EP2654991B1 - Arrangement for moulding a mixture of metal powder around a ceramic core - Google Patents

Description

TECHNICAL AREA

The invention proposes an arrangement for forming a two-part part comprising a core and an outer body made of a metal or ceramic powder-based polymer material partially covering the ceramic core.

The invention provides a mold for resisting injection constraints, while not damaging the core which is fragile.

STATE OF THE PRIOR ART

Powder injection molding , commonly known as "powder injection molding" (PIM) in English, makes it possible to produce complex-shaped ceramic or metal components using a tool that is used in the field of molding plastics.

Such a powder injection molding process comprises a first step consisting in producing a so-called "master" mixture, commonly called "feedstock", which consists of a mixture of an organic material forming a polymeric binder and an inorganic powder. ceramic or metallic. The proportion of inorganic powder in the feedstock is between 40% and 70% by volume.

A second step of the powder injection molding process consists of making the part (s) by injection of the feedstock, using conventional tools of the plastics industry.

Then, in a third step, the organic binder is removed and the resulting part is sintered.

The production of a part made of two different materials using such a molding process consists of producing a first part, or core, of a first material, for example ceramic, and then forming the external body of this part by injection molding of the feedstock containing the powder.

Depending on the component design, it is sometimes necessary for kernel faces not to be covered by the feedstock.

For this purpose, it is customary to make the mold in such a way that the faces that must remain uncovered by the outer body, are each in direct contact with a portion of the mold.

Also, to prevent the feedstock from seeping between the mold and each open face of the core, a certain pressure is applied between the core and the mold to maintain a good contact of the respective faces of the core and the mold.

The ceramic core is manufactured with certain dimensional tolerances. This requires to be careful not to cause breakage of the core when closing the mold because the pressure applied to the core could cause the breakage of it.

Document No. FR 2,485,987 discloses a mold made of elastomeric material.

Such a mold makes it possible to compensate for the dimensional variations of the core without the risk of breaking it.

However, because of the high pressure used during injection of the feedstock, the elastomeric material is deformed and the contact between the walls of the mold and the ceramic core is broken, so that part of the feedstock can be used. infiltrate between the core and the mold, covering a face to remain uncovered, or forming burrs.

Also, the powder contained in the feedstock is particularly abrasive, which can deteriorate the walls of the mold and thus cause defects on the component obtained.

The document FR.2.707.205 describes a mold of which only a part covering the core is made of synthetic material.

Although such a solution also makes it possible to avoid breakage by adapting the dimensions of the cavity to the dimensional variations of the core, the use of a synthetic material makes this part sensitive to injection pressure and abrasion.

It is an object of the invention to provide an arrangement for feedstock injection molding around a core which allows to adapt to the dimensional variations of the core and also which is able to withstand the injection pressure and the abrasion of the powder constituting the feedstock.

STATEMENT OF THE INVENTION

The invention relates to an arrangement for the overmolding by injection of a mixture based on metal and / or inorganic powder around a core comprising an upper horizontal face and a lower horizontal face, which comprises a mold delimiting a cavity in which the core is received, so that the upper face of the core is supported upwardly against an upper face vis-à-vis the cavity and the lower face of the core is supported downward against a lower face vis-à-vis the with respect to the cavity,
and which comprises an elastically deformable part on which at least one of the upper or lower faces of the core bears,
characterized in that the elastically deformable portion comprises a resiliently deformable metal blade on at least one face of which the core is directly in abutment.

The production of the elastically deformable portion from a metal plate allows this deformable portion to resist abrasion and pressure from the feedstock injection, while adapting the dimensions of the cavity to the dimensions of the ceramic core.

Preferably, the elastically deformable portion comprises a plastic plate which is compressed between the metal blade and a rigid part of the mold.

Preferably, the mold comprises a hollow housing open towards the cavity, which receives the elastically deformable portion such that the metal blade is located at a distance from the bottom of the hollow housing.

Preferably, the plastic plate is arranged between the metal blade and the bottom of the hollow housing.

Preferably, the metal blade is arranged at the opening of the hollow housing.

