GB2198977A - Melting and die-casting metal - Google Patents

Abstract

A die-casting machine has a shot sleeve (10, or 26) with a charge hole (9, or 25) to which a molten metal charge is delivered at the start of each casting operation. The charge is previously melted by induction heating, and in one form rests in a pivoted ladle (1) surrounded by the heating coil (2). The solid charge, cut from stock to the precise amount, is gravity delivered and, when melted, the ladle is tilted to pour directly into the charging hole (9). Alternatively, the charge is supported within an upright induction heating coil (21) by electromagnets (22) and, when molten, current switch-off causes the charge to drop directly into the charging hole (25). The apparatus can be inert gas shielded, and automated to run continuously, a charge being melted as the previous one sets in and is released from the die. <IMAGE>

Description

"Improvement,s relating to Die-Casting" This invention relates to die-casting and is concerned with melting the metal and charging it into a die-casting machine.

Conventionally, such a machine operating on the horizontal cold-chamber system has a shot sleeve projecting from it with a hole in its upper side into which a charge of molten metal is poured. A ram or plunger then carries this charge into the die. In many cases the ladling of the charge into the shot sleeve is still done by hand. The operator scoops out the approximate amount required from a nearby crucible and pours it into the shot sleeve hole. It is monotonous, dangerous and uncomfortable work. As with anything done by hand, accuracy and reliability can seldom approach that of automation.

There are mechanical systems for charging a diecasting machine, but they tend to simulate manual operations. Thus a ladle is fitted to the end of an articulated arm which is operated to dip the ladle into the crucible, lift it out and carry it across to the shot sleeve. While this does remove some of the drawbacks of manual operation, it involves complicated and expensive apparatus which may work well initially but which, after a long period in such a hostile environment, will tend to fail or lose its accuracy.

Also, it does not overcome the problems inherent in keeping a substantial quantity of molten metal in a crucible.

One particular drawback with a crucible heated by conventional means and with a sizeable capacity not needing constant recharging is that it takes a considerable time initially to melt a full charge of metal. Unless the machine is being worked continuously, with a make up supply being added gradually or a reserve crucible being heated, it is necessary to start the day or shift with firing the crucible and melting the charge. It cannot be left overnight in a molten state of readiness. This may take two hours or so, which is a considerable waste, particularly as it requires the presence of two operators for safety reasons.

Another snag is oxidation of the molten metal.

While some crucibles are largely covered and air can be prevented from reaching much of the surface, there inevitably has to be an open area into which the ladle can dip. Also as the ladle journeys from the crucible to the shot sleeve, further oxidation can occur. More problems arise from drips and from cooling metal clogging the tripping ladles, these being inherent in trying to scoop and transfer metal over distances.

A further problem is with trace elements and impurities in the metal, and in order to avoid concentrations of these it is necessary regularly to stir the contents of the crucible. this is fairly easily done when it is a small scale manual operation, but it adds to the complication and expense if it has to be done mechanically.

It is also important to maintain the metal in the crucible at an optimum temperature. This means temperature sensors, which are prone to damage and have a short life. Other sensors gauging molten metal levels in the furnace are also often needed, adding to the expense and complexity.

It is an aim of this invention to remove or at least substantially alleviate these problems.

According to the present invention there is provided apparatus for melting metal and charging a die-casting machine therewith, the apparatus comprising means for supporting a charge, induction heating means for heating and melting a charge when so supported, and means for relieving the support in a manner to cause the molten charge to feed by gravity to the inlet of a die-casting machine.

In one form, the supporting means is a receptacle with a pouring lip and the relieving means includes a pivotal mounting for the receptacle whereby it can be moved beteween a generally level supporting attitude and an inclined, pouring attitude. The heating means will then generally be a coil wound around the receptacle, and that in turn may be embraced by a cooling jacket. The relieving means will conveniently be a powered actuator for generating the pivotal movement of the receptacle.

In more detail, the receptacle will be of ceramic material with a bowl-shaped depression for receiving a slug of metal which is melted to form the charge for the die-casting machine. The induction coil may be water cooled, as in conventional practice with such coils, and the cooling jacket, designed not to interfere with the pouring lip, can also serve the purpose of reducing any extraneous eddy currents from the induction coil.

The powered actuator will preferably be hydraulically operated, and the apparatus may be mounted so that the molten charge is poured directly into the shot sleeve of the die-casting machine.

In another form, the supporting means comprises electromagnetic means for generating a field which can hold a magnetically susceptible charge in levitation, and the relieving means may then simply comprise a switch for cutting off the energising current to the electromagnetic means.

The induction heating means will then conveniently be a coil with its axis upright and centred on the supporting field, the coil ends being open respectively to receive a solid charge and to deliver a molten one.

Preferably, the coil is of spiral form, the turns increasing in diameter towards the top.

