Patent Application
20220250154
The present invention relates to additive manufacturing and in particular, but not exclusively, to post-processing of additively manufactured metal components.
Additive manufacturing (AM) is a process during which an object can be manufactured from a digital file using a layer-by-layer method. Fused Filament Fabrication (FFF) also called Fused Deposition Modelling (FDM) is a frequently used AM process during which heated material in the form of a paste is extruded through a printer nozzle to form a desired 3D shape. A variation of this technology involves the use of a metallic powder for sintering and a binder material, typically a polymer, for retaining the shape of the metallic powder during the extrusion process. In addition, a ceramic or other material interface layer may be used to support overhanging part structures while they are being printed.
Once the metallic powder together with the polymer binder are extruded to form a part, the part is in a so-called “green” state and requires de-binding and thermal post-processing. The de-binding process uses a solvent to dissolve a majority of the binder material supporting the metallic powder. During thermal post-processing, the metallic powder sinters together to form the final part whilst any remaining binder material is vaporised. The metallic powder may include two or more metals selected to form an alloy during the thermal post-processing stage.
However, the layer-by-layer nature of the FFF/FDM printing process results in a stepped surface which is undesirably rough and can promote cracking in use.
It is an aim of certain embodiments of the present invention to provide a method of post-processing an additively manufactured metal part to provide the part with a relatively smooth outer surface.
According to a first aspect of the present invention there is provided a method for smoothing a surface of an additively manufactured metal part, comprising:
In exemplary embodiments, the method comprises applying a chemical to a stepped surface of an additively manufactured part to at least soften a binder material supporting unprocessed powder particles of the part, but so as not to soften the powder particles of the part.
In other words, the powder particles comprise a material resistant to the chemical.
In exemplary embodiments, the method comprises allowing the binder material to cure and support the powder particles at the surface.
In exemplary embodiments, the method comprises thermally treating the part to remove the binder material from the part.
In exemplary embodiments, thermally treating comprises vaporising the binder material.
In exemplary embodiments, the method comprises sintering the part to fuse the powder particles together.
In exemplary embodiments, the method further comprises allowing the binder material to cure and support the powder particles at the surface of the part and sintering the part to fuse the powder particles of the part together, wherein, during sintering, the binder material is vaporised and removed from the part.
In exemplary embodiments, the method comprises drying the part to remove the chemical from the surface.
In exemplary embodiments, applying comprises vaporising the chemical and condensing the chemical on to the surface of the part.
In exemplary embodiments, vaporising comprises heating the chemical in a liquid state to a predetermined temperature.
In exemplary embodiments, condensing comprises creating an energy potential between the part and the vaporised chemical.
In exemplary embodiments, the method comprises cooling the part to create the energy potential and cause the vaporised chemical to condense on to the part.
In exemplary embodiments, applying comprises immersing the part in a reservoir of the chemical in a liquid state.
In exemplary embodiments, applying comprises dispensing the chemical in a liquid or vapour state on to the part via at least one dispending device.
In exemplary embodiments, the powder particles comprise a metal, ceramic, or polymer material resistant to the chemical.
In exemplary embodiments, the binder material comprises a thermoplastic polymer.
In exemplary embodiments, the chemical comprises 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), dimethylformamide, sulphuric acid, m-cresol, formic acid, trifluoroacetic acid, benzyl alcohol, 1,2,4 trichlorobenzene, tetrahydrofuran, 2-methyltetrahydrofuran, Xylene, or Dimethyl sulfoxide (DMSO).
According to a second aspect of the present invention there is provided use of a chemical to at least soften a binder material supporting unprocessed powder particles at a stepped surface of an additively manufactured part and allow the powder particles at the surface to flow under the influence of gravity into recesses defined by the stepped surface to thereby reduce a roughness of the surface.
In exemplary embodiments, the powder particles comprise a metal, ceramic, or polymer material resistant to the liquid.
According to a third aspect of the present invention there is provided apparatus for smoothing a stepped surface of an additively manufactured part, comprising:
In exemplary embodiments, the dispensing device comprises a plurality of nozzles located inside the chamber for spraying the chemical in a liquid or vapour state at the part.
