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(11) | EP 0 099 470 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Casting non-ferrous metals |
| (57) A method of casting non-ferrous metals having a melting point below that of ferrous
metals comprising the steps of making a mould predominantly of zircon sand and a chemically
hardenable organic resin binder which can be hardened at a temperature not exceeding
50°C, casting molten non-ferrous metal into said mould to make a casting and separating
the used sand from the casting. The mould and cores, if any, comprises, in addition
to a catalyst or hardening agent for the resin binder;
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[0001] This invention relates to costing non-ferrous metals having a melting point below that of ferrous metals such as aluminium, magnesium and copper and alloys based thereon.
[0002] The use of zircon sand as a foundry moulding material is well known in the production of iron and steel castings. US-A-3,212,144 and US-A-1,886,249 disclose the use of zircon sand for moulds for casting ferrous metals where clay is used as the binder. Both these specifications are concerned with mouldable mixtures and therefore incorporate "green sand" technology. As a result, large expensive capital plant is required involving a large sand re-cycling and re-conditioning plant and the formation of moulds by such processes as "jolt-squeeze" or "high pressure" or other variants.
[0003] US-A-2,535,662 discloses the use of zircon sand for moulds for casting ferrous metals where linseed oil is the binder. The hardening of the linseed oil bonded sand is carried out in an oven. This process is costly in energy and is accepted throughout the industry as being at a grave disadvantage because of distortion during baking.
[0004] BE-A-864,207 discloses that zircon sand is used mainly because of its high refractoryness, i.e. it has high thermal stability and it does not fracture nor melt under conditions of thermal shock and so provides improved casting surface finishes relative to silica sands when casting ferrous metals.
[0005] Usually such moulds are produced by the Croning (Shell) process, which uses phenolic based thermo-setting resins. Although the process has a reasonable reputation for the accuracy of the resulting castings, the accuracy is necessarily limited by the use of hot metal patterns, which are subject to thermal distortion and the distortion of the thin shell moulds.
[0006] In ferrous foundries using the Shell Process, the expensive zircon sand is re-ciaimed by a number of existing thermal reclamation systems, most of which heat the sand to a temperature in the range 800°C to I,000°C to burn off the remaining resin prior to re-coating with fresh resin. The high cost of such reclamation is usually recoverable in the relatively high price of such ferrous shell-moulded castings.
[0007] All of the above specifications relate to a different field to that of Applicant's invention being concerned with the casting of ferrous metals.
[0008] Because aluminium, magnesium, copper and other metals having a melting point below that of ferrous metals and alloys based thereon do not require the refractoriness of zircon sand, moulds for these metals have traditionally been made only in silica or other cheaper sands.
[0009] According to the present invention, we provide a method of casting non-ferrous metals having a melting point below that of ferrous metals comprising the steps of making a mould predominantly of zircon sand and a chemically hardenable organic resin binder which can be hardened at a temperature not exceeding 50°C, casting molten non-ferrous metal into said mould to make a casting and separating the used sand from the casting.
[0010] The mould and cores, if any, may comprise, in addition to a catalyst or hardening agent for the resin binder;
[0012] The organic resin binder is preferably a resin which can be hardened at or substantially at room temperature.
[0013] The organic resin binder may comprise phenolic, furane, or isocyanate base resin which can be hardened by a solid or liquid catalyst but which is preferably gas hardened.
[0014] Zircon sand preferably comprises the whole of the sand of which the mould and cores, if any, are made.
[0015] The zircon sand may have a particle grain size lying in the range 50pm to 500µm. An average grain size of approximately 100µm is common but as low as 75µm is experienced.
[0016] The mouid, and core if any, may be made by hand filling and ramming or by blowing on an automatic core or mould blowing machine. In the case of a mould, a moulding box or a boxless process may be used.
[0017] Preferabiy the method includes the steps of returning the sand to grain size and reclaiming the separated used sand such as by the method described hereinafter, which method and apparatus are the subject of our U.K. Application No. 8137797, and then using the thus reclaimed sand to make a mould in which molten non-ferrous metal is cast to make a casting followed by separating the used sand from the casting, again reclaiming the sand by the above described method and re-using the reclaimed sand to make a further mould.