With either form means may also be provided for creating an inert gas shield around the charge while being heated and being fed to the die-casting machine, thus preventing oxidation. Also, there may be means for delivering solid, measured charges to the supporting means, and'means for synchronising these delivery means with the relieving means whereby each charge is replaced in the supporting means after being fed to the diecasting machine.

More specifically, each charge can be accurately quantified so that it is the exact amount required by the die, and the delivery means can be automated to supply a fresh charge at the beginning of each cycle.

For example, a slug of metal may be cut to a predetermined length from a toleranced cross-sectioned ingot, and be delivered by chute to the receptacle.

This system allows virtually immediate start-up at the beginning of the day or shift. It can be fully automated so that there are no breaks in the production flow, and generally the time taken for the casting to cool down and be ejected from the die will be comparable to the time taken to melt the next charge.

With a regular and unvarying cycle of operation, temperatures are constant or vary little, and the die itself will not be excessively cooled between charges.

Also, the machine's expansion and contraction will even out, the viscosity of the oil in the hydraulic system will be maintained and the water cooled die temperature will remain steady. All this leads to long life for the machine and the dies and consistency of product.

The ability to use exactly the right quantity of metal at a constant temperature virtually eliminates flash. Also oxidation and slag losses may be reduced to negligible amounts even if the inert gas shield is not used. Furthermore, an identical charge each time, and the same wattage in the induction coil for the same time, will inevitably heat that charge up to the same temperature each cycle. Thus, there is no need for temperature sensing.

Another advantage with induction heating is that, when the metal is molten, it is kept in motion and is virtually self-stirring. This motion does not lead to break-up and splashing when the charge is magnetically supported. Thus, any impurities are kept well distributed and gases are expelled.

A further significant advantage is that it is a very compact piece of equipment, being mountable as a very close adjunct of the die-casting machine and not involving the occupation of floor space, such as that required by a conventional crucible. Its life expectancy is also very much greater than that of a crucible and its associated burners, and as a piece of capital expenditure it is expected to be more than competitive with existing full automated systems.

Running costs are also on a par with existing heating systems.

Also, there is the added bonus of virtually eliminating risk to the operator and substantially improved working conditions, particularly with magnesium alloys.

For a better understanding of the invention, some embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a vertical section of an induction heating appliance for charging a die-casting machine, Figure 2 is a plan view of the appliance of Figure 1, Figure 3 is a vertical section of another induction heating appliance for charging a die-casting machine, Figure 4 is a cut-away plan view of the appliance of Figure 3, and Figure 5 is a vertical section of a further induction heating appliance for charging a die-casting machine.

The appliance of Figures 1 and 2 has a ladle 1, preferably of ceramic material, surrounded by an induction heating coil 2, this being a hollow tube for water cooling. The ladle has a bowl-shaped depression 3 to receive a charge 4 of metal (shown in its melted state) and a pouring lip 5. Largely surrounding it is a cooling jacket 6, which leaves the pouring lip 5 free.

The jacket will be water cooled and internally equipped with numerous baffles to reduce the eddy-currents which emanate from the coil.

The ladle 1 and cooling jacket 6 have a common mounting on a rocker shaft 7 so that the whole assembly can be tilted in a direction to depress the pouring lip 5. This motion is generated by a hydraulic actuator 8.

When mounted at the feed end of a die-casting machine, it will be arranged so that the contents of the ladle 1 will be poured directly into the charging hole 9 of a shot sleeve 10. Thus there is minimal travel of the molten metal, with virtually no chance of oxidation occuring. To guard against this more thoroughly, the ladle may be equipped with means for creating an inert gas shield over the bowl 3.

The electrical and fluid connections to and from the coil 2 and jacket 6 can, of course, be made flexible to accommodate to the rocking motion.

The delivery of the charge 4 may be automated. As mentioned above, a slug of precise dimensions can be cut from tolerance cross-sectioned ingot and then mechanically pushed from above the bowl 3. This will be immediately activated after the ladle has poured and is being returned to its level condition. Heating and melting of this charge occurs while the casting in the die solidifies and is ejected, and spraying of the die is completed.

The coil arrangement shown in Figures 1 and 2, while making charging of the bowl 3 easy, may be too inefficient electrically to be acceptable. A different coil arrangement is shown in Figures 3 and 4, where parts 11 to 20 correspond respectively to parts 1 to 10.

Here the coil 12 is wound about a horizontal axis parallel to the shot sleeve 20 so that it can more closely embrace the elongated depression 13, and the cooling jacket 16 is more all-embracing, leaving a gap only for the pouring lip 15. The pivot 17 is above this lip and the actuator 18 is at the other end, although it will be understood that various different arrangements are possible.

