GB2339714A

GB2339714A - High speed casting

Info

Publication number: GB2339714A
Application number: GB9916691A
Authority: GB
Inventors: Crispin Alabaster Carney; Craig Grant
Original assignee: BI MEDICAST Ltd
Current assignee: BI MEDICAST Ltd
Priority date: 1998-07-16
Filing date: 1999-07-16
Publication date: 2000-02-09
Anticipated expiration: 2019-07-16
Also published as: GB9916691D0; GB2339714B

Description

2339714 High Speed Casting Process and-Apparatus This invention relates to

a high speed casting process and apparatus, and more specifically to a process and apparatus for casting precision orthopaedic implants and other prosthetic components which are required to be cast and for which there is a requirement that the cast item be of a certain purity.

Although the following description focuses in general on the application of the invention to orthopaedic implants and their casting from alloys, it is to be noted that the invention is not limited to this particular application, and further that by the word Ccalloy" is meant a metallic compound or composition which can adopt a molten condition.

There are currently available a number of metal and alloy casting processes in use depending on the desired qualities of the cast item. A number of different operating parameters including for example melt temperature, alloy composition, mould type and composition, melt solidification direction, rate of solidification, melt pour speed, melt time in crucible, and desired throughput or production rate, affect the resultant quality of the cast article. For example, if there is a requirement in the finished article for enhanced high temperature physical properties, then a directional solidification furnace may be employed.

Currently, the most important consideration in the casting of orthopaedic and prosthetic components is the purity of the cast article, as it is essential that a high degree of surface finish be attainable by polishing. Henceforth, the casting of such articles is generally currently conducted in a tilt/pour or rollover furnace wherein an alloy such as ASTM-F75 (a cobalt based alloy with chromium and molybdenum fractions) is inductively heated in a crucible and subsequently either:

i. the crucible is tilted and the molten alloy therein poured into an adjacent casting mould, or ii a casting mould is attached to the upper, open end of the crucible and both mould and crucible together are rotated through 180' such that the molten alloy falls under gravity into the casting mould.

Both crucible and casting mould are typically formed of a ceramic material, and the casting mould may be made by the lost wax process which enables the precise formation of recesses and internal passageways within the mould. In general, it is possible to provide a large number of such passageways and recesses within a casting mould, and therefore, a large number of separate components may be manufactured from a single mould.

Once the molten alloy is poured or otherwise deposited in the casting mould, and solidifies therein, the mould is broken open and the components are suitably removed from the central stem of solid alloy which exists as a result of the shape of the casting mould and its formation by the lost wax process. This central stem may be recycled in the formation of further components.

The primary disadvantages of this method of casting such components is that the average time taken to manufacture a component is too high on account of the two distinct steps of melting and casting which are involved, and that there is a requirement for personnel to oversee and control the casting process, and as a result the casting process cannot easily be automated and is poorly controlled.

3 A further disadvantage of conventionally cast components is that the resultant quality of these components depends to a large extent on the melt temperature and the pouring or flow characteristics of the melt as it passes from crucible to mould. In conventional casting processes, both these parameters are under human control, and therefore there can be great variation in the properties of conventionally cast articles.

A further method of casting, employed on a large scale by companies like Rolls Royce plc for the manufacture of turbine blades and other high precision components where a primary requirement is for excellent high temperature physical properties, is directional solidification. In this process, an alloy is melted within a crucible, the base of which is provided with an aperture through which the molten alloy is allowed to flow. To prevent premature flow of the molten alloy through said aperture, a disc of predetermined thickness and of the same alloy as that being melted is located in said aperture and melts as a result of heat conduction from the alloy above said disc and in contact therewith. The alloy within the crucible forms the core of a high power induction coil and the heat generated by the oscillating magnetic energy transferred into and out of the alloy is sufficient to melt said alloy. The crucible, induction coil and alloy are all encased in a first "melting" chamber.

Once the alloy disc has melted, the molten alloy flows under gravity into a second "casting" chamber in which the casting mould is located directly underneath the aperture of the crucible. The casting mould may have an enlarged upper aperture in the shape of a funnel to ensure that none of the molten alloy being poured thereinto is spilled or splashes in uncontrolled fashion. Furthermore, particularly where there is a requirement for high levels of purity, the recesses in the casting moulds in which the articles are cast may 4 be vertical such that molten alloy fills such recesses from their bases upwardly in an opposite direction to the flow of alloy from the crucible.

