A method of making a casting mould

Abstract

 A method for making casting moulds (18) which are formed by machining away the mould cavity from a block (2) of moulding material instead of ramming casting material around patterns. The gating system including runners and risers may also be machined out of the solid block. The mould cavity can be produced in an appropriate shape in a moulding box including a number of adjustable rods (R) to reduce the amount of material to be machined out of the block. A multi-­axis machining centre (12, 13) is incorporated in a casting plant and programmed to produce the required moulding cavities, including re-entrant cavities (34, 35, 36, 37). The gating system is machined into the block in any convenient position, not necessarily on the mould parting line. A plant incorporting the method of making casting moulds includes the machining out of mould cavities the metal filling of the cavities, the positioning of blocks adjacent to each other on a conveyor system and automatic removal of the runners and risers while the moulding blocks are in position on the conveyor.

Description

[0001] This invention relates to the production of castings of metals or other suitable materials, and in particular to a method of producing collapsible moulds for castings and casting plant for producing finished castings from collapsible moulds.

[0002] At present, castings are produced by the basic method of pouring molten material into a cavity in a collapsible sand mould, the cavity being formed by ramming sand round a rigid pattern, which is then removed from the mould.

[0003] However, pattern-making is a skilled operation and patterns are therefore expensive to produce, as well as taking up storage space, with time and effort expended in storing and retrieving them. These problems or other re-entrant shapes, are involved, as the patterns have to be complex to enable them to be extracted form the mould. The result of this is that it is difficult for small batch production of castings to be efficient.

[0004] Good castings cannot be made without good moulds and moulds are dependent on dimensionally accurate patterns without flaws or defects. The term "moulding process" hereinafter used refers to the method of making the mould and the materials used. The term "casting plant" refers to plant which includes the moulding process but also includes the method of introducing the metal into the mould cavity, and all the processes used in making the casting.

[0005] Known moulding processes have certain features in common:

1. The use of pattern (or core boxes);

2. Some types of aggregate mixture comprising a granular refractory and binders;

3. A means of forming the aggregate mixture around the pattern;

4. Hardening of the aggregate or developing its bond while in contact with the pattern;

5. Withdrawal of the pattern from the hardened aggregate mould;

6. Assembly of mould and core pieces to make a complete mould, metal then being poured into the mould.

[0006] The methods of making a casting mould, the casting plant and the production of a cast product by the methods or plant of the present invention largely eliminates features 1, 3, 4 and 5 and is believed to be particularly suitable for workshops with up-­to-date CNC machining equipment but without a foundry.

[0007] The casting method according to the present invention enables moulds to be made without the use of a rigid pattern, thus overcoming the problems associated with the use of patterns. It is relatively easy to form re-entrant shapes in the block, and of course, the castings are easily removed as the mould is collapsible.

[0008] The main purpose of the pattern is its use in moulding. However, to produce a casting successfully and render it suitable for further processing, the pattern may be required to perform other functions besides producing a mould cavity.

[0009] Good practice for castings generally requires that the system of channels and feeding reservoirs (gates and risers) for introducing metal into the mould cavity be attached to the pattern.

[0010] Accordingly, one aspect of the invention provides a method of making a casting mould comprising the steps of making a solid block of moulding material, hardening the block and removing material from said block to form the shape of at least part of a moulding cavity.

[0011] The material of the block is preferably removed by machining. A number of separate blocks with material removed form part of the moulding cavity are held together to form a complete moulding cavity. Short moulds with machined-out cavities passing through the blocks may be grouped together on a conveyor system to form a product whose length is equal to the number of short moulds grouped together and whose overall shape is determined by the cavities formed in the group of block moulds.

[0012] According to another aspect of the invention, a cast product is made from a group of mould blocks in which the mould cavities of the group vary from block to block to produce the required shape of the complete cavity and the blocks are of various overall dimensions to reduce the amount of machining to produce a cavity and to reduce the amount of moulding material to be removed after the casting is solidified.

[0013] An approximately profiled cavity may be formed in the block at the stage of making the solid block before further material is removed form said block to form the precise shape of the moulding cavity thereby reducing the amount of material to be removed.

[0014] A gating system preferably comprises channels and risers for the introduction of molten metal into the moulding cavity formed in the block by removal of material from the block. The channels and risers may be cut off from the required casting after solidifi­cation, along with any surrounding moulding material while the blocks are located on the conveyor.

