Composite Foundar Core and Casting Method Using Said Core

Abstract

A foundry shell core includes an outer shell made of a polymerised sand and resin mixture suitable for withstanding the heat and metallostatic pressure produced by molten metal during casting, and of an inner core body made of a material suitable for being thermally dissolved before using the shell core forming an inner cavity inside the core itself that facilitates the intake of smoke and gases produced during the casting. Alternatively, the inner core body is dissolved after the solidification of the casting. The shell cores are obtained by injection with the use of a conventional core box into which the core bodies, previously obtained by a die, have been inserted.

Description

The object of the present invention is a foundry core suitable for moulding a cavity or a surface inside or outside a casting. The invention further relates to a method for making such core, a casting method using such core and equipment for carrying out such method.

In the prior art, the foundry cores used for obtaining casting shape both inside and outside the same, are moulded with equipment called “core boxes”, which are filled (moulded) using various technologies, but always with sands of various types, mixed in advance with as many types of resins and catalysts to carry out the polymerisation thereof.

The use of sand as raw material for moulding foundry cores, according to the prior art is by far the best solution because since sand mainly consists of silicon and quartz, it exhibits a very high melting point and consequently, has a high refractory power.

Resins and catalysts used for hot polymerisation of the sand and resin mixture, or resins and catalysts with gases used for cold polymerisation, are all always highly polluting and toxic and so, also the sand used in moulding the cores becomes waste classified as toxic and noxious.

The partial recycling of the sand used in the core moulding, as currently happens, implies a high energy waste, whereas the disposal thereof, more or less expensive, depends on the type of binder used. For the reasons mentioned above, it is important that the amount of sand to be used for every single core and consequently, of resins, catalysts and gases, is the smallest possible. The object of the present invention is to propose a foundry core, equipment and a relevant casting method using such core which should allow obtaining castings of any type, material and shape, without having to use the current foundry cores fully consisting of a polymerised sand and resin mixture, so as to considerably limit the disadvantages mentioned above.

Another object of the invention is to reduce the environmental pollution caused by a casting process by the equivalent reduction by weight or volume of the use of all the current components that contribute to pollution, such as exhausted sand, resins, catalysts and gases, for carrying out the polymerisation.

Yet another object of the invention is to substantially reduce the costs in general in a casting process, and in particular the costs related to the production of foundry cores and to the recycling of sand, while achieving an overall energy saving, by the substantial reduction of the sand volume used.

Such objects are achieved by the use of a foundry core according to claim 1, with equipment according to claims 29 to 31 and with a casting method according to claims 14 to 28.

In accordance with an embodiment, the core according to the invention consists of an outer shell of sand and polymerised resin obtained with controlled thickness on an inner core body of foamed polystyrene suitable for dissolving, for example thermally, prior to the use of the core in the casting step, at a suitable temperature for dissolving the core body without having to damage the shell, or once the casting has been obtained, with different methods according to the type of material used in the core body moulding.

The core body is obtained with any shape, even complex, with the use of die. Said core body will support the shell of a polymerised sand and resin mixture, applied by controlled thickness injection with the use of a conventional core box, obtaining a foundry core therefrom which, besides the weight, and the smoke and gas permeability, is totally similar to the current ones. The thickness of the polymerised resin and sand shell, being obtained by injection after laying the core body in a core box, and thus with a controllable thickness, may vary from core to core and from zone to zone on the same core, allowing the reduction of said thickness to the minimum required.

The outer shell of polymerised sand and resin mixture has also the task of protecting the inner core body, if not dissolved before the casting, from the heat that may be transmitted by the molten metal through the coating shell, so that the core body does not collapse until the end of the metal solidification.

In other words, the inner core body, when present in the casting step, is not subject to the heat degradation process since the molten metal, thanks to the protective shell, cannot have any physical contact with the moulded core body of foamed polystyrene.

The shell consisting of a polymerised sand and resin mixture will preferably be used on all foundry cores, even on small-sized ones, since the primary objects, besides those of considerably reducing the current uses of sand and binders thereof and consequently proportionally reducing environmental pollution, production costs, weights, are also those of creating voids inside the cores, to be used for making smokes and gases produced in the casting step converging through the porosity of the sand and resin mixture shell and for facilitating their discharge through suction, thus obtaining better quality casts.

