) in this gravity casting process has generally been relatively low, commonly of the order of 45 % to 70 %, although the rate varies depending on the shape and other factors of the products. The gravity casting process has had many drawbacks, for example slag tends to be produced in large amounts due to oxidation of the melt while flowing through the sprue, runner and other parts of the casting apparatus, and also turbulence tends to occur in the melt flow since it is difficult to reduce the pressure head beyond a certain amount during casting.

Therefore, a complex casting plan has been required to obviate rejects, for example caused by inclusion of oxides in the products, and this requirement has resulted in a low yield rate of melt.

In an effort to obviate the aforementioned defects of the gravity casting process, a pres sure casting process which is called a lowpressure casting process or a low-pressure die casting process has been developed recently, according to which a holding furnace is provided, and air pressure at about 0.2 atmosphere is imparted to the surface of a metal melt reserved in the furnace to establish bottom pouring. This low-pressure casting process provides such merits that the yield rate is generally higher than about 90 %, less oxidation occurs on the melt, the laminar flow of the melt reduces the tendency of inclusion of oxides, and the casting procedure is simple.

However, in the case of this low-pressure casting process, the amount of melt reserved in the holding furnace decreases at the end of each casting cycle resulting in a variation of the liquid level, and the supply of fresh melt for the purpose of replenishment also causes a variation in the liquid level. This leads to constant variations in the pressure imparted to the melt, and non-uniformity tends to appear in the quality of products. Therefore, a holding furnace of large size is inevitably required, and provision of pressure controlling means of complex structure is required to attain mass production of products of uniform quality.

The low-pressure casting process has therefore been defective in that a high equipment cost is involved for the construction of a holding furnace of large size and the provision of the complex pressure controlling means, and a vast power cost is involved for maintaining the melt in such a large-sized furnace at the proper temperature. In addition, considerable expense is involved in cleaning, repair and maintenance of the furnace and much time for subsequent preparatory operation thereof. The prior art low-pressure casting process has further been defective in that the melt reserved in the furnace must be continuously heated even during non-working days such as holidays, once the operation is started, in order to maintain the designed efficiency of the system.

In the course of development of die casting machines, a so-called air machine adapted for casting a melt by air using a gooseneck melt feed pipe was utilized in this field. However, due to various circumstances, the die casting process of this kind has been replaced by plunger die casting and low-pressure die casting, and the application of the idea of the air machine to gravity casting has been ignored.

Noting the simplicity and high efficiency of the prior art gravity casting process and the high yield rate of the prior art low-pressure casting process, it is a primary object of the present invention to provide a novel and improved gravity casting process which obviates the aforementioned defects of the prior art processes.

According to one aspect of the present invention there is provided a gravity casting process using a communicating tube having first and second upwardly extending ends, the first end being connected to a gate formed at the bottom of a mold and the second end opening to the atmosphere at a level higher than the mold, the process comprising: a preparatory step of feeding a melt into the communicating tube to a level beneath the gate; a first step of feeding an additional amount of melt into the communicating tube and locating a plunger in the melt so as to fill the mold cavity of the mold with melt under gravity and to establish and maintain a gravity head of melt until a portion of the melt in the vicinity of the gate of the mold solidifies; and a second step of removing the plunger from the melt so as to release the gravity head and allow the level of liquid melt to fall from the gate.

According to a second aspect of the present invention there is provided a gravity casting process using a communicating tube having first and second upwardly extending ends, the first end being connected to a gate formed at the bottom of a mold and the second end opening to the atmosphere at a level higher than the mold, the process comprising: a preparatory step of feeding a melt into the communicating tube to a level beneath the gate and moving a plunger downwards into the communicating tube through the open end thereof to a position in which the plunger does not contact the melt; a first step of feeding an additional amount of melt into the communicating tube and locating the plunger in the melt so as to fill the mold cavity of the mold with melt under gravity and to establish and maintain a gravity head of melt until a portion of the melt in the vicinity of the gate of the mold solidifies; a second step of removing the plunger from the melt and stopping the plunger at a position in which the lower end of the plunger is located adjacent to the liquid level; and repeating a combined cycle of the first and second steps a plurality of times as desired.

