CORE FOR HIGH-PRESSURE DIE-CASTING AND METHOD IN HIGH-PRESSURE DIE-CASTING AND HIGH-PRESSURE DIE-CAST PIECE

The present invention relates to a core for high-pressure die- casting, which core is intended to form a cavity in a high- pressure die-cast piece, and which core includes a heart made from a disintegrable material, the external surface of which is formed of a reinforcement layer. The invention also relates =to a method in high-pressure die-casting and a high-pressure die- cast piece.

In high-pressure die-casting, molten metal is fed at high speed and pressure into a reusable die, generally made of steel. For example, when casting aluminium, the temperature of the molten metal is about 700 ⁰ C, the pressure being 400 - 600 bar, or even 1000 - 2000 bar. High-pressure die-casting is a mass-production method, by means of which it is possible to achieve complex but strong cast pieces with good electrical properties.

Usually, the die is at least two-part, which allows the high- pressure die-cast piece to be removed after casting. If cavities are desired inside the die-cast piece, one or more cores are used, which are placed in the die prior to casting. In order words, a core of the shape of the desired cavity is placed inside the die for the casting. The cores are made from some disintegrable material, which will, however, withstand the temperature of the metal being cast, until the metal has solidified. After this, the core is removed from inside the cast piece, by disintegrating it in some way. One known die material is sand, hardened with additives or by compaction.

In high-pressure die-casting, problems arise from the inade ^¬ quate strength of traditional cores. In practice, the cores do not withstand the loadings arising during die-casting, but instead disintegrate prematurely already when the die is being filled. In other words, the casting is spoiled. It has been

proposed to treat the surface of the heart of the core formed of sand with a liquid solution, to form a reinforcement layer. The liquid solution contains extremely small material particles, which improve the surface quality of the cavity and facilitate the removal of the core by acting as a detaching layer.

The invention is intended to create a new type of core for high-pressure die-casting, which can be relied upon to with- stand the stresses imposed on it without breaking. In addition, the invention is intended to create a new type of method for high-pressure die-casting, which can be applied independently of the material or die. The invention is also intended to create a new type of high-pressure die-cast piece, in which new and surprising properties can be obtained by means of cavities. The characteristic features of the core according to the invention are stated in the accompanying Claim 1. Correspondingly, the characteristic features of the method according to the invention are stated in the accompanying Claim 7. In addition, the characteristic features of the die-cast piece according to the invention are stated in the accompanying Claim 13. In the core according to the invention, a new type of reinforcement layer is used, with the aid of which the durability of the core in ensured at the present high pressures. In addition, by altering the properties of the reinforcing layer, it is possible, for example, to guide the flows of molten metal and thus help to reduce the stresses acting on the core. By means of the method, many kinds of new properties can be incorporated in a die-cast piece. This makes it possible, for example, to reduce the weight of a die-cast piece, or prevent porosity in the die- cast piece. In addition, problems caused by the heat can be avoided. It is also possible to create complicated, even branching cavities in a die-cast piece.

In the following, the invention is described in detail with reference to the accompanying drawings showing some embodiments of the invention, in which

Figure Ia shows a schematic cross-section of an apparatus for high-pressure die-casting,

Figure Ib shows the apparatus of Figure Ia opened,

Figure 2a shows a cross-section of a first embodiment of the core according to the invention,

Figure 2b shows a cross-section of a die-cast piece formed by the core of Figure 2a,

Figure 2c shows a cross-section of the die-cast piece of Figure 2b, along the plane A-A,

Figure 2d shows a cross-section of the die-cast piece of Figure 2b, along the plane B-B,

Figure 2e shows a cross-section of the die-cast piece of Figure 2b, along the plane C-C,

Figure 3a shows a cross-section of a second embodiment of the core according to the invention,

Figure 3b shows a cross-section of a die-cast piece formed by the core of Figure 3a,

Figure 3c shows a cross-section of the die-cast piece of Figure 3a, along the plane D-D,

Figure 4a shows a cross-section of a third embodiment of the core according to the invention,

Figure 4b shows a cross-section of a die-cast piece formed by the core of Figure 4a, along different planes,

Figure 4c shows part of a die-cast piece formed using cores according to the invention,

Figure 5a shows part of a fourth embodiment of the core according to the invention,

Figure 5b shows part of a fifth embodiment of the core according to the invention, along with cross-sections .