[0015] Another aspect of the invention preferably provides a casting plant comprising a block of compressed moulding sand, a drilling machine to produce aeration passages in the compressed block of moulding sand, a multi-axis machining system to remove material from the block to form cavities for the molten metal, conveyor means for guiding a row of block moulds through a molten metal feeding system to supply molten metal to the moulding cavity, cutting means to remove the solidified metal extraneous to the required casting when the metal-filled block is located on the said conveyor system, and shaker means for removing the rest of the block material from the finished castings.

[0016] Preferably, control means are included which position the machining system of the machining sytem in one of a number of axes, and further control means are provided to progress a tool into the block by the required amount along teh chosen axis to produce the required cavity.

[0017] According to yet another aspect, the present invention provides a casting system for making a solid block of moulding material with an approximate preformed moulding cavity including a moulding box containing moulding material has opposite sides of the box formed by an open-work frame to provide a number of through apertures, a ram to pass through the apertures, multi-axis robot means with which to position the ram above a number of selected apertures in a selected sequence, driving and control means to push the ram through the selected apertures of that sequence by the required amount whereby the moulding material may be profiled approximately to the required profile of a casting cavity, and machining means to remove moulding material to produce the required precise profile of the casting cavity.

[0018] All aspects of the invention will now be further described with reference to the accompanying drawings in which:

Figure 1 is a schematic layout of apparatus for performing a method of producing a casting moulding in accordance with the first aspect, and a method of producing a casting in accordance with the second aspect of the invention;

Figures 2 and 3 are cross-sections;

Figures 4 and 5 are plan views of part of the apparatus of Figure 1;

Figure 6 shows a side view of a component to be produced on the apparatus of Figure 1;

Figures 7, 8 and 8a show examples of components which may be cast; and

Figure 9 and 10 which show an approximate cavity moulding box to reduce the amount of machining.

[0019] The apparatus of Figures 1 to 5 is adapted to produce collapsible casting moulds and metal castings from these moulds, and is under the control of one or more computers (not shown) so that its operation is completely automated. the apparatus produces the moulds from moulding material in the form of a sand mixed with an alkaline-phenolic resin binder cured by gas.

[0020] The apparatus comprises a moulding machine 1 for making and hardening solid blocks 2 of the moulding sand. As shown in more detail in Figures 2 to 5, the machine 1 comprises a square vibratory mould 3, of which one side 4 is pivotally mounted at one edge 5. An ejector piston 6 is also mounted in the mould 3, and a hopper 7 connected to a weighing machine (not shown) is movable between a retracted position (Figure 3) and an operative position over the top of the mould 3 (Figure 2) in which it can deposit a predetermined weight of sand-resin mixture. An hydraulic ram 8 is similarly movable between a re­tracted position (Figure 2) and an operative position (Figure 3) in which it compresses the mixture in the mould 3. The ram 8 also includes a pipe 9 attached to a gas supply (not shown), and aperatures 10 allowing gas to flow into the mixture.

[0021] Returning to Figure 1, a three-axis CNC drilling machine is shown at 11. This machine may have a single drill or a multi-spindlea head, and is adapted to drill the blocks 2 for aeration. A CNC multi-­axis machining centre comprises two three-axis machines 12, 13 adapted to machine the blocks 2 to form cavities 14, comprising the shape to be cast and a gating system of runners, risers and feeds, together with core prints and line-up locations as required, or to form cores 15. A spraying machine 16 is adapted to spray the blocks and cores with a mould coating. A CNC assembly robot 17 is adapted to assemble the blocks 2 and cores 15, together with any other inserts, such as runner and riser sleeves, exothermic sleeves, strainers and chills (not shown), into the complete mould 18. A pouring ladle (not shown) is adapted to pour the required amount of molten metal into each mould 18, a pouring funnel 19 being provided for this purpose, in order to compensate for the variable trajectory of the ladle output, and the variation in position of the running in-feed in the moulds. A gantry-moulded CNC robot 20 operates cutting equipment 21 to cut off extraneous parts from a cooled casting. The apparatus is completed by a sand shake-­out conveyor (not shown), for removing the moulds 18 from the castings. The apparatus also has take­off points (not shown) where surplus sand is collected and returned to a sand reclaim/recycle circuit (not shown).