To this end, it should be noted that the presence of the core body in the use of the core, since it is highly permeable and further coated by a thinner shell, in any case allows the smoke and gas discharge. In any case, the discharge of said smokes and gases will be much better than a core completely made of sand and resin mixture. Since the foundry shell cores with controlled thickness exhibit the advantage of weighing, as regards the volume occupied by the core body of foamed polystyrene, which in medium and large sized cores is prevalent as compared to the total core volume, about 1/50 as compared to the sand mixture volume, they can be easily handled after the moulding in all the subsequent technological steps. The same applies to the cores wherein the core body has been thermally dissolved prior to their use for obtaining the casting. In this case, the weight recovered from the volume of the core body is total and the weight of the core will only derive from the volume given by the thickness imparted to the sand and resin mixture shell.

The inner core bodies of cores with shell, consisting of foamed polystyrene—EPS, will preferably moulded with dies. The cavity inside said dies suitable for receiving the material of the inner core body exhibits, at least for the portion corresponding to the core shape, smaller dimensions than the casting shape obtained with said core, and therefore compared to the cavity inside the core box. Such dimensional difference that equates a difference by volume, is equal to the thickness to impart to the coating shell of sand and resin mixture, thickness suitably selectable for every single core or for specific zones thereof. Since the shell of polymerised resin and sand mixture of the core has a controllable thickness, under the same casting pattern to be obtained the shell thickness will be smaller when the foundry core is used during the casting in combination with the core body, since this supports the shell. The shell thickness will be larger when only the shell of polymerised resin and sand shall withstand the metallostatic pressure generated by the molten metal. After the moulding with specific dies, the foamed core bodies must be inserted (assembled) in the conventional core box to be injection-coated with the shell of sand and resin mixture. After that, the mixture is polymerised.

The cores thus obtained, with the use of a much smaller amount of sand and resins than in conventional full foundry cores and therefore with a much lower weight, are suitable, with or without core body, for being used for any casting and thus capable of conveniently replacing the current foundry cores solely consisting of a polymerised sand and resin mixture.

It should be noted that the foundry core according to the present invention may be used, preferably with the inner core body, in the Lost Foam casting process. Advantageously, the cores proposed herein may be inserted in a specific die for obtaining by injection a foamed polystyrene casting pattern in a single piece, for example in the presence of complex casting geometries. The presence of the core body may determine the reduction of the shell thickness determining a further lower weight of the core by the effect of a smaller use of sand.

Moreover, there is the advantage of eliminating, in the Lost Foam casting technology, the need of having to compact the filling sand inside the casting pattern.

Further details and advantages of the foundry core with controlled thickness shell and of the casting method according to the present invention will appear more clearly from the following description of preferred embodiments thereof, made by way of indicative non-limiting examples with reference to the annexed drawings, wherein:

FIG. 1 shows a section view of a specific die X for the simultaneous moulding of the inner core body of two cores according to the invention;

FIG. 2 shows the inner core bodies of the cores placed in a core box Y for being coated by injection with the outer controlled thickness shell of sand and resin mixture;

FIG. 3 shows the core bodies coated with the shell, still in the core box Y;

FIG. 4 shows a conical container wherein there are inserted the composite cores of the previous figures and other composite cores, inside and outside the casting, for moulding an engine cylinder head;

FIG. 4a shows a view similar to the previous one, wherein the inner cores are hollow having undergone a core body dissolution before being placed into the conical container;

FIG. 5 shows composite cores laid in a semi permanent mould to produce an engine cylinder head; in the left side of the figure, the cores are depicted provided with inner core body; in the right side the cores are depicted hollow having undergone a dissolution of the core body before being laid into the semi permanent mould;

FIG. 5a shows an enlarged view of the central portion of FIG. 5 after the casting of the molten metal and with holes placed at the ends of all the mould core seats for discharging smokes and gases developed in the casting step;

FIG. 6 shows a section view of a specific die Z for making a pattern of foamed material of an engine cylinder head in a single piece by injection, wherein there are inserted the foundry cores with controlled thickness shell on a foamed core body, to be used for a Lost Foam casting process;

FIG. 7 shows the pattern obtained in a single piece with die Z of the previous figure; and

FIG. 8 shows the casting of an engine cylinder head obtained by casting molten metal into the conical container of FIG. 4 or 4a, in the semi permanent mould of FIG. 5, or from the polystyrene pattern of FIG. 7 used in the Lost Foam casting technology.

