FR3114258A1 - Improved process for manufacturing a part by additive manufacturing by deposition of molten wire - Google Patents

Abstract

The invention relates to a method for manufacturing a metal or ceramic part, comprising: - the production by additive manufacturing by deposition of molten wire of a green part (1) and at least one support structure (10) having a contact surface with the green part, the green part being made from a feedstock comprising a binder and a metal or ceramic powder whose filler content is between 50 and 65% by volume; - the elimination of each support structure (10); - the removal of the binder from the green part (1) to obtain a brown part; - the densification of the brown part by sintering. Each support structure is made of a thermoplastic polymer which is also present as a major component in the binder of the green part, the elimination of each support structure and the elimination of the binder of the green part being concomitant. Figure for abstract: Figure 4

Description

Improved process for manufacturing a part by additive manufacturing by deposition of molten wire

The invention relates to additive manufacturing by deposition of molten wire for the manufacture of metal or ceramic parts requiring one or more support structures.

PRIOR ART

Additive manufacturing by deposition of molten wire, or additive manufacturing FFF (from the English "Fused Fabrication Filament"), is a manufacturing by deposition of molten material which consists in building, layer by layer, a plastic part or a green part, by deposition of a molten filament which solidifies on cooling.

In the case of the additive manufacturing of a green part, the filament used results from the conditioning of a mixture, called feedstock, in the form of a filament with a diameter of between 1.5 and 2 mm and more generally between 1. 5 and 1.8mm.

Feedstock is a mixture of a powder (metallic or ceramic) and a binder; it is generally used in the manufacture of parts by injection using the MIM (Metal Injection Moulding) process.

The feedstock binder is a vector used for the implementation of the metal or ceramic powder, since it allows the implementation of the powder to give shape to the green part. Once the green part has been obtained, the binder must be removed (debinding).

The binder is a multicomponent system. It is mainly composed of two different thermoplastic polymers, namely a main polymer (that is to say having a mass percentage greater than the mass percentages of all the other components of the binder) and a secondary polymer in a lesser quantity, to which are added small quantities of additives (dispersants, stabilizers, surfactants, etc.). These additives make it possible to stabilize the distribution between the powder and the polymers; in particular, they make it possible to lower the viscosity of the feedstock in the molten state to facilitate its injection during the MIM process.

The binder comprising two different polymers, the debinding of the green part takes place in two stages. During the first stage, the main polymer is chemically or thermally removed. This first step aims to open pores between the powder particles from the outer surfaces towards the core of the green part. At the end of the first stage, the secondary polymer remains. This secondary polymer, also called “backbone”, allows the green part to keep its shape and gives the part a resistance that allows it to be manipulated. This secondary polymer is removed during a second stage, generally by applying a heat treatment at a higher temperature than that prevailing during the first stage. This second step is usually performed in a sintering furnace and is usually performed at the start of the part sintering step.

FFF additive manufacturing is illustrated in Figure 1; the 3D printing of a green part 1 by the FFF technique requires a printing plate 2, a spool 3 of feedstock filament 4 and at least one print head 5, also called "extruder" or "nozzle of extrusion”.

The filament 4 is shaped using the FFF technique and, from the molten and solidified feedstock, a so-called "green part" 1 is obtained. In Figure 1, an enlargement of the area around the extrusion nozzle ( delimited by a broken line) shows the feedstock filament which is in a molten state 6 at the exit of the extrusion nozzle, then the deposited filament is in a state in the process of solidification 7 and finally, in a solidified state 8. The green part is then subjected to a debinding operation, in order to eliminate the binder contained in the solidified feedstock. A so-called “brown part” 11 is then obtained. Finally, the brown part is subjected to a sintering operation, in order to obtain a 100% metallic or 100% ceramic part, depending on the type of powder used in the feedstock. .