Figure Ia shows a two-part die 10 in its closed state. The right-hand die piece 11 is attached to a support 12, which is fitted to an immovable attachment plate 13. The left-hand die piece 14 is fitted to a second support 15, which is attached to an attachment plate 16, which is arranged to move. This allows the die to be opened and closed. A cylinder 17 extends through the immovable attachment plate 13, at least as far as the die. Once the die 10 is firmly closed, the amount of molten metal necessary at the time, which is forced into the die 10 by a piston 18, is poured into the cylinder 17. The path of the molten metal is shown by an arrow in Figure Ia. In this case, the question is of the so-called cold-chamber technique. Once the cast has cooled, the die is opened and the die-cast piece is removed (not shown) . If necessary, detaching rods 19 are used. By changing the die pieces, different kinds of high- pressure die-cast pieces can be manufactured. Not only the opening and closing of the die, but also the dosing of the molten metal, the removal of the cast, and the cleaning and drying of the die are usually automated. This is thus mass production, in which the size of the series usually runs to tens of thousands, or even hundreds of thousands of pieces. In Figure Ib, the die 10 is shown opened and with the detaching rods 19 at the end of their stroke.

Figures 2a, 3a, and 4a show schematically a core 20 according to the invention for high-pressure die-casting. In the said figures, the white area surrounding the core 20 depicts the space that is filled by the casting metal inside the die in high-pressure die-casting. The core 20 itself is intended to form a cavity 21 in the die-cast piece 22, it being possible to fit even several cores at the same time to a single die, if required. Figure 2b, 3b, and 4b show schematically high-pres ^¬ sure die-cast pieces according to the invention, in which a cavity 21 of a desired shape is formed. The core 20 includes a heart 23 of a disintegrable material, the outer surface of which is formed of a reinforcement layer 24. The heart is made

from, for example, core sand and the reinforcement layer is formed around the heart, preferably as a sand or chill casting. In sand casting, the molten metal is poured into a cavity made in sand, where it cools. In chill casting, a permanent pattern is used, in which, as in sand casting, sand cores can be used.

According to the invention, the reinforcement layer 24 is an essentially unified metal layer 25, which is arranged to attach to the die-cast piece 22. In other words, the metal layer covers essentially the entire heart and is formed of metal or metal alloy poured around the heart in the pattern. Thus the metal layer forms a shell protecting the heart, so that a core made of a disintegrable material can surprisingly also be used in die casting. In addition, the metal of the reinforcement layer is selected to be such that it will attach to the die- cast piece during die-casting. By means of the aforementioned core, cavities that can be freely shaped are created in the die-cast piece. In addition, after die-casting, it is enough to remove the heart while the shape of the cavity remains pre- cisely as desired. The quality of the internal surface of the cavity is also excellent. In addition, the metal of the metal layer according to the invention is the same metal as that of the die-cast piece. Thus, the different layers are joined together seamlessly and the die-cast piece can be easily recy- cled, as, when it is eventually taken out of use, the scrap metal will contain only a single metal or metal alloy. At the same time, metal layers of even very different thicknesses can be used. Generally, the thickness of the said metal layer is 0,5 - 50 mm, preferably 5 - 40 mm. In that case, even in the most demanding applications the core will be sure to withstand the stresses of high-pressure die-casting. In addition, the cross-sectional shape and/or the thickness of the location in question of the metal layer can differ at different locations in the core. This makes it possible to create complicated cavities, while the core can be used, for example, to guide the molten metal in the die. The cross-sectional surface area of

the cavity formed by the core can also vary. For example, in a medium-channel application, throttling can be used to regulate the flow velocity of the medium locally. The cross-sectional shape of the channel can also be arranged as desired. In addi- tion, for example, by arranging finning heat exchange can be improved (Figure 2b) . The durability of the core can be further increased by increasing the thickness of the metal layer at the points at which the flow of molten metal strikes.

The core according to the invention permits the application of a new type of method in high-pressure die-casting. Depending on the application, one or more cores are used to form a cavity in a die-cast piece. Due to the demanding conditions, a core as described above is used, with a reinforcement layer arranged on its surface. According to the invention, the reinforcement layer is arranged of metal to form an essentially unified metal layer, which is arranged to attach to the die-cast piece. Thus, firstly, the core will withstand even high pressures and temperatures and, at the same time, the shape and internal-surface quality of the cavity will remain good. In addition, the cavities can be used even more versatilely than previously and thus new properties can be created in a die-cast piece. The method itself is also simplified, as the cavity is made ready at one time .