Preferably, the metal blade is made of spring steel.

Preferably, the plastic plate is compressed between the metal blade and the bottom of the hollow housing.

Preferably, the plastic plate is made of polytetrafluoroethylene (PTFE).

Preferably, the plastic plate is made of polytetrafluoroethylene (PTFE) loaded with 15% graphite.

Preferably, the elastically deformable portion is removably mounted in the hollow housing.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est une représentation schématique en perspective d'une pièce réalisée grâce à un agencement selon l'invention ;
• la figure 2 est une représentation schématique en perspective d'un éclaté d'un agencement selon l'invention ;
• la figure 3 est un éclaté vu en section axiale de l'agencement représenté à la figure 1 ;
• la figure 4 est une section similaire à celle de la figure 3, montrant les différences dimensionnelles entre la cavité du moule et le noyau céramique ;
• la figure 5 est une vue similaire à celle des figures 3 et 4, dans laquelle le moule est fermé et la partie déformable du moule est déformée.

Other features and advantages of the invention will appear on reading the following detailed description for the understanding of which reference will be made to the appended drawings in which:

• the figure 1 is a schematic representation in perspective of a part made by means of an arrangement according to the invention;
• the figure 2 is a schematic representation in perspective of an exploded of an arrangement according to the invention;
• the figure 3 is an exploded view in axial section of the arrangement shown in FIG. figure 1 ;
• the figure 4 is a section similar to that of the figure 3 showing the dimensional differences between the mold cavity and the ceramic core;
• the figure 5 is a view similar to that of figures 3 and 4 in which the mold is closed and the deformable part of the mold is deformed.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

In the following description, identical, similar or similar elements will be designated by the same reference numerals.

We have shown figure 1 a part 10 made in two parts which comprises a central core 12 of ceramic and an outer body 14 of metal.

The outer body 14 is made from a powder mixture of inorganic material, that is to say for example a metal or ceramic powder, combined with an organic binder. Subsequently, we will use the common name "feedstock" to designate this mixture.

For example, the feedstock is made from a metal powder consisting of spherical beads with a diameter of less than 45 .mu.m and the binder is a mixture based on polyethylene or paraffin and stearic acid.

The outer body 14 is made by overmolding the feedstock around the central core 12 and the binder of the feedstock is removed and the piece 10 is then sintered.

The core 12 is here of generally cylindrical shape of main axis A vertical. It is produced conventionally or in a powder injection molding process. As a result, dimensional variations, particularly concerning its axial length, are envisaged.

The outer body 14 is made by overmolding the feedstock around the core 12 so that the upper end horizontal face 12a and the lower end horizontal face 12b of the core 12 are not covered by the feedstock.

Representatives figures 2 and following an arrangement for making the outer body 14 of the part 10 by overmolding the feedstock around the core 12.

This arrangement is intended to be mounted in a metal casing of a conventional injection molding machine.

The arrangement mainly comprises a mold 18, which may also consist of a mold part or a frame, which consists of a first fixed lower part 20 and movable upper part 22.

The fixed part 20 and the mobile part 22 delimit a cavity 24 in which the core 12 is arranged and in which the feedstock is intended to be injected to form the outer body 14 of the part 10.

The fixed portion 20 and the movable portion 22 of the mold 18 further comprise fingers 26 for their positioning relative to each other, and means 28, consisting here of a screw system, for exerting a clamping force two parts 20, 22 of the mold 18 against each other, to resist the injection pressure.

The dimensions of the two parts 20, 22 of the mold are relatively large compared to the dimensions of the cavity 24 and the part 10 to made because the deformation of the two parts 20, 22 of the mold 18 must be the lowest possible during the injection feedstock.

As schematically represented in the figure 3 , the core 12 is inserted into the cavity 24 before the closure of the mold 18.

When the core 12 is in position in the cavity 24, the upper face 12a of the core 12 is in abutment against an upper face 24s of the cavity 24 and the lower face 12b of the core 12 bears downwards against a lower face 24i of the cavity 24.

Due to the dimensional tolerances on the core 12, the axial length of the core may be larger or smaller than a nominal dimension.