In FFF additive manufacturing, it is necessary to have a layer on which to deposit the material to deposit the next layer. Thus, to produce green parts having, for example, overhanging zones 9 at an angle greater than about 45° relative to a horizontal plane, or even elements such as internal cavities or channels, or holes, traversing or not, with an inclination greater than about 45°, it is necessary to have recourse to so-called “support” structures 10, in order to avoid, for example, depositing filament “in a vacuum”. The support structure 10 has at least one surface in contact with the green piece.

In Figure 2 is shown an example of a green room 1 that required the use of a support structure 10 during its construction, to support the area 9 of the overhanging green room 1.

Support structures can be of two types:
- they can be made with the same material as that with which the green part is made, and in this case their elimination is done mechanically (tearing off by hand and/or using tools); Or
- they can be made using another material, which requires that the device used for 3D printing by the FFF technique is equipped with a second extrusion nozzle that extrudes this other material, generally called carrier material. In this case, the second material is generally chosen so that it is soluble in a solvent which does not interact with the printed green part.

When support structures are printed with the same material used to print the green part, they are usually printed with a lower density than the green part to limit their impact on print time. A small space is also deliberately left between the support structures and the green part to prevent them from merging with the part and thus facilitate their removal. This method of making the support structures has the advantage of being fast, because no tool or parameter change is necessary to obtain the support structures.

However, support structures very often merge with the green part at their interface and thus leave marks on the green part.

Additionally, removal of support structures requires a post-print operation and support structures cannot be removed from a closed cavity.

Another technique (document [1]) consists of printing the support structures with feedstock identical to that used for the green part and interposing a thin layer of ceramic between the green part and each support structure. The set {green part + support structure(s) + ceramic interface(s)} is debinded, then sintered. The ceramic layer serves to prevent welding and diffusion between the part and each supporting structure during the sintering step. Note that here the support structure is used both as a construction support and as a sintering support; it is therefore not removed before the end of the sintering step. The disadvantage of this technique is that it can be relatively expensive for certain types of feedstock.

When the support structures are printed with a different material than the one used to print the green part, it is possible to print the support structures in a material whose degradation conditions are different from those of the part. But again, the removal of the support structures requires additional time, more or less depending on the thickness of the support structures.

In all cases, the use of support structures in the solutions proposed in the prior art has an impact on the operating range by adding a time necessary for their elimination, whether carried out mechanically, chemically (by dissolution ) or thermal.

There is a need to optimize the processes of the prior art in order to be able, when printing a green part by FFF additive manufacturing, to manufacture one or more support structures:
- which is (are) easily removable;
- less expensively than by making support structures with the feedstock itself;
- which does not cause(s) no increase in time in the post-print range;
- which is (are) compatible with the formulation of the feedstock used for printing the green part.

To meet this need, the subject of the invention is a method for manufacturing a metal or ceramic part, comprising:
- the production by additive manufacturing by deposition of molten wire of a green part and of at least one support structure having a contact surface with the green part, the green part being produced from a feedstock comprising a binder and a metal or ceramic powder, the filler content of the powder being between 50 and 65% by volume;
- the elimination of each support structure;
- removing the binder from the green part to obtain a brown part;
- densification of the brown part by sintering;
characterized in that each support structure consists of a thermoplastic polymer which is also present as a major component in the binder of the green part, the elimination of each support structure and the elimination of the binder from the green part being concomitant .

In the context of the present invention, it is considered that a component is predominant when its mass percentage is greater than the mass percentages of all the other components of the binder.

According to a possible variant of the invention, the method further comprises, during the production of the green part and of each support structure, the production by additive manufacturing by deposition of molten wire of at least one sintering support, a support structure being placed between each sintering support and the green part, each sintering structure being in a feedstock identical to the feedstock of the green part, each sintering structure being eliminated during the densification of the brown part.

Preferably, each support structure disposed between the green part and a sinter structure is in the form of a layer.

According to a preferred embodiment of the invention, the elimination of each support structure and the elimination of the binder from the green part are obtained by dissolving said thermoplastic polymer by joint applications of an acid and a heat treatment. This is a catalytic debinding.

