WO2018091881A1

WO2018091881A1 - Additive manufacturing

Info

Publication number: WO2018091881A1
Application number: PCT/GB2017/053428
Authority: WO
Inventors: Peter Choi; George Bonatsos
Original assignee: Developa2 Ltd
Priority date: 2016-11-15
Filing date: 2017-11-15
Publication date: 2018-05-24
Also published as: GB2556052A

Abstract

A heating apparatus for heating material deposited in an additive manufacturing device is disclosed, comprising: a body comprising a plurality of grooves formed in a surface of the body; and a plurality of optical guides disposed in the plurality of grooves, each of the optical guides comprising an optical fibre and a gradient index GRIN lens arranged to receive a beam of electromagnetic radiation from the optical fibre and focus the beam of electromagnetic radiation onto the material, so as to heat the material. Each optical guide may further comprise a spacer disposed between the optical fibre and the GRIN lens. A method of aligning a plurality of optical guides in the heating apparatus is also disclosed, comprising: inserting a plurality of optical guides into a plurality of grooves formed in a surface of a body; adjusting the relative positions of the optical guides along the respective grooves so as to align the optical guides to focus electromagnetic radiation onto a common focal plane; and clamping the plurality of optical guides against the body to secure the plurality of optical guides in the plurality of grooves.

Description

Additive Manufacturing

Field

The present invention relates to additive manufacturing. More specifically, the present invention relates to a drying apparatus for an inkjet-type deposition apparatus.

Background

Additive manufacturing is the process of building up a three dimensional article in layers. Typically, it is difficult to manufacture an article using more than one type of material at a rate which is consistent with high volume manufacture. There are several types of additive manufacturing devices. One such type is an inkjet deposition apparatus. In an inkjet deposition apparatus, a three-dimensional article is built up layer-by-layer by depositing droplets of a colloidal suspension of material and liquid carrier on a substrate from a number of nozzles. Various methods have been developed for fusing the material deposited by an inkjet-type deposition apparatus. In a thermal fusing method, a blanket layer of material is deposited and then a fusing agent is added at locations where fusing is required. The layer is then heated to cause the fusing agent to bind the loose material. However, thermal fusing techniques are relatively slow and require high temperatures, and consequently the cost of manufacturing complex multi- material parts is high.

In an optical fusing method, a laser or high power Xenon flashlamp is used to irradiate each layer after it has been deposited in order to evaporate the liquid carrier and fuse the particles of the material in all of the deposited droplets together. However, this rapid drying and melting causes rapid boiling of the liquid carrier, and hence splattering of the material. As a consequence, the created article is not of high quality due to inconsistency in the layers, reducing the definition that can be achieved in the finished article. As with thermal fusing techniques, optical fusing methods are also limited to depositing one layer composed of a single material at a time.

Aspects of the present invention aim to address one or more drawbacks inherent in prior art methods and apparatus for additive manufacturing.

Summary

According to a first aspect of the present invention there is provided a heating apparatus for heating material deposited in an additive manufacturing device, comprising a body comprising a plurality of grooves formed in a surface of the body, and a plurality of optical guides disposed in the plurality of grooves, each of the optical guides comprising an optical fibre and a gradient index GRIN lens arranged to receive a beam of electromagnetic radiation from the optical fibre and focus the beam of electromagnetic radiation onto the material, so as to heat the material.

In some embodiments according to the first aspect, each of the optical guides further comprises a spacer disposed between the optical fibre and the GRIN lens. The GRIN lens may, for example, be mounted in a tube configured to space the GRIN lens apart from the end of the optical fibre.

In some embodiments according to the first aspect, a first interface between the optical fibre and the spacer is configured to lie at an angle to a plane perpendicular to an optical axis of the optical guide, so as to suppress back-reflections along the optical axis.

In some embodiments according to the first aspect, a second interface between the spacer and the GRIN lens is configured to lie at an angle to a plane perpendicular to an optical axis of the optical guide, so as to suppress back-reflections along the optical axis.

In some embodiments according to the first aspect, the heating apparatus further comprises a first ferrule arranged to secure the optical fibre to the spacer at the first interface, and/or a second ferrule arranged to secure the spacer to the GRIN lens at the second interface.

