MXPA05002557A - Casting procedure, particularly for engine cylinder head. - Google Patents

Abstract

The present invention relates to a casting procedure for obtaining parts provided with inside cavities or holes, wherein said holes and/or cavities are obtained by laying into a mould or chill, intended to receive the molten metal, one or more cores made of sand or other material, and wherein a main core is used, realised into a special core box, along one or more secondary cores intended to be associated to said main core, comprising the following steps: coating at least one secondary core, only in the zones and by the pattern thickness with a layer of material intended to dissolve in contact with the molten metal; inserting the pre-coated secondary cores into the main core box yet to be moulded; moulding the main core box; and inserting the monolithic group obtained at the previous step, consisting of the main core, of the secondary cores and of the coating material that keeps them firmly connected, into the mould or chill intended to receive the molten metal. The cores to be coated can also be formed by hollow inserts (23) made of a heat resistant material, filled with sand polymerised resin, wherein the cavity represents the core pattern.

Description

PROCEDURE OF FOUNDRY, PARTICULARLY FOR HEAD OF CYLINDERS OF AN ENGINE FIELD OF THE INVENTION The present invention relates in general to the technology of production of castings. In particular, it is related to gravity shells and low pressure procedures that use cores to obtain interior cavities in the foundry. A typical example of such a casting process is that used to obtain engine cylinder heads, where internal cores are needed to obtain the water jacket for the engine cooling water, the intake and exhaust ducts and any another secondary cavity.

PREVIOUS TECHNIQUE Generally, for medium to large productions, the casting of an engine cylinder head is done using a fixed external mold, called a shell, while the inside and sometimes also the outside, nuclei are required, which are inserted (assemble) inside the shell to form a single body ready for casting. In a method of casting by sand mold and polymerized resin cores, the main difficulty consists in perfectly matching the interior of the part to be melted, ie the cores, with the outside, ie the shell, in such a way that that the required dimensional accuracy is obtained. The cores are obtained in corresponding molds, called core boxes and then usually preassembled in the vicinity of the shell. The group of preassembled cores is collected by automatic devices (fasteners and templates) and placed (assembled) in the shell. At this point it is possible to melt the molten metal which will fill the volume formed between the sand cores and the shell. Sand projections, called tap holders (molds) are obtained on the cores to keep the assembled core group in the desired position. Such tap holders are placed inside the mold and do not constitute part of the object resulting from the casting. In the specific case of cores for the intake and exhaust ducts of an engine cylinder head, whose surfaces from the shape of the casting end, such cores are inserted into the core of the water jacket and during the stage of movement of the group of cores towards the shell, if the assembling is done manually they are free due to the effect of the separations that will be occupied by the metallic thickness. In this way, they are placed by gravity within the lower zone of the corresponding passages that are provided within the core of the water jacket. When the duct nuclei are in contact with the rake (lower base) of the shell, they acquire their final position. When the core assembly is made with automatic systems, the duct cores are kept in suitable positions in relation to the water jacket by a special automatic device, but usually only by the side of the flange coupling to the manifolds of intake and escape. The entire operation requires an approach that must be carried out with caution. Since traditional technology provides water jacket cores that must be molded separately from each other, in addition to the other cores, the interior of the water jacket male box must also be provided with all of the other parts that result from the external thickness of the internal parts of the melt (ducts, etc.) and that are destined to house - during the subsequent assembly of the nucleus - the other nuclei. However, since the outer parts of the ducts do not undergo a shift since they are usually half the height of the water jacket, the moving parts are currently used, controlled by gears, camshafts, or at best case, by pneumatic cylinders, they almost always move on inclined axes. In the case of an engine cylinder head, in order to facilitate the removal of these moving parts, it is necessary to impart a greater inclination (sliding angle) and deform the external thickness of the ducts, providing excess material to obtain a minimum thickness required by the casting operation. This implies reducing the water jacket core, with the result of a greater fragility of the same and a lower efficiency of the refrigerant circuit. In other cases, the problem of developing the duct nuclei passes through the openings that are obtained in the water jacket cores is resolved by dividing the latter horizontally in two halves, which are then joined together by adhesive after insert the ducts. However, this implies higher production costs and lower quality of the finished product, mainly due to the defects (burrs) of foundry that can be generated in the water circulation compartment and due to the melting cavities that can develop from the possible contact of the molten metal with the semi-core fixing adhesive. Another casting process, called lost foam, consists of making multiple sectors of polystyrene using special dies. Once such sectors have joined together, the part to be melted is coupled. The polystyrene model obtained in this way is covered and then placed in a container, which is then filled by vibration with common sand or a similar material using a special pouring channel, also made of polystyrene, the molten metal is pour into the container. As the polystyrene burns, it is replaced by the metal so that the desired casting is formed. This procedure allows to eliminate the elaboration and placement of polymerized sand cores. However, on the other hand, in addition to the various technological problems, it also has the disadvantage - in the case of casting engine cylinder heads - that the shapes of the ducts, although coated, are not optimal since their surfaces are molded and therefore directly terminated by the polystyrene surfaces. They can even present unions resulting from the coupling of polystyrene sectors. The necessary use of glue, in addition, is the main cause of the cavities. In essence, this procedure is rarely used.