The second chamber and thus the casting mould is maintained at a temperature above the liquidus temperature of the alloy being cast to prevent any solidification thereof during the pouring phase of the process. Henceforth the molten alloy is allowed to completely occupy the casting mould recesses and settle therein without the formation of any metallic structure.

A third "solidification" chamber is provided into which the casting mould passes from the casting chamber and which is maintained at a temperature lower than the solidus temperature of the alloy. The alloy solidifies with a preferred crystallographically aligned structure and the articles are permanently cast in the casting mould, The primary requirement for excellent physical properties necessitates the use of a particular alloy and also that the casting process be conducted entirely under vacuum. Henceforth, each of the chambers described above is sealed from the atmosphere and a vacuum pulled therein. However, in directional solidification castings, the melt temperature and pouring characteristics are unimportant, and therefore little consideration is given to such parameters in the design of directional solidification furnaces.

Henceforth, it will be appreciated from the above that the process and apparatus for casting components with excellent physical properties do not immediately lend themselves to the casting of orthopaedic components for a number of reasons. Firstly, the apparatus is large as a result of the vacuum requirements and the third chamber which is required, and the method is too long-winded to produce orthopaedic components efficiently and cost-effectively.

Furthermore, the apparatus is too costly, and furthermore, there is no requirement for orthopaedic components to possess physical properties required for turbine blades.

A further disadvantage of the above casting method is the length of time which the molten alloy spends in contact with the contaminating crucible. It is desirable to reduce this time period to minimise contamination of the molten alloy.

It should additionally be pointed out that because of the negligible effect of melt pouring characteristics on the qualities of components cast by directional solidification, up until now,, little or no consideration has been given to aperture design in such processes.

It is an object of this invention to provide a method and apparatus for casting of alloy and metal components with a high purity level which is consistent, quick and efficient, and which enables the overall or average production rate of such components to be increased by reducing the requirement for personnel dedicated to overseeing and controlling the casting process.

It is a further object of this invention to provide a modified crucible design for component casting where the melt pouring characteristics have a noticeable effect on the physical properties of the resulting cast component.

According to the invention there is provided apparatus for casting orthopaedic components and the like comprising a first melting chamber, and a second casting chamber, said first chamber containing a crucible in which an alloy is heated, said crucible having an aperture in a base thereof which receives a disc of material which melts by conduction of heat through contact with 6 the melting alloy, said second chamber containing a casting mould which receives molten alloy through the aperture once the material disc has melted, characterised in that both first and second chambers are maintained at substantially atmospheric pressure and in that the second chamber is maintained at a temperature below the solidus temperature of the alloy being cast.

According to a second aspect of the invention there is provided a method of casting orthopaedic components and the like comprising the steps of i. heating an alloy in a crucible located in a first heating chamber, said crucible having an aperture in a base thereof which receives a disc of material which melts by conduction of heat through contact with the melting alloy, ii. allowing the molten alloy to flow under gravity through the aperture once the disc has melted and into a casting mould located underneath said aperture in a casting chamber, characterised in that said first and second chambers are maintained at substantially atmospheric pressure and in that said second chamber is maintained at a temperature lower than the solidus temperature of the alloy.

Preferably the aperture within the crucible is designed to constrain the melt to flow at a predetermined flow rate and with characteristic velocity profile through the aperture, and thus into the mould.

The applicant also foresees the use of specifically designed crucible inserts designed according to predetermined melt flow requirements depending on the melt temperature, alloy composition, and other like parameters. Such inserts may be designed to fit snugly within the apertures of conventional and widely available crucibles such that they can be adapted for use in this invention. Accordingly, such inserts are to be considered as within the scope of this application.

7 It is preferable that the alloy is inductively heated within a ceramic crucible by surrounding the crucible with a high power inductance coil through which passes rapidly oscillating current of large magnitude. A typical rating for the inductor of 60-8OkW with current oscillating at 912kHz is most preferable.

It is further preferable that the casting process described above and the operation of the apparatus is effected under the control of a computer program, and further preferably a programmed logic controller (PLC).

It is further preferable that the control of the process and apparatus is dependent on the measured temperature of the alloy.

It is further preferable during the casting process that the computer program or PLC imparts motion to the gas within at least the first heating chamber, and preferably the second casting chamber to condition or shield the melt.