Preferably, the thermoplastic polymer is a polyoxymethylene (POM). It may be a polyoxymethylene homopolymer, obtained by polymerization of formaldehyde, or a polyoxymethylene copolymer, obtained by polymerization of formaldehyde with ethylene oxide or dioxolane.

According to a preferred variant, the feedstock comprises 60% by volume of powder and 40% by volume of binder, the major component of the binder being polyoxymethylene (POM).

The principle according to the invention consists in producing, by the FFF technique, both the green part, the support structure(s), necessary for supporting the green part during its manufacture, and the possible sintering structures, using, for each support structure, a construction material corresponding to the thermoplastic polymer which is the main component of the binder and which acts as a vector for the powder in the feedstock used for the green part and, for each eventual sintering structure, the same feedstock used for the green part.

The construction material of each support structure will therefore be eliminated at the same time as the majority component of the green part during the step of debinding the green part.

This allows:
- to avoid the use of a third-party support material that could contaminate the green part;
- to avoid having to use very different part and support structure printing parameters; And
- above all to avoid an additional step of removing the post-printing support structures, which is time-consuming and risks damaging the part.

The solution proposed by the invention is innovative, because the removal of the support structures is carried out in masked time during the debinding operation necessary for the green parts printed with feedstock.

When there are one or more sintering structures, they are used to sinter the debinded part (brown part) and are eliminated during sintering.

The method according to the invention is applicable to any part that can be manufactured from feedstock by FFF additive manufacturing and requiring one or more support structures.

The method according to the invention has many advantages.

The manufacture of the support structure(s) is done during the printing process of the green part and under manufacturing conditions close to or even identical to those of the green part. Indeed, the parametry developed specifically for the feedstock is similar to those of the support structures insofar as the support structures are made with one of its compounds. The feedstock and the material of each support structure are, in fact, perfectly compatible.

The elimination of the support structures is carried out in masked time in an already existing operation (debinding) of the post-printing range of green parts in feedstock. Thus, there is no additional time for the removal of support structures or dedicated range. The same is true when there are one or more sintering structures, since they are eliminated during the sintering of the brown part.

Other advantages and characteristics of the invention will appear in the non-limiting detailed description below.

This description will be made with regard to the appended drawings, among which:

is a block diagram of the FFF process;

is a perspective view of an embodiment of a green part and its support structure, printed by FFF additive manufacturing;

is a perspective view of another embodiment of a green part, its support structure and its sintering structure, printed by FFF additive manufacturing;

is a cross-sectional schematic representation of a step of an embodiment of the invention, namely the step of producing a green part and its support structure by co-printing by FFF additive manufacturing of the green part with a feedstock containing as major component a thermoplastic polymer and the support structure consisting of this same thermoplastic polymer;

is a schematic sectional view of the green part and its support structure obtained at the end of the step of printing by FFF additive manufacturing of the method according to the invention, illustrated in FIG. 3;

is a schematic sectional view of the brown part obtained at the end of the debinding step of the process according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the context of the invention, it is possible to use metallic feedstocks (for example, feedstocks containing materials in powder form such as nickel- or cobalt-based superalloys, steels, titanium bases, copper, etc.) or ceramic feedstocks (for example, feedstocks containing materials in powder form such as SiC, Al ₂ O ₃ , ZrO ₃ , etc.). But the binder of these feedstocks must comprise, as the main component, a thermoplastic polymer, which could also be used to produce the support structure(s). Preferably, feedstocks are used whose binder has POM as its main component.

Referring to Figure 3, is produced on a printing plate 2, a green part 1 and its support structure 10, using a printing device comprising two extrusion nozzles 5. together {green part 1 + support structure 10} is shown in figure 5a.

This set {green part 1 + support structure 10} is then subjected to a debinding step which will make it possible both to remove the binder from the green part and eliminate the support structure. The elimination of each support structure is carried out in masked time during the elimination of the main component of the feedstock binder.