In some embodiments according to the first aspect, the heating apparatus further comprises a clamping member arranged to clamp the at least one optical guide against the body so as to secure the at least one optical guide in the at least one groove. In some embodiments the clamping member comprises at least one groove facing the at least one groove of the body. The clamping member may further comprise at least one second groove formed in a surface facing away from the at least one optical guide, the at least one second groove being configured to receive at least one second optical guide. In some embodiments according to the first aspect, the heating apparatus is included in an additive manufacturing device comprising: at least one reservoir for storing a colloidal suspension of material and a liquid carrier; at least one print head comprising a plurality of nozzles in fluid communication with the reservoir, each nozzle configured to deposit a droplet of the colloidal suspension onto a substrate; a drying unit disposed adjacent the at least one print head, configured to selectively supply a first energy pulse to a deposited droplet in order to evaporate the liquid from the deposited droplet; and a melting unit disposed adjacent the drying unit, and is configured to selectively supply a second energy pulse for melting the material in a droplet dried by the drying means, wherein the drying unit and/or the melting unit comprises the heating apparatus respectively arranged to supply the first or second energy pulse from one of the at least one optical guides.

In the additive manufacturing device, the heating apparatus may comprise a plurality of grooves spaced apart by a distance equal to a pitch of the nozzles of the print head. According to a second aspect of the present invention, there is provided a method of aligning a plurality of optical guides, the method comprising: inserting a plurality of optical guides into a plurality of grooves formed in a surface of a body, each of the at least one optical guides comprising an optical fibre and a gradient index GRIN lens configured to receive a beam of electromagnetic radiation from the optical fibre and focus the beam of electromagnetic radiation onto the material, so as to heat the material; adjusting the relative positions of the optical guides along the respective grooves so as to align the optical guides to focus electromagnetic radiation onto a common focal plane; and clamping the plurality of optical guides against the body to secure the plurality of optical guides in the plurality of grooves.

In some embodiments according to the second aspect, adjusting the relative positions of the optical guides comprises: transmitting electromagnetic radiation through the plurality of optical guides and onto a surface disposed on the intended common focal plane; and adjusting the relative positions of the optical guides so that the

electromagnetic radiation emitted by each of the optical guides forms a predetermined spot size on said surface.

In some embodiments according to the second aspect, the plurality of optical guides are configured to heat material deposited in an additive manufacturing device comprising a plurality of nozzles configured to deposit droplets of a colloidal suspension, and the predetermined spot size is selected to be greater than or equal to a size of a droplet deposited by one of the plurality of nozzles.

In some embodiments according to the second aspect, the optical guide further comprises a spacer disposed between the optical fibre and the GRIN lens.

Brief Description of the Figures

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure ι shows a system diagram of an additive manufacturing device according to an embodiment of the present invention;

Figure 2 shows a print head assembly according to an embodiment of the present invention;

Figure 3 shows a cross section of a melting unit according to an embodiment of the present invention;

Figure 4 shows a perspective view of a melting unit according to an embodiment of the present invention;

Figure 5 shows a cross section of a melting unit according to an embodiment of the present invention;

Figure 6 shows a cross section of a melting unit according to an embodiment of the present invention;

Figure 7 shows an optical guide according to an embodiment of the present invention; Figure 8 is a flow chart showing a method of aligning energy pulses according to an embodiment of the present invention; and

Figure 9 illustrates apparatus for aligning the optical guides in the respective grooves, according to an embodiment of the present invention.

In the drawings, like reference numerals refer to like features throughout. Detailed Description

With reference to Figure 1, an additive manufacturing device 100 is shown that includes a print head assembly 34 for manufacturing a three-dimensional article using an additive manufacturing process. It will be appreciated that Figure 1 is not drawn to scale, and relative dimensions of certain elements in Figure 1 are exaggerated so as to more clearly illustrate the inventive concept. Design instructions for creating the article may be generated in advance and stored in computer-readable memory. The print head assembly 34 comprises a deposition unit 18, a drying unit 22, and a melting unit 30 arranged adjacent to each other. In the present embodiment the deposition unit 18, drying unit 22, and melting unit 30 are adjacent in the X direction, which is the direction of relative movement, during deposition, between the print head assembly 34 and a substrate 46 onto which material is to be deposited. In this document, movement of the print head assembly 34 from right to left in Fig. 1, relative to the substrate 46, is referred to as movement the negative X direction. The deposition unit 18 has contained therein a reservoir 16 coupled to a print head 10. The reservoir 16 is coupled to the print head 10 by a suitable conduit 14. The conduit 14 maybe flexible to permit relative movement of the reservoir 16 and print head 10, or may be rigid if the reservoir 16 and print head 10 are to remain fixed relative to one another. In the present embodiment, the conduit comprises a flexible tube 14. The flexible tube 14 extends from the reservoir 16 to a nozzle 12 disposed facing the deposition area. The print head 10 may, for example, comprise 128 or more nozzles 12 arranged linearly in the Y direction in Figure 1, where the relative direction of motion between the print head assembly 34 and substrate 46 is in the X direction, and the substrate 46 is spaced apart from the print head assembly 34 in the Z direction. In the present embodiment each nozzle 12 is separated from the nearest neighbouring nozzle 12 by a distance larger than several times the diameter of a droplet. In the present embodiment the nozzle spacing is approximately 3θθμπι, but in other embodiments a different nozzle spacing may be used. The nozzle spacing can also be referred to as the pitch.