[0022] In operation the appratus is controlled wholly by the computer which is programmed for any given casting, in this case the component 22 of Figure 6, to operate all the machinery accordingly.

[0023] The first step in the operation is to form a solid block 2 of hardened moulding sand by discharging from the hopper 7 into the mould 3 a pre-determined weight of the sand-resin mixture (Figure 2). The mould 3 vibrates the mixture, the hopper 7 retracts and the ram 8 moves into teh operative position (Figure 3) to compress the sand mixture. Gas is then supplied to the block 2 to cure and harden it. The block 2 is then stripped from the mould 3 with the side 4 swinging open, and the ejector piston 6 pushing the block 2 clear (Figure 4 ). The piston and mould side 4 then retract (Figure 5) in readiness for a further cycle.

[0024] The block 2 is moved into the CNC drilling machine 11 for aeration andthen on to the CNC machining centre. Here, the machines 12, 13 machine it to the appropriate shape, either for a core 15 or for a mould 18 by forming cavities 14 of the shape to be cast, including a gating sysem of runners, risers and feeds as required. Where the component requires a mould parting line, this is formed by side of the block 2, with the remainder of the mould formed by an adjacent side of the next block.

[0025] The blocks 2 and cores 15 are moved on to the spraying machine 16 to be coated with a mould coating and then to the assembly robot 17 which assembles the blocks and cores in the appropriate sequence and orientation to complete a mould 18, including inserts such as runner and riser sleeves, chills, as required. The assembled moulds are accumulated in an accurate channel that guides the moulds 18 on three sides and indexes them for the pouring machine which inserts the funnel 19. The pouring ladle then pours the correct amount of metal into the mould 18, which moves into a cooling queue for the casting to solidify and then to the robot 20 which cuts off the runners and risers. This operation is performed with the casting still in the mould 18, which acts as a clamping and location device against the cutting forces. The mould 18 and casting then go to the sand shake-off conveyor.

[0026] The apparatus puts into effect first a new method of making collapsible moulds which does not require the use of rigid patterns and in which complex shapes can be formed relatively easily. Because of this, there will be greater choice for the location of moulding parting lines, leading to better quality castings. This is illustrated in Figure 7 which shows a section through a disc component 23 with various undercuts which would normally require a complex rigid pattern but which can easily be made using the present method with the parting line 24 as shown.

[0027] An example of the casting made in accordance with the invention is described more fully in Figure 8A. A block 31 of casting sand is drilled with through aperture 32 andthen with aperture 33. A tool or boring bar 38 is passed down the aperture 33 to the required distance along the axis, moved radially outwardly and rotated to cut away the groove 34. The tool 38 is then moved further along the aperture 33 to cut cavities 35, 36, 37, etc. Molten metal is fed into the mould via the right-hand end of the aperture 32 and surplus solidified metal cut off with the surrounding casting sand. The remaining sand is broken away to produce a casting to the required shape 38.

[0028] Complicated castings may be made without wooden patterns pinned together from two half patterns. There is no split mould and no tapers on the casting resulting from the need to withdraw the pattern from the sand. These tapers in pattern casting may need to be machined away or left to produce a part which is unnecessarily heavy. Re-entrants, namely flanges 34, 35, 36 and 37, cause no problem. Pattern sand casting may have required separate patterns and moulds held together which give rise to extra fettling because of the mould split lines.

[0029] It will be seen that the process and plant accord­ing to the invention may be particularly advantageous for casting crankshafts and engine cylinder blocks and many other complicated casting shapes, whether from cast iron or aluminium, or other metals.

[0030] The apparatus also puts into effect a new method of producing castings from collapsible moulds, by cutting off runners, risers and other extraneous parts before the mould is removed. This enables the whole casting process to be automated. Further advantages come form the combination of the two methods, enabling castings to be produced automatically and economically by using the computer. A central computer will normally hold the parameters for several castings so that it is very easy to change form one to another. It is also easy to alter the shape of any given cavity or core, which is particularly useful in speeding up the development of new castings. A further cap­ability of the apparatus is the production of variable length components such as that shown at 25 in Figure 8, by stacking appropriate moulds 18.

[0031] It is, of course, possible to use other types of moulding sand, the choice depending mainly on the material and shape of the castings and the length of the casting cycle.

