CREATION OF SLOTS ON THE SURFACE OF A CORE

Abstract

The invention relates to a foundry core for cast pieces of aluminium alloy in a mold, said core comprising a molding part, intended to be in contact with the molten aluminium alloy, and at least one non-molding part intended to be located outside the molten aluminium alloy, the core comprising at least one slot on the surface of the core, said slot extending from the molding part to at least one non-molding part for discharge of the gases generated in the molding part of the core during casting outside the molding part.

Description

FIELD OF THE INVENTION

The present invention relates to the field of foundry and casting of foundry pieces in aluminium alloy.

The invention relates more particularly to a foundry core for casting pieces in aluminium alloy in a mold. The relevant cores especially include a mixture of sand and binder.

TECHNOLOGICAL BACKGROUND

The foundry core constitutes part of the mold serving to produce a piece made of metal and in particular aluminium alloy.

The core is generally composed of a mixture of grains of sand and binder.

The foundry core enables creation of internal recesses of a piece. It is therefore immersed fully or partly in the molten metal.

When a core is enclosed by molten aluminium alloy, the phenomena of dilation of air, evaporation of the solvents of the binders and their combustion linked to the rise in temperature will boost the pressure in the core. This rise in pressure is accompanied by generation of gases inside the core. The resulting gases are most often evacuated to the exterior of the core via the porosity of the core which enables circulation of gases.

Gases escaping from the core are injected into the molten metal which encloses the latter, when pressure in the core exceeds metallo-static pressure (linked to the height of metal above the core). This phenomenon of gaseous discharge engenders in the molten metal the presence of bubbles which leave holes in the metal after solidification. This defect embrittles the piece and compromises its qualities.

Even if the bubbles manage to exit from the cast piece, gas discharges can leave traces of their passage in the solidified metal and are potentially responsible of porous aluminium sheet or even a priming zone for crack propagation.

Also, traces of gas discharges can appear even though the metallo-static limit pressure is not quite attained by the pressure inside the core.

Solutions for trying to avoid these disadvantages have been proposed.

The attempt was made to boost the average size of the grains of sand in the core to heighten the permeability of the sand and advance discharge of gases to the exterior of the core via the parts of the core which are not covered by aluminium (in practice, the core prints).

But this technique embrittles the cores due to decrease in the number of resin bridges between the grains.

Also, the use of large grains for the cores also changes the surface state of the piece to the point where the roughness measured on the surfaces of the piece can compromise their quality. Finally, this solution involves managing several sizes of grains of sand in a factory, which constitutes an additional inconvenience.

Another known attempt consists of adding contact forms between the core immersed in the liquid metal and the exterior of the mold. Evacuation of gases generated in the core can occur by these forms which serve as discharge chimneys.

This technique is widely used but requires different added operations restrictive on the cast pieces to replug the vacuums left by these forms: machining of the piece, adding one or more plugs to reclose the shape which enabled discharge and placing of a sealing control system.

It is also known to make closed gas suction conduits intended to be negatively pressurized in the core to suction the gases generated in the core and discharge them to the exterior of the mold. The negative pressure in the conduit is generally generated by suction of Venturi type.

This configuration is delicate to carry out and its configuration needs permanent control which constitutes a disadvantage per se. In particular, this type of system must suction enough to avoid the problem of gas discharges but not suction too strongly to prevent suction of metal in the core.

Also, the shape of the cores is often poorly adapted to their use. Their management and their maintenance represent complications of a technical and economic order.

It appears that the different attempts at resolving the problem of gaseous discharge of cores are all exposed to limitations.

The aim of the invention is to eliminate these limitations.

SUMMARY OF THE INVENTION

The invention proposes a foundry core for casting pieces of aluminium alloy in a mold, the core comprising a molding part, intended to be in contact with the molten metal, and at least one non-molding part, intended to be located outside the molten metal, the core comprising at least one slot on the surface of the core and the slot extending from the molding part to at least one non-molding part for discharge of the gases generated in the molding part of the core during casting outside the molding part.

The invention proposes a simplified alternative to techniques of the prior art by making slots on the surface of the cores. These slots create a space which discharges the gases generated in the core, and at the same time the slots are fine enough to prevent the aluminium alloy from entering within them. By drawing a preferred gas discharge path to the non-molding parts of the core, outside the molten metal (typically the core prints), these slots discharge gases which can be generated in the core without involving the disadvantages mentioned hereinabove.