In exemplary embodiments, the dispensing device comprises a heater element for vaporising the chemical in a liquid state and a perforated support member located above the heater element for supporting the part thereon.
Certain embodiments of the present invention will now be described with reference to the accompanying drawings in which:
As step S202 of the method, a chemical, such as a solvent, acid, ionic liquid or other component, suitable to soften/dissolve the polymer acting as a binder is applied to the surface/s of the AM part. Examples of suitable chemicals include, but are not limited, to 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), dimethylformamide, sulphuric acid, m-cresol, formic acid, trifluoroacetic acid, benzyl alcohol, 1,2,4 trichlorobenzene, tetrahydrofuran, 2-methyltetrahydrofuran, Xylene and Dimethyl sulfoxide (DMSO), or the like. The chemical is selected such that the metal or other material powder particles are resistant to the solvent, i.e. do not react to the chemical.
There are a number of ways the chemical may be applied to the surface of the part at the step S202. Aptly, the chemical may be vaporised using a hot plate or other suitable device and then condensed on to the AM part. As illustrated in
Optionally, the chamber 102 may have heated walls reaching temperatures up to around 70° C. Optionally, the apparatus 100 may contain a vacuum pump to be able to reduce a pressure in the chamber 102 and cause the chemical vapour to condense on to the part. Aptly, the part/s may be cooled to create an energy potential between the part and the chemical vapour and thereby cause the vaporised chemical to condense on to the part.
The applied chemical may alternatively be applied to the part/s in the liquid phase in which case the part would be immersed in a reservoir, such as a bath, flask, chamber or the like, containing a suitable chemical. As illustrated in
Alternatively, the chemical in a liquid or vapour state may be sprayed on to the part using a suitable nozzle/s, sprinkler/s, nebuliser/s, or other suitable dispensing device/s. As illustrated in
Aptly, the part/s is located in a chamber during the de-binding process which may be the same chamber as the part/s was printed in or a different chamber.
At step S204, the solvent, acid, ionic liquid or other chemical, dissolves/softens the polymer otherwise binding the metal powder together at the part's surface/s. The polymer softens just enough to allow the binding material to re-flow under the influence of gravity at the surface carrying metal powder particles with it.
In other words, the chemical allows the polymer binder material 20, and the metal particles 10 being carried by the binder material, to flow into and at least partially fill the stepped ‘recesses’ otherwise defined by the layering effect of the printing process (see arrows in
The amount of binder material softened and caused to re-flow, and in turn the final smoothness of the part, directly correlates to several parameters: contact time between the chemical and the part surface, type/strength/concentration of the chemical applied, method of application (condensing vapour or liquid immersion), and conditions of the method (temperature of the vapour/liquid and/or pressure of the system in case of the vapour method). The surface smoothness relationship can therefore be expressed by the following equation:
S=t×C×M×P
wherein S is the smoothness of the part (μ), t is the contact time between the chemical and the part (seconds), C is the constant adjusting for the type of chemical applied, M is the constant adjusting for the type of method, and P is the constant adjusting for the process parameters.
Optionally, after the smoothing process, the part is dried to remove any residual chemical or chemical trace from the part. The drying temperature is higher than the chemical boiling temperature but lower than the material melting temperature.
At step S206, the part undergoes thermal treatment to sinter the metal powder after the smoothing process. During sintering, the polymer binder material is vaporised out, whereas the metal powder is fused together to give the AM part its final shape. The resulting surface roughness of the final part is much improved because the layering/stepped effect has been desirably reduced during the smoothing process.
Certain embodiments of the present invention therefore provide a method of efficiently smoothing the surface/s of an additively manufactured metal part to improve the appearance of the part and to reduce its surface roughness and any potential notch effects otherwise caused by the rough, stepped surface/s of a conventional AM metal part which can undesirably lead to fatigue and fracture. A relatively smooth outer surface is also desirable for certain applications, particularly in the medical industry, where the potential for bacteria growth on the part must be kept to a minimum. The smoothing process according to certain embodiments of the present invention may also be applied to parts which have been additively manufactured from non-metal powder which is resistant to the chemical used to dissolve the binder, such as a glass, ceramic or polymer-based powder.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1908244.5 | Jun 2019 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2020/051391 | 6/9/2020 | WO |