According to a third aspect of the present invention there is provided gravity casting apparatus comprising: a mold having a gate formed at the bottom thereof; a communicating tube having first and second upwardly extending ends, the first end being connected to the gate and the second end opening to the atmosphere at a level higher than the mold; melt feeding means for feeding melt into the communicating tube; and a plunger movable upwards and downwards in the communicating tube by elevator means such that, when the plunger is moved downwards and submerged in the melt in the communicating tube, the level of the melt is raised to establish a given gravity head and, when the plunger is moved upwards out of the melt, the melt in the communicating tube drops to a level below the gate of the mold.

For a better understanding of the present invention and to show more clearly how it may be carried into effect. Reference will now be made, by way of example, to the accompanying drawings, in which: Figures 1 to 3 are schematic views of a casting apparatus showing a preparatory step, first step and second step respectively of a gravity casting process according to the present invention; Figures 4 and 5 are schematic views showing the preparatory step and first step respectively of an alternative gravity casting process according to the present invention; and Figure 6 is a sectional view showing in detail the structure of one embodiment of gravity casting apparatus according to the present invention.

A casting process according to the present invention will now be described in detail with reference to Figures 1 to 3.

Referring to Figures 1 to 3, a supporting frame structure 1 supports a metal mold 2 and a generally U-shaped communicating tube 3. The mold 2 is split into an upper mold section 2a and a lower mold section 2b. The communicating tube 3 consists of a melt feeding portion 3a and a metl pouring portion 3b.

The melt feeding portion 3a is generally in the form of a J as seen in Figures 1 to 3 and is disposed on one side of the mold 2. The upper end of the melt feeding portion 3a is located to open to the atmosphere at a level higher than the mold 2 to provide a melt feed port 3d, and its lower end is connected with the lower end of the melt pouring portion 3b. The upper end of the melt pouring portion 3b provides a melt pouring port which communicates with a gate 4 formed in the bottom of the mold 2. A melt 5 is fed into the communicating tube 3, and the mold 2 includes a core 6 located therein to define a mold cavity 7.

A ladle 8 feeds the melt 5 into the melt feed port 3d. A plunger 9 is located above the melt feed port 3d of the communicating tube 3 and can be moved upwards out of and downwards into the communicating tube.

In a preparatory step of the casting process, the melt 5 is fed from the ladle 8 into the communicating tube 3 through the melt feed port 3d. In this preparatory step, the melt 5 is fed into the communicating tube 3 until the liquid level of the melt in the melt pouring portion 3b attains a level benath the gate 4, as shown in Figure 1. In this case, the cavity 7 in the mold 2 together with the upper open end of the melt feeding portion 3a communicates with atmosphere, and the liquid level of the melt in the melt pouring portion 3b is the same as that in the melt feeding portion 3a.

Therefore, the liquid level of the melt pouring portion 3b can be easily and accurately controlled by controlling the liquid level of the melt in the melt feeding portion 3a.

In a first step, following the preparatory step, an additional supply of melt is fed into the communicating tube 3 through the melt feed port 3d, and the liquid level of the melt in the melt pouring portion 3b is gradually raised while maintaining substantially the same level as that of the melt in the melt feeding portion 3a.

Consequently, the melt partially fills the mold 2. The plunger 9 is then gently moved downards into the melt feeding portion 3a.

As the plunger 9 is submerged in the melt 5, the liquid level of the melt is further raised so that the melt completely fills the mold 2. The plunger 9 is further continuously moved down wards into the communicating tube 3 until a gravity head H of melt necessary for the feeding effect is established, as shown in Figure 2. This head H is maintained during the chill time until at least a portion of the melt in the vicinity of the gate 4 of the mold 2 solidifies. The casting plan is generally arranged so that the melt in the cavity 7 in the mold solidifies completely before the melt portion in the vicinity of the gate 4 solidifies.

The total amount of the melt fed into the melt feeding portion 3a in the first step shown in Figure 2 is substantially the amount fed into the cavity 7 in the mold 2. The design of the plunger and the velocity of downward movement of the plunger 9 can be selectively modified and adjusted.