By way of advantage the invention also proposes the following characteristics taken singly or in combination:

• • the non-molding part(s) are core prints, adapted to keep the core in position inside the mold;
  • the slots have a width adapted to prevent the molten aluminium alloy from entering inside said slot;
  • the slots on the surface have a width less than 1 mm and preferably 0.2 mm;
  • the slots have a rectangular geometric, U-shaped or V-shaped profile;
  • the slots have an average depth of between 0.2 and 2 mm, to allow the gases generated in the molding part of the core to circulate in the slots;
  • the slots are made by laser;
  • the slots are made by tooling with reliefs in the shape of blades.

The invention also proposes a mold adapted for casting foundry pieces, comprising a core such as described previously.

The mold can further comprise a suction system adapted to suction in the slots the generated gases, with suction occurring at the non-molding zones.

The invention also proposes a method for manufacturing a core such as described previously, characterized in that it comprises an etching step of said slots on the surface of the core by means of a laser.

The invention also proposes a method for manufacturing a piece of aluminium alloy by casting molten metal by means of a mold previously described.

Finally the invention proposes a cylinder head for an automobile obtained by a method for manufacturing a piece previously described, as well as an engine block for an automobile.

PRESENTATION OF THE FIGURES

FIG. 1a shows a core as per the invention immersed in a molten aluminium alloy.

FIG. 1b shows an enlargement of the core of FIG. 1a.

FIG. 2a illustrates a drawing in side elevation in section of a mold and a core such as defined in the invention, whereof the molding part is immersed in the molten metal.

FIG. 2b illustrates the core of FIG. 2a viewed from above, without the mold.

FIG. 3a illustrates a more exact representation of a core as per the invention.

FIG. 3b illustrates a sectional view along plane [AA′] of FIG. 3a of the core such as defined in the invention.

FIG. 4a illustrates different profiles of slots (not necessarily to scale).

FIG. 4b illustrates different types of slots with the flashes of molten metal entering inside these slots (not necessarily to scale).

FIG. 5 illustrates a simplified drawing of a plan view of the surface of the core immersed in the molten metal, within a mold, with possible circulation (arrows) in the slots of the gases generated in the core.

FIG. 6 illustrates the evolution of gaseous discharge as a function of the number of slots.

DETAILED DESCRIPTION OF ONE EMBODIMENT AT LEAST

In relation to FIGS. 1a, 1b, 2a, 2b, 3a, 3b a core 10 as per the invention is described.

The core 10 is adapted for casting foundry pieces in a mold 20. These cast foundry pieces are made of aluminium alloy and are typically intended for the automobile industry. These cast pieces are typically intended to be engine blocks or cylinder heads.

The core 10 comprises a molding part 11 which is intended to be in contact with the molten aluminium alloy 30.

The core 10 also comprises a non-molding part 12a, intended to be located outside the molten aluminium alloy 30. In general, a core 10 comprises several non-molding parts 12a separated especially by the molding part 11.

The core 10 is placed inside the mold 20 and held there immobile by the core prints 12b, which are generally non-molding parts 12a (see FIG. 2a). Generally, the mold 20 comprises a sole 21 which constitutes the bottom of the mold 20, and at least one movable guillotine 22 which constitutes a side wall of the mold 20 when said guillotine 22 is closed. The sole 21 and the guillotines 22 create the external shapes of the cast pieces.

The guillotines 22 are also adapted to immobilize the core 10. The core prints 12b are in contact with said guillotines 22 for this purpose.

The core 10 comprises at least one slot 13 located on the surface of the core 10. The slot 13 extends from the molding part 11 to at least one non-molding part 12a and produces a preferred discharge path of the generated gases to discharge the gases generated in the molding part 11 out of said molding part 11 and preferably then out of the non-molding parts 12a (see FIGS. 2a, 2b, 3a, 3b).

The main function of the slot 13 located on the surface of the core 10, and which extends from the molding part 11 to at least one non-molding part 12a, is to create a preferred discharge path of the gases generated to discharge the gases generated in the molding part 11 out of said molding part 11. These gases could be evacuated to a distance from the part of the core 10 which will effectively mold the piece to be made. They could accumulate at the non-molding parts 12, or be evacuated out of these parts 12 if the slot 13 is put in contact with free air.

Each slot 13 consists of a groove which connects the molding part 11 to a non-molding part 12a. Preferably, given the structure of the cores 10 whereof the molding part 11 is connected to two separate non-molding parts 12a, the slot 13 preferably connects the two non-molding parts 12a by passing through the molding part 11.