Generally, there is only one outlet in the cavity. However, according to the invention, the cavity 21 can also be arranged without an outlet, or at least two outlets can be arranged in it. Thus, a medium channel, support structure, and/or lighten- ing can be formed from the cavity. On the other hand, a shrinkage element, balance element, and/or thermal element can be formed from the cavity. Different applications are described hereinafter, with reference to the figures.

Figures 2b, 3b, 4b, and 4c show part of a high-pressure die- cast piece 22, in which a cavity 21 is formed with the aid of

a core. In a single die-cast piece, there can be even several cavities, which can also be for different purposes. In any event, each cavity has an internal surface 26. According to the invention, the internal surface is formed of an essentially unified metal layer, which has been as a reinforcement layer which, during die-casting has attached to the die-cast piece. In other words, the metal layer melts onto the die-cast piece while the shape of the cavity remains, however, as desired.

In the cavity 21 of Figure 2b, there is a single outlet 27 and the cavity 21 is elongated. However, in the cavity 21 there are three widenings in the middle area. Thus, the cavity can be used, for example, as a lightening, or to improve heat transfer, if the cavity is used as a channel for a medium. Figure 2c shows a cross-section of the die-cast piece 22 along the plane A-A of Fig. 2b. It can be easily seen from the cross-section how seamlessly the metal layer has attached to form part of the die-cast piece. The dotted line shows the outer surface of the metal layer of the core, prior to die-casting. In Figures 2d and 2e, the shapes of the cross-section are different, but the thickness of the metal layer is nearly the same over the entire area of the core. Through the same metal is used in the metal layer as in the die-cast piece, the different shadings are used in the figures to clarify the shape and dimensioning of the metal layer. In practice, it is impossible to distinguish the boundary surface between the layers in the finished die-cast piece.

In Figure 3b, the cavity 21 is arranged to form a channel for a medium, in which a cooling fluid, for example, can be circulated. Instead of cooling, heating can also be used. Of course, in electronics applications conducting heat away is often a problem, which is sought to be solved by finning and fans. However, finning increases the weight of a die-cast piece and wears the dies. In addition, blown cooling takes much energy and its effect is often insufficient. By using the core and

method according to the invention, it is now simple to obtain liquid cooling, which is also effective, in a die-cast piece. At the same time, finning can also be omitted entirely. On the other hand, in arctic conditions for example, a die-cast piece can be heated. In order to improve the transfer of heat, widenings according to Figure 2b, for example, can be made in the medium channel, in which case as much heat-transfer surface area as possible will be obtained at the desired locations. At the same time, the flow velocity will be reduced, thus increas- ing the delay time of the medium.

Figure 3a shows in addition part of the surface of the core 20. In the metal layer 25, there is preferably surface patterning 28, which ensures adhesion to the die-cast piece. The adhesion between the die-cast piece and the metal layer of the core can thus be improved by using suitable surface patterning and/or mechanical machining in the metal layer, for example, shot blasting or grinding. The cavity can also be shaped as desired. In addition, in the core 20 of Figure 3a, the thickness of the- metal layer 25 varies in different parts. In this way, the core can be made as light as possible, but nevertheless durable. In applications with outlets, the cavity opens to the outside of the die-casting. Figures 3a and 3b show a special application, in which the cavity 21 and its outlet 27 protrude outside the die-cast piece 22. Thus, a pipe or hose, for example, can be attached to the protruding part. On the other hand, an internal or external thread (not shown) can be machined in the protruding part. Figure 3c shows a cross-section on the plane D-D. In this case too, the thickness of the metal layer varies and can be freely selected.

Figure 4a shows a core 20, by means of which even complex cavities can be created in a die-cast piece. In Figure 4b, the cavity 21 acts as a medium connection, a support structure, and a lightening. According to the invention, in the cavity 21 without an outlet, the heart 23, or part of it, belonging to

the core 20 can surprisingly be left in the die-cast piece 22. An application of this kind is shown in Figure 4a, in which there is an auxiliary core inside the core 20, the material in the heart of which remains in the final die-cast piece. The auxiliary core is made beforehand and can be used in the manufacture of the actual core. In other words, the core according to the invention can be formed of two or more parts, which are joined to each other prior to be placed in the die. In addition, several hearts can be manufactured separately prior to the making of the metal layer. Thus, even very complex shapes and cross-sections can be created in the final core.