In the embodiment shown in Figure 1, the print head 10 comprises a single row of nozzles extending in the Y direction. However, in other embodiments the print head 10 may comprise one or more additional rows of nozzles 12 spaced apart from the first row of nozzles 12 in the X direction. When a plurality of rows of nozzles 12 are provided, the rows may be staggered so that no two nozzles 12 are directly opposite each other in the X direction.

The reservoir 16 contains a colloidal suspension of a material for manufacturing an article, and a liquid carrier. For example, the liquid carrier may be water or a solvent such as alcohol. The material may be an organic material or an inorganic material. The material maybe in the form of a polymer, compound or alloy. The material may include particles of ceramic, metal, plastic, or any suitable construction material capable of being suspended in the liquid carrier and deposited in droplet form through the nozzle 12. The reservoir 16 may be a secondary reservoir connected to a larger primary reservoir disposed outside of the deposition unit 18, to receive the colloidal suspension from the primary reservoir. For example, the reservoir 16 may be connected to a primary reservoir disposed on a translatable support member, such as a rail, that moves in parallel with the deposition unit 18. The support member may be above the print head assembly 34 or to the side of the print head assembly, to enable the reservoir 16 to move synchronously with the print head assembly 34. Alternatively, a primary reservoir may remain stationary while the deposition unit 18 is moving.

Furthermore, in some embodiments a secondary reservoir within the print head may be omitted, such that the print head receives the colloidal suspension directly from an external primary reservoir. The print head 10 comprises a driving mechanism for inducing the colloidal suspension to be ejected from the nozzles 12 as droplets 36. For example, the driving mechanism maybe a piezo-electric drive or a thermal drive. Each nozzle 12 ejects a droplet 36 of the colloidal suspension onto the substrate 46. In the present embodiment, each droplet 36 is typically 30 micrometres (μιτι), but in other embodiments a different droplet size may be used. The driving mechanism may be controlled to set the diameter of each droplet according to the type of material being deposited in the droplet.

In further embodiments, the deposition unit 18 includes a plurality of print heads 10. When a plurality of print heads 10 are provided, different ones of the print heads 10 may be coupled to different reservoirs 16, or to the same reservoir. In some

embodiments the deposition unit 18 further includes a plurality of reservoirs 16, each coupled to respective ones of the plurality of print heads 10, and each containing a different material suspended in a liquid carrier. As previously explained, in some embodiments, the plurality of reservoirs 16 may be coupled to the print heads 10 without being provided in the deposition unit 18 itself. When a plurality of print heads 10 and reservoirs 16 are installed, the print head assembly 34 is capable of

manufacturing an article comprised of a plurality of materials.

Each print head 10 may further include an optical positioning mechanism 44 to detect the position of the print head 10 with respect to the print head assembly 34, enabling the print heads to be spatially aligned with respect to one another. Positioning mechanisms for aligning print heads are well-known in the art of inkjet-type devices, and a detailed description will not be repeated here. However, in brief, the optical positioning mechanisms 44 may be arranged to transmit a light beam, such as a laser beam, and detect the light beam's reflection to determine the position of the print head 10. It will be understood that this is merely an example of one such positioning mechanism, and other suitable positioning mechanisms may be used in embodiments of the invention. A kinematic stage 52, on which the deposition unit 18 is mounted, can be controlled in order to align the print heads 10 by translating each print head 10 in the X and/ or Y direction.

In the present embodiment, each drying unit 22 and melting unit 30 further comprises an optical positioning mechanism 44, and is separately and independently aligned with droplets deposited from one of the print heads 10. The drying units 22 and melting units 30 are also mounted on kinematic stages 52, by which accurate positioning can be carried out. In other words, the drying unit 22 and the melting unit 30 are calibrated to be aligned with the print heads 10, using the optical positioning mechanisms 44 and kinematic stages 52. The kinematic stages 52 are preferably 5-axis positioning mechanisms configured to control positioning in the X direction, Y direction, Z direction, as well as pitch and yaw (rotation around X and Y axes).