In accordance with a general embodiment, the foundry core according to the present invention comprises an outer shell 13 made of a polymerised sand and resin mixture and an inner core body 5 made of a material having a specific weight lower than that of said mixture and suitable for being dissolved before using the core, or as an alternative, after the melting and solidification of the casting.

The thickness of said outer shell 13 is selected so at to withstand at least the metallostatic pressure produced by the molten metal during the casting.

When the inner core body 5 is present in the molten metal casting step, that is, it has not been dissolved before, the thickness of the outer shell 13 is selected so as to prevent also a transmission of the heat generated by the molten metal to the inner core body 5, so as to prevent the thermal collapsing of said core body.

In accordance with an embodiment, the material forming core body 5 is suitable for being thermally dissolved. For example, said material is foamed polystyrene, for example of the traditional type known as EPS.

As an alternative and advantageously, the material forming the core body is a water-soluble and biodegradable foamed material, for example obtained from corn. The water solubility of core body 5 can only take place after the melting and solidification step of the casting.

The inner core body 5 may have any geometrical shape and may advantageously be obtainable with a die X. The presence of the core body has the primary object of replacing the equal volume of the mass of polymerised sand and resin currently used in moulding conventional foundry cores.

For its ecological features and for the fact that core body 5 must basically act as support for the protective shell 13 in the shell forming step by injection, the main material taken into consideration for being coated with shell 13 preferably but not exclusively is a new-generation biodegradable and water-soluble foam, obtained from renewable natural materials, such as for example the foam called Mater-bi™ manufactured by the Company Novamont.

As an alternative, the use of the conventional foamed polystyrene derived from hydrocarbons—EPS, currently used for several other purposes, is as much effective.

Of course, using such material will not give all the environmental/ecological advantages that on the other hand can be better obtained with the use of biodegradable and water-soluble foam.

It should be noted that the water solubility of the foamed material of core body 5 can only be used after the molten metal casting step and the casting solidification, when shell 13 of sand and resin mixture has ended its function.

The castings of any metal obtained by any process, but with the use in the casting step of cores consisting of an outer shell 13 of resin and sand mixture on an inner core body 5, will then be freed from the residues of said cores, and therefore if the core bodies 5 used are made of biodegradable foam, the castings may also be placed in water since besides being biodegradable, said foam is also water-soluble and therefore the core bodies will dissolve quickly. The residual material derived therefrom can be disposed of through water filtering systems, in the sewage or composted to produce humus, but not before the residues of sand and resin mixture released by the coating shell 13 protecting core bodies 5 have been recovered by decantation, since the disposal of such residues is conditioned by the type of components used.

If the foundry cores used to obtain the casting have been obtained from a shell 13 on a foamed core body 5 derived from hydrocarbons—EPS, on the other hand, said castings will be placed in an oven. The fumes and the volatile products derived from the thermal degradation of the foamed core bodies 5 will be sucked in and eliminated, as normally occurs with this type of products, and the residues of sand mixture of shell 13 will be recovered for disposal thereof.

In accordance with a variation of the casting method using composite cores according to the invention, the cores are freed from core body 5, for example by thermal degradation, prior to their use in the casting step. The casts thus obtained will then proceed to the subsequent steps in a traditional manner.

As already mentioned hereinbefore, core bodies 5 are advantageously obtained by moulding with the use of a die X. Taking as a reference an inner or outer shape of the casting, which shape must be obtained by the use of a core according to the invention, the corresponding shape of the die X suitable for receiving the material of core body 5 exhibits a smaller volume, or dimensions, or a width, for keeping into account the fact that the core body must be coated by the outer shell 13.

Since dies X are not very stressed, they can advantageously be made of a light alloy, for example aluminium. Dies X are specific for the required type of production, even though in general, they can be functionally compared to those used in the production of foamed polystyrene—EPS items, and must be fitted on a conventional press, preferably with horizontal division, used for moulding foamed polystyrene items. The production environment of core bodies 5 therefore is not conditioned to be that of foundries.