Figure 4b shows a cross-section of a high-pressure die-cast piece along a different plane to that in Figure 4a. At this location, the auxiliary core has been unified, though with small empty places in the metal layer, due to the support of the core during manufacturing. The left-hand cavity 21 of

Figure 4c is also filled. For example, the total weight of the

^■ die-cast piece can be reduced by using a material that is lighter than the casting metal. The cavity functions at the same time as a shrinkage element. In other words, places that are thicker than the rest of the structure can be made hollow with the aid of the core, or filled with some material, in which case detrimental concentrations of mass will be avoided. Porosity caused by shrinkage will also be avoided while less casting metal than before will be required. In a corresponding manner, the location of the mass in different places in the die-cast piece can be planned, in which case the cavity will act as a balancing element. A sand-filled cavity will also act as insulation. Instead of sand, rock wool, for example, can be used, in which case the cavity will act as a thermal element. It will then be possible to limit the spread and conduction of heat. In addition, a tight, but light filling of the cavity will also attenuate vibration.

The right-hand cavity 21 of Figure 4c depicts a medium-channel application. In this case, for example, hot electronics components can be placed on a flat surface, under which a medium is circulated. The heat will then be effectively transferred from the electronic components to the outside of the die-cast piece.

Figure 5a shows part of a core 20 according to the invention. According to the invention, the core 20 includes at least one branch 29. The creation of smoothly branching cavities, which can be achieved by means of such a core, has been impossible up until now. In Figure 5a, a unified heart 23 branches into two branches, which then reunite. A cavity that is advantageous in terms of flow is thus formed in the finished die-cast piece. Around the heart 23 is already a metal layer 25 according to the invention, which has been created by casting in the die. Two additional branches, that continue separately, branch out of the heart 23 of Figure 5b.

The method permits present and even future commercial cores intended for sand, chill, and die-casting to be used when die- casting metal alloys. The core according to the invention, eguipped with a metal layer, will withstand conditions, in which known cores are not, as such, suitable for use, due to the high mechanical stress. A core equipped with a metal layer is placed in a casting die, which, when it closes, locks the core in place. Molten metal is then fed at high pressure into the die-casting die. In other words, the die-casting die is filled by casting normally in a die-casting machine, when the metal shell cast around the core protects the heart of the core inside from the stresses caused by the flowing die-casting metal. These stresses are, for example, the mechanical stress caused by the flow of the metal and the impulse. As the metal layer is of the same metal as the casting metal, when the casting metal solidifies, the shell metal adheres seamlessly to form part of the die-cast piece. It is thus preferable to use the same base material as the material in the metal layer as in

the final die-casting alloy, i.e. the casting metal. The joint between the metal layer and the casting metal will then be chemically and metallurgically as strong as possible and the die-cast piece can later be recycled when it is taken out of use. The support of the core can be implemented by casting in the die to the metal layer the required support and the support structures for installation in the die-casting die. In the method, it is also possible to use separate core supports and/or permanent supports integrated in the die-casting die. By means of the core, closed cavities too can be made in a die- cast piece, from which the core material is removed by machining a hole in the die-cast piece.

The core material inside the metal layer withstands compression stress well and thus prevents the core from being crushed by the effect of high temperature and pressure. The binder in the core will disintegrate thermally after a sufficiently long time, so that it can be removed from the cavity formed inside the die-cast piece, for example, with the aid of compressed air and shot blasting. Additional heating of the die-cast piece after casting may be necessary in order to disintegrate the binder of the core. In the method, it is also possible to utilize commercial water-soluble core materials.

With the aid of the core and the method, freely shapable cavities can be formed inside die-cast pieces, with the aid of which a cast product can be lightened, or they can be used, for example, as cooling channels. In the lightening application, the weight of the die-cast piece is reduced while at the same time using less casting metal. One common die-casting metal is aluminium, though zinc and magnesium, for example, are also used. The method disclosed is suitable for alloys that can be die-cast. In addition, the metal layer of the core is preferably shaped in such a way as to maintain a laminar flow in the molten casting metal. For example, the location receiving the flow is thicker than elsewhere in the core. The shaping is

otherwise too arranged to be streamlined. However, during die- casting, the metal of the metal layer melts to form part of the die-cast piece, with the heart of the core defining the shape of the cavity.

Thus, according to the invention, the reinforcement layer is cast from casting metal in a die. In other words, in the core, the heart and the reinforcement layer form a piece cast in a mould, which is then fitted to a die-casting die. Earlier in the description, reference has been made to sand or chill castings as examples, which are, as such, known mould-casting methods, but other mould-casting methods can also be used. In practice, two different casting methods are now combined in a new and surprisingly manner, so that entirely new types of cavity are obtained in die-cast pieces. According to the invention, the heart of a core made from a weak, disintegrable material is protected by a metallic reinforcement layer, which, in addition, is attached to the finished die-cast piece.