CASTING PROCESS AND SAND MOULD PROVIDED WITH A GATING SYSTEM FOR PRODUCING AT LEAST PARTLY THIN WALLED ALUMINIUM CASTS WITH SAND MOULDING TECHNOLOGY BY MEANS OF GRAVITY CASTING

Abstract

The subject of the invention is a process for the production of at least partly thin-walled and aluminium castings with sand moulding technology by gravity casting, which allows producing casts with 100 times or favourably 200-400 times larger overall dimensions in case of 1-3 mm wall thickness. The main idea of the process is that sand mould containing mould cavity is provided, melt of aluminium content is produced, the melt is introduced into the mould cavity at several points through a gating system of narrowing cross section. A further subject of the invention is a sand mould fitted with a gating system to produce at least partly thin-walled castings with sand moulding technology, by gravity casting. The wall thickness of thin-walled segments is 1-3 mm and the largest dimension is more than a 100 but favourably at least 200-400 multiple of the wall thickness. The main idea behind the sand mould with a gating system is that it contains a mould cavity allowing the production of at least partly thin-walled castings, and is equipped with a gating system, which is composed of at least two sprues and one ingate to each having a porthole into the mould cavity and in liquid contact with the sprues.

Claims

1: Casting process for producing at least partly thin-walled aluminium castings with sand mould technology by gravity casting, wherein wall thickness of a thin-walled part is 1-3 mm, characterised by: providing a sand mould comprising a mould cavity, producing a melt of aluminium content, introducing the melt into the mould cavity at multiple points through a gating system of narrowing cross section.

2: The process according to claim 1 characterised by producing a casting the largest overall dimension of which is more than 100 times of the wall thickness.

3: The process according to claim 1 characterised by providing a gating system of narrowing cross section for the mould cavity which contains at least two sprues each being in liquid communication with at least one gate respectively, each gate having an inlet opening into the mould cavity.

4: The process according to claim 3 characterised by providing liquid communication between at least two gates and at least one sprue with a runner, and the overall cross section of the inlets of the gates opening from one runner is at least two times smaller, more preferably 3-5 times smaller than the overall cross section of branches of the given runner.

5: The process according to claim 3 characterised by providing 2-4 gates per each runner, and selecting the number of gates and runners such that a thin-walled casting portion having a dimension of 100-1000 cm.sup.2 is cast from one gate.

6: The process according to claim 2 characterised by selecting the height of the sprue such that the drop between the upper inlet opening of the sprue and a parting plane of the sand mould is at least 0.1 times greater, preferably 0.6-1.3 times greater than the largest dimension of the casting to be produced.

7: The process according to claim 1 characterised by preheating the sand mould before starting the casting at least in the segments of the thin-walled parts of the casting to at least 100° C.

8: The process according to claim 7 characterised by providing a cooling metal insert in the sand mould and heating the cooling metal insert prior to starting the casting until vapour is precipitated and the insert's surface becomes dry.

9: The process according to claim 1 characterised by that prior to being poured into the gating system, the melt is overheated by at least 100° C. with respect to melting point of the melt.

10: Sand mould having a parting plane and provided with a gating system for producing at least a partly thin-walled aluminium casting with sand moulding technology by gravity casting, wherein wall thickness of a thin-walled portion of a casting is 1-3 mm and the largest dimension of the casting is at least 100 times greater, characterised by that the sand mould defines a mould cavity for producing the at least partly thin-walled casting, and is provided with a gating system of an overall narrowing cross section, which contains at least two sprues each being in liquid communication with at least one gate respectively, each gate having an inlet opening into the mould cavity.

11: Sand mould provided with a gating system according to claim 10 characterised by having a runner providing liquid communication between at least one sprue and at least two gates.

12: Sand mould provided with a gating system according to claim 11 characterised by that the inlets of gates from a runner have a cross section at least two times smaller than the overall cross section of branches of the runner.

13: Sand mould according to claim 11 characterised by comprising 2-4 gates per runner and at least one gate is provided for producing a thin-walled casting portion having an area of 100-1000 cm.sup.2.

14: Sand mould according to claim 10 characterised by that the sprue comprises a sprue part formed in the sand mould and an attachment joined to it from above.

15: Sand mould according to claim 10 characterised by that the drop between the upper inlet opening of the sprue and the parting plane of the sand mould is at least 0.1 times greater than the largest dimension of the mould cavity.

16: Sand mould according to claim 10 wherein the largest dimension of the casting is at least 200-400 times greater.