The outer coating shell 13 of core body 5 is obtained by injection using the conventional “core blowers”, with sand and resin mixture, which is then polymerised. The coating occurs by inserting (assembling) the core body 5 in the cavity of core box Y, said cavity matching the volume and dimensions of the cavity to be obtained in the casting. Once the core body has been laid in the core box Y, an air space is formed around it which is filled by the injected sand and resin mixture.

Advantageously, the core box Y must not be modified relative to that normally used for moulding conventional cores that are completely made of sand, if not, optionally, in the length of the core prints. For this reason, it is only necessary to check for every single core box that the length of the core prints are sufficient for ensuring perfect centring of core body 5 into the core box and thus ensure also the positioning of core body 5 in the step of injection of shell 13.

The use of the process described above for obtaining the composite core according to the invention achieves the following surprising advantages.

Shell 13 of polymerised resin and sand can have different thickness according to the zone and according to the need. In other words, the outer core shape remaining unchanged (which must match the cavity to be obtained in the casting), the thickness of shell 13 can vary according to the needs as compared to the interior of the core. In other words, in certain zones it may be larger and in others more limited to the disadvantage or to the advantage of the component material of the inner core body 5. Of course, the value of the thickness of the outer shell 13 must be decided during the design step so as to make die X for the inner core body accordingly. In fact, it is the shape of core body 5 relative to the cavity of the core box Y that determines the thickness of the outer shell 13.

The value, also variable, of the shell thickness is selected, as mentioned above, so as to minimise the use of polymerised resin and sand while ensuring that the shell opposes the metallostatic pressure to which the core is subject during the casting of the molten metal. Since in most cases such pressure is not constant on the entire surface of the core, the shell thickness may advantageously be selected accordingly. For this reason, shell 13 can be defined as having a controlled thickness, unlike other types of core coatings that for example envisage the immersion in a bath or the laying with a brush without the possibility of making, in any case, a core with a sand and resin mixture shell.

If the inner core body 5 is present in the step of casting the molten metal, that is, it has not been dissolved in advance, the thickness of shell 13 must be selected also for protecting core body 5 from the transmissible heat of the molten metal, so that the foam core body cannot degrade in the melting and solidification step of the casting.

It should be noted that if the core dissolution prior to the casting is chosen, the thickness of shell 13 must be larger for making up for the lack of the support to core body 5 in the casting step in order to control the metallostatic pressure.

Another surprising advantage offered by obtaining the shell through injection in core box, consists in the fact that the surface finish of shell 13 does not derive from the quality of the surface of the inner core body, but reflects the finish of the inner surface of the cavity of core box Y. Such surface finish could therefore be made as best as possible, irrespective of how the outer surface of the inner core body looks.

It is suitable that all core boxes Y, either existing or newly constructed, wherein the foamed core body 5 has to be inserted (assembled) to be coated by injection with shell 13 consisting of sand and resin mixture, are arranged for being subject to cold polymerisation or, if hot, to a polymerisation temperature that must be less than the thermal collapsing point of the foam so as to not damage core body 5. Since core boxes Y are more easily subject to wear, it is suitable that newly constructed ones are made of sand abrasion resistant materials such as medium hardness hardened steel.

If the making of a foamed polystyrene pattern 47 in a single piece is now considered, to be used in the Lost Foam casting technology, when the casting exhibits complex geometries, rather than moulding several sectors (slices) to be glued to one another, it is advantageously possible to use, thanks to their very low weight, the cores consisting of a foamed material core body 5 inside, coated with a shell 13 of a mixture of sand and binders thereof. Said cores must be assembled in a single specific die Z (FIG. 6), which besides being unique for the entire external shape, is capable of seating all the inner cores required for making the casting shape therein.

Said die Z is further characterised in that it is provided with seats 5′ for the core prints 20 of the cores for the realization of the inner shape of the casting, and along with said cores, it represents the entire casting shape (internal and external). It is suitable that the use of the cores with shell 13 in the Lost Foam casting technology takes place with the core body 5 to avoid having to make the filling sand converge, as it currently happens with the use of a pattern consisting of sectors (slices) which sand contrasts in the voids which would otherwise form with the degradation of the core body inside said cores.