Geometric Properties of Slots

In reference to FIG. 4a (given by way of illustration, not to scale), the profile of the slots 13 is preferably rectangular, in U-shape or V-shape. These forms offer a good compromise between the simplicity of manufacture and efficacy, i.e., the capacity of the generated gases to circulate and be drained. A profile enlarging as it moves away from the surface of the core 10, of trapezoidal type, for example, makes easier circulation and drainage of the generated gases and prevents the molten aluminium alloy 30 from entering inside the slot 13. Other profiles are also possible.

In general, the dimensions of the slots respond to restrictions linked to the molten metal and the generated gases. The volume of the slot 13 has to be optimized.

The slots 13 have a width 13a such that the molten aluminium alloy 30 does not produce any visible flashes 31 on the surface of the piece after cooling (see FIG. 4b). For this, either the molten aluminium alloy 3 must not enter inside the slots 13 or it must enter by a given distance less than a quality criterion defined by a specification. Typically, the impact on the surface state of the aluminium alloy has to be zero or the value of the roughness of the cast piece unchanged with and without slots 13. This means optimizing the width 13a to maximize drainage of the generated gases and minimize the flashes 31. Optimizing of this width 13a makes easier optimization of the volume of the slot 13, particularly in the case of fairly simple geometric profiles. The width 13a is a useful width, i.e., it is measured at the surface (see the widths 13a in FIG. 4b).

The width 13a is especially a function of the types of aluminium alloy used.

Typically, the width of the slots 13a is less than 1 mm and preferably less than or equal to 0.2 mm.

But the person skilled in the art could adapt the width 13a of the slots 13 to obtain the results mentioned previously.

FIG. 4b (given by way of illustration, not to scale) illustrates different profiles of flashes 31 obtained for different widths 13a of the slot 13.

The slots 13 have an average depth 13b such that the generated gases can circulate inside the slots 13. The depth 13b of the slots 13 theoretically has no influence for the flashes 31 but the complexity, cost and fragility of the core, inter alia, increase with depth 13b.

On principle, this means optimizing the volume of the slot 13, at a constant width of slot 13a, to maximize drainage of the generated gases, minimize manufacturing complications of the slots 13 and limit the fragility of the core 10. In the case of fairly simple geometric profiles, this volume optimization will optimize the average depth 13b.

A depth value 13b greater than 0.2 mm in practice means that the gases generated in the molding part 12a can circulate inside said slots 13. Preferably and for the reasons cited hereinabove, the depth 13b is greater than 0.2 mm and less than 2 mm.

The path of the slots 13 is traced so as to drain as much generated gases as possible by offering a preferred circulation path of the generated gases. For this, the tracing plan is optimized so as to favour circulation and drainage of the generated gases to limit the pressure due to said gases to a maximum.

An advantageous tracing plan can for example consist of having the fewest zones of the molding part 11 without slots 13, i.e., ensure that no point of the surface of the molding part 11 of the core 13 is at a distance greater than a given limit value, relative to the closest slot 13. In the case of several slots 13, the minimal and/or maximal spacing between the slots 13 can be determined.

The slots 13 are preferably traced by maximizing the radii of curvature and limiting the angles, even more so acute angles, to facilitate circulation and drainage of the generated gases (FIG. 3a).

According to an embodiment comprising at least two slots 13, said slots 13 do not cross each other to maximize the surface covered for a total given length of slots 13.

According to another embodiment comprising at least two slots 13, said slots 13 cross each other to offer a circulation alternative and better drainage of the generated gases (FIG. 2b).

The non-molding parts 12a are advantageously core prints 12b since the core prints 12b are already acting as non-molding parts 12a.

But it is possible to create a particular core 10 in which the non-molding parts 12a are not carried 12b and present a structure favouring circulation of the generated gases, for example a branched structure.

Evacuation of the Generated Gases

The non-molding parts 12a which receive the slots 13 comprise means 40 adapted to enable discharge of the gases generated in the molding part 11 (FIG. 2a). These means 40 can be made in different ways.

In particular, the means 40 can simply consist of a fluid connection 41 between slots 13 of the non-molding parts 12a and a certain volume of pressurized air less than the pressure of the generated gases to be discharged (this pressure is typically atmospheric pressure), said volume being typically far greater than the volume of generated gases. In the case of the mold 20 with sole 21 and guillotines 22, a bore or an opening 42 in the guillotine 22 allows said connection 41 between slots 13 of the non-molding parts 12a (here typically core prints 12b) with the volume of air.