[0018] The use of zircon sand for complete moulds and cores for non-ferrous metals is not an obvious choice, indeed the cost of zircon and the cost of reclaiming zircon blocked the concept of its use in this way until the present invention.
[0019] The present invention provides a number of extremely important and unlooked for benefits as follows.
[0020] The accuracy of cored holes and wall thicknesses defined by cores, are improved by a factor of up to twenty times. External features of castings are typically five times more accurate than their silica sand cast counterparts. This improvement in accuracy follows from the low expansion combined with the high thermal capacity of zircon sand compared to silica. It enables the accuracy to exceed the accuracy of all other casting methods known to date including investment and pressure die casting. The high thermal capacity increases freezing rate and also improves mechanical properties.
[0021] Sand expansion defects, such as scabs, rat-tails, finning, flash and the like are eliminated.
[0022] The levels of addition of the resin binder are significantly lower than those used for the Croning Shell Process. Because the patterns are used at or near to room temperature, they retain their accuracy and so produce accurate moulds and cores. Also, the moulds can be made of any convenient thickness as a thick shell, or in the form of block moulds, or can be made in steel boxes or frames. In this way, also, accuracy can be conserved compared and contrasted for example with a thin Croning Shell mould which is easily distorted.
[0023] In the present invention, the zircon sand is reclaimed by maintaining a mass of the sand without agitation of sand in the container whilst permitting the gas to percolate through the sand and whilst the sand is held in a treatment teperature range of 250°C to below 600°C for a time to achieve good reclamation.
[0024] The treatment temperature may lie in the range 2500C to 400°C, preferably in the range 250°C to 350°C.
[0028] It has been found that no mixing or agitation of the sand is required, the desired reclamation occurring solely due to residence of the sand at the above mentioned temperature and for the above mentioned time. There is therefore no need to provide any mechanical mixing or agitating device in apparatus for performing the method.
[0030] The invention will now be described in more detail by way of example, with reference to the accompanying drawings, wherein:-
FIGURE I is a diagrammatic plan view of an apparatus for performing a method embodying the invention; and
FIGURE 2 is a diagrammatic cross-sectional view of an apparatus for reclaiming used zircon sand.
[0031] Referring to Figure I, a foundry includes a moulding station 1 where moulds are made of zircon sand bonded with a furane resin. The moulds may be made by hand filling or ramming but, in the present example, are blown using an automatic mould blowing machine. The mould may be used in conventional mould boxes or a boxless process, as in the present example, may be used. If desired a phenolic resin, isocyanate or similar resin which can be hardened by soiid or liquid catalyst but which is preferably gas hardened at room temperature or up to 500C may be used. A core or cores are similarly made of such resin bonded zircon sand, although if desired a different binder system may be used compared with that of the remainder of the mouid. The core or cores is/are positioned, as necessary, within the mould cavity at a mould assembly station 2 where the cope and drag halves of the mould are closed. In the present example, the mould comprises:
98% zirconium silicate sand;
0.55% furane resin, by weight of sand;
0.20% MEKP (methyl ethyl ketone peroxide) by weight of sand;
2.0% usual impurities such as oxides of transition elements.
[0032] The resin was hardened using SO2 gas amounting to about 0.25% S02 equivalent by weight of sand.
[0034] These moulds are transferred by a conveyer 3 to a casting station 4 where molten metai, in the present example magnesium alloy, is cast through ingates in the mould into the mould cavity and around the core or cores when present.
[0035] After casting, the full mould is transferred by the conveyor 3 to a shakeout station 5 where the sand of the mould and core or cores, when present, are shaken out of the casting by a shakeout machine. The used lumps of zircon sand initially shaken out of the casting are returned to grain size by the vibratory action of the shakeout machine at the shakeout station 5. Alternatively, the sand lumps can be returned to grain size by a separate attrition or crushing unit, not shown. The sand, returned to grain size, is fed by a conveyor 6 to a sand reclaiming plant 7 where the sand is reclaimed and then supplied by a conveyor 8 to the moulding station I where binder is mixed with the reclaimed sand and new cope and drag parts of a mould made and transferred to the mould assembly station 2.
[0036] The reclaiming plant 7 comprises, referring to Figure 2, a closed hopper 10 to which sand is supplied by the conveyor 6. Sand is fed from the bottom of the hopper 10 by a screw conveyor I into a container 12 containing electrical heating element 13 and sparge tubes 14. Fluidising air is fed to the sparge tubes 14 via a duct 15 by a fan 16.