Since the liquid level of the melt being poured can easily be detected visually and the gravity head of the melt can easily be adjusted the gentle pouring of the melt into the mold 2 is assured in a simple manner without turbulence occurring in the melt.

The second step proceeds upon complete solidification of the melt portion in the vicnity of the gate 4 of the mold 2. As shown in Figure 3, the plunger 9 is moved upwards, thereby releasing the head H which imparted the feeding effect. At the same time, the liquid level of the melt in the melt pouring portion 3b, including the unsolidified melt in the vicinity of the gate 4, is completely separated from the solidified casting product at the gate 4 and drops to a level beneath the gate 4 of the mold 2. That is, the liquid level of the melt in the melt pouring portion 3b is restored to the original level which is the same as that of the melt in the melt feeding portion 3a, as shown in Figure 1.

After completion of the casting cycle, the upper section 2a of the mold 2 is separated from the lower section 2b, also as shown in Figure 3, to take out the casting product 10.

Then, the mold 2 is cleaned to prepare it for the next casting cycle, or the mold 2 is replaced by another mold prepared previously Thereafter, the first step for replenishment of the melt, as shown in Figure 1, is intiated to repeat the casting cycle, the preparatory step being unnecessary because of the melt already present in the communicating tube 3.

The amount of melt replenished into the communicating tube in this first step is the same as the amount necessary for casting the product and is maintained at a constant amount in each casting cycle. Thus, the replenishing operation of the melt becomes simple and easy.

In an alternative version of the process according to the invention, the first step of the casting cycle is modified. In the first step, the additional supply of melt is fed into the communicating tube 3 through the melt feed port 3d. The amount of melt fed into the melt feeding portion 3a in this tep is such that the liquid level of the melt in the melt pouring portion 3d attains substantially the level of the lower end of the gate 4, but does not enter the gate 4. The plunger 9 is then gently moved downwards into the melt feeding portion 3a.

As the plunger 9 is submerged in the melt 5, the liquid level of the melt in the melt pouring portion 3b is raised so that melt completely fills the cavity 7 in the mold 2. The gravity head H of the melt necessary for the feeding effect is subsequently established by the continued downwards movement of the plunger 9. Since the cavity 7 is filled with melt by the continued downwards movement of the plunger 9, the velocity for pouring the melt into the mold 2 is kept constant without interruption, and production of oxides in the melt can be prevented.

In a further alternative version of the process according to the invention, the first step of the casting cycle is again modified. This further alternative version is explained with reference to Figures 1 to 3.

In the first step of the casting cycle, the plunger 9 is firstly gently moved downwards into the communicating tube 3 through the melt feed port 3d: The liquid level of the melt in the melt pouring portion 3b is raised as well as that of the melt feeding portion 3a until the liquid level of the melt in the melt pouring portion 3b attains substantially the level of the lower end of the gate 4 but does not enter the gate 4. The additional supply of melt is then fed from the ladle 8 into the communicating tube 3 through the melt feed port 3d. As the liquid level of melt in the melt pouring portion 3b is raised, the melt 5 enters the cavity 7 in the mold 2 from the gate 4 and completely fills the cavity 7. The melt is continuously fed from the ladle 8 into the communicating tube 3. When the liquid level of the melt in the melt feeding portion 3a attains a level at which the gravity head H of melt necessary for the feeding effect into the mold 2 is established, the melt feeding operation is stopped. Since the cavity 7 is filled with melt by feeding the melt into the communicating tube 3, the velocity of pouring the melt into the mold 2 is kept constant by maintaining the velocity of feeding the melt into the communicating tube 3 at a constant value, and the production of oxides in the melt can be prevented.

In the first step of the casting cycle, reduction of time required for performing the casting operation may be achieved by feeding the additional supply of the melt and moving the plunger downwards simultaneously.

In another alternative version of the process according to the invention shown in Figures 4 and 5, the same reference numerals are used to denote substantially the same elements as those denoted in Figures 1 to 3.

Referring to Figures 4 and 5, in the preparatory step of the casting operation, the melt 5 is fed from the ladle 8 into the communicating tube 3 through the melt feed port 3d. In this preparatory step, the melt 5 is fed into the communicating tube until the liquid level of melt in the melt pouring portion 3b attains a level beneath the gate 4, as shown in Figure 4.