OBJECTIVES AND ADVANTAGES OF THE INVENTION The object of the present invention is to eliminate the disadvantages of the prior art mentioned above by proposing a new casting process that allows to obtain higher quality fuses and in this way reduce the number of melt scrape due to the dimensional defects and introduce additionally new design prospects. Another object of the invention is to provide a casting method that allows a perfect relative positioning between each core and an easy insertion of the cores into the mold or into another core, whatever its shape. Another object of the invention is to provide a casting process that will allow a considerable simplification of the core boxes, that is, without any complex shape, cuts and moving parts connected and which therefore is cheaper, more reliable and easier to maintain. Another object of the invention is to provide a die-casting method for motor cylinder heads which allows obtaining cores for the intake and exhaust ducts without any deformation in the outer thickness and with the most varied and complex shapes, which it can result in better engine performance and the engine is ecologically more advanced with respect to exhaust gases, as allowed by the new casting technology. A further object of the invention is to provide a casting process for engine cylinder heads that allows embedding inserts for the ducts inside the casting, made of a material capable of resisting the heat generated by the molten metal in order to obtain ducts. perfectly smooth which can contribute to improve the efficiency of the engine. These and other objects of the invention are obtained by a casting process according to the subsequent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The additional features of the invention will appear more clearly with reference to the appended indicative and non-limiting drawings. In the drawings: Figure 1 shows cores of sand and polymerized resin to elaborate the intake and exhaust ducts of the engine cylinder head; - Figure 2 shows a sectional view of the core boxes for molding the cores of Figure 1, in a variant with inserts around the intake and exhaust ducts; - Figure 3 shows the duct cores with inserts obtained with the core box of Figure 2; - Figure 4 shows a section of duct cores inserted inside the die for coating with foamed material; - Figure 5 shows the group of valve housing and duct cores coated with foamed material; - Figure 6 shows the box of males of the water jacket still empty; - Figure 7 shows the box of cores (figure 6) with the group of duct cores of figure 5 inserted therein; Figure 8 shows the group of cores, valve housings and foamed cover that is obtained by molding the core of the water jacket in the core box of the previous figure with the valve guides inserted in the cover; - Figure 9 shows a complete core assembly assembly scheme of the core group of the previous figure; Figures 9a and 9b show two enlarged details of the core assembly scheme, wherein the black parts indicate the difference in shape and volume of water jacket dimensions that can be obtained by the casting process under discussion, in comparison with the current technique; - Figure 10 shows the scheme of core assembly in the variant with inserts.