The substrate 46 onto which droplets are deposited may be a previous layer of the article being manufactured. In other words, where the article comprises n layers of deposited material, the substrate 46 is layer n-i. Alternatively, the substrate 46 may be a base layer not forming part of the completed article. The substrate 46 to be printed is mounted on a high stability base plate 42. In one embodiment, the base plate 42 is provided with a precision vertical movement mechanism, to allow it to traverse along the Z direction. The limit of travel of the base plate along the Z direction controls the height of the article that can be manufactured. The remaining dimensional limits are defined by a surface bounded by the extent of the nozzles 12 in the Y direction, and the limit of travel of the print head assembly 34 in the X direction.

In a further embodiment, the base plate 42 is additionally provided with a precision movement mechanism along the Y direction. In this embodiment, the printable area is limited only by the limit of travel of the base plate 42 along the Y direction. Furthermore, in some embodiments the base plate 42 can be connected to a micro- movement mechanism configured to move the base plate 42 by a distance in the Y direction that is less than the separation between the nozzles 12. This enables a higher resolution to be achieved, by enabling a droplet to be deposited at a location between two droplets previously deposited by adjacent nozzles. For example, the base plate 42 may be moved by a distance equal to half the nozzle spacing to achieve a 2x resolution increase.

The print head assembly 34 is configured to move at a near constant velocity across the printable area in the X direction. Typically, the print head assembly 34 moves with a velocity of 1 metre per second (m/s), although other velocities are also possible.

The drying unit 22 comprises a plurality of drying elements. Each drying element comprises an optical guide 50 having an energy emitting part 28, such as a focussing lens, configured to focus a pulse of electromagnetic radiation onto a deposited droplet. The optical guide 50, which in the present embodiment comprises a silica glass optical fibre, will be described in more detail later with reference to Figure 7. In other embodiments a different type of optical guide 50 may be used, for example a polymer fibre.

The energy source 24 may be configured to emit energy in various forms. For example, the energy source may be a source of electromagnetic radiation. The energy source 24 is used to dry the material in a deposited droplet 36 by evaporating the liquid carrier. For example, the energy sources 24 may be photon energy sources 24 that are configured to generate and emit pulses of incoherent or coherent electromagnetic radiation. In the present embodiment the photon energy sources 24 are high power infrared light emitting diodes (LEDs). However, in other embodiments other types of energy source may be used as described above. In some embodiments, the number of drying elements 24 is equal to the number of nozzles 12 on each corresponding print head 10. The focusing lenses 28 are positioned such that each lens 28 will be directly above a droplet 36 deposited from a

corresponding one of the nozzles 12 when the drying unit 22 is aligned with the print head 10 and the print assembly 34 moves in the X direction. In the present

embodiment, the drying unit 22 is mounted in the print head assembly 34 on a kinematic stage 52 configured to provide at least 5-axis adjustment, to allow the optical axes of drying energy pulses generated by the energy sources 24 to be aligned with the centres of the corresponding deposited droplets 36. This allows each deposited droplet 36 to be individually illuminated by a drying energy pulse generated by a corresponding one of the energy sources 24.

Each of the drying elements is individually controllable to generate an energy pulse for drying, without melting, the material deposited on the substrate. Example temporal and intensity profiles include a box function, a ramp-down function, a ramp-up function, and a comb function.

In further embodiments, the drying unit 22 may comprise a smaller number of drying elements than the number of print head nozzles 12. For example, the drying unit 22 may only comprise a single drying element, which takes the same form as previously described. In such embodiments, the drying unit 22 can be configured to be moveable in the Y direction in order to selectively illuminate each individual droplet. In other words, the drying unit 22 can be configured to raster across the substrate 46.

The diameter of each droplet immediately after being ejected from a nozzle 12, before contacting the substrate, is dictated by the nozzle geometry, the ejection process and properties of the droplet. As shown in Figure 1, the droplet spreads after landing on the substrate 46. Typically, the diameter of a droplet as-deposited on the substrate is between 1.25 and 2 times the diameter of the airborne droplet 36. In the present embodiment, each focussing lens 28 is configured such that each drying element illuminates an area that is large compared to the droplet size. This ensures that all liquid in the droplet will be heated and evaporated by the drying energy pulse. For example, the Full Width Half Maximum of the drying energy pulse, taking the form of a Gaussian beam, maybe between 1.25 and 2 times the diameter of the deposited droplet. However, in other embodiments each drying element may illuminate a smaller area, for example when the ambient temperature is sufficiently high to evaporate any residual liquid around the edge of the droplet before the droplet reaches the melting unit 30. The spot size may, for example, be elliptical or rectangular. When an elliptical spot is used, the major axis in an ellipse may be selected to be greater than or equal to a size of a droplet deposited by one of the plurality of nozzles. When a rectangular spot is used, the longer of the two axes of the rectangle may be selected to be greater than or equal to a size of a droplet deposited by one of the plurality of nozzles. The melting unit 30 comprises a plurality of melting elements. The melting elements comprise an optical guide 50 having a focusing lens 28, and an energy source 32 for melting the material after it has been dried by evaporating the liquid carrier.

