Sand core and sand mold

Abstract

And a sand core and a sand mold. The utility model relates to a sand core (1) for casting a screw rotor, the screw rotor comprising a body having a contoured surface with helical blades extending between the two head surfaces and two head surfaces, the sand core (1) comprising a unitary piece (2) with an internal cavity (3), the internal cavity (3) is at least partially delimited by the unitary part (2) by a rotationally symmetric surface formed by a spiral groove (5) around a rotationally symmetric axis, the spiral groove (5) having a profile for forming a spiral blade during casting of the screw rotor, the outer surface of the unitary part (2) being provided with one or more openings (4) extending into the internal cavity (3), the one opening or each of the plurality of openings (4) is adjacent to the helical groove (5).

Description

Technical Field

[0001] The utility model relates to a sand core and a sand mold for casting a screw rotor (for example, a screw rotor for a compressor, a vacuum pump or an expander element). The screw rotor comprises a main body having a contour surface and two head surfaces, wherein the contour surface has spiral blades extending between the two head surfaces.

[0002] More specifically, the utility model relates to a sand core, which comprises an integral part having an internal cavity, and the internal cavity is used to form the spiral blades in the process of casting a screw rotor. Background Art

[0003] Casting processes are known from the prior art in which molten casting material, usually grey cast iron or ductile iron, is poured via runners into the inner mold cavity of a sand core in a sand mold.

[0004] When the mold cavity in the sand core is filled with this molten casting material, the casting is at least partially formed by the surfaces of the mold cavity.

[0005] In order to accurately cast the casting and avoid extensive finishing of the casting, the sand core usually has a closed structure so that the mold cavity surrounds the casting material in the sand core as completely as possible during the casting process.

[0006] For this purpose, the outer surface of the sand core usually includes only a limited number of openings to provide access to the internal mold cavity, such as:

[0007] - one or more inlets for pouring molten casting material into the mold cavity; and

[0008] - One or more outlets for degassing and for connecting the mould cavity to one or more risers in the sand mould.

[0009] Due to the limited number of openings on the outer surface, it may be difficult to grasp the sand core on the outer surface to place the sand core into the sand mold or to remove the sand core from the sand mold.

[0010] It is also difficult to remove loose sand from the sand core or clean other contaminants from the internal mold cavity due to the limited accessibility of the mold cavity and the complex geometry of the mold cavity for a screw rotor with helical blades as a casting.

[0011] For the same reason, it is also difficult to provide a coating to the internal mold cavity prior to the casting process.

[0012] The coating protects the sand core from erosion and burn-in in the mold cavity during the casting process due to the high temperature of the molten casting material, wherein erosion and burn-in can lead to rough areas on the outer surface of the screw rotor resulting from the casting process.

[0013] After the coating has been applied to the internal mold cavity and dried, excess coating material must be drained from the mold cavity to prevent it from accumulating during the casting process at locations in the mold cavity that should actually be filled with molten casting material, which would be detrimental to the quality and dimensional accuracy of the final casting. It goes without saying that the generally limited number of openings on the outer surface of the sand core leading to the internal mold cavity is not conducive to draining excess coating material from the sand core.

[0014] When designing the sand core, it is also necessary to consider the position, number and size of the inlet or inlets, which will have a decisive influence on the uniform and complete filling of the mold cavity with casting material during the casting process and thus the quality of the casting produced during the casting process.

[0015] The location, number and size of the outlet or outlets connected to the riser or risers or to one or more degassing channels in the sand mould during the casting process may also affect the quality of the resulting casting.

[0016] During the casting process, the feeder or each of the feeders is firstly used as a venting channel, so that air inclusions in the resulting casting can be avoided.

[0017] Then, once the mold cavity is completely or nearly completely filled with casting material, the one or each of the plurality of risers acts as a reservoir for excess molten casting material from which molten casting material can be drawn back into the mold cavity as the casting shrinks during solidification of the casting. In this way, the formation of pores in the casting that are not filled with casting material due to shrinkage of the casting during solidification is avoided.

[0018] Therefore, the location, number and size of the outlet or outlets are crucial to reduce the number and / or volume of these holes in the casting that are not filled with cast material or to avoid these holes.

[0019] Due to the complex geometry of the mold cavity, when casting screw rotors horizontally, with a limited number of outlets in the sand core, it is difficult or impossible to reduce and / or eliminate the formation of pores that are not filled with the casting material during casting.