DETAILED DESCRIPTION OF THE INVENTION As stated in the above, the present invention relates to a casting process to obtain fuses that are provided with interior cavities. As it is known, such cavities are obtained when placing in a mold, such as a shell, designed to receive the molten metal, one or more cores made of sand and polymerized resin or other material. In turn, such cores are obtained previously in special molds, called boxes of cores. In the case of more cores, these are obtained separately, each within a box of relative cores, and then joined together before being placed (assembled) inside the mold or shell. For this purpose, the cores are usually provided with projections and complementary cavities, called positive and negative taps, to support each other and with other sand projections designed to be placed within the appropriate housings in the shell, which are not part of the structure. foundry The process according to the present invention provides a coating of one or more cores made of sand or other material with a layer of foamed material, such as polystyrene, only in the shaped zones, using a special die and then placing them inside the mold . The core coating material is designed to dissolve on contact with the molten metal, which replaces it and thus determines the required casting thickness, so that the finished casting surface will be determined by the quality of the surface of the casting. core. In particular, the process under discussion has been conceived and is especially useful in casting processes that require the elaboration of a main core and one or more secondary cores. According to the invention, after they are processed in a conventional manner in respective core boxes, such secondary cores are placed inside a die and are covered with foamed material only in the forming zone with the thicknesses required by the casting and then they are placed (pre-assembled) inside the main core box that is still to be molded, that is, it is empty. In order to receive the secondary cores already covered with foamed material, the main core box will be empty in the shapes of the secondary cores since the shapes and thicknesses have been replaced by the cores and by the coating layer. As a consequence, the main core box is much easier and cheaper to produce since it allows to eliminate any internal trim and any moving part necessary to produce the secondary core containers. In addition, the main core box only has the outer tap holders of the secondary cores, which will be preassembled inside them. After molding the main core box with sand and polymerized resin, a single monolithic body is obtained, already assembled and showing considerable geometric precision, consisting of the main nucleus of the secondary cores, which are integral with the main core through the coating that forms the casting thickness. Such a monolithic body can then be transported easily and placed inside the mold or mold. In addition to cores of sand or other material, the smelting process under discussion can be applied to shallow, shallow inserts consisting of heat-resistant material such as metal or composite material, and designed to be embedded within the smelter for constitute the interior surfaces of perfectly smooth cavities. From the dimensional point of view, the main core box is capable of receiving both the sand cores or the inserts coated with foamed material. If the inserts of metal or other material to be embedded within the casting and these inserts have an interior hollow and where such gap corresponds to the design of the core, these should be placed in the specific core box which only considers the insert thickness in addition, and then they are molded. The resulting core will be provided with tap holders and embedded inserts, only in the shaped zone and also serves as a support for the inserts, such a core will also prevent the molten material from penetrating into the hollow part of the embedded inserts. The property that allows pre-assembly of the coated cores in advance inside the box of cores that is still to be molded (hollow) allows such secondary cores to conform to any geometric shape which is not otherwise possible. In the melt it is therefore possible to obtain even several passages, labyrinths and so on, which was not possible before, and these since it is not necessary to carry out more a successive assembly, only after the molding of all the cores. Consequently, the described procedure results in an outer thickness of all the secondary cores that do not have deformation and that have been shaped perfectly as the drawings, which is not always possible according to the traditional technology, given that frequently the interior shapes of the main core box requires moving parts for shifting, which can only be obtained with special deformation. In any case, the foundry designer has a new technology that allows to obtain foundries that can be even embedded within other parts currently cast separately, according to the limitations of the current traditional casting technology. Such new technology can also be used to obtain reinforcement by pre-covering fragile cores with foamed material in order to facilitate handling or for greater protection against breakage after pre-assembly in the mold or to limit the effect of metallostatic pressure.

The above is obtained by making both the pre-assembled directly in the die, and in this case the thickness of the coating can be equal to or less than the thickness of casting, or by placing the pre-coated cores in another box of cores yet to be molded, and in this case the coating must be equal to the casting thickness. In order to coat both cores and inserts with polystyrene or other equivalent material, it is necessary to provide them with a specific die consisting of a single negative lower half and an upper negative half, since the positive patterns consist of the cores of the inserts that are going to be coated. The die is constructed with all the housings of core taps equal to the core boxes, the molds or shells, considering the specific tolerances and the thermal expansions.