Optionally, a portion of the substrate 46 beneath the dried material may also be melted so that the material is bonded to the substrate 46. This process may also be referred to as fusing, since the deposited material is fused to the substrate. To fuse the deposited material to the substrate, the substrate may be melted to a relatively shallow depth, for example about 0.1 μπι. The energy source 32 may take any of the forms as described above with reference to the drying unit 22. The energy source 32 is configured to generate energy pulses having a higher intensity than the drying energy pulses. For example, the energy source 32 maybe an ion beam source, electron beam source, or a high power photon source. In some embodiments, the energy source 32 maybe a high intensity photon energy source 32 configured to generate and emit coherent electromagnetic radiation, and may for example be a high power laser diode. Although coherent electromagnetic radiation is preferable, as previously explained, incoherent electromagnetic radiation may be generated. Alternatively, the melting unit 30 may comprise a single energy source 32 coupled to the plurality of focusing lenses 28 by a multiplexer. In the present embodiment each high intensity photon energy source 32 is coupled to the respective focusing lens 28 through an optical fibre, as described later with reference with Figure 7. The number of high intensity photon energy sources 32 is equal to the number of nozzles 12 on each corresponding print head 10. The focusing lenses 28 are positioned such that each lens 28 will be directly above a droplet 36 deposited from a correlated nozzle 12 when the melting unit 30 is aligned with the print head 10 and the print assembly 34 moves in the negative X direction.

In the present embodiment, each of the melting elements is individually controllable. The temporal and intensity profiles of the melting energy pulses, or beams, produced by the energy sources 32, are programmable to take any of the forms previously described.

As with the drying elements described above, the melting elements may be configured to provide various spot shapes and sizes in embodiments of the present invention. For example, the melting elements may be configured to provide circular, elliptical or rectangular spots. In some embodiments, the spot size may be smaller than the diameter of a droplet, meaning that the melting pulse covers only a fraction of the dried spot of deposited droplet. For example, in one embodiment the energy source 32 is a commercially available laser diode based on a strip geometry, configured to provide a rectangular spot with a width narrower than that of a single droplet. In embodiments in which the spot produced by a melting element only covers a fraction of the total area of an as-deposited droplet, multiple consecutive pulses from the melting element may be used to melt all, or substantially all, of the material in the entire droplet. This arrangement allows the use of lower-power laser diodes to melt a complete droplet.

For example, in one embodiment each melting element is used with a plurality of pulses in the same position over a deposited droplet, and is then moved to another position over the same droplet, wherein the spatial separation between the two positions is less than the width of the spot produced by the melting element. This approach provides overlap between the spots for consecutive series of pulses, ensuring no strong boundary effect, as well as averaging out the spatial non uniformity of the laser focal spot. In this way, the spot is rastered across the whole droplet to fuse all, or substantially all, of the material in the droplet. This may be referred to as a 'stitching' process. In other embodiments in which the spot size is smaller than the droplet diameter, the spot may be moved between each pulse rather than applying a plurality of pulses at the same location. In further embodiments, the melting unit 30 may comprise a smaller number of melting elements than the number of nozzles, for example only one melting element, which takes the same form as previously described. In such embodiments, the melting unit 30 is configured to move in the Y direction in order to selectively illuminate each individual droplet. In other words, the melting unit 30 is configured to raster across the substrate 46.

