Continuous casting apparatus and method

Abstract

Continuous casting apparatus which comprises a crucible 2 within which metal or a metal alloy material is melted, a die 3 through which cast material exits the crucible, a drive system 5 for withdrawing cast material through the die and means 4 for rapidly cooling the cast material as it is pulled through the die 3. The drive system 5 can move the casting in both forward and reverse directions as it is exits the die 3 and comprises a servo motor 6, a gearbox 7, a drive shaft 8 and at least one roller 9 & 12 for supporting the casting. The drive system 5 has a total backlash of up to 3 Arcminutes (0.05 degrees). In use the casting can be kept in motion at all times, with a reverse speed, time and distance all being less than the forward speed, time and distance.

Description

This application relates to an apparatus and method for continuous casting of metal rod, strip and tube. The invention may be particularly but not exclusively aimed at casting of copper, tin, lead, zinc, gold or silver or alloys of these metals. The apparatus and method may be applied to vertical continuous casting or horizontal continuous casting. BACKGROUND OF THE INVENTION It is known to produce metal rod, strip and tube by both vertical continuous casting and horizontal casting methods. For example, copper cathode and additional metal alloy ingredients are fed into a charging chamber of a graphite furnace casting crucible where the copper is melted and conditioned by heat from the graphite heating elements. In some cases, metal alloy ingredients may be melted in the same crucible from which continuous casting takes place, or alternatively melted in a separate furnace and transferred to the casting crucible in a molten state. The feedstock may be fed manually or automatically into the crucible. The charging chamber is linked to an adjacent casting chamber by a passage at the bottom of the chambers. Molten metal alloy such as for example copper alloy passes through the passage into the casting chamber, where it is passed through a filter bed to form a reservoir of conditioned melt at the top of the casting chamber. One or more water-cooled cooling tubes extend into the conditioned melt, and the molten copper alloy is pulled through an orifice in a casting die in the cooling tube to form a copper alloy strand of the required diameter. Heat is extracted from the casting die by a high efficiency cooler. Continuous casting only hardens completely after the casting has exited the die; however it spends enough time in the casting apparatus to develop a protective solidified skin of an adequate thickness on the outside which is in contact with the inner walls of the casting chamber whilst the centre of the casting remains molten and only hardens during the withdrawal process. The casting produced from the die increases in length as new molten metal is supplied to the casting chamber. The rate of adding molten metal is controlled to keep up with the solidifying casting as it exits the die. Rollers are typically used to guide the casting out of the die and to assist in providing a smooth flow of metal casting out of the die along its given path. A cutting device, typically a torch or saw is provided at the required distance from the die to cut through the casting thereby producing castings of a required length. The cutting device moves with the casting at the same speed as it makes a cut. Alternatively the casting is guided out of the die and coiled onto a spool. The cast product is pulled through the casting die in a repeatable sequence to achieve solidification of the metal at the die area. Traditionally, this repeatable sequence is of a forward-stop, forward-stop format. The movement is achieved by the use of DC motors, gearboxes, lateral indexers and servomotors. The period of time during which the cast product is stopped or held at zero speed is referred to as dwell. It is important to control the movement of the cast rod through the die in order to avoid surface cracks in the cast rod. Traditionally, production rates in the industry can range from several KG / hr to several tonnes / hr depending on materials used and the cross-section of the cast rod. In terms of linear speed this can be from a few mm / min to many meters / s. The cast product can then be used to for creating finished products. More recently the use of indexers and more laterally servo motors has allowed for casting speeds to increase and also to allow casting diameters to reduce, typically to around 8mm, but the limitation of indexers and early servo motors were exposed at higher casting speeds. Furthermore, the introduction of alloy materials can also increase surface cracking in the cast rod. Thus, improving continuous casting apparatus and casting methods through controlling the movement of the casting through the die has been identified by the present applicant as an important factor in improving the integrity of the casting. In particular, if the rate of acceleration of the casting during the sequence of forward and reverse motions can be maximised, the casting can reach the target forward and reverse velocities more rapidly, which allows the casting to move forward by greater increments in each sequence as less time is taken to accelerate the casting to the target velocity in each movement. Backlash in the motors and gearbox controlling movement of the casting can adversely affect the movement of the casting. Backlash generally refers to the amount of play between cooperating teeth in a gearing system. This affects the ability