Alternatively, the devices 40 further comprise a suction system 43 for favouring circulation and drainage of the gases generated in the slots 13 (FIG. 5).

Creation of the Slots

The slots 13 are preferably produced by a laser 50. This technique is minimally invasive and enables fine precision in manufacture despite the complex forms of cores 10.

It is also possible to make these slots 13 by means of specialized tooling which can consist of an apparatus with reliefs in the shape of blades (the slots 13 whereof the profile is V-shaped are typically obtained this way).

Example of Use

In an example, the core 10 is installed in the mold 20 and fixed to the guillotines 22 of the mold 20 by the core prints 12b. The molten aluminium alloy 30 is poured into the mold 20 and encloses the molding part 11 of the core 10. The heat causes generation of gases inside the core 10. These gases preferably circulate via the slots 13 and are drained to the non-molding parts 12a (FIG. 4) to then be discharged by the discharge means 40. There are no traces of gas discharges in the piece. Selecting the width 13a of the slots as adapted further avoids the flashes 31 on the cooled piece.

Result and Comparison

Use of the slots 13 on the core 10 helps reduce pressure due to the gases generated inside the core 10 by allowing the gases generated to circulate and be drained outside the molding part 11 via a preferred circulation route.

FIG. 6 illustrates results obtained.

A comparison is made between a core so-called reference “test piece” (test piece without slot 13 on the surface) and a series of cores “test pieces” so-called test, each test piece of this series comprising on its surface a different number of slots 13 of width 13a of 0.2 mm.

All the test pieces (including the reference test piece) have the same general geometry outside the possible slots 13 and consist of the same material.

Use of a Manometer Shows:

• • for the test piece of the series test which comprises a slot 13 a drop in the gas discharges by 22% relative to the reference test piece,
  • for the test piece of the series test which comprises two slots 13: a drop of 40%,
  • and for the test piece of the series test which comprises sixteen slots 13: a drop of 46%.

Claims

1. A foundry core (10) for casting pieces of aluminium alloy in a mold (20), said core (10) comprising: a molding part (11), intended to be in contact with the molten aluminium alloy (30),at least one non-molding part (12a), intended to be located outside the molten aluminium alloy (30),characterized in that the foundry core (10) comprises at least one slot (13) on the surface of the core (10), said slot (13) extending from the molding part (11) to at least one non-molding part (12a) for discharge of the gases generated in the molding part (11) of the core (10) during casting outside the molding part (11).

2. The foundry core (10) according to claim 1, wherein the non-molding part(s) (12a) are core prints (12b), adapted to keep the core (10) in position inside the mold (20).

3. The foundry core (10) according to any one of the preceding claims, wherein the slots (13) have a width (13a) adapted to prevent the molten aluminium alloy (30) from entering inside said slot (13).

4. The foundry core (10) according to any one of the preceding claims, wherein the slots (13) on the surface have a width (13a) less than 1 mm and preferably 0.2 mm.

5. The foundry core (10) according to any one of the preceding claims, wherein the slots (13) have a rectangular geometric, U-shaped or V-shaped profile.

6. The foundry core (10) according to any one of the preceding claims, wherein the slots (13) have an average depth (13b) of between 0.2 and 2 mm, to allow the gases generated in the molding part (11) of the core (10) to circulate in the slots (13).

7. The foundry core (10) according to any one of the preceding claims, wherein the slots (13) are made by laser.

8. The foundry core (10) according to any one of claims 1 to 6, wherein the slots (13) are made by tooling with reliefs in the shape of blades.

9. A mold adapted for casting foundry pieces, comprising a core (10) according to any one of the preceding claims.

10. The mold according to the preceding claim, further comprising a suction system (43) adapted to suction in the slots (13) the generated gases, with suction occurring at the non-molding zones (12a).

11. A method for manufacturing a core (10), according to any one of claims 1 to 8, characterized in that it comprises an etching step of said slots (13) on the surface of the core (10) by means of a laser (5).

12. The method for manufacturing a piece of aluminium alloy by casting molten metal by means of a mold according to one of claims 9 to 10.

13. A cylinder head for an automobile obtained by a method according to the preceding claim.

14. An engine block for an automobile obtained by a method according to claim 12.