[0020] In this case, “horizontal casting” means that the rotational symmetry axis of the mold cavity extends in the gravity horizontal plane, which rotational symmetry axis coincides with the rotational symmetry axis of the cast screw rotor after the screw rotor has been formed in the sand core by filling the mold cavity with casting material.

[0021] The "gravity horizontal plane" refers to the plane perpendicular to the direction in which the main gravity (usually the earth's gravity) acts on the cast material. In other words, the "gravity horizontal plane" refers to a horizontally oriented plane.

[0022] When horizontally casting spiral rotors, impurities present in the molten casting material and / or inhomogeneities arising when the casting material solidifies in the mold cavity cannot be removed from the mold cavity by flotation on the casting material still molten in the mold cavity through a limited number of outlets. For example, for rotors cast from EN-GJS-450-10 ductile iron, the above-mentioned inhomogeneities appear in the form of oxide slag and graphite flotation.

[0023] When these impurities and / or inhomogeneities are trapped in the material of the screw rotor produced during the casting process during the solidification of the casting material in the mold cavity, they can lead to uneven weight distribution and thus imbalance around the axis of rotational symmetry of the screw rotor. The presence of impurities and / or inhomogeneities in the material of the resulting screw rotor may also have a negative impact on the surface quality and / or mechanical properties (such as strength and / or stiffness) of the screw rotor.

[0024] Due to the above reasons, the quality of the screw rotors cast with a limited number of outlet levels usually cannot meet certain predetermined requirements, and / or the outer surface of the screw rotor still needs to be machined to a considerable depth after the casting process. This leads to the formation of scrap and increases the finishing time and finishing cost of the screw rotor.

[0025] Furthermore, a disadvantage of a sand core having a monolith for forming the spiral blades of a screw rotor is that the surface of the internal cavity of the monolith for forming the spiral blades usually includes slightly tapered spiral grooves with a constant pitch. The reason for this is that the internal cavity in such a monolith is usually formed by a mold or master mold having a shape corresponding to the screw rotor. The spiral grooves must be slightly tapered and have a constant pitch to allow the mold to rotate the monolith unhindered after the monolith is formed.

[0026] Therefore, the screw rotor produced by the casting process naturally also has slightly conical helical blades with a constant pitch. In order to make the helical blades parallel helices, that is, helices with a constant radius, it is necessary to carry out additional extensive finishing steps on the produced screw rotor. Such finishing steps require a certain finishing time and finishing costs and lead to unnecessary scrap formation.

[0027] In order to remove the mold from the monolithic piece, the outer surface of the monolithic piece must also include a large opening having a diameter equal to or greater than the largest diameter of the mold.

[0028] Furthermore, manufacturing molds is an expensive, time-consuming, and energy-intensive process due to the numerous process steps involved, such as purchasing materials; heat treating the materials; turning, milling, and grinding the materials; nickel plating; post-measuring; and shipping the molds.

[0029] Therefore, manufacturing the mold is usually a one-time, non-iterative activity, in which a buffer allowance is provided to compensate for deviations and shrinkage behavior of the cast material during the casting process. Due to this buffer allowance, the screw rotor produced by the casting process needs to be further finished after the casting process.

[0030] Finally, a disadvantage of horizontally casting the screw rotor is that the vertical height of the sand mold is the limiting factor for the outer diameter of the cast screw rotor.

[0031] The vertical dimension of the sand core must be smaller than the vertical height of the sand mold. The outer diameter of the screw rotor produced by the casting process is limited by this vertical dimension of the sand core and the minimum thickness of the wall of the integral piece of the sand core surrounding the internal cavity.

[0032] On the other hand, when horizontally casting a screw rotor having a limited outer diameter, the vertical height of the available sand mold may be much greater than the vertical dimension of the sand core, thereby losing a large amount of useful space in the available sand mold.

[0033] In other words, when casting screw rotors horizontally, the vertical height of the sand mold is usually not adaptable or not flexibly adaptable to the outer diameter of the screw rotor to be cast, resulting in efficiency losses in the casting process in terms of production time and production costs. Utility Model Content

[0034] The present invention aims to solve at least one of the above disadvantages and / or other disadvantages.