17: Sand mould according to claim 10 wherein the drop between the upper inlet opening of the sprue and the parting plane of the sand mould is 0.6 to 1.3 times greater than the largest dimension of the mould cavity.

18: The process according to claim 1 characterised by that prior to being poured into the gating system temperature of the melt is at least 200° C. above melting point.

19: The process according to claim 1 characterised by that prior to being poured into the gating system temperature of the melt is at least 350° C. above melting point.

20: Sand mould for producing at least partly thin-walled aluminium casting with sand moulding technology by gravity casting, wherein wall thickness of a thin-walled portion of the casting is 1 to 3 mm and the largest dimension of the casting is at least 100 times greater, which comprises a mould cavity defined by the mould, and a gating system provided with at least one gate having an inlet opening into the mould cavity, and at least one sprue in liquid communication with each gate; said gate having a progressively smaller cross sectional area toward said inlet opening.

Description

[0039] Further details are described in examples through figures.

[0040] FIG. 1a shows a rough perspective sketch of the inner structure of a sand mould fitted with a gating system according to the invention.

[0041] FIG. 1b shows a rough cross sectional sketch of the sprues of the sand mould fitted with a gating system as regards FIG. 1 a.

[0042] FIG. 1c shows a perspective image of a casting produced by means of a sand mould.

[0043] FIG. 2 shows another rough perspective sketch of the inner layout of the sand mould with a gating system.

[0044] Materials, tools and terms applied in the invention:

[0045] ‘Aluminium casting’: a casting part which is made of Aluminium or Aluminium alloy.

[0046] ‘Aluminium alloy’: generally ‘silumin’ alloys specifically (ENAC or other) alloy groups according to the patent such as AlSi12MgTi, AlSi7Mg, AlSi10Mg, AlSi9Cu (these are mainly used in the technology relative to the invention, but in certain cases other Al alloys can also be used).

[0047] ‘Articulated, rich in details’: alloy with complex geometry in which in a given case stiffening ribbing is applied.

[0048] ‘Thin-walled’: 1-3 mm average wall thickness of the alloy, the size is larger than 50 times or a 100 times of the wall thickness and change in the wall thickness will not exceed 50% of that.

[0049] ‘Pressure casting’: A casting process in which liquid metal is poured by a casting machine during extremely short times (0.01-0.05 m/s) and at a very high flow rate (20-80 m/s in the channel) into the cooled (150-200° C.) mould cavity where extremely high pressure is applied while the metal is solidified.

[0050] ‘Gravity casting’: A casting procedure in which liquid metal is poured into the mould cavity by gravity energy under atmospheric pressure. This casting technology operates by the principles of communicating vessels in which no further energy (e.g. centrifugal force effect) aids the molten metal fill in the mould cavity.

[0051] ‘Sand mould’: refractory sand of 0.2-0.4 mm grain sizes (typically quartz sand but other kind of artificial sand can also occur) hardened with organic or non-organic bonding agent system in cold state (with chemical bond) or by heat (under the effect of being heated)

[0052] ‘Sand mould casting’: pouring liquid metal into any kind of sand mould.

[0053] With the sand mould fitted with a gating system and by the process to be introduced in the following, such—at least partly—thin-walled aluminium castings can be produced by gravity casting, in which the wall thickness of one or more parts is 1-3 mm and the largest dimension is multiplied by 100 times, or even 200-400 times compared to the wall thickness.

[0054] The largest dimension means the largest linear dimension of the given part of a cast, i.e. the longest side of the smallest prism which can involve the given part of the cast.

[0055] In FIGS. 1a and 1b the inner structure of the sand mould (12) fitted with a gating system (10) is shown. In FIG. 1 a the outer edges of the sand mould are shown only as illustration around the inner formation in a perspective view.

[0056] The sand mould (12) includes an upper half (12a) and a bottom half (12b), which are joined in a joint surface (13) and these two parts form the mould cavity (16). In the present structure the mould cavity (16) is completely thin walled and provides casting parts (14) with 1-3 mm wall thickness, which is also separately shown in FIG. 1 c. In FIG. 1 c the cast part is not separated from the complete casting (14′), which means that the solidified parts of the melt in the gating system (10) are joined to the casting part (14), which can be separated, e.g. by cutting them off the casting (14).

[0057] In the present case the gating system (10) consists of two sprues (18), one runner (20) by each sprue and 5 gates (22) with portholes (22) opening from each runner into the mould cavity (16).