The plunger 9 is subsequently moved downwards into the communicating tube 3 so that the lower end of the plunger 9 is positioned slightly above the liquid level of melt in the melt feeding portion 3a, as shown in Figure 4.

In the first step following the preparatory step, the plunger 9 is moved further downwards to be submerged in the melt 5 in the melt feeding portion 3a. At the same time, the additional supply of melt is fed from the ladle 8 into the communicating tube 3 through the melt feed port 3d. The melt is further continuously fed into the communicating tube 3 after the plunger 9 has been moved downwards through its full stroke and stopped.

When the liquid level of melt 5 in the melt feeding portion 3a attains a level at which the gravity head H of melt necessary for the feeding effect into the mold 2 is established, as shown in Figure 5, the supply of melt from the ladle 8 into the communicating tube 3 is terminated. This state is maintained until the melt 5 in the cavity 7 of the mold 2 solidifies.

In the second step, when the melt 5 in the gate 4 solidifies (at this time the melt in the cavity 7 has already solidified), the plunger 9 is moved upwards so that the lower end of the plunger 9 is moved away from the liquid level of the melt in the melt feeding portion 3a.

The plunger 9 returns to the initial position in which the lower end of the plunger 9 is po toned slightly above the liquid level ofthemelt, as shown in Figure 4. Consequently, the liquid level of the melt in the melt pouring portion 3b drops away from the gate 4.

After completion of the casting cycle, the casting product 10 is taken out of the mold 2.

Then, the mold 2 is cleaned to prepare it for the next casting cycle, or the mold 2 is replaced by another mold newly prepared.

Thereafter the first and second steps are repeated.

In this version of the process, the plunger 9 moves upwards and downwards through a short stroke, and the supply of melt into the communicating tube 3 takes place simultaneously with the downwards movement of the plunger 9. The result is that the time required for performing the casting operation is greatly reduced.

In another version of the process according to the invention, the first step of the casting cycle in the preceding version is modified.

This version of the process is explained with reference to Figures 4 and 5. The plunger 9 is firstly moved downwards to be submerged in the melt 5 in the melt feeding portion 3a until the liquid level of the melt in the melt pouring portion 3b attains substantially the level of the lower end of the gate 4, but does not enter the gate 4. Then, the additional supply of melt is fed from the ladle 8 into the communicating tube 2 through the melt feed port 3d so that the cavity 7 in the mold 2 is completelyfilled with melt 5. The melt is further continuously fed into the communicating tube 3 until the liquid level of the melt in the melt feeding portion 3a attains a level at which the gravity head H of melt is established, thereby providing the feeding effect into the mold 2.

Also, in the first step of another modification of the casting cycle in the previous version, the additional supply of melt is firstly fed into the communicating tube 3 until the liquid level of the melt in the melt pouring portion 3b attains substantially the level of the lower end of the gate 4, but does not enter the gate 4. Then, the plunger 9 is moved downwards so as to fill the mold 2 with melt and subsequently to establish the gravity head H necessary for the feeding effect.

According to the casting process of the present invention, the liquid level of the melt is previously located adjacent to the gate 4 of the mold 2 within the communicating tube 3, and in the first step of the casting cycle an additional amount of melt is fed into the communicating tube 3 and the plunger 9 is located in the melt in the melt feeding portion 3a, whereby the liquid level of the melt in the melt pouring portion 3b is gently and substantially continuously raised to feed the melt into the cavity 7 in the mold 2. Therefore, no turbulence occurs in the melt being fed into the mold 2, and oxidation of the melt being fed into the mold 2, and oxidation of the melt being fed and the possibility of inclusion of oxides in the melt are substantially eliminated, thereby reducing the number of reject castings to less than that according to the prior art gravity casting process as well as the prior art low-pressure casting process. That is, by utilization of the plunger for submerging in the melt, gentle feeding and pouring of the melt take place until the mold is completely filled with melt, and thereafter the gravity head of melt is subsequently established.

Therefore, no abrupt pressure variation occurs in the course of pouring of the melt.