The distance between the upper face 24s and the lower face 24i of the cavity 24, when the mold 18 is closed, is defined so that whatever the axial length of the core 12, the upper faces 12a and 12b of the lower core 12 are always bearing against the upper 24s and lower faces 24i of the cavity 24.

That is to say that this distance between the upper 24s and lower 24i of the cavity 24, is always less than or equal to the axial length of the core 12. This ensures that the feedstock can not cover the upper faces 12a and 12b of the core 12.

As can be seen at figure 4 since this distance is smaller than the actual length of the core 12, during the closure of the mold 18, the upper faces 12a and 12b of the lower core 12 are respectively in abutment against the upper 24s and lower faces 24i of the cavity 24 before the lower 20 and upper 22 of the mold 18 are in abutment against each other.

The core 12 is ceramic and may break if the clamping force of the two parts 20, 22 of the mold 18 is applied to the core 12.

To prevent the core 12 from breaking, the mold 18 comprises a portion 30 which is deformable elastically and which is able to deform when the mold 18 is closed to compensate for the difference between the length of the core 12 and the distance between the upper faces 24s and lower 24i of the cavity 24.

Here, the deformable portion 30 is arranged in a hollow housing 32 formed in the fixed portion 20 of the mold 18 so that an upper face of the deformable portion 30 delimits at least part of the lower face 24i of the cavity 24.

Here, the upper face of the deformable portion 30 is flush with the upper face 20s of the fixed portion 20 of the mold 18.

The hollow housing 32 is of complementary shape to the shape of the deformable portion 30.

As can be seen at figure 5 when the mold 12 is closed, the core 12 is supported on the deformable portion 30 which deforms elastically, so that the core 12 is subjected to a limited pressure force.

During the overmolding operation to form the outer body 14, that is to say during the injection of the feedstock, the axial compression force of the core 12 between the upper portion 22 of the mold and the deformable portion 30, resulting from the elastic deformation of the deformable portion 30, is large enough to prevent the feedstock from infiltrating between the upper faces 12a and 12b of the lower core 12 and the faces 24s, 24i vis-à-vis the cavity 24 , respectively, as well as between the lower face 22i of the upper part 22 of the mold 18 and the upper face 24s of the deformable part 30.

The deformable portion 30 comprises a metal blade 34 which is elastically deformable, of which an upper face forms at least part of the lower face 24i of the cavity 24.

Preferably, the upper face of the metal blade 34 completely forms the lower face 24i of the cavity 24.

The core 12 is directly downwardly supported on the upper horizontal face of the blade 34 and causes an elastic deformation towards the bottom of the blade during the closure of the mold 18, to compensate for the difference in values between the height of the core 12 and the distance between the faces 24s, 24i vis-à-vis the cavity 24.

In return for this elastic deformation, the blade 34 produces on the core 12 an elastic force corresponding to its deformation.

The blade 34 is made of spring steel of the type known under the name X10. The hardness of such a metal is particularly advantageous in the present case of the injection of the mixture based on metal powder. Indeed, the powder forming the feedstock is particularly abrasive, and such a constituent metal of the blade 34 is sufficiently resistant to abrasion so that the blade 34 is not damaged during the injection of the feedstock.

Also, this constituent metal of the blade 34 limits the deformations resulting from the injection pressure.

The deformable portion also comprises a plastic plate 36 which is compressed between the metal blade 34 and the bottom of the hollow housing 32 receiving the deformable portion 30.

The plastic plate 36 functions as a damper to the deformation of the blade 34, it deforms elastically by compression during the closure of the mold 18 and produces an elastic force on the metal plate 34 which is transmitted to the core 12.

The elasticity of the plastic plate 36 is determined in such a way that regardless of the dimensions of the core 12, the elastic force produced by the plastic plate 36, combined with the force produced by the blade 34 after the closure of the mold 18 , is large enough to prevent any feedstock infiltration on the upper faces 12a and 12b of the lower core 12, and so that this elastic force is less than a force likely to cause the breakage of the core 12 by compression.