[0058] The runners (20) allow the liquid metal to run in the joining surface (13) of the mould parts (12a and 12b) or in its surroundings from the sprues (18) to the runner gates (22). According to the process segmented shaping allows complete filling of the mould cavity as well as reduces the formation of turbulence and foaming, and aids the formation of steady flow. These runners are trapezoidal in various sizes; e.g. upper width 10 mm, bottom width 21 mm, height 17 mm.

[0059] Ingates (22) are channels connecting the runners (20) and the mould cavity (16) with the aim of allowing the liquid metal to run into the mould cavity, controlling flow rate and eliminating the formation of turbulence and foaming. They come in various sizes; e.g. gate width 42 mm, gate height adjusted to the wall thickness of the cast, e.g. 2 mm, widening towards the runners: width e.g. 10 mm, height 16 mm.

[0060] The sprue (18) is composed of the sprue itself, which is formed in the sand mould, and the riser (26) fitted to it from the outside. The upper part of the latter one is a pouring cup (28) to allow easier pouring of the melt into the sprue (18).

[0061] The gating system is of narrowing cross section thus the flowing cross section is getting narrower (including even a transitional increase) towards the runners (22a). In this manner the flow rate of the melt will increase towards the runners (22a) and will reach its highest rate there. This arrangement is in contrast to the conventional sand mould technology, in which gating systems of expanding cross sections are applied since slow and laminar flow will result in higher cast quality in case of thick walled casting parts.

[0062] In the context of the current invention, a gating system of narrowing cross section (10) is any gating system that can achieve the highest flow rate at the runner (22a) by narrowing the flowing cross sections. For this reason at least the ingates are to be of narrowing cross section, i.e. the inner cross section of ingates is narrowing towards the runner (22) and becomes the narrowest at the runner (22a). The flow rate is at least twice or more advantageously 3-5 times higher than the average flow rate in the runners (20), or when no such are applied, in the down sprues (18). This can be achieved by providing at least twice or even 3-5 times wider total cross section for the runners (22a) than that of the runners (20). In this case both runners (20) have 2 branches respectively, which start from the sprue (18).

[0063] Attachments (26) also contribute to the increase of flow rate. The gradient height between the upper port (i.e. the upper edge of the cup) of the sprue (18) and the joining surface (13) of the sand mould (12) is to be 0.3 or even 0.6-1.3 times multiple of the largest dimension of the mould cavity.

[0064] Risers (30) are also incorporated in the sand mould (12). Their task is to exhaust from the mould the gases that are formed during casting as well as to exhaust air accumulated in front of the liquid metal. To the purpose they have cylindrical shape. Their typical diameter is double the wall thickness of the cast (2-6 mm).

[0065] In the case of thick-walled cast parts (14), a cooling metal insert, e.g. a cooling iron bar is applied (not shown). This cooling iron bar will allow thick walled segments to solidify at an identical rate with the thin walled segments.

[0066] Feeders can also be applied to feed the thick walled segments.

[0067] In FIG. 2 the inner layout of the sand mould (12) and gating system (10) is illustrated, which includes four sprues (18) and four runners (20). 5 ingates (22) belong to each runner along the longer sides of the mould cavity (16), while along the shorter sides 4 ingates (22) are joined to each runner (20).

[0068] The sand mould (12) has 2-5 ingates (22) on each runner (20). The number and layout of the ingates and runners is designed in a way that at least one runner gate (22) should belong to each 100-1000 cm.sup.2 segment of the mould cavity, which provides thin-walled casting parts. This arrangement will enable the melt to fill in the whole mould cavity (16) before getting solidified.

[0069] In order to produce castings of smaller dimensions (14) the runner can even be neglected. In this case the ingates (22) are directly connected to the bottom of the sprue (18). In order to produce casting parts of larger dimensions (14) a number of segmented runners (20) or branching runners may be applied, or in a given case several sprues can be connected to one single runner (20).

[0070] The sand mould (12) fitted with a gating system (10) can be applied in the following way.

[0071] Plastic patterns and mould cores provide the manufacturing tool, which is applied to produce the mould halves (12) and cores. From moulding sand suitable for pre-heating the mould halves (12a, 12b) are produced, which will form the cavity related to the casting. The sand mould is typically a chemically bonded dry mould which can tolerate heating.

[0072] Cross section, height and width of the gating elements are always determined by the features and casting position of the cast (14).