It is further preferable that the weight of alloy charge deposited in the crucible is less than 6kg. Such a charge weight of alloy allows for particularly rapid melting thereof thus minimising the duration of contact between the molten alloy and the crucible which releases unwanted impurities into the melt.

It is further preferable that the density of the alloy is lower than 10000 kg/m'.

According to a further aspect of the invention there is provided apparatus for casting orthopaedic components and the like comprising a first melting chamber, and a second casting chamber, said first chamber containing a crucible in which an alloy is heated, 8 said crucible having an aperture in a base thereof which receives a disc of material which melts by conduction of heat through contact with the melting alloy, said second chamber containing a casting mould which receives molten alloy through the aperture once the material disc has melted, characterised in that both first and second chambers are maintained at substantially atmospheric pressure and in that the second chamber is maintained at a temperature below the solidus temperature of the alloy being cast, and in that the profile of the aperture in the crucible is such as to ensure a predetermined flow characteristic of molten alloy therethrough.

According to a still further aspect of the invention there is provided a method of casting orthopaedic components and the like comprising the steps of i. heating an alloy in a crucible located in a first heating chamber, said crucible having an aperture in a base thereof which receives a disc of material which melts by conduction of heat through contact with the melting alloy, ii. allowing the molten alloy to flow under gravity through the aperture once the disc has melted and into a casting mould located underneath said aperture in a casting chamber, characterised in that said first and second chambers are maintained at substantially atmospheric pressure and in that said second chamber is maintained at a temperature lower than the solidus temperature of the alloy, and in that the profile of the aperture in the crucible is such as to ensure a predetermined flow characteristic of molten alloy therethrough.

The profiling of the aperture through which the molten material flows during the process has the effect of ensuring that the molten alloy flows therethrough in a substantially uniform manner. Henceforth the purity and integrity of the resulting cast component is not compromised by the rapid solidification of the molten alloy 9 because the said molten alloy will settle within in the mould in a uniform manner without imperfection.

It will be understood that the casting method according to the invention combines the advantage of automation of the precision component casting method with the simplicity and efficiency of the tilt/pour and rollover casting methods to provide a much enhanced casting method for orthopaedic components. Components cast as described above have improved purity levels, and can be produced more, consistently, efficiently and quickly than previously possible. Furthermore, the reduction in cost of the casting apparatus which is no longer required to be provided with vacuum apparatus reduces the overall cost of casting.

A specific embodiment of the invention is now provided by way of example with reference to the accompanying drawings wherein:

Figure 1 shows a schematic representation of the casting apparatus according to the invention, Figures 2a, b, c show respectively a section, a side elevation and an end elevation of the crucible used in the apparatus of figure 1.

Figure 3 shows an enlarged sectional view of the aperture in the crucible of Figures 2a, b, c through which molten alloy flows during casting, and Figures 4a, b, c, show schematic representations in plan, side elevational and end elevational views respectively of the machinery used in the casting process, and Figure 4d shows an enlarged sectional view of the crucible and its disposition above a mould.

Referring firstly to Figure 1, there is shown a schematic representation of a casting apparatus 2 which is provided with two independent chambers, the first of these being a melting chamber 4 and the second being a mould chamber 6.

The melting chamber is surrounded by a heat shield 8 and is comprised of wall members 10 which are capped by a lid 12 which incorporates a window 14 behind which an infrared thermometer 16 may be disposed to assess the temperature within said melting chamber 4. Additionally, one of the wall members 10 is provided with an inlet 18 through which a conditioning or shielding gas may enter the melting chamber 4 either to prevent unnecessary oxidation of the melt with the surrounding gaseous atmosphere within the chamber, or to ensure that the surface of the melt remains in contact with and is conditioned by the said conditioning gas. An exhaust port 20 is also provided to allow for the escape of the gas from within the chamber.

The gas shielding is intended to eliminate any oxidation (i.e. chemical reaction of the reactive elements of the alloy with Oxygen). The two preferred forms of gas shielding are inert gas shielding where the primary concerns are the elimination of oxidation and the achievement of a cleaner melt, and conditioning gas shielding wherein oxidation of the reactive components is prevented and also the mechanical properties of the melt are enhanced by absorption of Nitrogen.