Also in this case, the quality of the internal casting shape, unlike what happens with the current Lost Foam casting technology, is realised specularly by the quality of the surface of shell 13 of the cores, since shell 13 of sand mixture that encloses core body 5 is defined in the moulding, as much specularly by the quality of the surface of the core box Y. Such surface quality is further improvable with optional subsequent coating of shell 13. In any case, it does not depend on the quality of the polystyrene pattern surface anymore as currently happens, with the consequent elimination of all surface flaws existing in this process.

Since the use of the foamed polystyrene pattern used in the Lost Foam casting technology is totally different from the use of the cores with foamed core body, the pearls to be used for moulding the polystyrene pattern in a single body will be the conventional ones derived from hydrocarbons—EPS. Since the new-generation ecological material is derived from natural products, in the thermal degradation, essential in the Lost Foam casting process, if used, it would release carbon residues in the casting step, harmful for the casting.

The following description and the annexed figures refer to a practical example of application of the new casting method using cores with shell 13 with controlled thickness, to a casting process for obtaining an engine cylinder head 42, both in the case of casting by gravity (or at low pressure) in a semi permanent mould or in a core container, and in the case of Lost Foam casting process.

FIG. 1 shows a cross section of an example of die X, consisting of two half dies 23, 24, for moulding the foamed core bodies 5. For simplicity of illustration, the picture is that of a section of an intake and discharge duct of an engine cylinder head 42. To simplify the description, the shell cores 13 for the intake and discharge ducts and the relevant parts are indicated with the same reference numerals even though, of course, they are not perfectly equal to each other.

Going back to die Z, all the shape of core bodies 5 for the core of the intake and discharge ducts, in correspondence at the casting shape and the first portion of the side core prints, adjacent said shape, or excluding end 20 of such prints, is moulded by die X with reduced dimensions as compared to the corresponding shape of the cavity of the core box Y, and therefore of the casting. Such reduction is equal to the dimensions of air space 34 wherein it generates the necessary void for obtaining with the injection of the sand and resin mixture the controlled thickness imparted to the coating shell 13 on the core prints and shape. With the injection step, the volume difference existing between the shape of core body 5 generated by die X and the volume of core box Y which in turn matches the geometrical casting shape, is thus compensated.

Moreover, it should be noted that end 6 of core body 5 of the ducts facing downwards in die X, which acts as a core print, will be reduced in height by an amount equal to the thickness of coating 13 (FIG. 3), but only on the lower surface of said end and not on the remaining negative conical seat 20′ which is intended for centring on the corresponding positive centring pawl 20″ of the core box.

By analogy, the same technological concepts must be applied to all the core prints of every specific foundry core when the centring seat of the core which also serves as print, is negative rather than positive.

Reference numeral 28 indicates the seats for the injectors of the material of core body 5, whereas reference numeral 26 indicates pins suitable for creating holes 36 into core bodies 5, preferably in correspondence at injectors 32 of the core box Y (FIG. 2). Said holes 36 facilitate the subsequent coating in the coating step of core body 5 with shell 13 on all parts thereof.

Once moulded, core bodies 5 are laid in a core box Y, of the type and dimensions usually used for moulding a conventional full core fully made of polymerised resin and sand mixture.

FIG. 2 shows a cross section of an example of core box Y, made of two parts, a top one 30 and a bottom one 31, with the core bodies 5 already inserted (assembled), before the coating with shell 13.

It has to be noted the interspace 34 suitable for receiving the sand and resin mixture that will then be polymerised. Said air space 34 is equal to the dimensional and volume difference existing between core bodies 5, as compared to the shape of the core for the intake and discharge ducts of casting 42, and therefore to the core box Y.

As explained before, to save on sand and resin, shell 13 may have variable thickness according to the needs from core to core and when required, from zone to zone, on the same core so as to ensure the consistency required to oppose the metallostatic pressure generated by the molten metal on the coating of sand mixture 13 and when present, protect the core body 5 inside the core from the heat that develops during the casting step, and consequently on all the remaining cores required for obtaining the casting.

