| WO/2012/040963 | CLEAN METAL INGOT MOLD |
| JP55008321 | REPAIR METHOD OF STEEL INGOT CASTING MOLD AND MOLDING BOARD |
| JP62227550 | PRODUCTION OF INGOT |
Grandfield, John (37 Mattingley Crescent, West Brunswick, VIC 3055, AU)
Nguyen, Vu (15 Ruby Place, Springvale, VIC 3171, AU)
Nguyen, Thang (64 Queens Parade, Glen Iris, VIC 3146, AU)
Rohan, Patrick (318 Myers Street, East Geelong, VIC 3219, AU)
Alguine, Vladimir (16/3 Vatutina Street, kv 49 Moscow, 7, 12135, RU)
Grandfield, John (37 Mattingley Crescent, West Brunswick, VIC 3055, AU)
Nguyen, Vu (15 Ruby Place, Springvale, VIC 3171, AU)
Nguyen, Thang (64 Queens Parade, Glen Iris, VIC 3146, AU)
Rohan, Patrick (318 Myers Street, East Geelong, VIC 3219, AU)
| 1. | A mould for solidifying metal into an ingot comprising a rigid base, end walls and side walls wherein the rigidity of the base provides minimal distortion of the base of the mould and minimal air gap between the base and the solidifying metal during the heating and cooling cycle of the mould. |
| 2. | The mould of claim 1 wherein the base has a maximum vertical thermal distortion of 400 μm during the heating and cooling cycle of the mould. |
| 3. | The mould of claim 1 wherein the heating and cooling cycle of the mould is within the range of 25°C to 3000C. |
| 4. | The mould of claim 1 wherein the minium thickness of the base is 25 mm. |
| 5. | The mould of claim 1 wherein the exterior surface of the base of the mould is provided with at least one strengthening rib. |
| 6. | The mould of claim 5 wherein the strengthening rib is tapered towards the ends of the mould. |
| 7. | The mould of claim 1 wherein the side walls are adapted to experience an outward displacement prior to the ingot solidifying, thereby minimising the air gap produced between the side wall and solidifying ingot during the subsequent ingot contraction and mould wall inward displacement phase. |
| 8. | The mould of claim 1 wherein the outward thermal displacement of the side walls is at least 750 μm. |
| 9. | The mould of claim 7 or 8 wherein splash guards are provided along the top edge of the side walls, at least one mould guard is provided with at least one slot extending from the outer edge of the mould guard towards the side wall of the mould. |
| 10. | The mould of claim 9 wherein a plurality of slots are provided in at least one mould guard. |
| 11. | The mould of claim 9 wherein a plurality of slots are provided in each mould guard. |
| 12. | The mould of claim 9 wherein the splash guard comprises a flanged member which extends from the upper edge of the side wall. |
| 13. | A mould for solidifying molten metal comprising a base, end walls, at least two side walls and two flanged members extending outwardly along the top of the side walls, each flange member being provided with a plurality of slots extending from an outer edge towards the respective side wall. |
| 14. | The mould of claim 13 wherein the slots extend substantially across the flange member. |
| 15. | The mould of claim 13 wherein the side walls are adapted to experience outward displacement prior to ingot solidification, the outward thermal displacement being at least 750 μm. |
| 16. | The mould of claim 1 wherein the base is provided with a lateral ridge. |
| 17. | The mould of claim 16 wherein the lateral rib is formed on the internal surface of the mould, the external surface of the mould in proximity of the internal rib being provided with a corresponding indent. |
| 18. | The mould of claim 17 wherein the regions of the base adjacent the internal lateral ridge are provided with a concave shape. |
| 19. | A mould for solidifying metal into an ingot comprising a base, end walls and side walls wherein the side walls are adapted to experience outward displacement prior to the ingot solidifying thereby reducing the air gap produced between the side wall and solidifying ingot. |
| 20. | The mould of claim 19 wherein the outward thermal displacement of the, side walls is at least 750 μrn. |
| 21. | The mould of claim 18 or 19 wherein the rigidity of the base provides minimal distortion of the base during the heating and cooling cycle of the mould. |
| 22. | The mould of claim 18 or 19 wherein the vertical thermal distortion of the base is less than 400 μm during the heating and cooling cycle of the mould. |
This invention relates to an ingot mould for casting of non-ferrous metals and in particular, to an ingot mould for use in chain conveyor ingot casting processes.