of the gears to change direction of rotation quickly and can lead to hammering of gear teeth leading to damage over time to the gears. Backlash can be reduced by using precision components in a drive system such as gear boxes and servo motors. The focus on manufacturing in precision components is tighter tolerances, so all around the gear will be a tighter providing more precise fit between engaging gear teeth. Tighter fit means less plan in the gear teeth which is a main cause of backlash in the drive system. Backlash can also be reduced by shortening the distance between the centres of two gears to ensure that they move into a tighter mesh. It is an object of the present invention to overcome or at least mitigate at least one of the above mentioned disadvantages. It is a further object of the present invention to provide an apparatus for and method of continuous casting of rod, strip and tube with a good surface finish close to the intended shape of the finished product particularly of copper and copper alloys by controlling the movement of the rod during the casting process. It is a further object of the present invention to provide an apparatus for and method of continuous casting of rod, strip and tube, which is more efficient in terms of time and man power than previously known apparatus and methods and more environmentally friendly as the rod, strip or tube can be cast closer to the intended shape of the finished product, thus less time and energy is required in finishing the product after the material has been cast. It is a further object of the present invention to provide an apparatus for and method of continuous casting of rod, strip and tube, which allows for increased casting speeds without sacrificing the integrity of the cast product. The present applicant has determined that by reducing and or limiting the backlash in the system for a continuous casting apparatus, it is possible to increase the acceleration rates of the casting without damage to the gearing and simultaneously providing an improved casting product. Additionally, the present application has determined that by modifying the sequence of forward and reverse movements in the continuous casting method, a casting product with improved surface finish can be achieved. According to a first aspect of the present invention there is provided an apparatus for continuous casting, the apparatus comprising a crucible within which metal and or metal alloy material is melted, a die through which cast material exits the crucible, a drive system for withdrawing cast material through the die and means for rapidly cooling the cast material as it is pulled through the die, the drive system comprising a servo motor and gearbox for moving the casting in both forward and reverse directions as it is exits the die, wherein the drive system has a total backlash of up to 3 Arcminutes. Preferably, the drive system has a total backlash of below 2.5 Arcminutes. By limiting backlash in the drive system to be up to 3 and preferably below 2.5 Arcminutes, the rate of acceleration of the casting in the forward and reverse directions can be increased over that which is possible with a standard drive system. Advantageously the drive system further comprises a drive shaft which is rotatable in response to operation of the drive system in either forward or reverse directions. Advantageously also, the apparatus further comprises one or more rollers mounted on the drive shaft, said rollers rotatable with the drive shaft to support and direct the cast material as it passes through the casting die. According to a second aspect of the present invention there is provided a method of continuous casting comprising the steps of, melting metal and or metal alloy materials in a crucible, pulling molten metal from the crucible through an orifice in a casting die and operating a drive system to control the movement of the casting in a repeating sequence comprising one or more forward movements followed by one or more reverse movements, wherein the method comprises limiting backlash in the drive system to up to 3 Arcminutes. Preferably the method comprises limiting backlash in the drive system to below 2.5 Arcminutes. Advantageously the method of casting includes the step of maintaining the casting in motion at all times such that the casting is not held stationary at any time. Eliminating dwell in the repeating sequence of movements of the cast material through the casting die leads to improvements in the surface finish of the cast material. Conveniently, the speed of the casting through the die during the reverse movement(s) is less than the speed of the casting through the die during the forward movement(s). Conveniently also, the casting is moved in the reverse movement(s) for smaller periods of time than in the forward direction in each sequence. Conveniently, the casting is moved in the reverse movement(s) by a smaller distance than in the forward movement(s). Preferably, also, the method further comprises the step of extracting heat from the casting die as the molten metal is pulled through the die to rapidly cool the casting. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are described, by way of example, with reference to the accompanying drawings in which:- Fig. 1 is a schematic diagram of a continuous casting apparatus according to one aspect of the present invention. Fig. 2 is a graph plotting the velocity chart for a standard casting process together with an embodiment of the invention for comparison; DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a continuous casting apparatus according to one aspect of the present invention. The apparatus (1) comprises a crucible (2) within which metals and or metal alloys are introduced and melted. One or more casting dies (3) extends into the crucible by a sufficient amount that they extend below the surface of the molten metals within the crucible. In some embodiments the height of the one or more dies above or within the crucible may be adjusted to ensure that they maintain the required position within the crucible. A cooling device (4) is mounted adjacent the casting die, and in the embodiment shown in figure 1, this is above the casting die, so that the casting is cooled immediately after passing through the die (3). In the apparatus of figure 1, an array of 3 casting dies are illustrated, each mounted side by side and extending into the crucible. Each casting die has an associated cooler mounted on the apparatus to cool a casting drawn through each individual casting die. In other, non-illustrated examples, the number of casting dies and associated coolers may be greater or less than that shown in the illustrated embodiment. The apparatus further comprises a drive system (5) comprising a servo motor (6) and a gearbox (7) mounted adjacent to the crucible. A shaft (8) extends from the servo motor, through the gearbox and extends across the crucible (2) behind the array of coolers. Rollers (9) are mounted along the shaft via hydraulic couplings (not shown) and are rotatable with the shaft. The rollers are positioned in line with the end of each cooler remote from the respective casting die associated with that cooler. The shaft is mounted at an appropriate distance from the end of the cooler such that a casting produced through a die will exit the cooler and pass over the roller. In the embodiment shown in figure 1, a secondary drive shaft (10), parallel to the main drive shaft (8) is also provided at a position slightly in front of the main drive shaft. A belt and pulley system (11) is mounted on the main drive shaft and secondary drive shaft to enable rotation of the secondary drive shaft together with rotation of the main drive shaft. A further set of rollers (12) is mounted on the secondary drive shaft, again via hydraulic couplings (not shown), each of the rollers on the secondary drive shaft being aligned with a roller (9) on the main drive shaft. In the embodiment illustrated a touch screen or user interface (13) is provided on the apparatus and connected either by hard wiring or wirelessly to the servo motor (6) to enable the user to provide input controls to the servo motor for rotation of the main and secondary drive shafts (8, 10) through the gear box (7) in both the forward and reverse directions. The drive system (5) and specifically the servo motor and gear box are selected to minimise backlash in the apparatus. The present applicant has appreciated that limiting the backlash in the servo motor and gearbox to up to 3 Arcminutes, but more preferably below 2.5 Arcminutes has a significant effect on the operation of the apparatus and the quality of the cast material as will be described more specifically below. As with a standard continuous casting operation, the method of the present invention involves melting materials metals and or metal alloys within the crucible (2) and drawing molten metals in the form of a casting through the casting dies (3) , through the coolers (4) and over the rollers (9, 12). However, where known continuous casting methods tend to follow a repeating sequence of a forward-stop, forward-stop format in which the stop element of the sequence is a period of time during which the casting is held at zero speed, by limiting the backlash in the drive train of the continuous casting apparatus as noted above to be up to 3 Arcminutes but preferably below 2.5 Arcminutes, the present invention facilitates a continuous casting method in which the repeating sequence of forward and reverse motions eliminate any period of dwell. In other words, the casting is not held at zero speed for any period of time within the sequence. Furthermore, by limiting the backlash in the drive train (5) as mentioned above, the present applicant has established that the acceleration of the casting in the forward and reverse directions within each sequence to reach a target velocity, can be significantly increased above that which is possible with a standard drive train in which the backlash is not limited, without loss of quality of the surface finish of the casting. As the acceleration of the casting, particularly in the forward direction, is significantly increased beyond that which was previously possible, the overall distance that the material moves in each repeating sequence is increased, and therefore the casting speed is significantly improved. As noted above, the components of the drive system, and particularly the servo motor and the gearbox are selected to minimise backlash. Examples of a suitable servo motor and gearbox which are available and provide the required limited backlash would be for example a SINAMICS® S210 servo motor available from Siemens AG in Germany and an Economy (E) servo worm reducer gear box available from Atlanta Drive Systems, Inc. of the US. Examples of suitable belts and pulleys for the drive system would be for example high performance belts such as AT5, AT10 or AT20 and zero backlash pulleys such as T5 or T10 available from HPC Gears Limited in the UK. Other equivalent suitable components could, of course, be used. Example Table 1 below illustrates a typical velocity chart for a casting an 8mm rod comprising