The melting unit 30 is mounted in the print head assembly 34 on a kinematic stage configured to provide at least 5-axis adjustment, to allow the optical axis from each high intensity energy source 32 to be aligned with the centre of the corresponding deposited droplet 36. This allows each deposited droplet 36 to be individually illuminated by a melting energy pulse (which may comprise a plurality of sub-pulses) generated by the high intensity energy sources 32, after the liquid carrier has been evaporated by the drying unit 22. As shown in Figure 1, the drying unit 22 is spaced apart from the print head 10 in the X direction. The drying unit 22 is disposed so as to trail the print head 12 in the direction of movement of the print head assembly 34 relative to the substrate 46 while material is being deposited. The melting unit 30 is also spaced apart from the drying unit 22 in the X direction, and is disposed to trail the drying unit 22 in the direction of movement of the print head assembly 34 relative to the substrate 46 while material is being deposited. The separation between the drying unit 22 and melting unit 30 may be determined in accordance with the relative velocity of the deposition unit 18 to the substrate 46 during deposition, and/or in accordance with the thermal properties of the material being deposited and the liquid carrier. The components of the print head assembly 34 are controlled by a controller 20 coupled to a user input device. The controller 20 comprises a memory for storing control instructions. The controller 20 is configured to control the position of the print head assembly 34, print head(s) 10, drying unit 22 and melting unit 30. Additionally, the controller 20 controls the spatial positions and rate at which droplets 36 are ejected from the nozzles 12 and the temporal and intensity profiles of the drying and melting energy pulses generated by the energy sources 24 and 32. Temporal and intensity profiles are a measure of energy intensity as a function of time.

In the present embodiment, the controller 20 is configured to select the temporal and intensity profiles of the drying energy pulses based on the thermal properties of the material being deposited and liquid carrier and the thermal properties of the substrate 46 on which the droplet has been deposited, so that the liquid carrier is heated to a temperature below its boiling point. This causes the evaporation of the liquid carrier without splattering. In some embodiments, the temporal and intensity profiles of the drying energy pulses can be programmed to cause flash evaporation of the liquid carrier. Flash evaporation is a process whereby a liquid is heated to a superheated state.

In addition, in the present embodiment the controller 20 is configured to select the temporal and intensity profiles of the melting energy pulses based on the thermal properties of the material being deposited and the substrate 46, so as to melt the underlying substrate 46 to a predetermined depth through the deposited material. By melting both the substrate and the newly-deposited material, the material in the droplet can be fused to the substrate, improving the structural integrity of the finished article. The melting operation may therefore also be referred to as a fusing operation. The predetermined depth may be of the order of 0.1 micrometres. By controlling the depth to which the substrate is melted during the fusing operation, the physical and mechanical properties in 3-D of the finished article can be controlled with high precision. Alternatively, as will be explained later, it is not essential to melt the underlying substrate 46. For example, it may be desired to produce a movable or flexible layer that can slide over the substrate 46, in which case the material in the newly-deposited layer can be melted without fusing the material to the underlying substrate 46. In operation, different print heads 10 are activated when the print head assembly 34 is positioned over a designated area, as determined by the controller 20 according to the required material. A plurality of print heads 10 can eject droplets 36 of colloidal suspension of different materials simultaneously, as the print heads 10 will be over different spatial positions. In subsequent passes of the print head 10, different patterns of droplets can be ejected to create complex three dimensional structures comprising multiple individual material components.

In addition, positional feedback mechanisms can be incorporated on the print head assembly 34 to allow the controller 20 to determine exactly when each print head 10 is over a designated area where the material corresponding to that particular print head 10 is to be deposited. The controller 20 may also use the positional feedback mechanism to determine when the drying unit 22 is over a deposited droplet of a particular material, and control the respective energy sources 24 to deliver the necessary energy to evaporate the liquid carrier. Similarly, the controller 20 may use the positional feedback mechanism to determine when the melting unit 30 is over a deposited droplet of a particular material and control the respective photon sources 32 to deliver at least one pulse of energy with a suitable temporal and intensity profile to melt and fuse the colloidal clusters remaining in the deposited droplet, and a thin surface layer of underlying substrate 46, after the liquid carrier has been evaporated. As described above, if the surface layer of the underlying substrate 46 needs to be melted during the fusing operation in order to fuse the deposited material to the substrate 46, the substrate may be melted to a depth of the order of 0.1 micrometres.

Referring now to Figure 2, a print head assembly 34 is illustrated according to an embodiment of the present invention. In Figure 2, a cross-sectional view of the print head in the X-Y plane is shown. In the present embodiment, the deposition unit 18 comprises a plurality of print heads 10, each coupled to a respective reservoir 16 by a respective conduit 14. Each reservoir 16 may contain a different material in colloidal suspension. The print heads 10 are positioned close to each other and are adjacent in the X direction, which is the direction of relative movement, during deposition, between the print head assembly 34 and a substrate onto which material is to be deposited. A single drying unit 22 and a single melting unit 30, each having the same number of focusing lenses 28 as the number of nozzles in each print head 10, are configured to be adjustable in the Y direction so that they can be aligned with the print heads 10. In alternative embodiments, the number of focussing lenses 28 is less than the number of nozzles 12, and the drying unit 22 and/ or melting unit 30 can traverse in the Y direction in order to sequentially illuminate individual droplets in turn.