[0035] More specifically, the object of the present invention is to provide a sand core and / or sand mold for casting a screw rotor in a fast, cheap, precise and / or flexible manner, for example to achieve a certain desired final quality with a minimum finishing allowance.

[0036] The present invention also aims to effectively utilize the available space in a sand mold to minimize the time and cost required for casting one or more screw rotors.

[0037] For this purpose, the utility model relates to a sand core for casting a screw rotor, the screw rotor comprising a body having a profile surface and two head surfaces, the profile surface having a spiral blade extending between the two head surfaces,

[0038] wherein the sand core comprises a monolithic part having an inner cavity, wherein the inner cavity is at least partially delimited by the monolithic part by a rotationally symmetrical surface formed by a helical groove around an axis of rotational symmetry, wherein the helical groove has a profile for forming a helical blade during a casting process of a screw rotor,

[0039] The outer surface of the integral part is provided with an opening or a plurality of openings extending into the inner cavity,

[0040] It is characterized in that

[0041] The one opening or each of the plurality of openings is adjacent to the spiral groove.

[0042] Due to the opening or openings, the mold cavity of the sand core according to the present invention can be easily accessed, in particular the spiral grooves of the inner cavity, thereby forming the spiral blades of the screw rotor during the casting process.

[0043] Since loose sand and impurities in the mold cavity are mainly accumulated in these spiral grooves, they are easily removed from the sand core through the opening or openings.

[0044] Instead, the coating can be easily applied to the surface of the internal cavity of the monolithic piece through the opening or openings to reduce or even eliminate the rough areas on the outer surface of the screw rotor produced by the casting process. After the coating is applied in the internal cavity, excess coating material accumulated in the spiral grooves of the internal cavity can also escape through the opening or openings.

[0045] Therefore, the sand core according to the utility model can be used for casting screw rotors having complex geometries and higher quality and dimensional accuracy than in the case of known sand cores. As a result, when the sand core according to the utility model is used for casting screw rotors, less waste is formed, less energy is consumed, and less finishing time and cost are required than in the case of known sand cores.

[0046] By providing the one or each of the plurality of openings adjacent to the spiral groove of the internal cavity, the distance between the outer surface of the monolithic piece of the sand core and the internal cavity of the monolithic piece can also be eliminated.

[0047] In other words, when casting the screw rotor horizontally, the integral piece of the sand core has a minimum vertical dimension for a given outer diameter of the screw rotor to be cast.

[0048] Therefore, the sand core according to the invention is lighter than known sand cores. Together with the advantageous position of the opening or openings in the outer surface of the monolithic piece, this makes the sand core according to the invention easy to handle.

[0049] Furthermore, when casting the screw rotor horizontally, for a given outer diameter of the screw rotor to be cast, the sand core will fit in the sand mold with a smaller vertical height. If the vertical height of the sand mold is fixed, when casting the screw rotor horizontally, the sand core according to the utility model leaves more freedom in the sand mold for vertical space for casting other castings in the sand mold.

[0050] In a preferred embodiment of the sand core according to the invention, the inner cavity is additionally delimited by the integral part by two end surfaces, which are configured to form two head surfaces during the casting of the screw rotor.

[0051] This can be achieved, for example, by manufacturing the sand core entirely by means of additive production techniques, rather than forming the internal cavity in the sand core by means of a mold having conical spiral blades corresponding to the spiral blades of the screw rotor to be cast.

[0052] Therefore, when making the sand core, there is no need to remove the mold from the monolith. Therefore, the monolith also does not need to include a large opening with a diameter equal to or greater than the maximum diameter of the mold. The presence of such a large opening in the monolith would prevent at least one of the head surfaces from being formed by the internal cavity of the monolith.

[0053] Therefore, an advantage of this preferred embodiment of the sand core according to the present invention is that, in addition to the spiral blades of the profile surface, the two head surfaces of the screw rotor can also be formed by the internal cavity in the integral piece.

[0054] In this case, the monolithic piece is preferably provided with two cylindrical holes, wherein each of the two cylindrical holes is adjacent to the outer surface and one of the two end surfaces.

[0055] Two journals of the screw rotor can be cast through these cylindrical holes, wherein each of the journals protrudes from one of the head surfaces of the body of the screw rotor. At the height of these journals, the screw rotor can be supported by bearings in the housing of the compressor element, vacuum pump element or expander element.