DETAILED DESCRIPTION OF A MODE OF THE INVENTION The described casting process is adapted, although not exclusively, to a method of die-casting a head of engine cylinders. In this case, with reference to the accompanying drawings, the main core 11 is the core of the water jacket that is designed to produce the refrigerant circulation passages, while the secondary cores are mainly those that are related to the pipeline. intake 12 and exhaust 13. The latter, plus any other secondary core such as those designed to create the exhaust gas circulation compartment and which somehow involve the core of the water jacket are molded into boxes of cores respective in the traditional way. Once molded, such cores are placed inside a single die 19 (Figure 4) to be coated with foamed material 18, such as polystyrene. The valve housings 14, 15 for the intake and exhaust valves can be pre-positioned inside the die, in special references. In addition, the punch can be provided with movable cylindrical bolts 16, 17 designed to produce housings 16 ', 17' (FIG. 5) for the valve guides 16", 17". The polystyrene 18 or an equivalent material injected into the die only envelops the shaped areas of the cores inserted therein with the required casting thickness and thus the core tappers 12 ', 13' are excluded. In addition, the valve housings on the outside diameter are embedded, while the interior they will be aligned to - le ¬ the conical edges of the duct nuclei. For this specific application, the valve housings 14, 15 must have the appropriate machining material in the inner diameter. The outer diameter of the valve housings is produced with a taper equal to that of the interior, and such tapering is required for the coating material to support and hold the valve housings in position during subsequent handling, until assembly into the shell or die The metal will eventually block the valve housings over the casting. The group consists of ducts 12, 13 and valve housings 14, 15, all coated with foamed material 18 and therefore consists of a single body (figure 5) and then placed (preassembled) inside the box. male 20 of water jacket (figures 6, 7). As said before, the water jacket box 20 shows a very simple structure since it is free of shapes corresponding to the outer thickness of the preassembled secondary cores. In fact, instead of such forms there are reference housings 20 'and negative taps 20' '(FIG. 6) designed to receive the secondary cores with the relevant taps and the valve housings covered with foamed material. Therefore, the core box is free of any clipping and moving parts. In this point, the male box of the water jacket is filled with sand and polymerized resin, and in this way a very precise monolithic group is obtained in which the water jacket core 11 wraps and retains the outer thickness of the pipe cores that they consist of foamed material on the conformed zones (figure 8). In this way you also get a perfect relative placement between each core. When all of the groups consisting of secondary cores, the valve housings and the foamed material have been molded in a single body with the water jacket core, the valve guides 16 '', 17 '' can be automatically inserted inside. of suitable accommodations 16 ', 17' which are obtained in the foamed material. The special sealing members are applied to the joints between the foamed material and the box of cores of the water jacket of the upper half in order to avoid infiltrations of sand inside the guide housings during molding. The valve guides will be solid (without center holes) since the mechanical machining to insert the valve stems is done with valve guides embedded within the cast iron. Among other things, this allows to avoid the use of traditional reinforcement projections around the valve guide within the duct cores. The valve guides will be provided with a negative circular groove in the portions embedded in the foamed material which will hold the valve guides in position in the molten metal when the latter replaces the foamed material. In the upper portion of the valve guide there is often another core 21 for the oil gallery, as in the case shown in Figure 10, or a core for the rocker compartment which produces the riser tubes (power supply). cast metal during shrinkage by cooling). As a consequence, the upper end of the valve guides will always be guided within the suitable housing and produced within the upper core 21 or within the rocker compartment core, and will therefore be locked in the correct position, even when the metal The cast iron has dissolved or is dissolving the foamed material around the valve guides, without causing the same collapse of the valve guides. In the lower portion, the valve guides are inserted and stopped within suitable housings 22 obtained in the duct cores (Figure 3).

At this point, the monolithic group comprising the water jacket core 11, the secondary cores, the valve housings, the valve guides, the foamed material together with other cores such as the core 21 can be placed assembled within the Coquilla (figure 9, 10). During casting, the molten metal will dissolve and replace the foamed material, determining the required thickness and embedding the valve housings and valve guides. Figure 10 shows the same assembly scheme described in the above, but where the duct cores consist of hollow metal inserts 23 (or made of another material capable of resisting the heat generated by the molten metal), filled with sand and polymerized with resin that has a support function as well as the function of avoiding any penetration of molten metal inside the inserts. The interior of such inserts has the same dimensional characteristics of the sand cores. The polymerized sand cores and the inserts 23 are molded in the specific core box 10 which must take into consideration the thickness of said inserts (Figure 2). At one end, the inserts terminate against the valve housings while at the opposite end, they end in the same plane as the molten raw flange.

Since the exhaust addition duct cores 12, 13 and any other secondary core are placed (pre-assembled) after they have been coated with the foamed material 18 in the male jacket 11 of the water jacket, there are no limits of design for ducts or for other secondary cores. For example, the addition ducts can be connected together with a single chamber without any interruption in the horizontal direction to the upper parts of the valve housings. Such a chamber can even reach the addition manifold coupling flange and form a single integral chamber with the same manifold without implying any problem of assembling with the water jacket. Such a concept can also be extended to inserts made of other material and embedded in the foundry. As a consequence, the head designer will have a broad freedom of design given that the current design limitations such as the forced passage of the ducts through the water jacket are eliminated. For example, as shown in Figures 9a and 9b, the water jacket compartment can be produced with a more rounded design (black parts) instead of the current sloped surfaces at the sharp edges to allow for shifting. The thickness of the outer duct core is also free of deformations, with a constant and perfect thickness exactly as in the drawing specification. Briefly, the method of die casting proposed and applied for the materialization of a motor cylinder head allows to obtain the following advantages: - intake and exhaust ducts without any limitation of internal design and without deformations in the external thickness, with thicknesses of casting consistent constants; - intake and exhaust ducts consisting of heat-resistant inserts and embedded during casting; - greater geometrical precision in the position of the intake and exhaust ducts and the water jacket in relation to the combustion chambers; - a water jacket with a greater volume of water passage in the most critical areas; - valve housings embedded during casting; Valve guides embedded during casting; - possibility to eliminate the holes created by the tap holders to support the water jacket core in the pouring stage, and in this way the mechanical machining required to plug said holes is eliminated.