Charging of this device must be through the pouring lip gap, and to receive the ingot it can be arranged for the ladle to be tilted back beyond the horizontal after discharging into the shot sleeve. It is then returned to the horizontal for melting. For convenience and compactness of illustration, the charging appliance and shot sleeve are shown rather close and out of proportion, but it will be understood that an actual device will allow the tilting described.

A different approach, avoiding the need for a ladle altogether, is illustrated in Figure 5.

A water cooled induction coil 21 is of spiral form with its axis vertical and its largest turn uppermost, resembling a bed spring. As well as increasing in diameter, the turns also increase in pitch towards the top. This coil is flanked by electromagnets 22 which, when energised, create a high magnetic flux within the coil, the pitch of the turns being sufficiently large not to cause the coil to have a shielding effect. Also associated with this coil is an overhead slug delivery device 23 and, at its lower end, a sensor 24, conveniently an infra-red one, to detect a charge falling into the charging hole 25 of a shot sleeve 26.

This whole assembly, apart from the delivery device, is shrouded by a casing or curtaining, or a mixture of rigid and flexible shields, as indicated by broken line 27 so that the space where molten metal is exposed can be filled with inert gas to suppress oxidation. The shroud does of course have a "trap-door" facility for allowing the slug to drop through from the delivery device 23.

In use, a slug is delivered by being pushed into the coned void within the coil. It is probably too much to expect the electromagnetic field to "catch" the slug and so the latter is dimensioned to be arrested by one of the lower, smaller turns of the coil 21. But when the current is switched on (and it may be before the slug actually drops into the coil) the slug is supported by the magnetic field while being heated by the coil. The time taken to achieve the molten state and temperature appropriate for casting depends on the size of the slug and may be empirically determined by experiment. The slug remains cohesive and levitated while molten.

When the melting time expires, current to both the coil and the electromagnets 22 is automatically switched off and the slug falls through the lower end of the coil into the shot sleeve 26. When molten it will assume a different shape from the original slug, and as it drops it will elongate and narrow to pass with clearance through the lowermost turn of the coil 21. Its fall is picked up by the sensor 24 and the latter's response can be used to trigger the plunger operation which carries the molten charge into the die.

Current is then restored and the next slug is delivered, to repeat the cycle. Throughout its melting, the metal is shielded by inert gas within the shroud 27.

Instead of relying on the coned coil 21 to catch the slug, which in time would lead to wear, distortion or other damage to the lower turns, there could be a movable platform inserted laterally through the coil, or positioned immediately below it, once a molten charge had been delivered. The new slug would drop onto this, and when the magnetic support was effective, the platform would be withdrawn. This could be automated with the rest of the system, and while it would mean extra equipment and expense, the coil could be cylindrical and embrace the slug closer and more effectively, and the electromagnets could also be closer and less powerful for the same effect.

Claims (11)

Claims

1. Apparatus for melting metal and charging a diecasting machine therewith, the apparatus comprising means for supporting a charge, induction heating means for heating and melting a charge when so supported, and means for relieving the support in a manner to cause the molten charge to feed by gravity to the inlet of a diecasting machine.

2. Apparatus as claimed in Claim 1, wherein the supporting means is a receptacle with a pouring lip, and the relieving means includes a pivotal mounting for the receptacle whereby it can be moved between a generally level supporting attitude and an inclined pouring attitude.

3. Apparatus as claimed in Claim 2, wherein the induction heating means is a coil wound around said receptacle.

4. Apparatus as claimed in Claim 2 or 3, and further comprising a cooling jacket embracing said coil.

5. Apparatus as claimed in Claim 2, 3 and 4, wherein the relieving means comprises a powered actuator for generating the pivotal movement of the receptacle.

6. Apparatus as claimed in Claim 1, wherein the supporting means comprises electromagnetic means for generating a field which can hold a magnetically susceptible charge in levitation, and the relieving means comprises switch means for cutting off the energising current to the electromagnetic means.

7. Apparatus as claimed in Claim 6, wherein the induction heating means is a coil with its axis upright and centred on the supporting field, the coil ends being open respectively to receive a solid charge and to deliver a molten one.

8. Apparatus as claimed in Claim 6 or 7, wherein the coil is of spiral form, the turns increasing in diameter towards the top.

9. Apparatus as claimed in any preceding claim, and further comprising means for creating an inert gas shield around the charge while being heated and being fed to the die-casting machine.

10. Apparatus as claimed in any preceding claim, and further comprising means for delivering solid, measured charges to the supporting means, and means for synchronising these delivery means with the relieving means whereby each charge is replaced in the supporting means after being fed to the die-casting machine.

11. Apparatus for melting metal and charging a diecasting machine therewith, the apparatus being substantially as hereinbefore described, with reference to Figures 1 and 2, Figures 3 and 4 or Figure 5 of the accompanying drawings.