[0056] In existing sand cores that use a mold or master pattern to manufacture a monolithic part, only one first journal can be formed from the monolithic part because the mold must be removed from the monolithic part on the side of the monolithic part where the second journal is located. In order to remove the mold on this side of the monolithic part, the monolithic part must have an opening on this side with a diameter at least equal to the outer diameter of the mold, which is usually larger than the outer diameter of the second journal, so that the second journal cannot be formed from the monolithic part.

[0057] By forming the two journals on the body of the screw rotor in one piece, the alignment can be more easily measured, checked and, if necessary, corrected, thereby ensuring clamping stability of the casting in the clamping device for further processing steps.

[0058] In another preferred embodiment of the sand core according to the present invention, the spiral groove is a parallel spiral.

[0059] In this context, "parallel spiral" means that the spiral groove has a constant outer radius, or in other words, the spiral groove is not a tapered spiral in which the outer radius of the spiral groove would converge or diverge.

[0060] This can be achieved, for example, by manufacturing the sand core entirely by means of additive production techniques, rather than forming the internal cavity in the sand core by means of a mold having conical spiral blades corresponding to the spiral blades of the screw rotor to be cast.

[0061] The advantage of this preferred embodiment of the sand core according to the present invention is that the screw rotor produced by the casting process no longer needs to be finely machined to make the spiral blades of the contour surface parallel spirals.

[0062] As a result, waste formation can be reduced, the production time of the screw rotor can be shortened, and the production cost of the screw rotor can be reduced.

[0063] In yet another preferred embodiment of the sand core according to the present invention, the spiral grooves have a variable pitch.

[0064] This can be achieved, for example, by manufacturing the sand core entirely by means of additive production techniques, rather than forming the internal cavity in the sand core by means of a mold having conical spiral blades corresponding to the spiral blades of the screw rotor to be cast.

[0065] This has the advantage that the sand core according to the present invention can be used to cast a screw rotor having a spiral blade profile surface with a variable pitch, and the screw rotor does not need to be finished after casting to achieve the variable pitch of the spiral blades.

[0066] The advantage of using such a screw rotor with helical blades with variable pitch in a vacuum pump element is that pulsations in the gas flow through the vacuum pump element are reduced. This improves the overall efficiency of the vacuum pump element through lower energy consumption, quieter operation, more uniform temperature distribution and therefore lower cooling water consumption.

[0067] The advantages of using such a screw rotor having helical blades with variable pitch in a compressor element are reduced throttling losses and a smaller sealing line at the high-pressure end of the compressor element.

[0068] In another preferred embodiment of the sand core according to the present invention, the surface of the inner cavity is at least partially provided with a coating, and the sand core is provided with a discharge hole for discharging excess coating material from the inner cavity after providing the coating on the surface of the inner cavity.

[0069] The coating protects the sand core in the mold cavity from erosion and sintering due to the high temperature of the molten casting material during filling of the sand core with the molten casting material, thereby reducing or eliminating rough areas on the outer surface of the screw rotor resulting from the casting process.

[0070] By providing drainage holes in the sand core, it is possible to prevent excess coating material from accumulating in the sand core after providing the coating on the internal cavity surface. In fact, this will lead to a decrease in quality and weight imbalance of the screw rotor produced by the casting process.

[0071] The utility model also relates to a sand mold for casting a screw rotor, wherein the sand mold comprises a sand core according to any of the above embodiments.

[0072] It goes without saying that this sand mold according to the present invention has the same advantages as the above-mentioned embodiments of the sand core according to the present invention.

[0073] In a preferred embodiment of the sand mold according to the present invention, the sand mold is provided with one or more degassing channels for degassing the internal cavity in the sand core during casting of the screw rotor, wherein the one or each of the multiple degassing channels is connected to the one or at least one of the multiple openings in the outer surface of the integral part of the sand core.

[0074] This has the advantage that the degassing channel or channels are in fluid communication with a location or locations in the internal cavity (i.e. one or more spiral grooves of the rotationally symmetrical surface) where air or other gases accumulate when the spiral rotor is cast horizontally and must be discharged from the sand core.

[0075] This eliminates the possibility of gas inclusions in the screw rotor produced during the casting process, which is beneficial to improving the mechanical properties and weight balance of the resulting screw rotor.