In addition, the melt can be replenished by a constant amount in each casting cycle, so that no variation occurs in the gravity head imparting the casting pressure. Thus, defectfree castings having uniform quality can be mass-produced. Further, the yield rate of the melt can be improved over that of the prior art low-pressure casting process.

A practical structure of an apparatus preferably used for the practice of the casting process of the present invention will be described in detail with reference to Figure 6.

In Figure 6, the same reference numerals are used to denote the substantially same members and parts appearing in Figures 1 to 3 to dispense with any detailed description.

Referring to Figure 6, the melt feeding portion 3a of the communicating tube 3 consists of a lateral path extending gradually downwards from the junction with the melt pouring portion 3b and an upright path extending from the lowermost end of the lateral path. A drain port 3c is disposed adjacent to the lowermost end of the lateral path and is normally closed by a plug or valve member 11 during the casting operation. This drain port 3c is opend to drain completely the melt 5 from the communicating tube 3 when the interior of the communicating tube 3 is to be inspected or cleaned after a predetermined period of operation. The drain port 3c is also opened when a suitable lining is to be applied to the inner wall of the communicating tube 3 of, for example, steel to prevent undesirable dissolved constituents of steel from mixing with the melt which may be, for example, an aluminium alloy. The drain port 3c is also opened when the casting operation is to be interrupted for a relatively long period of time. The disposition of the lateral path of the melt feeding portion 3a in the manner illustrated in Figure 6 is advantageous in that the melt can be rapidly and completely drained from the communicating tube 3. The melt feed port 3d is formed by spreading the upper end of the melt feeding portion 3a rightwards in Figure 6. The melt feed port 3d has an opening area sufficiently larger than an axial sectional area of the upright path of the melt feeding portion 3a so as to facilitate the feeding of the melt from the ladle 8 into the communicating tube 3.

An electric heating coil 12 is wound around the communicating tube 3 so as to prevent cooling of the melt 5 which is always maintained at the predetermined level within the communicating tube 3 during the continuous casting operation. It is apparent that such an electric heating coil 12 may be replaced by any other suitable heating means, for example a low-frequency induction heating coil, a gas heater or an oil burner. The supporting frame structure 1 is hollow, and its internal wall is lined with a heat shielding layer 13 of, for example, refractory brick so as to prevent dissipation of heat from the communicating tube 3. The plunger 9 is preferably made from silica if the melt is aluminum. The plunger 9 is fixed at its upper end to a rod 14 which is in turn connected with an air cylinder 15 by way of a joint. The air cylinder 15 is fixed on the upper face of a lateral extension of an Lshaped strut 16. The lower end of the strut 16 is fixed to the supporting frame structure 1.

The ladle 8 forming melt feeding means is reciprocated between a furnace and the melt feed port 3d of the communicating tube 3 by carrier means, not shown. The ladle 8 is also arranged to be located above the melt feed port 3d of the communicating tube and inclined by the carrier means, as shown in Figure 6, so that the melt supplied from the furnace is fed into the communicating tube 3 through the melt feed port 3d.

In the apparatus, the plunger 9 may previously be located at a position in which the lower end of the plunger is close to, but does not contact the liquid level of the melt in the melt feeding portion 3a. The plunger 9 is adapted to be moved downwards from such a position and returned to the position after completion of each casting cycle.

However, the stroke of the air cylinder 15 in the apparatus may be extended to such an extent that the plunger 9 is firstly located at a position far from the melt feed port 3d of the communicating tube, and is adapted to be moved downwards to locate the lower end of the plunger 9 adjacent to the lowermost end of the communicating tube 3 at the lowest position of the plunger.

The apparatus of the present invention shown in Figure 6 is small in size and simple in structure compared with the prior art lowpressure casting apparatus since its principal components are the simple communicating tube 3 and the plunger 9 associated therwith.

Bulky equipment such as a holding furnace is unnecessary, and the apparatus requires a smaller area for installation than the prior art apparatus. In addition, the equipment cost, power cost, repair and maintenance cost, etc.

of the apparatus are far lower than the prior art apparatus. Further, the apparatus can perform the casting process with improved efficiency over the prior art low-pressure casting apparatus due to the fact that it does not require a holding furnace which must be continuously heated for a long period of time before starting the operation.