Background of the invention
The chain conveyor ingot casting process is widely used to cast non-ferrous metals including aluminium, zinc, magnesium and lead. The chain conveyor process has a series of open cast iron moulds for casting trapezoid cross section ingots, on a conveyor system. The conveyor may be linear or circular and up to 40 metres long. The moulds are filled at one end of the conveyor and as they move along the conveyor, the ingot cools and at the end of the conveyor the solid ingot is ejected. The method of cooling the ingot may be via a cooling medium such as air or water. The machine productivity is dependent on the number of ingots delivered at the end of the line per unit time and their weight. The rate of delivery depends on the line speed and the spacing between moulds. The line speed in turn is dependent on the time required to solidify the ingot and the length of the conveyor line.
In order to increase production rate, it is necessary to increase the length of the conveyor if the solidification times remains the same, or reduce the ingot solidification time and allowing a line speed increase with the same line length. Increasing the length of the conveyor is expensive, requiring additional building space and infrastructure and there is a practical limit to the line length.
The option of reducing the solidification time increases productivity of an existing conveyor by requiring less time between filling the mould and ejecting the ingot from the mould.
The applicant has measured the nature of the heat transfer and mould deformation during solidification of non-ferrous metal ingots and found that the predominant heat transfer mechanism is conduction and is limited by the formation of a gap between the ingot and the mould as the ingot shrinks during solidification and cooling. This gap contains various low thermal conductivity gas species including air and any decomposition products from the lubrication or mould parting agent that may have been used, and is found to be the biggest heat flow resistance. The dimension of this gap thus determines the solidification time.
Summary of the invention
According to one aspect of the invention, there is provided a mould for solidifying metal comprising a base end walls and side walls, wherein the rigidity of the base provides minimal distortion of the base of the mould and minimal air gap between the base and solidifying metal. The rigidity of the base and side walls is controlled to achieve mould thermal distortions during casting such that the air gap between the ingot and the mould is reduced or minimized, thereby reducing solidification times. The applicant has found that by providing a rigid base and thereby minimising the distortion of the base of the mould during the heating and cooling cycle, a smaller air gap on the base of the mould results. This is contrary to conventional heat flow theory which would suggest that thickening of the base would increase the heat flow resistance of the mould.
Furthermore, the applicants have found that the mould wall goes through a heating and cooling cycle and displaces outward and then inward during solidification of molten metal. If the mould is designed such that outward displacement of the walls in the early stages is maximised before the ingot becomes rigid, the air gap dimension is minimised because the mould contracts by a greater degree compared to the ingot when the ingot solidifies and begins to contract inwardly.
According to a second aspect, the invention provides a mould having walls which experience significant outward displacement prior to solidification and with maximised inward expansion after the air gap forms.
Accordingly one embodiment of the invention provides an ingot mould, a base end walls and side walls wherein the base has a maximum vertical thermal distortion of 400 μm and the walls have a outward thermal displacement of at least 750 μm over a typical heating and cooling cycle of 250C to 3000C. The applicant has found that by providing an ingot mould construction in which there is only a small vertical thermal distortion in the mould base and maximum outward thermal displacement in the mould walls over the heating and cooling cycle of the mould, then smaller gaps between the surfaces of the solidifying metal ingot and the mould result. These smaller air gaps greatly increase the heat transfer from the forming ingot and the mould, thereby decreasing solidification time.
One means of achieving a mould construction of minimal base distortion is to provide a mould in which the base is thicker than that typically used. Preferably, the mould base thickness is at least 75% greater for a typical base thickness of 14-20mm. The minimum base thickness of the invention would preferably be 25mm. The maximum mould wall outward displacement enables the air gap to be reduced to as small a dimension as possible. During the early stages of the solidification cycle of the ingot, the ingot has no strength being substantially liquid and remains in contact with the mould wall as the mould wall displaces outward. Once the outer surface of the ingot has solidified the ingot begins to shrink as it solidifies and cools, the air gap forms and the mould wall consequently cools and shrinks inward toward the ingot. During this cooling cycle, the inwardly receding mould walls help to minimise the air gap dimension between the ingot wall and the mould wall. The greater the outward displacement in the early stage before the ingot is rigid, the greater the inward displacement when the gap forms and consequently the smaller will be the air gap. Furthermore, it was also found that the base displacement should be small during the cycle to keep the gap dimension small and aid the heat transfer through the base.