In FIG. 3, the core bodies 5 are shown coated with shell 13, except for the end portion 20 of the side core prints, which ends are geometrically coupled with seats 25 of the core box Y. As it can be seen, the coating material 13 is present also in holes 36 obtained from die X with pins 26 on core bodies 5, preferably in correspondence at the seats of injectors 32. Side ends 20 of the core prints and the internal negative seats 20′ act as centring and core print of core body 5 in core box Y. The latter is provided with conical centring pawls 20″ suitable for inserting into the negative internal seats 20′ of the core bodies.

In accordance with an embodiment of the casting method according to the invention, the inner cores 5, 7, 8 plus other outer cores 4, 9, 10, 11 are inserted in a metal container 1 with conical walls (FIGS. 4 and 4a).

The assembly forms what needed for obtaining a casting for the engine cylinder head 42 molten by gravity.

In the embodiment of the method illustrated in FIG. 4a, the inner cores 5, 7, 8 are used without core body 5 since it has been dissolved prior to their use in the casting step. In this way, a void is created inside the cores which acts as a plenum chamber and discharge for the gases developed in the molten metal pouring step. The outer cores 4, 9, 10, 11 are depicted with the use of shell 13 on core body 5. Also said cores, reinforced by suitable ribs, may be used with only the shell with increased thickness 13 of resin and sand mixture.

For convenience, and for opposing the metallostatic pressure, in particular on the side cores 9, 10, the entire group of cores, both internal and external, have been assembled in a metal container that is provided with conical walls 1′. The metallostatic pressure on the top core 11, when this is not a metal pattern, will be opposed by a counterweight or a mechanical device 12, which will also act on the side cores 9, 10 to prevent floating thereof. The process described above may be conveniently applied also to obtain smaller and above all larger castings of a cylinder head, when these are completely obtained with shell cores, both internal and external or also partly in combination with only external metal patterns in place of the cores.

Container 1 is provided with hooks 2 to which a rocker arm can be hooked for allowing the rotation of the metal container for removing the casting. Reference numeral 3 indicates the metal shape that generates the combustion chamber in the casting with the shape without the coating shell 13 since not needed. Said shape 3 is incorporated in core body 5 in the moulding step of the core body for the bottom core 4 external to the casting which, for further saving of sand and binders thereof, must not necessarily be coated in the bottom and side zones by shell 13.

Also the external side cores 9, 10 for the sides must not necessarily have the core body 5 coated in the rear side by shell 13.

In accordance with a variation of the method, cores 5, 7, 8, 11 are inserted in a semi permanent mould 50 (FIGS. 5, 5a), consisting of the metal parts 14, 15, 16, 17.

In fact, the cores object of the invention can be used also and especially to be laid (assembled) in a fixed pattern such as a metal mould. Said cores, depicted with or without inner core body 5, have the outer shell 13 but not on the ends of the core prints 20. However, the cores cannot undergo any damages for collapsing process when they come in direct contact with the mould operating heat. To this end, ends 25 of the seats of the mould core prints, coinciding with those of core body 20 of the cores and not coated with the protective shell 13, may be easily modified by providing them with an insert of insulating material 18. To this end it is noted that ends 20 of the core prints consisting of core body 5 cannot be coated by the protective shell 13 because in the step of injection of the coating shell 13, the ends of the core prints 20 rest into seats 25 of the core box Y for allowing the centring of core body 5 and thus also allow the protection with shell 13 of sand mixture to the portion of core prints adjacent the casting shape.

If core body 5 is present, the core centring into the mould will occur by the assembly in the mould on the entire length of the core prints 20, core body 5 included.

Since the portion of the core prints adjacent the portion of casting shape is coated by shell 13 of sand mixture, an excellent thermal insulation is guaranteed. The technology of protection of core body 5 located at the ends of the core prints for the intake and discharge ducts and thus not protected by the polymerised sand and resin shell 13, by analogy must be used on any other type of cores, when these must be protected from the mould operating heat.

It should be noted that also the bottom conical seats 20′ of the cores of the intake and discharge ducts and the core prints of feet 7′ of core 7 for the circulation of the coolant are protected by inserts of insulating material 21, 22.

In the embodiment of the casting method that envisages the use of cores in mould without core body 5, as thermally dissolved before, the smokes and gases produced in the casting, through the porosity of shell 13, converge into the voids obtained inside the core and can easily be sucked outside to obtain a better quality casting.