[0076] In another preferred embodiment of the sand mold according to the present invention, the sand mold is provided with one or more flow channels for pouring liquid casting material into the internal cavity during casting of the screw rotor, wherein the one or each of the multiple flow channels is connected to the one or at least one of the multiple openings in the outer surface of the integral part of the sand core.

[0077] By connecting the one or more flow channels to the one or at least one of the openings, the liquid casting material in the inner cavity first ends up in the spiral groove, so that the casting material fills the inner cavity in a uniform and complete manner. This is conducive to improving the mechanical properties and weight balance of the resulting screw rotor.

[0078] In another preferred embodiment of the sand mold according to the present invention, the sand mold comprises a plurality of sand cores.

[0079] This has the advantage that the sand mould can be used to cast a plurality of screw rotors simultaneously, which means that the production time for producing each screw rotor with the sand mould is reduced.

[0080] In this case, the sand cores are preferably arranged in the sand mould according to a triangular arrangement pattern.

[0081] This has the advantage that the available space in the sand mould can be utilised to the maximum extent possible and the sand mould can accommodate as many sand cores as possible.

[0082] In this way, the production time for producing each screw rotor in a sand mold is reduced to a minimum. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Hereinafter, in order to better illustrate the features of the present invention, some preferred applications of the sand core and sand mold for casting a screw rotor according to the present invention will be described with reference to the accompanying drawings, wherein:

[0084] Figure 1 is an external view of the sand core according to the utility model;

[0085] Figure 2 Shows Figure 1 Longitudinal section of the medium sand core;

[0086] Figure 3 shows a longitudinal section of a sand mould according to the invention having a sand core according to the invention; and

[0087] Figure 4 A cross section of a sand mold having multiple sand cores according to the present invention is shown. DETAILED DESCRIPTION

[0088] Figure 1 A sand core 1 for casting a screw rotor according to the present invention is shown. The screw rotor comprises a main body having a profile surface and two head surfaces. The profile surface has spiral blades extending between the head surfaces.

[0089] The sand core 1 comprises a monolithic part 2 having an inner cavity 3 .

[0090] The outer surface of the integral member 2 is provided with a plurality of openings 4 , wherein each of the openings 4 extends into the inner cavity 3 .

[0091] Figure 2 Shows Figure 1 Longitudinal section of the middle sand core 1.

[0092] In this longitudinal section of the sand core 1 it can be seen that the inner cavity 3 is at least partially delimited by the monolithic part 2 by a rotationally symmetrical surface formed by a helical groove 5 around an axis of rotational symmetry, wherein the helical groove 5 has a profile configured to form a helical blade during casting of a screw rotor.

[0093] In this case, it can also be seen that each of the openings 4 in the outer surface of the monolithic piece 2 is adjacent to one of the helical grooves 5 .

[0094] Furthermore, the internal cavity 3 is delimited by the monolithic piece 2 by two end surfaces 6 configured to form two head surfaces of the screw rotor to be cast.

[0095] In this case, the monolithic piece 2 is provided with two cylindrical holes 7 , wherein each of the two cylindrical holes 7 is adjacent to the outer surface of the monolithic piece 2 and one of the two end surfaces 6 .

[0096] Two journals of the screw rotor can be cast through these cylindrical holes 7 , wherein each of the journals protrudes from one of the head surfaces of the screw rotor body for a bearing of the screw rotor in a housing of a compressor element, vacuum pump element or expander element.

[0097] Spiral groove 5 Figure 2 The spirals are parallel and have a constant pitch.

[0098] However, within the scope of the present invention, it is not excluded that the helical groove 5 is a conical helix and / or has a variable pitch.

[0099] The surface of the inner cavity 3 may be at least partially provided with a coating in order to reduce or even eliminate rough areas on the outer surface resulting from the casting process.

[0100] After the coating has been provided on the surface of the inner cavity 3 , excess coating material can be discharged through discharge holes (not shown) in the sand core 1 to prevent excess coating material from accumulating at certain locations in the sand core 1 .

[0101] Figure 3 A longitudinal section of a sand mold 8 for casting a screw rotor according to the present invention is shown, wherein the sand mold 8 includes a sand core 1 according to the present invention.