[0032] The CNC drilling and machining centres shown are three-axis versions, but additional axes, for example with variable tilt spindles, may be used. Further, a CNC turning centre may be more appropriate for the production of certain shapes. The machines would, of course, differ form the machines used for metal because of the different characteristics of the blocks. They would have greater axis traverse rates as they would need to remove sand at a minimum rate of 1500 cc per minute. This would be compensated by the lower cutting forces incurred and the position­ing accuracy could be of the order of +/- 0.05 mm instead of +/- 0.01 mm for metal cutting machines.

[0033] As well as forming moulds, the moulding machine 1 and CNC machining centres 11, 12, 13 may also be used to form a conventional pattern from a block 2. Such a pattern would not, of course, be used in the rest of the system, but would be drawn off the apparatus and covered with heated plastic film blown or vacuumed on to the sand pattern. The film would protect the sand pattern and act as a release agent. The pattern could then be used to produce a mould, using any existing known techniques using patterns. Forming a pattern in this way of re-usable material means that it is easy and cheap to make and modify the pattern. Furthermore, the physical pattern does not have to be stored as the instructions for making it are, of course, stored in the computer.

[0034] In some applications an excessive amount of sand may have to be machined out of the block to form the desired cavity. If the proposed equipment is to be used frequently on this type of application, then the block moulding station Figures 2, 3, 4, 5 should be replaced by the Pre-formed Approximate Cavity moulding device shown in Figures 9 and 10.

[0035] Essentially, this device is capable of being programmed to produce an approximate cavity thereby minimising the sand to be removed by machining. Figures 9 and 10 show a platen incorporating this device. One or more of these platens may replace the sides of the block-forming machine shown at station 1 of Figure 1. The device works by having a series of rods (R) in a grid and a programmable actuator (A) to push each rod out to a specific distance as required. This then permits the creation of a 2-­dimensional profile that approximates to the exact shape required. The moulding of the sand may be done with one or more of these platens forming the side of the block moulding machine. In practice, two opposing sides are all that would be required for most applications. If the variation in the depth of the cavities is so great as to inhibit the ramming of the sand, it may be advisable to have only a single such platen. This should be quite adequate for most applications and should give reasonable sand utilisation. The use of the single platen also simplifies the platen withdrawal mechanism which would be complicated where more than one form platen is involved.

[0036] The operation of a typical form platen is des­cribed below, though there is a whole range of electric, magnetic, hydraulic or pneumatic gripping and actuation devices that could be used.

[0037] The proposed device, as shown, has a grid composed of plain (L₁) and slotted (L₂) locators. These are fixed to a U-clamp (F) by flexible fixings (metal or rubber). A cross-bar (b) completes the clamping arrangement. B and F are held together by dual-pressure hydraulic cylinders (C₁ and C₂). The whole clamping arrangement holds rods (R) in the grid. When the rods are waiting to be set to be programmed length by the actuator (A) the whole arrangement is clamped under low clamping pressure. In this state the rods will only move axially in the grid under the actuator force. The actuator itself is programmable in X and Y directions which covers all the rods in the grid, and in the Z direction to set the axial position of each rod. When the rods are set to length, the hydraulic pressure switches to high clamp force and the platen is used for moulding (the sand block) in this condition. The various split lines in the platen are sealed against the entry/exit of sand and curing agents. The choice of the dimensions of the rods depend largely on how well one can forecast the components on which the machine will be used.

[0038] By incorporating this refinement into the line, it is now possible to machine all moulds in a single pass because the machining is only finishing a cavity that has already been partly formed. As can be seen more clearly the required profile is formed by varying the setting of the rods. Machining to the required profile only involves finishing to the required curve of the shape required, for example, blending off the square edges left by the rods. The output of the moulding line could be largely independent of the volume of the cavities to be produced in the sand.

[0039] At the end of the moulding cycle for a batch of castings, the platen is withdrawn, switches to low clamp force and all the rods pushed back by a flat surface, flush with the platen, ready for re­setting for the next component.