Claims (23)

1. CLAIMS 1. A casting process to obtain parts that are provided with interior cavities or holes, where the holes or cavities are obtained by placing inside a mold or shell designed to receive the molten metal one or more cores made of sand or other material, wherein each core is manufactured separately in a suitable core box, and where before being placed in the mold or shell, is provided for the step of coating at least one core with a layer of material adapted to dissolve in contact with the casting metal, characterized in that before said coating step at least one core with a layer of material adapted to dissolve in contact with the casting metal, comprises the step of placing at least one valve housing for the valve of admission or exhaust in a die to coat at least one core with a layer of material adapted to dissolve in contact with the metal of foundry that has the valve housing embedded.
2. 2. The casting process, as described in claim 1, wherein before the step of coating at least one core with a layer of a material adapted to dissolve in contact with the casting metal, comprises the step of producing at least one housing for a valve guide.
3. The casting process, as described in claim 1 or 2, wherein the coating is applied to the core only in the zones and by the thickness of the casting form.
4. 4. The casting process, as described in claim 3, wherein the coating is performed by injection molding on the core to be coated.
5. A method of casting an engine cylinder head, as described in any of the preceding claims, wherein a main core is provided, which is the water jacket core designed to produce the refrigerant circulation passages , and secondary cores, which are mainly duct cores for the exhaust intake ducts.
6. The casting method, as described in claim 5, wherein the main core is manufactured in a special core box, together with one or more secondary cores designed to be associated with the main core, characterized in that it comprises the following stages : - coating at least one secondary core, only in the zones and for the formed thickness, with a layer of material designed to dissolve in contact with the molten metal; - inserting the group consisting of at least one secondary core and at least one valve housing, all coated with the layer of material designed to dissolve in contact with the molten metal within the main core box yet to be molded; - molding the main core box; e - inserting the monolithic group comprising the water jacket core, the secondary cores, at least one valve housing, the layer of material designed to dissolve in contact with the molten metal that keeps it firmly connected, inside the mold or shell designed to receive the molten material.
7. The method of die-casting a motor cylinder head as described in claim 6, wherein the main core is that designed to produce the water jacket for circulation of the engine coolant, and wherein the cores Secondary elements comprise at least the cores of the intake and exhaust ducts, characterized in that the duct cores are placed inside a single die which is to be coated with the coating material layer in such a way that they form a single body which leaves to place inside the box of males of the special water jacket.
8. The casting process, as described in claim 7, wherein the valve housings for the intake and exhaust ducts are first placed inside the die, the injected material wraps the valve housings over the outer diameter.
9. The casting process, as described in any of the preceding claims, wherein prior to the assembly of the monolithic group within the mold or shell, at least the valve guides for the intake or exhaust valves are inserted within the group monolithic comprising the main core and the secondary coated cores.
10. The casting process, as described in any of the preceding claims, wherein the cores to be coated consist of hollow inserts made of a heat resistant material, wherein the cavity represents the shape according to the drawing .
11. The casting process, as described in any of the preceding claims, wherein the cores to be coated consist of a hollow insert only for the shaped areas, made of a heat-resistant material filled with sand and polymerized resin to produce the impressions and avoid metal infiltrations.
12. The casting process, as described in any of the preceding claims, wherein the sand coating material and the polymerized resin cores of the inserts is a foamed material, such as polystyrene.
13. 13. The main core box for the casting process according to any of claims 5-12, characterized in that it consists solely of two portions designed to be closed one on the other, so that it is free of cuttings and therefore of movable parts adapted to perform the shifting, and where it shows lodges and negative impressions to receive and block in position the secondary cores pre-coated with the coating material.
14. 14. A die for coating the intake and exhaust duct cores in a motor cylinder head casting process, as described in any of claims 5-12, characterized in that it is provided with cylindrical moving bolts designed for to produce at least seats within the coating material for the valve guides of the intake or exhaust valves.
15. 15. A die for coating the intake and exhaust duct cores in a motor cylinder head casting process, as described in any of claims 5-12, characterized in that it is configured to receive at least one housing. valve for the intake or exhaust valves.
16. 16. The motor cylinder head characterized in that it has embedded at least one hollow insert made of metal or other heat-resistant material, the interior of which forms the design of the corresponding intake and exhaust duct.