[0073] In order to avoid early solidification, the sand mould (12) is pre-heated at least in the thin walled segment of the casting up to 100° C., better to 100-600° C., or even more advantageous to 300-500° C. (or 0.5-0.8 times of the solidification temperature of the Aluminium alloy). Heating can be performed with gas flame. The moulded cooling iron bars, supposing there are any, are also heated until vapour precipitates and dries from the surface (the surface of the cooling iron bars must remain pure); then the mould surfaces (16), runner gates (22) and the runners (20) and sprues (24) are heated up again before the mould halves are closed.

[0074] After the sand mould (12) has been closed and the risers (26) of the gating system have been mounted, the Aluminium melt is produced by heating Aluminium (or Al alloy). Before being fed into the gating system, the melt is over-heated by 100° C., advantageously at least by 200° C., or even by 200-350° C., which will further contribute to avoiding too early solidification.

[0075] The liquid metal (melt) is introduced through the gating system (10) into the mould cavity of the preheated mould halves (12a, 12b).

[0076] The mould cavity is filled with liquid metal by means of pouring ladles, preferably through the pouring cup (12a) fitted on the sprue of the upper mould half (12a).

[0077] Moulding sand and excess parts are removed from the casting (14), which is followed by the casting being applied according to the purpose.

[0078] The most difficult task with thin walled castings (14) by gravity casting is to force the molten metal to completely fill in the mould without solidification. How to feed and cool at the same time so that the whole casting should become cooled and evenly solidified approximately at the same time? For this purpose the following methods are applied in order: significant size difference of trapezoidal formation of the ingates (in practice this means a decrease in the flow cross section), segmenting of the runners, increasing pouring height (significant increase in the “liquid” column height relative to the dimensions of the piece), significantly increasing the mould temperature, an increased over-heating temperature of the melt, and application of the above even simultaneously, in the same period of time.

[0079] To summarise all the above, prior to the casting process applied in the invention, firstly the manufacturing tool, which enables producing a replica of the part from sand, is made according to a virtual model created by a generally used 3D design program or other suitable programs. This is followed by distributing the tool related to the user's demand, as well as shrinkage of the casting and moulding inclination is concerned. The next step is to determine the pouring position, which can be either vertical or horizontal, according to the geometry of the model then the coring position of the possible cavities of the part will be given. When the part has been assembled from the mould halves (12a, 12b) and the possible cores, the required ingates, sprues, risers, hidden feeders are designed, which will all be installed in the mould frame and be moulded together with the mould halves (12a, 12b), which will allow producing identical parts for the casting. The manufacturing tool is then treated with mould remover and filled in with washed and sized, chemically bonded sand. During the filling up the designed cooling iron is moulded in the sand as well as the upper part of the core is made rigid with iron strands. The mould halves (12a, 12b) are precisely joined by positioning devices, which are also included in the manufacturing tool. Overflow preventers prevent flow-off occurring from the mould buckling on the joining surface, followed by the mould halves (12a, 12b) being treated, heated and closed. In certain cases the upper mould half (12a) is designed with an increased height and balanced. After casting the melt is allowed to cool and then the cores will be carefully removed from the casting, which is then cut off from the ingates and finely purified. This process is followed by checking the main dimensions and delivered to the supplier for testing. After the casting has been tested, small quantity production will be launched.

[0080] Advantages of the Application of the Invention Filed for Patent:

[0081] A direct economic benefit of the process applied in the invention is that it allows manufacturing castings of almost identical properties related to the technical parameters, with low financial investment and during a fraction of time as compared to the production of large scale casts with metal mould of otherwise high production cost.

[0082] In comparison with the castings produced with pressure technology, the quality of casts will be practically identical, however at lower cost and in a simpler way.

[0083] The process is suitable for producing cast parts to be made costly even on large scale, which can be applied for practical purposes.

[0084] The products of this kind are mainly indoor and outdoor luminaires, engine parts—spare parts, cylinder-heads, machine components, spare parts for mechanical and precision engineering, fittings, etc. All these products can be produced through this invention in such quality that they will be suited for practical application under operational conditions. This will result in cost-effective development and more effective testing before the manufacture on large scale is launched.

[0085] Aluminium castings for different purposes are to be produced with this process applied in the invention on small to medium scale, even in several hundred pieces.

[0086] By this innovative process castings can be produced with lower machining tolerance and with less moulding incline, therefore dimensional accuracy is in compliance with the related standard, moreover this process allows achieving higher dimensional accuracy compared to that of gravity sand mould casting.