EP1046725A1

EP1046725A1 - Spray deposition of metals

Info

Publication number: EP1046725A1
Application number: EP20000303022
Authority: EP
Inventors: Gregory John Gibbons
Original assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Current assignee: Bayerische Motoren Werke AG
Priority date: 1999-04-23
Filing date: 2000-04-10
Publication date: 2000-10-25
Also published as: GB2349393A; GB9909268D0

Abstract

It is known to remove a ceramic pattern from a spray cast metal shell cast about the ceramic pattern by mechanical breaking up of the ceramic pattern by the use of pneumatic drills. It is not uncommon that damage may be caused to the metal shell by this method requiring that the metal casting be scrapped. A method of removing such a spray cast metal object 6 from a ceramic pattern 8 is disclosed comprising dissolving at least a part of the ceramic pattern 8 in an alkali solution 4. This has as an advantage that the method of the present invention is a much less skilled operation than known methods. It is a further advantage that the risk of damaging the spray cast metal object in performing the method of the present invention is substantially reduced.

Description

The present invention relates to the spray deposition of metals, and in particular, but not exclusively, to the use of spray deposition of metals in the manufacture of tooling for use in the production of plastic mouldings and metal castings.
It is currently known to use freeze cast ceramic patterns onto which sprayed metal is deposited to form spray cast metal shells. The ceramic pattern is then removed by mechanical breaking up of the ceramic pattern by the use of tooling such as pneumatic drills. This is necessary since the ceramic pattern is very hard and also adheres well to the metal shell. It will be appreciated that removal of the ceramic pattern is a difficult and time consuming task. It is not uncommon, in the case of smaller or more complex castings that damage may be caused to the metal shell. Often, the damage is of a nature that scrapping of the metal casting may be necessary. Accordingly, the removal of the ceramic pattern will be understood to be a skilled and time consuming task.
According to a first aspect of the present invention, a method of removing a spray cast metal object from a ceramic pattern on which the spray cast metal object has been cast comprises dissolving at least a part of the ceramic pattern in an alkali solution. This has as an advantage that the method of the present invention is a much less skilled operation than the method noted above. It is a further advantage that the risk of damaging the spray cast metal object in performing the method of the present invention is removed, or at least substantially eliminated in comparison with the method noted above. This may conveniently be achieved by immersing the ceramic pattern and spray cast metal object in a bath of the alkali solution.
This has as an advantage that it enables the relatively quick removal of the ceramic pattern from the spray cast metal object without damage being caused to the metal object, and without adversely affecting the integrity of the metal object. A further advantage is that the ceramic pattern can be removed from complex areas of the metal object and also from areas of the metal object to which access would be relatively difficult when using a mechanical tool.
In a first preferred embodiment, the ceramic pattern is coated with a metal hydroxide powder prior to spray casting of the metal object, and after spray casting of the metal object the ceramic pattern and the metal object are immersed in a bath of water to allow the metal hydroxide powder to dissolve in the water to form the alkali solution. This has as an advantage that the alkali solution is concentrated in a region of the ceramic pattern adjacent to the cast metal object. Accordingly, less of the ceramic pattern need be dissolved before the spray cast metal object can be removed, thereby allowing the spray cast metal object to be removed from the ceramic pattern in a shorter amount of time than might otherwise be possible.
In an alternative preferred embodiment, passageways are formed in the ceramic pattern through which the alkali solution may pass. This has as an advantage that the alkali solution contacts a greater surface area of the ceramic pattern and so has a greater opportunity to dissolve the ceramic pattern. This in turn means that the ceramic pattern dissolves relatively quickly allowing the spray cast metal object to be removed from the ceramic pattern in a shorter amount of time than might otherwise be possible.
The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 shows a diagrammatic side view of a bath for use in accordance with a first aspect of the present invention;
Figure 2 shows a diagrammatic side view of a ceramic pattern at a first stage of a method according to a second aspect of the present invention;
Figure 3 shows a diagrammatic side view of a second stage of the method according to the second aspect of the present invention;
Figure 4 shows a diagrammatic side view of a third stage of the method according to the second aspect of the present invention;
Figure 5 shows a diagrammatic side view of a final stage of the method according to the second aspect of the present invention;
Figure 6 shows a diagrammatic side view of a ceramic pattern at a first stage of a method according to a third aspect of the present invention;
Figure 6A shows an enlargement of a portion of Figure 6;
Figure 7 shows a diagrammatic side view of a second stage of the method according to the third aspect of the present invention;
Figure 8 shows a diagrammatic side view of a third stage of the method according to the third aspect of the present invention;
Figure 9 shows a diagrammatic side view of a final stage of the method according to the third aspect of the present invention;