[0102] The sand mould 8 is provided with a plurality of degassing channels 9 for degassing the inner cavity 3 in the sand core 1 during the casting of the screw rotor. In this case, the degassing channels 9 are connected to the openings 4 in the outer surface of the monolithic piece 2 of the sand core 1 .

[0103] Furthermore, the sand mold 8 is provided with a casting device 10 having a flow channel or a plurality of flow channels 11 along which molten casting material can be poured into the sand core 1 to fill the sand core 1 with the casting material and thus form the screw rotor to be cast.

[0104] Outside the sand mould 8, the casting device 10 may comprise a pouring funnel 12 and / or a feed chute 13 at the inlet of the sand core 1. The feed chute 13 may be equipped with a metal filter 14. The advantages of such a metal filter 14 are described in Belgian patent BE 1027183 B1.

[0105] It is not excluded within the scope of the invention that the sand mould 8 is provided with a plurality of flow channels connected to the openings 4 in the outer surface of the monolithic piece 2 of the sand core 1 so that the casting material is poured into the inner cavity 3 along the openings 4 (not shown in the figures).

[0106] Figure 4 A cross section of a sand mold 8 having a plurality of sand cores 1 according to the present invention is shown.

[0107] In this case, the sand cores 1 are arranged in the sand mold 8 according to a triangular arrangement pattern, thereby making maximum use of the available space in the sand mold 8 .

[0108] The present invention is by no means limited to the sand cores, sand molds and methods described by way of example and shown in the drawings, but the sand cores or sand molds according to the present invention may be realized in various forms and sizes without exceeding the protection scope of the present invention defined in the claims.

Claims

1. A sand core for casting a screw rotor, the screw rotor comprising a main body having a contour surface and two head surfaces, the contour surface having a spiral blade extending between the two head surfaces, The sand core (1) comprises a monolithic part (2) having an inner cavity (3), wherein the inner cavity (3) is at least partially delimited by the monolithic part (2) by a rotationally symmetrical surface formed by a helical groove (5) around an axis of rotational symmetry, wherein the helical groove (5) has a profile for forming the helical blades during casting of the screw rotor, The outer surface of the integral part (2) is provided with one or more openings (4) extending into the inner cavity (3), It is characterized in that The one opening or each of the plurality of openings (4) is adjacent to the spiral groove (5).

2. The sand core according to claim 1, characterized in that: The inner cavity (3) is additionally delimited by the monolithic piece (2) by two end surfaces (6) which are configured to form the two head surfaces during the casting of the screw rotor.

3. The sand core according to claim 2, characterized in that: The integral member (2) is provided with two cylindrical holes (7), wherein each of the two cylindrical holes (7) is adjacent to the outer surface and one of the two end surfaces (6).

4. The sand core according to any one of claims 1 to 3, characterized in that: The spiral groove (5) is parallel spiral.

5. The sand core according to any one of claims 1 to 3, characterized in that: The spiral groove (5) has a variable pitch.

6. The sand core according to any one of claims 1 to 3, characterized in that: The surface of the inner cavity (3) is at least partially provided with a coating, and wherein the sand core (1) is provided with a drain hole for draining excess coating material from the inner cavity (3) after providing the coating on the surface of the inner cavity (3).

7. A sand mold for casting a screw rotor, characterized in that: The sand mold (8) comprises a sand core according to any one of claims 1 to 6.

8. The sand mold according to claim 7, characterized in that The sand mold (8) is provided with a degassing channel or multiple degassing channels (9) for degassing the internal cavity (3) in the sand core (1) during casting of the screw rotor, wherein the one degassing channel or each of the multiple degassing channels (9) is connected to the one opening or at least one of the multiple openings (4) in the outer surface of the monolithic piece (2) of the sand core (1).

9. The sand mold according to claim 7 or 8, characterized in that: The sand mold (8) is provided with a flow channel or multiple flow channels (11) for pouring liquid casting material into the internal cavity (3) during casting of the screw rotor, wherein the one flow channel or each of the multiple flow channels (11) is connected to the one opening or at least one of the multiple openings (4) in the outer surface of the integral part (2) of the sand core (1).

10. The sand mold according to claim 7 or 8, characterized in that: The sand mold (8) comprises a plurality of sand cores.

11. The sand mold according to claim 10, characterized in that The sand cores are arranged in the sand mould (8) according to a triangular arrangement pattern.

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