[0037] Sand is heated to a temperature lying in the range 250-400°C to the fluidised bed. As a result, little or no reclamation occurs in the fluidised bed. However, if desired the sand may be heated to a higher temperature, for example up to 600°C in which case some reclamation of the sand occurs in the fiuidised bed, the extent of reclamation depending upon the dwell time of the sand in the bed.
[0038] Sand flows from the container 12 through a duct 16 into a relatively large container 17 where the sand dwells, unagitated, as indicated at 29. In the example illustrated, the temperature of the sand entering the container 17 is only slightly below the temperature in the fluid bed. If it is necessary to feed the sand over a greater distance, for example as a result of location of the container 12 remote from the container 17, suitable thermal insulation and/or auxiliary heating means may be necessary to prevent excessive cooling of the sand.
[0039] The sand is allowed to dwell unagitated in the container 17 at a temperature lying in the range 250-400°C, although the maximum temperature may be 600°C, for a time sufficient to achieve reclamation. Where oxygen for the slow combustion process occurring within the container 17 is obtained from air rising from the exit 19 and percolating through the mass of sand 18 in the container 17, the dwell time typically lies in the period four to twenty four hours.
[0040] However, if desired air may be caused to pass through the sand at a flow rate of at least 400/L/min m2 in which case the dwell time is measured in minutes.
[0041] In each case the used air is removed from the container 17 by an updraught through a conduit 20, extending to a cyclone 21 where the extracted dust fines and the like are withdrawn as indicated at 22 whilst invisible fumes are discharged to atmosphere as indicated at 23.
[0042] The table below sets out the operating conditions in respect of two reclaiming operations carried out on zircon sand which has been used to manufacture non-ferrous metal castings. After treatment under conditions set out in the table, the sand was re-used and found to produce high quality moulds in which non-ferrous metals again cast.
I. A method of casting non-ferrous metals having a melting point below that of ferrous
metals comprising the steps of making a mould predominantly of zircon sand and a chemically
hardenable organic resin binder which can be hardened at a temperature not exceeding
50°C, casting molten non-ferrous metal into said mould to make a casting and separating
the used sand from the casting.
2. A method according to Claim I wherein the mould and cores, if any, comprises, in
addition to a catalyst or hardening agent for the resin binder;
3. A method according to Claim I or Claim 2 wherein the organic resin binder is a
resin which can be hardened at or substantially at room temperature.
4. A method according to any one of the preceding claims wherein the organic resin
binder comprises phenolic, furane, or isocyanate base resin.
5. A method according to Claim 4 wherein the resin is hardenable by a solid or liquid
catalyst.
6. A method according to Claim 4 wherein the resin is hardenable by means of a gas
hardener.
7. A method according to any one of the preceding claims wherein the zircon sand comprises
the whole of the sand of which the mould and cores, if any, are made.
8. A method according to any one of the preceding claims wherein the zircon sand has
a particle grain size lying in the range 50 to 500µm
9. A method according to any one of the preceding claims wherein the mould is made
by hand filling and ramming or by blowing on an automatic core or mould blowing machine
using a moulding box or a boxless process.
10. A method according to any one of the preceding claims wherein the non-ferrous
metal comprises aluminium, magnesium, copper or alloys based thereon.
11. A method according to any one of the preceding claims wherein the method includes
the steps of returning the sand to grain size and reclaiming the separated sand and
then using the thus reclaimed sand to make a mould in which molten non-ferrous metal
is cast to make a casting followed by separating the used sand from the casting, again
reclaiming the sand and re-using the reclaimed sand to make a further mould.
12. A method according to Claim I wherein the sand is reclaimed by placing a mass
of the sand in a stationary container and providing a supply of combustion supporting
gas to the container and maintaining the mass of sand in a treatment temperature range
for a time sufficient to reclaim the sand, wherein the mass is maintained without
agitation in the container whilst permitting the gas to percolate through the sand
and whilst the sand is held in a treatment temperature range of 2500C to below 600°C for a time to achieve good reclamation.
13. A method according to Claim 12 wherein the treatment temperature lies in the range
250°C to 400°C.
14. A non-ferrous casting when made by a method according to any one of the preceding
claims.