Method for winding a filament for an additive manufacturing device

Description

Technical field of the invention

[0001] This document relates to a method for winding a filament with a high metal powder loading rate. This filament is intended for an additive manufacturing device. State of the prior art

[0002] When producing parts using additive manufacturing, a filament is melted and solidifies to create the parts layer by layer. The consumable filament is stored around a spool.

[0003] Traditionally, in a fused filament deposition process, also called "Fused Filament Fabrication" (FFF), filaments are used comprising a binder containing polymers such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polycarbonate, nylon, polyethylene terephthalate glycol (PETG), etc. These filaments are generally not loaded with metal powder. They may be lightly loaded, such as carbon fibers, to enhance the final properties of the part. In the case of additive manufacturing of metal parts via the FFF process, the filament reaches an extremely high metal powder loading rate because it is intended to produce a composite part called a "green part". This part will undergo several post-printing operations to remove the polymer part of the part and densify it to finally obtain a fully metallic part.

[0004] The FFF process typically includes an amount greater than or equal to 80% and preferably between 85 and 91% by mass of metal powder. The filaments typically have a diameter between 1.5 and 5mm and preferably between 1.65 and 1.85mm. This process is very similar to a metal powder injection molding process, also called "Metal Injection Molding" (MIM). The difference between the FFF process and the MIM process is that in the FFF process, the part is printed and not injected using a press and a mold as in the MIM process.

[0005] It is important to understand the MIM process as a whole in order to understand the necessity and importance, in the FFF process, of having a filament with a quantity greater than or equal to 80% by mass of metal powder.

[0006] There figure 1 classically illustrates the MIM 1 process. In this MIM 1 process, the binder 2 and the metal powder 4 are mixed in a mixer 6, then the whole is extruded in an extruder 8. A filament 10 is obtained and is injected with a press 12. A green part 14 is obtained.

[0007] There figure 2 illustrates the FFF 3 process. In this FFF 3 process, a filament 10 is obtained in the same way as in the MIM 1 process. However, in the FFF 3 process the filament 10 does not pass through a press 12 but a green part 14 is printed by means of a 3D printer 16 in which a filament 10 loaded with metal powder is loaded.

[0008] Then, both in the MIM 1 process illustrated in figure 1 than in the FFF 3 process illustrated in figure 2 , the green part 14 passes into a first debinding furnace 18. As its name indicates, this first operation will remove the polymer binder. In this step, the polymer part contained in the part is removed. At the end of this step, the debinded part 20 is almost exclusively made of metal powder. There is just a little binder left between the powder grains to hold the part. If all the binder were removed at this step, the part would collapse. At the end of this step, sintering 22 takes place. At this stage, we speak of a brown or brown part 24. Once 90% of the binder has been removed, the fragile and porous part passes into a second sintering furnace which will densify the part. At the end of this sintering step, the part will be entirely metallic and the porosities left by the evacuation of the binder during the debinding step will almost completely disappear. The counterpart is that the volume of the part will decrease.Thus, the greater the proportion of binder in the green part, the greater the final volume variation of the part will be. This is why we try to print or inject a part with the highest metal content to minimize volume variations and more easily predict the final geometry of the part. Overall, shrinkage is isotropic on all 3 axes.

[0009] So, the figure 3 illustrates a variation in volume between a green 14 piece in figure 3A and a sintered piece or brown piece 24 in figure 3B The higher the metal powder loading rate, the less volume variation there will be. But a high metal powder loading rate makes the filament very fragile.

[0010] There figure 4 illustrates the evolution of the bending stress σ in Mega Pascal (MPa) as a function of the strain rate ε.

[0011] Three-point bending tests on such a filament with a high metal powder loading rate show that at room temperature, i.e. at a temperature between 18 and 22°C, the elasticity of the filament is very low and less than or equal to 1%.

[0012] From the figure 4 , we see that the bending stress σ increases rapidly until reaching 17 MPa for a strain rate of 0.4%. Then, for a strain greater than 0.4%, the bending stress σ suddenly drops indicating a breakage of said filament.