lead and lead alloy using a continuous casting apparatus with a standard drive train with backlash of 13.05 Arcminutes and with a forward-stop cycle of movement. Table 2 sets out an embodiment of the present invention for casting the same 8mm lead / lead alloy rod using a low backlash drive train, with backlash of 2.27 Arcminutes and with a forwards-reverse cycle of movement in which the casting is in motion at all times. Backlash in both systems was measured over 20 measurements and the figures above are the averages of those measurements. For the standard drive train, the deviation of measurement was 3.98 Arcminutes. For the low backlash drive train, the deviation of measurement was 1.09 Arcminutes. Acceleration and deceleration is represented in the tables as the time taken to reach a target speed of the casting in either the forward or reverse directions. The output of both tables is plotted on the graph of figure 1. Table 1 Move 1 Move 2 Move 3 Move 4 Acceleration (s) 0.025 n / a n / a n / a Move time (s) 0.025 n / a n / a n / a Move Distance (mm) 6.25 n / a n / a n / a Deceleration (s) 0.025 n / a n / a n / a Dwell (s) 0.008 n / a n / a n / a Table 2 Move 1 Move 2 Move 3 Move 4 Acceleration (s) 0.01 0.01 n / a n / a Move time (s) 0.05 0.001 n / a n / a Move Distance (mm) 20 -1 n / a n / a Deceleration (s) 0.01 0.01 n / a n / a Dwell (s) 0 n / a n / a n / a As can clearly be seen in the graph of Figure 1, the velocity chart of table 2 shows movement of the casting with significantly more rapid acceleration when compared with the typical velocity chart in the forwards direction leading to a significantly greater forwards movement of the casting. This is followed by a similarly rapid deceleration of the casting than when compared to the typical velocity chart. However, where table 1 sets out a standard casting with a distinct period of dwell in which the casting is held at zero velocity at end of the forward motion, in other words this is a standard forwards - stop motion, the above embodiment of the present invention as shown in table 2 introduces a second distinct movement in the reverse direction during which the moving time of the casting is less than in the forward direction. The drive train of the continuous casting apparatus of the present invention is controlled to ensure that the casting is in motion at all times. In this example, the casting is accelerated in the forwards direction for only 0.01 seconds but reaches the target speed where it is held for a period of 0.05 seconds before being similarly rapidly decelerated over a period of 0.1 seconds. During this forwards motion stage, the casting moves 20mm in the forward direction. By limiting the backlash in the drive train of the apparatus as described above through selection of components, or through adjustment of backlash in components where such adjustment is provided for, greater control of the movement of the casting as it exits the casting die is achieved, allowing for a higher target speed to be achieved and this target speed to be reached more quickly than standard casting processes. This allows the casting to move forward by a larger distance during the forward movement of the casting sequence than standard casting processes which equates to a significantly higher casting speed for the same material. The direction of movement of the drive train can also be changed more efficiently which allows for the rapid deceleration and reversal in movement of the casting that is set out in table 2 above. It will be appreciated by the skilled person that the present invention provides an improved continuous casting apparatus and an improved method of continuous casting of rod, strip or tubing. This results in a better quality of product with no material defects which the present application has evidenced by tensile testing of samples of as cast-material. A better quality product exhibits no material cracks during tensile tests other than necking and tensile break area. By carefully controlling and limiting the backlash in the drive train of the continuous casting apparatus, and by using this to facilitate significantly more rapid acceleration than was previously achievable using standard components in the drive train, the continuous casting sequence can be tuned to provide a quality cast product in significantly less time than was previously achievable. By reducing the defects in the material, less material waste is also produced which the skilled person would recognise as a further advantageous effect of the present invention. This allows for casting to be formed in closer to the intended shape of the finished product, thus less time and energy is required in finishing the product after the material has been cast. This provides a significantly more efficient process leading to less waste of materials, less time required in manufacture, less time and effort in reshaping cast materials. The person skilled in the art will recognise the present invention provides an apparatus and method which is more environmentally friendly than known apparatus and methods. As noted, the present invention may be particularly but not exclusively aimed at casting of copper, tin, lead, zinc, gold or silver or alloys of these metals. The apparatus described in figure 1 is mounted for vertical continuous casting however the skilled person will be able to apply the same modifications to an upwards continuous casting apparatus and operate the same method as described above to achieve the same results in terms of increased casting speeds but without sacrificing quality of casting product.