The cores without inner core body 5 have the core prints only consisting of the first portion of shell 13 and rest into the seats of the core prints of the mould. With the use of this embodiment, also mould 50 does not need being modified for applying the inserts of insulating material 18, 21, 22 graphically illustrated in the left portion of FIGS. 5, 5a.

FIG. 5a shows casting 42, the casting runner 42′, the protections with insulating material 18, 21, 22 and reference numerals 7′, 20 and 20′ indicate the centring references of the core prints in vertical, horizontal and lower centring on the conical pawls 20″. Reference numeral 35 further indicates the horizontal and vertical holes located at the ends of the core prints on the equipment for conveying smokes and gases produced in the casting step outwards.

In accordance with a further embodiment of the casting method, cores 5, 7 and 8 are inserted preferably complete with core body (the presence of the core body could determine a smaller thickness of the shell and thus a lower total weight of the core) in a die Z (FIG. 6) for moulding a polystyrene casting pattern 41 in a single piece (FIG. 7) to be used in the Lost Foam casting technology.

FIG. 6 shows an example of said die Z, comprising the die portions 37, 38 and 39, for the casting of an engine cylinder head 42. For obtaining the casting pattern 41 in a single piece, all the necessary moulded cores with shell 13 of sand mixture on foamed core body 5 have been inserted (assembled) in die Z.

In particular, note seats 5′ and the positive conical pawls 20″ suitable for seating both prints 20 of the shell cores 13 moulded on core body 5 as well as the bottom conical seats 20′ for centring the cores themselves.

Die Z exhibits seats 40 for the injectors of foamed polystyrene pearls.

FIG. 7 shows a section of the casting pattern of foamed polystyrene 41 obtained in a single piece from die Z, with all the shell cores 5, 7 and 8 incorporated therein for being used in the Lost Foam casting technology. The casting pattern is shown after the withdrawing from die Z complete with all the cores for the inner shapes and before being immersed in the refractory mixture for receiving the traditional coating, in this case on the external shape only because the internal shape fully consists of the shell cores.

With such further coating, also the negative conical seats 20′ for centring the lower ends of the cores of ducts 5 facing downwards and the ends of the lower feet 7′ that centre the core prints 7 for the circulation of the coolant will be protected.

Finally, FIG. 8 shows a section view of an engine cylinder head 42 obtained according to one of the three processes described above, but always with the use of cores consisting of a shell of sand and resin mixture with controlled thickness 13, for moulding both internal and external shape and wherein said cores may be used both with core body 5 incorporated into the core and without said core body. Note risers 43 for feeding casting 42, which in the case of low pressure casting technology are not provided.

It should be noted that, at least as regards casting in semi permanent mould 50 (FIG. 5) or in the core container 1 (FIGS. 4, 4a) the cores proposed herein can be used in combination with cores consisting of a shell of sand mixture 13 on core body 5, with the same cores where the core body 5 has been dissolved before use and both, even if not depicted herein in the diagrams, in combination with conventional full cores fully made of polymerised resin and sand.

In the practice, the casting method proposed herein allows replacing the current cores fully made of a polymerised sand and resin mixture with composite cores consisting of an inner core body 5 coated with an outer shell of a polymerised sand and resin mixture 13, so that only the polymerised sand contacts the molten metal even if core body 5 has optionally been dissolved or not, prior to the use in the casting step of the shell core 13. Advantageously, a natural biodegradable and water-soluble foamed material and thus derived from renewable sources can be used for moulding core bodies 5 in which case, the thermal biodegradable ecological principle can be used both before and after the melting and solidification step of the casting, whereas the water-solubility can only be used after the melting and solidification of the casting.

A further advantage of the invention is to reduce to about 1/50 the weight of the current cores solely made of sand and binders thereof, as regards the volume which can be prevalent on medium and large sized ones as compared to the total volume of the core, occupied by the foamed polystyrene core body 5. The weight reduction results from the difference given by the specific weight of the foamed polystyrene and that of the sand mixture. If the core is used without core body 5, as thermally dissolved before, the weight of said core will match the volume occupied by the coating shell of sand and resin mixture 13.