Figure 10 shows a diagrammatic side view of a ceramic pattern at a first stage of a method according to a second embodiment of a third aspect of the present invention;
Figure 11 shows a diagrammatic side view of a second stage of the method according to the second embodiment of the third aspect of the present invention;
Figure 12 shows a diagrammatic side view of a third stage of the second embodiment of the method according to the third aspect of the present invention;
Figure 13 shows a diagrammatic side view of a final stage of the method according to the second embodiment of the third aspect of the present invention;
Figure 14 shows a diagrammatic side view of a ceramic pattern at a first stage of a method according to the forth aspect of the present invention;
Figure 15 shows a diagrammatic side view of a second stage of the method according to the forth aspect of the present invention;
Figure 16 shows a diagrammatic side view of a third stage of the method according to the forth aspect of the present invention;
Figure 17 shows a diagrammatic side view of a final stage of the method according to the forth aspect of the present invention; and
Figure 18 shows a diagrammatic side view of a method according to a fifth aspect of the present invention.
Referring first to Figure 1, there may be seen a bath 2 for use in accordance with the method of the present invention. The bath 2 is formed of any suitable material able to resist the effects of a strongly alkali solution. Conveniently, the bath 2 may be made of a steel, for example a stainless steel. The bath 2 is filled with an alkali solution 4 at an elevated temperature.
Conveniently, the alkali solution 4 may be prepared by dissolving an alkali metal hydroxide in water. The alkali metal hydroxide may be sodium hydroxide (NaOH) or potassium hydroxide (KOH). The water may be distilled and de-ionised. The molarity of the solution is preferably between 2M and 10M. More preferably, the molarity of the solution is 5M. The water temperature is preferably greater than 60°C. The water temperature is more preferably 80°C.
A sprayed metal object, in the form of a metal shell 6, produced in accordance with known spray deposition techniques, together with an associated ceramic pattern 8 on which the metal shell has been spray cast, is immersed in the heated alkali solution 4 within the bath 2. The ceramic pattern 8 is conveniently manufactured from a freeze cast silica or a freeze cast alumina. The metal shell 6 is left immersed in the heated alkali solution 4 until the ceramic pattern 8 has dissolved partially or totally, so as to enable the metal shell to be separated from the ceramic pattern. Any part of the ceramic pattern that remains may then be easily removed.
The alkali solution 4 may be agitated by any suitable means, for example by rocking, by the use of a chemical pump or by ultrasonic means. It will be understood that a combination of any or all of these means is possible. Agitation of the alkali solution 4 ensures that the solution is able to fully contact and penetrate the ceramic pattern 8 within the bath 2.
Such a method is particularly suitable for removing ceramic patterns from a metal shell where together the ceramic pattern 8 and the metal shell 6 represent a comparatively small volume. For larger volume ceramic patterns, the following method illustrated with reference to Figures 2 to 5 is useful.
A ceramic pattern 12 is produced according to known methods. A layer 14 of alkali metal hydroxide in the form of a powder is then applied to a surface of the pattern 12 onto which a metal object is to be spray cast (Figure 2). The powder may conveniently adhered to a surface of the ceramic by using a suitable inorganic binder. Preferably, the binder is water soluble, for example salt-glass.
The metal object in the form of a metal shell 16 is then oversprayed onto the coated ceramic pattern 12 (Figure 3). The ceramic pattern and associated metal shell are then soaked in water 18 by immersion in a bath 20 (Figure 4). The alkali metal hydroxide powder dissolves in the water to produce an alkali solution which dissolves the ceramic pattern 12, in particular in a region adjacent to the metal shell 16. When sufficient of the ceramic pattern at an interface between the ceramic pattern 12 and the metal shell 16 has been dissolved, the metal shell 16 may be removed from a remaining portion of the ceramic pattern 12 (Figure 5). The metal shell 16 may be washed to remove any small particles of ceramic pattern detritus that remain in the spray cast metal object.
A third alternative method will now be described with reference to Figures 6 to 9. A ceramic pattern 22 is produced according to known methods. The ceramic pattern 22 is provided with a network of drilled passageways 24. The passageways may be drilled either before or after spray casting of the metal object. In the illustrated embodiment, the passageways 24 are formed prior to spray casting (Figure 6). When the passageways are formed prior to spray casting, it may be necessary to use plugs 26 of a suitable material to seal at least some ends of the passageways (Figure 6A).
It will be appreciated that drilling the passageways after spray casting of a metal object has as an advantage that it removes the need to use such plugs. However, it is a disadvantage that the drilling of the passageways requires skill and care to prevent unwanted damage to the spray cast metal object.
Once the metal object, in the form of a metal shell 28, has been cast, a suitable alkali solution 30, such as that described previously, is allowed to pass through the passageways. This may conveniently be achieved by immersing the ceramic pattern 22 and the associated spray cast metal shell 28 in a bath 32 of the alkali solution 30 (Figure 8). The passageways 24 allow a large amount of the alkali solution to penetrate the ceramic pattern 22 quickly. Accordingly, the spray cast metal shell 28 may be removed from any remains of the ceramic pattern 22 relatively quickly (Figure 9).