Claims

1. An apparatus for continuous casting, the apparatus comprising a crucible within which metal and or metal alloy material is melted, a die through which cast material exits the crucible, a drive system for withdrawing cast material through the die and means for rapidly cooling the cast material as it is pulled through the die, the drive system comprising a servo motor and gearbox for moving the casting in both forward and reverse directions as it is exits the die, wherein the drive system has a total backlash of up to 3 Arcminutes.

2. An apparatus according to claim 1, wherein the drive system has a total backlash of below 2.5 Arcminutes.

3. An apparatus according to claim 1 or 2, wherein the drive system further comprises a drive shaft which is rotatable in response to operation of the drive system in either forward or reverse directions.

4. An apparatus according to claim 3, wherein the apparatus further comprises one or more rollers mounted on the drive shaft, said rollers rotatable with the drive shaft to support and direct the cast material as it passes through the casting die.

5. A method of continuous casting comprising the steps of, melting metal and or metal alloy materials in a crucible, pulling molten metal from the crucible through an orifice in a casting die and operating a drive system to control the movement of the casting in a repeating sequence comprising one or more forward movements followed by one or more reverse movements, wherein the method comprises limiting backlash in the drive system to up to 3 Arcminutes.

6. The method of claim 5 wherein the method comprises limiting backlash in the drive system to below 2.5 Arcminutes.

7. The method of claim 6 or 7, wherein the method further includes the step of maintaining the casting in motion at all times such that the casting is not held stationary at any time.

8. The method of any of claims 6-8, wherein the speed of the casting through the die during the reverse movement(s) is less than the speed of the casting through the die during the forward movement(s).

9. The method of claim 8 wherein the casting is moved in the reverse movement(s) for smaller periods of time than in the forward direction in each sequence.

10. The method of claim 9, wherein the casting is moved in the reverse movement(s) by a smaller distance than in the forward movement(s).

11. The method of any of claims 6-10, wherein the method further comprises the step of extracting heat from the casting die as the molten metal is pulled through the die to rapidly cool the casting.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:07 05 24CLAIMS1. A method of continuous casting comprising the steps of, melting metal and or metal alloy materials in a crucible, pulling molten metal from the crucible through an orifice in a casting die and operating a drive system to control the movement of the casting in a repeating sequence comprising one or more forward movements followed by one or more reverse movements, wherein the method further includes the step of maintaining the casting in motion at all times such that the casting is not held stationary at any time and wherein the speed of the casting through the die during the reverse movement(s) is less than the speed of the casting through the die during the forward movement(s) and wherein the method comprises limiting backlash in the drive system to up to 3 Arcminutes.

2. The method of claim 1 wherein the method comprises limiting backlash in the drive system to below 2.5 Arcminutes.

3. The method of claim 1 or 2 wherein the casting is moved in the reverse movement(s) for smaller periods of time than in the forward direction in each sequence.

4. The method of claim 3, wherein the casting is moved in the reverse movement(s) by a smaller distance than in the forward movement(s).

5. The method of any of claims 1-4, wherein the method further comprises the step of extracting heat from the casting die as the molten metal is pulled through the die to rapidly cool the casting.

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