[0013] Under such conditions, as illustrated in figure 5 , the filament is particularly brittle at room temperature. It then becomes impossible to wind and unwind the filament onto a spool of an additive manufacturing device without the risk of cracks 102 or filament breakage. Filament breakage or filament cracking while winding the filament around the spool renders it unusable. Indeed, in the FFF process, the print head and the drive system need a continuous filament with a constant cross-section to ensure a regular flow rate.

[0014] To overcome this difficulty, it is known to protect said filament 104 by placing a polymer skin 106 around the filament as illustrated in figure 6 . The addition of this 50 µm thick polymer skin 106 around the filament 104 makes it possible to achieve the necessary curvature radius for its winding and facilitates the printing of the material because it makes it less brittle and less viscous. However, this solution increases the matrix content in the filament 4 which induces other difficulties such as the increase of residual stresses in the part formed from such a filament. A higher proportion of binder causes more shrinkage and makes it less predictable. The presence of a polymer skin around the filament can cause an uneven distribution of the powder and binder in the part.

[0015] It is therefore important to provide a technical solution that does not modify the composition of the filament and guarantees its integrity, i.e. that no cracks or breaks are present in this filament.

[0016] Documents WO2018106733 and KR101533998 are considered prior art. Presentation of the invention

[0017] This document relates to a method of winding a filament for an additive manufacturing device comprising the steps: providing a filament loaded with at least 80% by mass of metal powder; heating said filament to a temperature of at least 70°C and maintaining said filament at said temperature; winding said filament around the axis of a spool, preferably metal, the diameter of the spool being, when empty, greater than or equal to a diameter of 120mm.

[0018] By placing oneself in such conditions, it becomes possible to wind the filament without generating breakage or cracking of said filament. This allows one to place oneself in conditions of favorable elasticity. In addition, the spool being preferably made of metal, this prevents it from deforming with heat.

[0019] The said diameter of the coil can be between 100 and 140 mm, preferably between 120 and 140 mm.

[0020] Said filament can be heated to a temperature between 70 and 140°C, preferably between 70 and 90°C.

[0021] This document relates to an installation for winding a filament onto a reel for an additive manufacturing device comprising: a filament extruder; stretching means; means for heating said filament to a temperature of at least 70°C; drive means; means for winding said filament around the spool, preferably metallic, the diameter of the empty spool of which is greater than or equal to a diameter of 100 mm.

[0022] Heating means may be provided between the stretching means and the winding. The stretching means may include a drawing belt.

[0023] The means for heating said filament may comprise means for blowing air at said temperature.

[0024] The heating means of said filament may comprise an infrared heating means.

[0025] The drive means may include at least one drive roller. Brief description of the figures

[0026] [ Fig. 1 ] represents a metal powder injection molding process, also called “Metal Injection Molding” (MIM); [ Fig. 2 ] represents a fused filament deposition process, also called “Fused Filament Fabrication” (FFF); [ Fig. 3 ] represents a variation in volume between a green part in figure 3A and a sintered part in figure 3B . [ Fig. 4 ] represents a three-point bending test on a filament at room temperature and more precisely the evolution of the bending stress σ of said filament in Mega Pascal (MPa) as a function of the strain rate ε; [ Fig. 5 ] represents an example of a damaged filament with cracks; [ Fig. 6 ] represents a filament surrounded by a polymer skin; [ Fig. 7 ] represents an extrusion device or industrial extrusion line for a filament according to the invention; [ Fig. 8 ] represents in figure 8A a coil according to the invention and in figure 8B a classic reel. [ Fig. 9 ] represents the evolution of the elastic modulus G' as a function of the temperature T for a filament during dynamic thermomechanical analysis (DMTA) tests according to the invention; [ Fig. 10 ] represents three-point bending tests on a filament at a temperature T of 80°C and more precisely the evolution of the bending stress σ of said filament in Mega Pascal (MPa) as a function of the deformation rate ε according to the invention; [ Fig. 11 ] represents in figure 11A the evolution of the bending stress σ of said filament in Mega Pascal (MPa) as a function of the strain rate ε and in figure 11B , the evolution of the minimum radius of curvature R in millimeters (mm) of the central roller of the spool such that the filament is not damaged as a function of the deformation rate ε according to the invention; [ Fig. 12 ] represents a diagram illustrating the obtaining of a radius of curvature as a function of a deformation during a three-point bending test. Detailed description of the invention