Alternatively, the alkali solution 30 may be introduced under pressure into the passageways 24. This enables the alkali solution 30 to pass through the ceramic pattern 22 quickly and also induces the alkali solution 30 to permeate into the ceramic pattern 22. In this way, the alkali solution 30 will reach an interface between the metal shell and the ceramic pattern to dissolve the ceramic pattern 22 at the interface enabling the metal shell 28 to be separated from the ceramic pattern 22 without the need to wait for all of the ceramic pattern 22 to be dissolved.
A variation of the third alternative method will now be described with reference to Figures 10 to 13. A ceramic pattern 33 is produced according to known methods. The ceramic pattern 33 incorporates a plurality of integrally formed non-communicating passageways 35.
The passageways are formed with the aid of an array of pins 34 (Figure 10). Conveniently, the pins 34 can be configured to any suitable array. Preferably, an outer face of the ceramic is spaced from the ends of the pins. The array is removed following forming of the ceramic pattern and prior to casting of a metal shell 36 (Figure 11).
Once the metal shell 36 has been cast, a suitable alkali solution 37, such as that described previously, is allowed to pass through the passageways 35. This may conveniently be achieved by immersing the ceramic pattern 33 and the associated spray cast metal shell 36 in a bath 38 of the alkali solution 37 (Figure 12). The passageways 35 allow a large amount of the alkali solution to penetrate the ceramic pattern 33 quickly. Accordingly, the spray cast metal shell 36 may be removed from any remains of the ceramic pattern relatively quickly (Figure 13).
It will be appreciated that this variation, in which the passageways are integrally formed in the ceramic, avoids the need to drill holes in the ceramic to form the passageways. A further advantage of this variation is that the array of pins may be readily reused
A forth alternative method will now be described with reference to Figures 14 to 17. This method is similar to the third alternative method. A ceramic pattern 42 is produced according to known methods with passageways formed integrally with the ceramic pattern. The passageways may be formed as part of a lost core method, or alternatively may be formed by tubular members 44 cast into the ceramic pattern (Figure 14). The tubular members 44 include walls that allow the alkali solution to pass therethrough. This may be achieved by any suitable means, for example by providing holes in the walls of the tubular members or by constructing the walls of the tubular members of a material sufficiently porous to allow the passage of an alkali solution 46 therethrough.
Once the metal object, in the form of a metal shell 48, has been spray cast about the ceramic pattern 42 (Figure 15), the suitable alkali solution 46, such as that described previously, is allowed to pass through the passageways. This may conveniently be achieved by immersing the ceramic pattern and the associated spray cast metal shell in a bath of a suitable alkali solution. Alternatively, the alkali solution 46 may be poured through the passageways 44 (Figure 16).
The passageways 44 allow a large amount of the alkali solution to penetrate the ceramic pattern 42 relatively quickly. The alkali solution 46 will reach an interface between the metal shell and the ceramic pattern to dissolve the ceramic pattern 42 at the interface enabling the metal shell 48 to be separated from the ceramic pattern 42 without the need to wait for all of the ceramic pattern 42 to be dissolved. Accordingly, the spray cast metal shell 48 may be removed from the remains of the ceramic pattern relatively quickly (Figure 17).
Alternatively, the alkali solution 46 may be introduced under pressure into the passageways. This enables the alkali solution 46 to pass through the ceramic pattern 42 more quickly and also induces the alkali solution 46 to permeate into the ceramic pattern. In this way, the alkali solution 46 will more quickly reach the interface between the metal shell and the ceramic pattern 42 to dissolve the ceramic pattern at the interface enabling the metal shell 48 to be separated from the ceramic pattern without the need to wait for all of the ceramic pattern 42 to be dissolved.
A further embodiment is illustrated diagrammatically in Figure 18. A ceramic pattern 52 and an associated spray cast metal object in the form of a metal shell 54 are produced in accordance with known methods. A suitable alkali solution 56 of the kind described previously is then directed under pressure in the form of a spray 58 at the ceramic pattern 52. This enables the alkali solution 56 to permeate the ceramic pattern 52 to dissolve the ceramic pattern 52 while the mechanical action of the spray 58 agitates the ceramic pattern and removes those portions of the ceramic pattern 52 dissolved or loosened from the ceramic pattern. The spray 58 is applied until the spray cast metal shell 54 can be removed from all of the ceramic pattern 52 with which it is in contact.
While this method, when compared with some of other methods disclosed herein, is relatively slow and requires the dissolution or disintegration of a large part of the ceramic pattern, it can be used with existing ceramic patterns and their associated spray cast metal shells without the need for modification of the ceramic pattern. This method is also relatively quick in comparison to the method disclosed in relation to Figure 1, and also may be used on comparatively large ceramic patterns and their associated spray cast metal shells.