One embodiment of a means of achieving a mould construction of minimal base distortion is to provide at least one strengthening rib or fin, preferably extending longitudinally along the exterior surface of the base of the mould. As the mould is preferably sized to be used with existing equipment, the rib may be tapered towards the ends of the mould to allow clearance for other equipment in the chain conveyor, such as water piping and jets in the water bath.
In another embodiment, a means to achieve a mould construction of minimal base distortion is to provide at least one strengthening rib or fin which extends laterally of the base of the mould. The lateral rib or fin may project from the base of the mould but it is preferable that the rib or fin is formed on the interior surface of base of the mould. A corresponding indent in the exterior surface of base of the mould is provided to ensure that thickness of the base does not vary. The lateral rib or fin is beneficial in reducing distortion of the base about the longitudinal axis.
A further embodiment of a method of achieving a mould construction of minimal base distortion would be to use a lower expansion coefficient material to form the mould body and preferably one with a coefficient of thermal expansion of substantially zero over the operating range of the mould.
One embodiment of maximising the thermal outward displacement of the mould walls is to use thinner walls than conventionally used. As mentioned above, reducing the thickness of the wall and increasing the ability of the mould wall to expand is contrary to the teachings in the industry. The mould wall outward displacement could also be increased by using two layers of material in the mould wall; an outer material with a greater coefficient of thermal expansion than the inner mould material.
In one design of a chain conveyor line, there needs to be an overlap between the moulds on the conveyor to stop any water coming up between the moulds onto the metal and any metal dropping between the moulds during filling into the water bath. This is commonly achieved by using splash guards on the mould. These splash guards are found to heat up by only a few degrees during casting while the rest of the mould heats up to between 250 0C to 3000C. These cold splash guards were found to restrain the outward displacement of the hot mould resulting in a restricted bowing of the walls and an increased bowing of the base. Removal of the splash guards enables increased outward displacement of the walls which reduces the air gap, with the consequence of a reduced solidification time as mentioned above. However in ingot mould designs which require splash guards, the applicant has found that by providing a series of slots along the splash guard, there is minimal restraint by the cold splash guard on the outward displacement of the side walls, thereby maximising bowing of the mould side walls and minimising bowing of the mould base. According to this aspect of the invention, there is provided an ingot mould comprising a base, end walls, at least two side walls and two flanged members extending outwardly along the top of the side walls, each flange member being provided with a plurality of slots extending substantially across the flanged member.
Brief description of the drawings
Figure 1 is a schematic diagram of the general layout of an ingot casting machine;
Figure 2 is a schematic diagram of an ingot mould design of the prior art using splash guards;
Figure 3 shows two views of a mould construction in accordance with an embodiment of the invention in which figure 3(a) is a perspective view from below the mould and figure 3(b) is a perspective view from above the mould;
Figure 4 is a cross sectional view through a typical mould design (4(a)) and a 32mm thick base mould (4(b)). Note the absence of splash guards on the mould shown in 4(b);
Figure 5 is a bottom perspective view of a second embodiment of the invention;
Figure 6 is a sectional view through line 6-6 of the embodiment of figure 5;
Figure 7 is a sectional view through line 7-7 of the embodiment of figure 5;
Figure 8 is a sectional view through line 8-8 of the embodiment of figure 5;
Figure 9 is a sectional view through line 9-9 of the embodiment of figure 5;
Figure 10 is a graph showing the mould temperature at various positions within the mould wall and base during the heating and cooling cycle experienced in the standard mould when the liquid metal is poured into the mould; and Figure 11 is a graph of the measured outward displacement on a standard mould commonly used in the industry.