[0027] This document is placed in the context of an additive manufacturing device allowing a part to be built layer by layer, by depositing a molten filament which solidifies upon cooling.

[0028] There figure 7 illustrates an installation 108 for extruding a filament 104 and winding it onto a spool 110 for subsequent use in an additive manufacturing device.

[0029] In such an installation, the filament 104 is produced by means of an extruder 112. This filament 104 has a diameter of between 1.5 and 5 mm. It comprises between one and three polymers and is loaded with metal powder at least 80% by mass.

[0030] The filament 104 is then drawn by drawing means 114. These drawing means 114 comprise a drawing belt 116. The filament 104 is then heated by heating means 118 to a temperature of at least 80°C. These heating means 118 comprise air heating means at said temperature 120 and / or an infrared heating means 122. The filament 104 is thus heated and maintained at said temperature: one of the binder compounds is maintained in the molten state to soften the filament. The filament is then driven by drive means 124 towards a spool 110. These drive means comprise at least one drive roller 126.

[0031] The filament 104 is then wound by winding means 127 around a spool 110. This spool comprises a central roller 128 or cylindrical part with a circular base around which the filament is wrapped. The central roller 128 has an outer perimeter of its base which is inscribed in a circle so that this central roller 128 can be a cylindrical part with a polygonal base. This central roller 128 has a diameter greater than or equal to 100 mm, preferably between 100 and 140 mm, even more preferably between 120 and 140 mm.

[0032] There figure 8 illustrates a comparison between a classic 140 coil in figure 8B and a spool 110 used for winding the filament loaded with metal powder according to the invention into figure 8A The diameter of the central roller 128 is 120mm on the spool 110 according to the invention which is preferably made of metal and which is illustrated in figure 8A while the diameter of the central roller 128 is smaller for the classic 140 reel in figure 8B The coil 110 according to the invention comprises a first cylindrical rim 144 and a second cylindrical rim 146 which each have a diameter greater than that of the central roller 128.

[0033] In operation, the filament, due to its elastic properties, can be wound without breaking or cracking from a temperature of at least 70°C, preferably between 70 and 90°C. Once wound hot, the filament retains its winding shape as it cools. To unwind it without breaking or cracking, it is necessary to heat the filament again to a temperature of at least 70°C, preferably between 70 and 140°C, even more preferably between 70 and 90°C.

[0034] There figure 9 illustrates the evolution of the elastic modulus G' in Mega Pascal (MPa) of said filament as a function of temperature T during a dynamic thermomechanical analysis test, also called DMTA. This elastic modulus G' provides information on the rigidity and the elastic component of the material. The elastic modulus G' has a value of 9.5×10 3< MPa for a temperature of approximately 50°C and decreases as the temperature increases. Softening occurs at temperatures between 80 and 140°C, giving said filament the properties necessary for winding without breakage or cracking around the spool. At a temperature T of 80°C, the elastic modulus G' drops to 4.4×10 3< MPa and to less than 10 3< MPa at a temperature T of 140°C.

[0035] There figure 10 illustrates the three-point bending tests carried out on the said filament at a temperature of 80°C and more precisely this figure 10 shows the evolution of the bending stress σ in Mega Pascal (MPa) as a function of the strain rate ε. Four curves are presented corresponding to the same tests carried out four times. Unlike the results of three-point bending tests carried out at room temperature in figure 4 , the bending stress σ is lower here. A plateau is reached from about 1% strain rate for which the bending stress σ is between 7 and 9 MPa. Thus, even at strain rates of 5%, there is no rupture, breakage of the filament in response to these stresses.