Also, the plastic plate 36 makes it possible to limit the extent of the deformation of the blade 34 in an area centered on the core contact surface 12 on the upper face of the blade 34. Thus, the peripheral body 14 formed will comprise a slight excess thickness located at the lower end 12b of the core 12.

According to an alternative embodiment not shown, the deformable portion 30 of the mold consists solely of a metal blade 34.

An empty space is maintained between the blade 34 and the bottom of the hollow housing 32, for example by means of shims to allow the blade 34 to deform elastically downwards in the hollow housing 32.

The dimensions of the blade 34 are then determined so as to produce an elastic force on the core 12 which prevents the feedstock from covering the faces of the core 12 and which does not cause the core 12 to break.

Despite the fact that the blade 34 is resistant to the abrasion of the metal powder, it can be damaged as the mold 18 is used.

For this purpose, the blade 34, and possibly the plastic plate 36, is adapted to be removed from the hollow housing 32 to be replaced by a new blade 34, or a new plastic plate 36, respectively.

To facilitate this exchange, the blade 34 and the plastic plate 36 are not fixed in the hollow housing 32, their holding in position is performed on the one hand by the movable part 22 of the mold and on the other hand by the pressure d injection of the feedstock.

By way of non-limiting example, the blade 34 is made of spring steel X10 CrNi18-8, its hardness HV is between 170 and 250, its Young's modulus is 195 GPA and the limit of tensile strength (Rm ) is 690 to 900 MPa.

Also, the plastic plate 36 is in a first example made of unfilled polytetrafluoroethylene (PTFE), its Shore D hardness is 50 to 65, its Rockwell hardness is 25, its Young's modulus is 300 to 800 MPa, its resistance to tensile strength: 10 to 40 MPa, its elongation at break is between 100 and 400%.

In a second example, the plastic plate 36 is PTFE loaded with 15% graphite. Its Shore D hardness is greater than 55, its density 2.14 to 2.19 g / cm3 and its elongation at break is greater than 200%.

Claims (10)

1. A configuration for overmoulding by injection of a blend consisting of a metal and/or inorganic powder around a core (12) including an upper horizontal face and a lower horizontal face,
   which includes a mould (18) delimiting a cavity (24) in which the core (12) is received, such that the upper face (12a) of the core (12) is pressed against a facing upper face (24s) of the cavity (24) and the lower face (12b) of the core (12) is pressed against a facing lower face (24i) of the cavity (24),
   and which includes an elastically deformable portion (30) on which at least one of the upper or lower faces (12a, 12b) of the core (12) is pressed,
   characterised in that the elastically deformable portion (30) includes an elastically deformable metal blade (34) on at least one face of which the core (12) is pressed directly.
2. A configuration according to claim 1, characterised in that the elastically deformable portion (30) includes a plastic plate (36) which is compressed between the metal blade (34) and a rigid portion of the mould (18).
3. A configuration according to claim 1 or 2, characterised in that the mould (18) includes a hollow recess (32) open towards the cavity (24), which receives the elastically deformable portion (30) in such a way that the metal blade (34) is located at some distance from the bottom of the hollow recess (32).
4. A configuration according to the previous claim, characterised in that the plastic plate (36) is positioned between the metal blade (34) and the bottom of the hollow recess (32).
5. A configuration according to the previous claim, characterised in that the metal blade (34) is positioned at the opening of hollow recess (32).
6. A configuration according to any one of the previous claims, characterised in that the metal blade (34) is made of spring steel.
7. A configuration according to any one of the previous claims, in combination with claim 2, characterised in that the plastic plate (36) is compressed between the metal blade and the bottom of the hollow recess (32).
8. A configuration according to any one of the previous claims, in combination with claim 2, characterised in that the plastic plate (36) is made of polytetrafluoroethylene (PTFE).
9. A configuration according to the previous claim, characterised in that the plastic plate (36) is made of 15% graphite filled polytetrafluoroethylene (PTFE).
10. A configuration according to any of the previous claims, characterised in that the elastically deformable portion (30) is installed in a non-permanent manner in the hollow recess (32).

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