Detailed description of the embodiments
As mentioned earlier, Figure 1 is a chain conveyor 1 comprising a series of open cast iron moulds 2 for casting trapezoidal cross section ingots on a conveyor belt. A liquid metal pourer fills each of the ingot moulds 2 as they pass under the filler along its path along the upper line of the conveyor belt. As the ingot solidifies, the solidified ingot wall inwardly contracts away from the mould wall enabling the ingot to be easily unloaded under gravitational forces as the conveyor belt rounds the end. The solid ingots 4 are then transported away by a further conveyor belt.
As shown in Figure 2 and 4(a), the typical ingot mould 10 comprises a base 11 , two side walls 12, 13 and two end walls 14, 15 with a cavity 16 defined within those walls. For moulds cooled by a water bath, splash guards 17, 18 are provided to stop any water coming up between the moulds and into empty moulds before filling causing molten metal explosions and also to prevent any metal dropping between the moulds into the water bath during filling of the ingot. As mentioned earlier, these splash guards 17, 18 further provide mechanical strength to the side walls 12, 13. The applicant has found that by minimising the deflection in the base 11 , during the heating and cooling cycle of the ingot and by designing the walls of the ingot mould so that the walls expand much more than in a conventional ingot mould during the heating and cooling cycle, an ingot mould can be produced which has a minimum air gap between the base and side walls during the heating and cooling cycle of the ingot moulds and thereby provides greater thermal conduction of heat from the ingot through the ingot mould wall to the heat sink or heat transfer medium such as a water bath.
For the purpose of exemplification, the moulds of the invention were produced from cast iron. The coefficient of thermal expansion for cast iron can vary between 4 to 18 micrometers per meter per degree Kelvin depending on the grade of iron. For the purpose of testing, a ductile iron having a coefficient of thermal expansion of approximately 11.5 micrometers per meter per degree Kelvin was commonly selected. While the absolute dimensions of the airgap between the solidified metal and mould can be reduced by producing moulds of material having a low or substantially zero coefficient of thermal expansion, and it would be appreciated by those skilled in the art that the features of the invention are able to minimise the airgap produced using, not only, more conventional mould materials such as cast iron, but also other types of mould materials. Hence the invention should not be considered to be restricted to those materials which have been used by way of example.
In the embodiments shown in Figures 3(a) and 3(b), an ingot mould 20 is shown having side walls 21 , 22, a base 23 and end walls 24, 25. The ingot mould of Figures 3(a) and 3(b) is designed to be used with a water bath and so is provided with improved splash guards 27, 28. The splashguards are provided with a plurality of slots 26, preferably extending across the width of the splash guards. The slots are spaced at a distance which will allow the respective side walls 21 , 22 to expand substantially unaffected by any structural constraints imposed by the splash guard. In one preferred form, the slots are equi-spaced along a length corresponding to the side wall in contact with the ingot mould cavity.
A further feature of reducing the thermal deflection of the base is a strengthening means disclosed in Figure 3(a) by the use of a strengthening longitudinal rib or fin 29 extending centrally along the exterior surface of the base 23 of the mould.
In order to measure the mould temperatures, thermocouples were positioned inside the mould wall. Figure 10 is a graph showing the mould temperature at various positions within the mould during the heating and cooling cycle. As shown in Figure 10, pouring the molten metal into the mould immediately raises a temperature of the moulds internal surface towards the temperature of the molten metal. A thermal gradient is thus established through the mould wall with those thermocouples positioned closer to the internal surface being hotter than those closer to the water cooling. The thermocouples placed within the mould wall show a peak temperature reached within 60 seconds of pouring. Then the mould wall cools rapidly over the next 30 seconds and then slowly cools to ambient temperatures within approximately 15 minutes. The temperatures peak around 150-22O0C at thermocouple positions within the mould wall. In a further embodiment 30 of the invention shown in figures 5-9, a lateral rib 31 is provided on the base of the mould. In the figures, the lateral rib 31 is formed on the inside of the mould with a corresponding indent 32 formed into the external surface of the base. As shown in figure 6, the thickness of the base either side of the lateral rib at sections A and C is the same thickness as at section B.