17. 17. The engine cylinder head casting, characterized in that at least one valve housing is embedded.
18. 18. The cast iron cylinder head, characterized in that at least one valve guide is embedded.
19. 19. A casting process for obtaining parts that are provided with interior cavities or holes, wherein the holes or cavities are obtained by placing inside a mold or shell designed to receive the molten metal one or more cores made of sand or other material , wherein each core is manufactured separately within a suitable core box, and where before placing them inside the mold or shell, it is provided for the step of coating at least one core with a layer of a material adapted to dissolve in contact with the casting metal, characterized in that, before the step of coating at least one core with a layer of material adapted to dissolve in contact with the casting metal, it comprises the step of producing at least one housing for a valve guide.
20. 20. A process for casting an engine cylinder head, wherein the cylinder head has at least one water jacket for the circulation of engine coolant, at least one intake or exhaust duct, at least one valve housing for the intake or exhaust valve, wherein the water jacket and the duct are obtained by placing within a mold or shell designed to receive the molten metal one or more cores made of sand or other material and produced separately in boxes of appropriate cores, and wherein less the core of the duct is covered with a layer of a material adapted to dissolve in contact with the casting metal before being placed inside the mold or shell, characterized in that it comprises the step of embedding at least the valve housing within the layer of a material adapted to dissolve in contact with the cast metal, such that during casting, the molten metal will dissolve and replace the layer of a material adapted to dissolve in contact with the cast metal, determining the required thickness and embedding the valve housing.
21. 21. The casting process of the engine cylinder head, wherein the cylinder head has at least one water jacket for the circulation of engine coolant, at least one intake or exhaust pipe, at least a valve guide for the intake or exhaust valve, where the water jacket and the duct are obtained by placing inside a mold or shell designed to receive the molten metal one or more cores made of sand or other material and manufactured separately in boxes of suitable cores, and wherein at least the core of the duct is covered with a layer of a material adapted to dissolve in contact with the casting metal before being placed inside the mold or shell, characterized in that it comprises the stage of embedding at least one valve guide within the layer of a material adapted to dissolve in contact with the casting metal, such that during casting, the metal f It will dissolve and replace the layer of a material adapted to dissolve in contact with the molten metal, determining the required thickness and embed the valve guide.
22. 22. The casting equipment of a motor cylinder head, wherein the cylinder head has at least one intake or exhaust duct, at least one valve housing for the intake or exhaust valve, wherein the duct is obtained with one or more cores made of sand or other material, and wherein at least the core of the duct is covered with a layer of a material adapted to dissolve in contact with the casting metal, characterized in that it provides for embedding. minus the valve housing within a layer of a material adapted to dissolve in contact with the cast metal, such that during casting, the molten material will dissolve and replace the layer of material adapted to dissolve in contact with the metal. casting metal, determine the required thickness and embed the valve housing.
23. 23. Casting equipment for an engine cylinder head, wherein the cylinder head has at least one intake or exhaust duct, at least one valve guide for the intake or exhaust valve, wherein the duct is obtained with one or more cores made of sand or other material, and wherein at least the core of the duct is covered with a layer of a material adapted to dissolve in contact with the casting metal, characterized in that it provides for embedding. minus the valve guide within the layer of a material adapted to dissolve in contact with the casting material, such that during casting the molten metal will dissolve and replace the layer of material adapted to dissolve in contact with the metal of casting, determining the required thickness and embedding the valve guide. SUMMARY The present invention relates to a casting process for obtaining parts that are provided with interior cavities or holes, wherein the holes or cavities are obtained by placing within a mold or shell, designed to receive the molten metal, one or more cores made of sand or other material and where a main core is used, materialized in a special core box, together with one or more secondary cores designed to be associated with the main core, comprising the following steps: coating at least one secondary core, only in the zones and by the distribution thickness with a layer of material designed to dissolve in contact with the molten metal; insert the pre-coated secondary cores into the main core box yet to be molded; molding the main core box; and inserting a monolithic group obtained as in the previous step, which consists of the main core, of the secondary cores and of the covering material which keeps them firmly connected, inside the mold or mold designed to receive the molten metal. The cores to be coated can also be formed by hollow inserts (23) made of a heat-resistant material filled with resin polymerized with sand, where it represents the core distribution.