In the apparatus of the present invention, an automatic melt feeding mechanism such as an automatic ladle and an electromagnetic device may be provided instead of the ladle 8, so that the melt is fed automatically. The communicating tube 3 may include a plurality of branched melt feeding portions connected with a plurality of melt pouring portions communicating with a plurality of molds respectively so as to obtain a plurality of castings simultaneously. Also, the portion of the communicating tube adjacent to the mold may be independent of the remaining portion, to be assembled with the latter on the construction site.

According to the present invention, further, a plurality of prepared molds or a plurality of apparatus according to the invention may be arranged on a horizontal conveyor or a turntable to be sequentially moved to the stations at which the individual steps of the casting cycle are performed or preparation of the mold and/or delivery of the casting may be separated as other steps to be carried out at other stations. A mass production system which may be more efficient than that above described may be provided by such combinations.

Further, the plunger is preferably made from ceramics or heat resisting material. A mechanical, hydraulic or electrical operating device may be selectively utilized for moving the plunger upwards and downwards.

WHAT WE CLAIM IS:- 1. A gravity casting process using a communicating tube having first and second upwardly extending ends, the first end being connected to a gate formed at the bottom of a mold and the second end opening to the atmosphere at a level higher than the mold, the process comprising: a preparatory step of feeding a melt into the communicating tube to a level beneath the gate; a first step of feeding an additional amount of melt into the communicating tube and locating a plunger in the melt so as to fill the mold cavity of the mold with melt under gravity and to establish and maintain a gravity head of melt until a portion of the melt in the vicinity of the gate of the mold solidifies; and a second step of removing the plunger from the melt so as to release the gravity head and allow the level of liquid melt to fall from the gate.

2. A casting process according to claim 1, wherein a combined cycle of the first and second steps is repeated for successive molds.

3. A casting process according to claim 1 or 2, wherein the first step comprises moving the plunger downwardly through the open end of the communicating tube and into the melt subsequent to the feeding of the additional melt into the communicating tube.

4. A casting process according to claim 3, wherein the first

Claims (21)

**WARNING** start of CLMS field may overlap end of DESC **. continuously heated for a long period of time before starting the operation. In the apparatus of the present invention, an automatic melt feeding mechanism such as an automatic ladle and an electromagnetic device may be provided instead of the ladle 8, so that the melt is fed automatically. The communicating tube 3 may include a plurality of branched melt feeding portions connected with a plurality of melt pouring portions communicating with a plurality of molds respectively so as to obtain a plurality of castings simultaneously. Also, the portion of the communicating tube adjacent to the mold may be independent of the remaining portion, to be assembled with the latter on the construction site. According to the present invention, further, a plurality of prepared molds or a plurality of apparatus according to the invention may be arranged on a horizontal conveyor or a turntable to be sequentially moved to the stations at which the individual steps of the casting cycle are performed or preparation of the mold and/or delivery of the casting may be separated as other steps to be carried out at other stations. A mass production system which may be more efficient than that above described may be provided by such combinations. Further, the plunger is preferably made from ceramics or heat resisting material. A mechanical, hydraulic or electrical operating device may be selectively utilized for moving the plunger upwards and downwards. WHAT WE CLAIM IS:-

1. A gravity casting process using a communicating tube having first and second upwardly extending ends, the first end being connected to a gate formed at the bottom of a mold and the second end opening to the atmosphere at a level higher than the mold, the process comprising: a preparatory step of feeding a melt into the communicating tube to a level beneath the gate; a first step of feeding an additional amount of melt into the communicating tube and locating a plunger in the melt so as to fill the mold cavity of the mold with melt under gravity and to establish and maintain a gravity head of melt until a portion of the melt in the vicinity of the gate of the mold solidifies; and a second step of removing the plunger from the melt so as to release the gravity head and allow the level of liquid melt to fall from the gate.

2. A casting process according to claim 1, wherein a combined cycle of the first and second steps is repeated for successive molds.