Claims

A method of removing a spray cast metal object (6;16;28;36;48;54) from a ceramic pattern (8;12;22;33;42;52) on which the spray cast metal object (6;16;28;36;48;54) has been cast comprising dissolving at least a part of the ceramic pattern (8;12;22;33;42;52) in an alkali solution (4;30;37;46;56).
A method according to claim 1, characterised in that the alkali solution (4;30;37;46;56) is a solution of an alkali metal hydroxide.
A method according to claim 2, characterised in that the alkali metal hydroxide comprises sodium hydroxide or potassium hydroxide.
A method according to any previous claim, characterised in that the ceramic pattern (8;12;22;33) and spray cast metal object (6;16;28;36) are immersed in a bath of the alkali solution (4;30;37).
A method according to claim 4, characterised in that the bath of the alkali solution (4;30;37) is agitated, at least while the ceramic pattern (8;12;22;33) and spray cast metal object (6;16;28;36) are immersed in the bath.
A method according to any of claims 1 to 3, characterised in that the ceramic pattern (12) is coated with a metal hydroxide powder prior to spray casting of the metal object (16), and after spray casting of the metal object (16), the ceramic pattern (12) and the metal object (16) are immersed in a bath of water (18) to allow the metal hydroxide powder to dissolve in the water to form the alkali solution.
A method according to any of claims 1 to 3, characterised in that passageways (24;35) are formed in the ceramic pattern (22;33;42) through which the alkali solution may pass.
A method according to claim 7, characterised in that the passageways (24) are formed by drilling the ceramic pattern (22) prior to spray casting of the metal object (28).
A method according to claim 7, characterised in that the passageways (24) are formed by drilling the ceramic pattern (22) after spray casting of the metal object (28).
A method according to claim 7, characterised in that the passageways (24;35) are formed as a part of a process of manufacture of the ceramic pattern (22;33;42).
A method according to any of claims 7 to 10, characterised in that the alkali solution (46) is directed under pressure through the passageways.
A method according to any of claims 1 to 3, characterised in that the alkali solution (56) is directed under pressure at the ceramic pattern (52).