[0036] From the results of the three-point bending tests illustrated in figure 9 , it is possible to determine a minimum radius of curvature R that the filament can accept before breaking for a given temperature and in this case for a temperature of 80°C. This is illustrated in figure 11 .

[0037] There figure 11B illustrates the radius of curvature R of the central roll of the coil in millimeters (mm) as a function of the deformation rate ε. The figure 11B is obtained knowing the position of three red points as illustrated in figure 12 during the three-point bending test. A machine records a displacement of the central point 160 and two other support points 162, 164 remain at a fixed position. It is thus possible to determine the radius of curvature R of a part illustrated by a curve 166 during the three-point bending test via a small geometric calculation thanks to the positions of the three points 160, 162, 164 and this as a function of the displacement of the central point.

[0038] Since these displacements and stresses are recorded throughout the three-point bending test, it is therefore possible to express the radius of curvature R by the displacement of the central point as a function of the stress in the part. Finally, for each position of the central point, which therefore corresponds to a radius of curvature R, a curve can be plotted expressing the radius of curvature R as a function of the strain rate ε, and thus determine the maximum radius of curvature R accepted by the filament, as illustrated in Figure 10B.

[0039] From the calculations carried out, a critical deformation rate ε was thus deduced which should not be exceeded in order not to damage the filament. This critical deformation is between 2 and 4% and is preferably less than 4%. figure 11A corresponds to the figure 6 and allows to make the link with the figure 11B . As illustrated in figure 11B , a filament break occurs between 5.5 and 6.5% as indicated by the two dotted lines 150, 152. The graph of the figure 11B shows that the greater the deformation rate ε, the smaller the minimum radius of curvature R that must not be exceeded in order not to damage said filament. Under the aforementioned conditions, that is to say with a deformation rate of less than 4% as indicated by the limit line 154, the minimum diameter of the central roller of the reel is between 100 and 140 mm and preferably greater than or equal to 120 mm.

Claims

1. Method for winding a filament (4) for an additive manufacturing device comprising the steps of: providing a filament (4) filled with at least 80% metal powder by mass; - heating said filament (4) up to a temperature of at least 70°C and keeping said filament (4) at said temperature; - winding said filament (4) around the axis of a coil (110), preferably metallic, the diameter of the coil (110) being in the empty state larger than or equal to a diameter of 100 mm.

2. Method for winding a filament (4) according to claim 1, wherein said diameter of the coil is comprised between 100 and 140 mm, preferably between 120 and 140 mm.

3. Method for winding a filament according to one of the preceding claims, wherein said filament (4) is heated to a temperature comprised between 70 and 140°C, preferably between 70 and 90°C.

4. Installation (8) for winding a filament (4) onto a coil (110) for an additive manufacturing device comprising: - a filament (4) extruder (12); - stretching means (14); - means (18) for heating and keeping said filament (4) at a temperature of at least 70°C; - drive means (24); - means (27) for winding said filament (4) around the coil (110), preferably metallic, the diameter of the coil (110) of which in the empty state is larger than or equal to a diameter of 100 mm.

5. Installation for winding a filament (4) onto a coil (110) according to claim 4, wherein the means (18) for heating and keeping said filament at a temperature of at least 70°C intervene between the stretching means (14) and the winding means (27).

6. Installation for winding a filament onto a coil according to claim 4 or 5, wherein the stretching means (14) comprise a drawing belt (14).

7. Installation for winding a filament onto a coil according to one of claims 4 to 6, wherein the means (18) for heating said filament comprise means for blowing air at said temperature (20).

8. Installation for winding a filament onto a coil according to one of claims 4 to 7, wherein the means (18) for heating said filament include an infrared heating means (22).

9. Installation for winding a filament onto a coil according to one of claims 4 to 8, wherein the drive means (24) include at least one drive roller (26).