3. A casting process according to claim 1 or 2, wherein the first step comprises moving the plunger downwardly through the open end of the communicating tube and into the melt subsequent to the feeding of the additional melt into the communicating tube.

4. A casting process according to claim 3, wherein the first step comprises moving the plunger downwadly through the open end of the communicating tube and into the melt subsequent to the feeding of the additional melt into the communicating tube to a level at which the melt partially fills the mold cavity of the mold.

5. A casting process according to claim 3, wherein the first step comprising moving the plunger downwardly through the open end of the communicating tube and into the melt subsequent to the feeding of the additional melt into the communicating tube substantially to the level of the gate, but such that it does not enter the gate.

6. A casting process according to claim 1 or 2, wherein the first step comprises moving the plunger downwardly through the open end of the communicating tube and into the melt simultaneously with the feeding of the additional melt into the communicating tube.

7. A casting process according to claim 1 or 2, wherein the first step comprises moving the plunger downwardly through the open end of the communicating tube and into the melt prior to the feeding of the additional melt into the communicating tube.

8. A casting process according to claim 7, wherein, when the plunger is moved into the melt, the melt attains substantially the level of the gate, but does not enter the gate.

9. A gravity casting process using a communicating tube having first and second upwardly extending ends, the first end being connected to a gate formed at the bottom of a mold and the second end opening to the atmosphere at a level higher than the mold, the process comprising: a preparatory step of feeding a melt into the communicating tube to a level beneath the gate and moving a plunger downwards into the communicating tube through the open end thereof to a position in which the plunger does not contact the melt; a first step of feeding an additional amount of melt into the communicating tube and locating the plunger in the melt so as to fill the mold cavity of the mold with melt under gravity and to establish and maintain a gravity head of melt until a portion of the melt in the vicinity of the gate of the mold solidifies; a second step of removing the plunger from the melt and stopping the plunger at a position in which the lower end of the plunger is located adjacent to the liquid level; and repeating a combined cycle of the first and second steps a plurality of times as desired.

10. A casting process according to claim 9, wherein the first step comprising moving the plunger downwards into the melt in the

communicating tube while simultaneously feeding the additional melt into the communicating tube, thereby filling the mold cavity of the mold with melt.

11. A casting process according to claim 9, wherein the first step comprises moving the plunger downwards into the melt in the communicating tube subsequent to the feeding of the additional melt into the communicating tube.

12. A casting process according to claim 11, wherein the first step comprises moving the plunger downwards into the melt in the communicating tube, to fill the mold cavity of the mold with melt, subsequent to the feeding of the additional melt into the communicating tube such that the melt attains substantially the level of the gate, but does not enter the gate.

13. A casting process as claimed in claim 9, wherein the first step comprises moving the plunger downwardly into the melt in the communicating tube prior to the feeding of the additional melt into the communicating tube to fill the mold cavity of the mold with melt.

14. A casting process as claimed in claim 13, wherein, when the plunger is moved downwards into the melt, the melt attains substantially the level of the gate, but does not enter the gate.

15. A gravity casting process according to claim 1 and substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

16. A gravity casting process according to claim 1 and substantially as hereinbefore described with reference to Figures 4 and 5 of the accompanying drawings.

17. Gravity casting apparatus comprising: a mold having a gate formed at the bottom thereof; a communicating tube having first and second upwardly extending ends, the first end being connected to the gate and the second end opening to the atmosphere at a level higher than the mold; melt feeding means for feeding melt into the communicating tube; and a plunger movable upwards and downwards in the communicating tube by elevator means such that, when the plunger is moved downwards and submerged in the melt in the communicating tube, the level of the melt is raised to establish a given gravity head and, when the plunger is; moved upwards out of the melt, the melt in the communicating tube drops to a level below the gate of the mold.

18. Casting apparatus as claimed in claim 17, wherein heating means is provided to maintain the melt in the communicating tube at a constant temperature.

19. Casting apparatus as claimed in claim 17 or 18, wherein a melt drain port is provided at the lowest level of the communicating tube.

20. Gravity casting apparatus as claimed in claim 17 and substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

21. Material cast by a process according to any one of claims 1 to 16 or in an appratus as claimed in any one of claims 17 to 20.