Centrally located within the melting chamber 4, there is a compressed ceramic fibre crucible 22 which is surrounded by an induction coil 24 and is provided with a pout nozzle 26 at a base end thereof.

In use, a small charge of an alloy, for example 5 kg of the ASTMF75 alloy, is deposited in the crucible 22 from above as shown generally by the dotted lines 28 whereupon the lid 12 of the melting chamber 4 is sealed against the wall members 10, and an operator of the apparatus can initiate the casting operation as controlled by a programmed logic controller (not shown).

Additionally, a metal disc 30 is located within the pour nozzle 26 to block said pour nozzle and thus prevent the charge 28 from flowing under gravity into the mould chamber 6 immediately as it becomes molten. The thickness of said metal disc 30 is chosen such that the charge 28 can become completely molten before said metal disc 30 itself melts as a result of heat conduction from the molten charge in the crucible above. Additionally, the size of the charge 28 and the power rating of the inductor coil 24 allows for a very fast melt time of said charge 28, and also a reduction in the melt time of the metal disc 30. This has the effect of reducing the contaminant effect of the ceramic crucible 22 which in turn results in a lower inclusion level in the resulting cast articles.

It should be pointed out that the ceramic crucible 22 is designed only for a single use, after which it is disposed of. Such crucible design further reduces the contamination of the melt thereby as compared to multiple-use, denser ceramic crucibles which are commonly used in casting processes.

The mould chamber 6 comprises a mould 32 which is surrounded by thermal insulation 34 within the mould chamber 6 which is in turn mounted on a base plate or spill containment means 36.

Referring now to Figures 2a, b, c in which the compressed ceramic fibre crucible 22 is shown in greater detail, it can be seen from Figure 2a that the crucible has an open end 22A and a closed end 12 22B in which is provided an aperture 22C which allows molten alloy to flow from within the crucible cavity 22D under gravity as shown by dotted arrow 22E.

The profile of the aperture 22C provides a seat 22F on which a metal disc 30 as shown in Figure 1 may be placed to initially prevent the flow of molten alloy as shown by arrow 22E however, as the material disc 30 is typically of identical composition to the said alloy, heat conduction between the molten alloy charge 28 and said material disc 30 melts said material disc opening the flow path shown in Figure 2a.

The profile of the aperture 22C can be seen more clearly in Figure 3. The crucible is provided with a protrusion 22X in its lowermost surface through which the aperture 22C passes The diameter of the aperture 22C reduces gradually through the said protrusion and such tapering of the aperture ensures uniform, steady flow of the melt therethrough after the disc 22F has melted.

Automation of the apparatus may be achieved by the programmed logic controller which detects both the temperature of the molten alloy within the crucible by analysing the melt colour using the infrared thermometer 16, and also by detecting the power consumption of the inductance coil which changes markedly as the bulk of the alloy charge drains through the pour nozzle 26 of the crucible after the material disc 30 has melted. As a result of this control, the requirement for operating personnel is minimised.

The flow chart provided on the following page of the specification identifies the important steps in the entire casting process.

13 Open meting chamber lid I Position sealing disc/charge/crucible assembly in the induction coil Close melting chamber fid - interlock I Open mould chamber Place mould in the mould chamber and close door Interlock 2 S T PLC checks interlocks 1 & 2 are complete and starts shieldingloonditioning gas flow I - A fixed time after the start of gas low, induction current is applied to melt the charge] Once the charge has fUlly melted, the seeing disc is melted by conduction and the charge pours directly into the mould at a rate determined by nozzle design The falling level of metal in On crucible changes the inductance of the meting coil PLC ttakes temperature reading from infrared thermometer which =Aes the met power system to tip (process monitor) After a fixed delay. shielding/oonditioning gas is turned off and the door locks role 1 -1 CYCLE COMPLETE -F -;7-e Used crucible and cast mould are removed ready n A Ycyde Referring to Figures 4a, b, c, d, a casting unit 40 is shown having a control unit 42 with a control panel 44 at the front thereof which controls the operation of a pair of casting stations 46, 48 each of which is provided with inductive heating coils 50, 52 between which can be placed crucibles 54, 56. Moulds 66, 68 provided with funnels 14 70, 72 may be firstly deposited in loading stations 58, 60 having doors 62, 64, whereafter said loading stations are displaced laterally within the machine 40 such that the said funnels register directly underneath apertures 74, 76 provided at the bottom of the crucibles 54.1 56. This configuration allows extremely speedy operation of the machine and the speed of casting of components is considerably increased. A user simply places alloy ingots in the crucibles, and the desired moulds in the loading stations, and proceeds to the control panel 44 where the machinery operates according to user selectable pre-set routines stored within the control unit.