In embodiments of the present invention, print heads 10 for depositing different materials may be provided in different spatial arrangements within the print head assembly 34. For example, the print heads may be adjacent in the X direction as shown in Figure 2, or alternatively may be adjacent in the Y direction. However, arranging the print heads 10 in a row in the X direction provides an advantage over arranging the print heads 10 in a row in the Y direction in that it avoids the whole print head assembly 34 from having to translate in the Y direction to deposit different materials on the same area. Therefore, the embodiments described with reference to Figure 2 allow different types of material to be deposited in a single pass of the print head assembly 34 over the substrate 46.

In the present embodiment, each energy pulse is aligned accurately with each droplet 36. For example, where a droplet is 30 μπι in diameter, the centre of the energy pulse should ideally be within +/- 1.5 μιτι of the centre of the droplet. Figures 1 and 2 are generalised schematics to aid understanding of the inventive concept.

In reality, each drying unit 22 and melting unit 30 may comprise several hundreds of energy sources 32 each having associated focussing lenses 28. The coupling and optical positioning of several hundreds of energy sources 32, such as high power diode lasers, in a tight volume, critically aligned with the nozzles 12 is a challenging problem that requires a complex and expensive fabricating process. In an exemplary melting unit 30, 128 nozzles 12 are arranged in a row. Prior art systems employ optical focusing and precision beam steering in an effort to solve the problem. However, this too requires a complex fabrication process. Embodiments which address these problems will now be described with reference to Figures 3 to 8, where no beam steering is needed. Figures 3 to 7 illustrate heating apparatus according to various embodiments of the present invention, which can be suitable for use as the drying unit 22 and/ or melting unit 30 in an additive manufacturing device such as the one shown in Figure 1. Figure 8 is a flowchart showing a method of aligning optical guides in the heating apparatus.

Referring to Figure 3, a cross sectional view of a melting unit 30 across the line A-A in Figure 1 is shown. Here, a plurality of grooves 304 are formed in a body 302. The body 302 may be made from any suitable material, for example, plastic or glass. Each groove 304 contains a single optical guide 50. When the optical guides 50 are inserted into the grooves 304, they are prevented from moving laterally by the walls of the groove 304 and by a clamping member 306 arranged opposite the body 302. As shown in Figure 3, in the present embodiment each groove 304 is arranged to have a "V" shape in cross-section, while the optical guides 50 are circular in cross-section. However, in other embodiments a different cross-sectional shape may be chosen for the grooves, for example a "U" shaped cross-section or a square cross-section. The distance between successive peaks of grooves 304, d, otherwise known as the pitch, is preferably the same as the pitch of the nozzles 12. In an exemplary embodiment, the pitch of the grooves is 3θθμπι and the diameter of the optical guide is 25θμπι. In other words, the pitch of the grooves 304 allows precision stacking of the array of optical guides 50 to match the pitch of the nozzles 12.

The clamping member 306 may be secured to the body 302 in various ways, for example, using adhesive or suitable mechanical fixings such as screws. The clamping member 306 shown in Figure 3 has a flat bottom surface, where the bottom surface faces the grooves 304 in the body 302. Figures 5 and 6 show alternative embodiments. In the embodiment of Figure 5, the lower surface of the clamping member 306 comprises a row of second grooves 504 that are aligned with respective first grooves 304 in the body 302, such that each optical guide 50 is trapped between the faces of one first groove 304 and the corresponding one of the second grooves 504. In the embodiment of Figure 6, a row of third grooves 604 are formed in the upper surface of the clamping member 306, where the upper surface is the surface facing away from the first grooves 304 in the body 302. Here, a second row of optical guides 50 is inserted into the row of third grooves 604 in the upper surface of the clamping member 306. A second clamping member 606 is used to secure the second row of optical guides 50 in the row of third grooves 604.