[0040] The methods of making a casting mould and the casting plant according to the present invention offer a number of advantages over casting systems dependent on the use of patterns. Changing from one casting to another does nto involve a delay or stoppage of the line, whereas with conventional casting, even if that is readily available from the pattern sand or other material is packed around the new pattern store. Changing from one casting to another involves re-programming of the multi-axis machining facility or selection from a memory of programs. The production of moulds and cores is without hardware in the form of patterns or core boxes or expendible patterns, i.e. wax and polystyrene types. Reverse tapers, undercuts and other re-entrant shapes are possible by suitable selection of tool and direction of cutting axis.

[0041] A number of different castings may be produced from one mould without the problem of inserting "loose" patterns to fully occupy a mould. Long, variable length components can be readily produced that could only otherwise be produced conventionally by floor moulding. Moulding sand is soft and can be readily removed at high speed by machining by suitable tooling. Machining into moulding sand does not need to be so accurately controlled to produce adequate casting cavities as the control required for final finish machining of metal workpieces.

[0042] New prototype casting may be produced quickly as compared with conventional pattern casting which involves the preparation of patterns. Adjustments can be readily made in line rather than stoppage of the line while patterns are modified or re-made.

Claims

1. A method of making a casting mould comprising the steps of making a solid block of moulding material, hardening the block and removing material from said block to form the shape of at least part of a moulding cavity.

2. A method of making a casting mould as claimed in Claim 1 in which the material of the block is removed by machining.

3. A method of making a casting mould as claimed in Claim 1 in which a number of separate blocks with material removed forming part of the moulding cavity are held together to form a complete moulding cavity.

4. A method of making a cast product using short moulds with machined-out cavities passing through the blocks and grouping the block moulds together to form a product whose length is equal to the number of short moulds grouped together and whose shape is determined by the cavities formed in the group of block moulds.

5. A method making a cast product as claimed in Claim 4 in which the mould cavities in each block vary from block to block to produce the required shape of the complete cavity and the blocks are of various overall dimensions to reduce the amount of machining to produce a cavity and reduce the amount of moulding material to be removed after the casting is solidified.

6. A method of making a casting mould as claimed in any of the preceding claims 1-3 in which an approx­imately profiled cavity is formed in the block at the stage of making the solid block and further material is removed from said block to form the precise shape of the moulding cavity thereby reducing the amount of material to be removed.

7. A method of making a casting mould as claimed in any of the preceding claims 1-3 and 6 in which a gating system comprising channels and risers for the introduction of molten metal into the moulding cavity is formed in the block by removal of material from the block.

8. A method of making a casting mould as claimed in Claim 7 in which the channel and risers are machined into the block in a position away from a parting line between adjacent mould cavities to facilitate subsequent removal of material solidified in the channels and risers from the castings while they are still located on the conveyor system and in such a manner to reduce subsequent fettling of the casting.

9. A method of casting as claimed in any preceding claim comprising making a solid block of moulding material, hardening the block, removing the material from the block to form a moulding cavity and channels and risers for the introduction of molten metal into the cavity and cutting off the channels and risers from the required casting after solidification, along with any surrounding moulding material.

10. A casting plant comprising a moulding machine for forming and hardening comprising a block of mould­ing sand, a drilling machine to produce aeration passages in the compressed block of moulding sand, a multi-axis machining system to remove material from the block to form cavities for the molten metal, conveyor means for guiding a row of block moulds through a molten metal feeding system to supply molten metal to the moulding cavity, cutting means to remove the solidified metal extraneous to the required casting when the metal filled block is located on the said conveyor system and before removing from the conveyor belt, and shaker means for removing the rest of the block material from the finished castings.

11. A casting plant as claimed in Claim 10 in which control means which further comprises positioning the machining system of the machining system in one of a number of axes and further control means to progress a tool into the block by the required amount.

12. A casting system for making a solid block of moulding material with an approximate preformed mould­ing cavity including a moulding box containing moulding material has opposite sides of the box formed by an open-work frame to provide a number of through apertures, a ram to pass through the apertures, multi-­axis robot means with which to position the ram above a number of selected apertures in a selected sequence, driving and control means to push the ram through the selected apertures of that sequence by the required amount whereby the moulding material may be profiled approximately to the required profile of a casting cavity, and machining means to remove moulding material to produce the required precise profile of the casting cavity.

13. A method of making a casting mould substantially as hereinbefore described with reference to Figures 1-8 of the accompanying drawings.

14. A casting plant as hereinbefore described with reference to Figures 1-10 of the accompanying drawings.

Drawing