Thanks to the considerable weight reduction, as already said, the cores with controlled thickness shell 13 on foamed core body 5 can advantageously also be used for obtaining the polystyrene casting pattern 41 to be made in a single piece, to be used in the Lost Foam casting technology, especially in the presence of complex geometries, such as those for the casting of an engine cylinder head 42. This allows eliminating the need of splitting up the pattern into several sectors, the need of having to use any type of adhesive and as a consequence eliminating all the gas developed thereby in the casting step (about 2500 cm3/g), which causes a widespread porosity of the casting, eliminating any signs of junction currently generated by the union of the various sectors and in the practice obtain a casting of better quality.

Advantageously, moreover, with the several economic and technological advantages, the new technology can be applied with minimal economic investments. To achieve such object, in fact, all the existing “core blowers” are used, without having to make any changes, as well as all the existing core boxes, with optional minimal changes only for elongating the core prints on the horizontal plane. The existing moulds may be modified, even if not necessarily, with the addition of the inserts of insulating material 18 at the ends of the core prints 20 unless core body 5 is dissolved prior to the use of the core, in which case it is sufficient to use the portion consisting of only the shell of sand and resin mixture 13 as core print and therefore the semi permanent mould needs no modification either.

Claims

1-17. (canceled)

18. A foundry core for obtaining a cavity or an inner or outer surface of a casting, comprising an outer shell made of a polymerized sand and resin mixture and an inner core body made of a material having a specific weight lower than that of said mixture and suitable for being dissolved before using the core or after casting solidification, the thickness of said outer shell being selected so as to at least withstand metallostatic pressure produced by the molten metal during casting.

19. A core according to claim 18, wherein the thickness of the outer shell is selected so as to prevent transmission of heat generated by the molten metal to the inner core body, for preventing thermal collapsing of said inner core body, when it has not been dissolved prior to the casting of the molten metal.

20. A core according to claim 18, wherein the outer shell has an uneven thickness according to the different metallostatic pressure it is subject to.

21. A core according to claim 18, wherein the material forming the inner core body is suitable for being thermally dissolved before or after its use.

22. A core according to claim 21, wherein the material forming the inner core body is foamed polystyrene.

23. A core according to claim 22, wherein the material forming the core body is a water-soluble foamed material.

24. A core according to claim 18, wherein the inner core body has at least one through hole for the passage of the material of the outer shell consisting of sand and resin mixture so as to facilitate the inner core body sheathing operation.

25. A method of making a core according to claim 18, comprising the steps of:

arranging a core box having a cavity corresponding to the shape of the cavity or surface of the casting to be obtained with the core;

molding the inner core body so that it has a smaller volume than that of said cavity of the core boxes;

arranging the inner core body into the cavity of the core box;

injecting a mixture of sand and resin into the air space between the inner surface of the cavity of the core box and the core body; and

carrying out the polymerization of the sand and resin mixture.

26. A method according to claim 25, wherein the shape of the inner core body is selected so as to obtain a thickness of the outer shell suitable for withstanding the metallostatic pressure the core is subject to.

27. A casting method, comprising the steps of:

making at least one core according to the method of claim 25;

laying the core in a container suitable for receiving the molten metal;

casting the molten metal;

removing any cores from the casting obtained at the previous step.

28. A casting method according to claim 27, wherein after carrying out the casting of the molten metal, the inner core body is dissolved.

29. A casting method according to claim 27, wherein before positioning the core in the container suitable for receiving the molten metal, the inner core body is dissolved.

30. A casting method according to claim 27, wherein the core shell protects the inner core body from any contact with the molten metal.

31. A casting method according to claim 27, wherein, before laying the at least one composite core into the container suitable for receiving the molten metal, said core is incorporated in a pattern of foamed polystyrene representing the casting for a lost foam casting process.

32. A container for carrying out the method according to claim 27, wherein in the core print zones intended to contact the inner core body of a core, said container has inserts of an insulating material suitable for preventing the degradation of the material of said inner core body.

33. A die for carrying out the casting method according to claim 31, wherein the die is constructed only with the external shapes of the pattern corresponding to the molten cast, in that it has seats suitable for seating the core prints of all the cores making up the internal shape of said pattern.