Such efficiency of casting is unprecedented for orthopaedic and other surgical implants.

It is to be pointed out that modifications and alterations may be made to the invention without exceeding its scope or departing from its spirit, and it will be instantly seen by a person skilled in the art that these modifications and alterations are covered by this application.

Claims

1. Apparatus for casting orthopaedic components and the like comprising a first melting chamber, and a second casting chamber, said first chamber containing a crucible in which an alloy is heated, said crucible having an aperture in a base thereof which receives a disc of material which melts by conduction of heat through contact with the melting alloy, said second chamber containing a casting mould which receives molten alloy through the aperture once the material disc has melted,, characterised in that both first and second chambers are maintained at substantially atmospheric pressure and in that the second chamber is maintained at a temperature below the solidus temperature of the alloy being cast.

2. A method of casting orthopaedic components and the like comprising the steps of i. heating an alloy in a crucible located in a first heating chamber, said crucible having an aperture in a base thereof which receives a disc of material which melts by conduction of heat through contact with the melting alloy, ii. allowing the molten alloy to flow under gravity through the aperture once the disc has melted and into a casting mould located underneath said aperture in a casting chamber, characterised in that said first and second chambers are maintained at substantially atmospheric pressure and in that said second chamber is maintained at a temperature lower than the solidus temperature of the alloy.

3. Apparatus according to claim 1 or a method according to claim 2 characterised in that the aperture within the crucible is designed to constrain the melt to flow at a predetermined flow rate and with characteristic velocity profile through the aperture, and thus into the mould.

16 3. Apparatus according to claim 1 or a method according to claim 2 characterised in that an insert having a profiled aperture therethrough is deposited in the base of the crucible, the profile of said aperture being chosen to achieve a predetermined melt flow through said insert depending on the melt temperature, alloy composition, and other like parameters

4. Apparatus or method according to claim 3 characterised in that said insert fits snugly within the aperture the crucible.

5. Apparatus or method according to any of the preceding claims characterised in that the alloy is inductively heated within a ceramic crucible by surrounding the crucible with a high power inductance coil through which passes rapidly oscillating current of large magnitude.

6. Apparatus or method according to any of the preceding claims characterised in that the casting process described above and the operation of the apparatus is effected under the control of a computer program, specifically a programmed logic controller (PLC).

7. Apparatus or method according to any of the preceding claims characterised in that the control of the process and apparatus is dependent on the measured temperature of the alloy.

8. Apparatus or method according to claims 6 or 7 characterised in that the computer program or PLC imparts motion to the gas within at least the first heating chamber, and preferably the second casting chamber to condition or shield the melt.

17

9. Apparatus or method according to any of the preceding claims characterised in that the weight of alloy charge deposited in the crucible is less than 6kg.

10. Apparatus for casting orthopaedic components and the like comprising a first melting chamber, and a second casting chamber, said first chamber containing a crucible in which an alloy is heated, said crucible having an aperture in a base thereof which receives a disc of material which melts by conduction of heat through contact with the melting alloy, said second chamber containing a casting mould which receives molten alloy through the aperture once the material disc has melted, characterised in that both first and second chambers are maintained at substantially atmospheric pressure and in that the second chamber is maintained at a temperature below the solidus temperature of the alloy being cast, and in that the profile of the aperture in the crucible is such as to ensure a predetermined flow characteristic of molten alloy therethrough.

11. A method of casting orthopaedic components and the like comprising the steps of i. heating an alloy in a crucible located in a first heating chamber, said crucible having an aperture in a base thereof which receives a disc of material which melts by conduction of heat through contact with the melting alloy, ii. allowing the molten alloy to flow under gravity through the aperture once the disc has melted and into a casting mould located underneath said aperture in a casting chamber, characterised in that said first and second chambers are maintained at substantially atmospheric pressure and in that said second chamber is maintained at a temperature lower than the solidus temperature of the alloy, and in that the profile of the aperture in the crucible is such as to ensure a predetermined flow characteristic of molten alloy therethrough.