At the end of the debinding step, a brown part 11 is obtained (FIG. 5b) which is porous and fragile. To be consolidated, the brown part must then be sintered. As a reminder, sintering is a heat treatment carried out at a temperature below the melting temperature of the powder constituting the brown part, so as to densify the part.

According to a preferred embodiment of the invention, a feedstock is used whose thermoplastic binder is polyoxymethylene (POM) and polyoxymethylene is also used to build the support structure(s) necessary for the construction of the green part.

Preferably, the thermoplastic binder being POM, the debinding step of the process according to the invention is carried out under nitric acid and at a temperature between 115 and 125°C. This is a catalytic debinding. It should be noted that the melting point of POM is higher than this temperature; it is generally between 164°C and 175°C.

As is known, a feedstock is a mixture of a metal or ceramic powder and a thermoplastic binder, the mixture containing between 50 and 65% by volume of powder. Preferably, the feedstock according to the invention is a mixture of 60% by volume of a metal or ceramic powder and 40% by volume of binder, the major component of the binder being polyoxymethylene (POM). For example, one can use a feedstock whose binder is composed of POM, as the main component, of polypropylene (PP) as a secondary component and other compounds in very small proportions. The POM will be eliminated during the debinding of the green part; the PP, on the other hand, constitutes the backbone of the brown part and is eliminated during the sintering step of the brown part.

To illustrate the process according to the invention, a green part is produced using an FFF additive manufacturing machine having two extrusion nozzles. The manufacturing chamber of the machine can be temperature regulated at a temperature between 80 and 150°C, and more precisely between 110 and 130°C. The feedstock filament has a diameter between 1.5 and 2 mm. In this embodiment, the feedstock binder used contains between 7 and 10% by mass of POM and the support structure consists solely of POM.

Only one or more support structures can be made, as well as the green part.

According to another embodiment, it is also possible to produce, in addition to the support structure(s) 10 and the green part, a sintering structure 12 for each support structure 10, each support structure 10 being in the form of 'a layer disposed at the interface between the green part and each sintering structure 12, as shown in Figure 3. Each sintering structure 12 is made of a feedstock identical to that used for the green part.

The green part can have through openings, internal blind channels, or both.

The support structures 10 are eliminated during the catalytic debinding operation.

If there are one or more sintering structures 12, these remain at the end of the catalytic debinding operation and can be used for sintering the piece. Each sinter structure will be removed during the debinding part sintering step.

CITED REFERENCE

[1] US 10,035,298 B2

Claims (6)

A method of manufacturing a metal or ceramic part, comprising:
- the production by additive manufacturing by deposition of molten wire of a green part (1) and of at least one support structure (10) having a contact surface with the green part, the green part being made from a feedstock comprising a binder and a metal or ceramic powder, the filler content of the powder being between 50 and 65% by volume;
- the elimination of each support structure (10);
- removing the binder from the green part (1) to obtain a brown part (11);
- densification of the brown part by sintering;
characterized in that each support structure consists of a thermoplastic polymer which is also present as a major component in the binder of the green part, the elimination of each support structure and the elimination of the binder from the green part being concomitant . Method according to claim 1, further comprising, during the production of the green part and of each support structure (10), the production by additive manufacturing by deposition of molten wire of at least one sintering support (12) , a support structure (10) being disposed between each sintering support (12) and the green part (1), each sintering structure (12) being of a feedstock identical to the feedstock of the green part, each sintering structure being eliminated during the densification of the brown part. A method according to claim 2, wherein each support structure (10) disposed between the green part (1) and a sinter structure (12) is in the form of a layer. Process according to any one of Claims 1 to 3, in which the elimination of each support structure and the elimination of the binder from the green part are obtained by dissolving the said thermoplastic polymer by the joint applications of an acid and a thermal treatment. A method according to any of claims 1 to 4, wherein the thermoplastic polymer is polyoxymethylene (POM). Process according to Claim 5, in which the feedstock comprises 60% by volume of powder and 40% by volume of binder, the major component of the binder being polyoxymethylene (POM).