In this embodiment, the base 33 of the mould on either side 34, 35 of the lateral rib 31 is provided with slight concave shape from the external surface. As shown in figures 7, 8 and 9, the thickness of the base is uniform across the mould. In the preferred embodiment, the thickness is 32 mm which is consistent with a thicker base. At section B in the region of the lateral rib, the external and internal surfaces of the base are flat across the base. In section A and C in regions longitudinally juxtaposed either side of section B, the internal and external surfaces are of uniform thickness but with an arcuate shape, the radius of the arc being 318 mm. This gives the base in these regions a concave shape.
This base structure minimises the gap which forms between the base and solidifying metal as the metal cools.
In the embodiment shown in Figure 4(b), the mould is provided with a thicker base. The old mould design which has a base thickness of 13-14mm, provides an average air gap at the ends of the base in the range of 210μm to 286μm and an average air gap at the base centre of 73-118μm. The average air gap measured with the mould in accordance with the invention having a 32mm base thickness is less than 125μm at the base end and less than 90μm at the wall end top.
In this embodiment of the invention, the deflection in the base of the ingot mould is limited by providing a thicker base. The object of increasing the thickness of the base is to reduce the deflection which occurs. Table 1 is a comparative example of the effect of thickening the base on the average air gap formed in the ingot mould during the heating and cooling cycle of the ingot mould. Figure 11 is a graph showing outward displacement on a conventional mould in which position A is the wall at the lower position in the centre of the mould, position B is higher position on the wall at the centre of the mould, position C is the base towards the centre of the mould and position D is the base toward the end of the mould. As can be seen from Table 1 , the effect of a more rigid thicker base and walls having a greater outward displacement are most notable at the ends. The effect of the thicker base is further reflected in the solidification times of the different mould constructions shown in Table 2. Table 1 Average air gaps over the 50-250 second period
In regard to the wall deflection, the applicant has found that the moulds which currently exist in the prior art have a deflection of approximately 0.67-0.7mm at the top centre position. By reducing the wall thickness, and removing or placing slots in the splash guards the minimum deflection of the wall is preferably 0.75mm at the top centre position and can be as much as 1.1mm or greater. This increased deflection of the wall enables the wall to expand outward much more than moulds currently in use so that while the ingot metal is molten, the mould wall can expand outward with the molten metal, maintaining contact between the wall and the ingot surface and as the metal begins to solidify at the mould wall and contract inward producing the air gap, the mould wall is able to contract with the ingot wall thereby minimising the air gap with the ingot mould. This reduced air gap greatly increases the rate of heat transfer from the ingot metal and thereby reduces the solidification time.
The mould wall thickness should be less than 20mm and preferably less than 15mm in order to produce an ingot mould having the required level of thermal outward displacement to comply with the invention. In conjunction with the minimum base deflection, the invention is able to provide an ingot mould which is able to solidify non- ferrous metal ingots such as aluminium much more quickly than conventional moulds under similar conditions and thereby greatly improve the productivity of current chain conveyor ingot casters.
The current deflection experienced in the ingot mould base of existing ingot moulds is around 1.6mm in the centre and 1.3mm at the end of the base. In accordance with the present invention, it is preferable that the deflection of the base be less than 0.4mm across the length of the base. By incorporating a 32mm thick base into the existing mould construction, the applicant has achieved a measured deflection of 0.12mm in the centre and 0.29mm at the end of the base. The mould deflection is measured using a linear variable differential transformer (LVDT) mounted on a reference frame with the inner displacing rod spring loaded against the mould.
A typical ingot used in the above examples is 113 mm high, 170 mm wide at the top, 116 mm wide at the bottom and 537 mm long. Additionally, these ingots have two ears or protrusions in proximity to both ends of the ingot. These ears or protrusions extend 121 mm and are 40 mm thick.
For a typical 22.5 kg ingot the solidification time in the old mould is approximately 280- 310 seconds depending on the casting conditions such as water cooling, mould coating and cast metal temperature. Under these similar conditions, the new mould is able to provide solidification times of less than 250 seconds. For a typical machine operating at one tonne/per hour, per metre of line length using the old mould (solidification time less than 305 seconds), this reduction in solidification time to 234 seconds (based on the average above) allows an increase of the productivity of the line by 31 % producing 1.31 tonnes/per hour per metre of line length. Table 2 Solidification times for different mould designs
^Normalised for variations in individual ingot weights and pouring temperatures.