In the embodiment shown in Figure 6, the second and third grooves 504, 604 are offset from one another such that the first and second optical guides 50 are offset. In the present embodiment, the offset between adjacent second and third grooves 504, 604 is equal to half of the pitch of the row of first grooves 304 in the body 302. This arrangement can produce an array of spots with a pitch equal to d/2, where d is the spacing between adjacent optical guides 50 in the same row. Accordingly, the distance between consecutive spots can be less than the diameter of a single optical guide 50. For example, when the deposition unit 52 is configured to deposit droplets of material 36 which are separated laterally by a distance that is less than the diameter of one optical guide 50, an arrangement such as the one shown in Figure 6 can be used to produce an array of spots with a small enough pitch to be capable of selectively heating and/ or melting the material in each individual droplet.

Although in the embodiment of Figure 6 an offset of d/ 2 is used, in other embodiments a different offset may be chosen. For example, in another embodiment three rows of optical guides may be provided, with the offset between neighbouring rows of optical guides being equal to d/3. It will be appreciated that these numbers are merely examples of values that can be chosen. In other embodiments any number of rows of optical guides may be provided, with the offset between neighbouring rows of optical guides being selected to give the desired spacing between spots.

Figure 4 shows a perspective view of part of the melting unit 30 shown in Figure 3. Here, an optical guide 50 is shown being inserted into a groove 304. The optical guide 50 has a high length to diameter aspect ratio. This high aspect ratio of the optical guide 50 allows the melting unit 30 to have a relatively simple mechanical design for housing and positioning the optical guide 50 to deliver high beam pointing accuracy and precision. The optical guide 50 is a composite structure shown in more detail in Figure 7- The grooves 304 shown in Figure 4 extend the whole length of the optical guide 50.

However, in some embodiments, the body 302 may be short relative to the length of the optical guide 50. Here, the grooves 304 may support the bottom end of the optical guide 50, where the bottom end is the end closest to the droplets 36 when they are deposited. In some embodiments, where the grooves 304 support the bottom end of the optical guides 50, a second relatively short body having a second array of grooves 304 may be arranged to support the top end of the optical guides 50, where the top end of the optical guides 50 is end closest to the energy source 32.

Referring to Figure 7, each optical guide 50 comprises an optical fibre 701 coupled at one end to the high intensity energy source 32. A single high intensity energy source 32 may be associated with a respective optical guide 50, or each optical guide 50 may be coupled to the same high intensity energy source 32. The optical fibre 701 may comprise a fibre pigtail for coupling with the energy source 32, such as a diode laser energy source. At the other end, the optical fibre 701 is coupled with a spacer 702. In some embodiments, the interface between the spacer 702 and the optical fibre 701 is normal to the longitudinal axis of the optical fibre 701. However, in other

embodiments, the interface between the spacer 702 and the optical fibre 701 is an angled facet arranged to lie at an angle to a plane perpendicular to the longitudinal axis of the optical fibre 701. For example, the interface may form an angle of between about o degrees and about 8 degrees with a plane perpendicular to the longitudinal axis of the optical fibre 701. Angling the interface in this way has the effect of reducing back- reflections along the optical axis. The spacer 702 is for example a glass rod.

The spacer 702 is coupled at its other end to a focussing lens 28. In some embodiments the focussing lens 28 is a gradient index (GRIN) lens, although in other embodiments a different type of lens may be used. The GRIN lens 28 may, for example, be about 1 mm long and have a diameter of about 0.25 mm. In other words, the GRIN lens has a high length to diameter aspect ratio. The GRIN lens 28 may be mounted in a protective tube for ease of handling, to avoid damage to the GRIN lens 28 during assembly of the heating means. The GRIN lens 28 shown in Figure 7 has the same diameter as the spacer 702.

The strength of the GRIN lens 28 and its focal length are predetermined to form a desired spot size. The spot size is chosen relative to the size of the droplets deposited from the nozzles 12. A ferule, such as a glass ferule, may be provide around the circumference of the spacer 702 where the spacer 702 meets the GRIN lens 28 to increase the bonding between the GRIN lens 28 and the spacer 702. Alternatively or additionally, a ferule, such as a glass ferule, may be provided around the circumference of the spacer 702 or the optical fibre 701 where the spacer 702 meets the optical fibre 701 to increase the bonding between the optical fibre 701 and the spacer 702.

When the heating apparatus is installed in the additive manufacturing device, it is desirable for the beam of electromagnetic radiation emitted by each optical guide to be perpendicular to the substrate on which material is being deposited. This ensures that the spot formed on each deposited droplet has a symmetric energy profile. In the present embodiment, the heating apparatus is configured such that the optical axes of the optical guides will be perpendicular to the substrate on which material is deposited, when the heating apparatus is installed in the additive manufacturing device. To put it another way, once the heating apparatus is installed in the additive manufacturing device the optical axes of the optical guides are arranged to be parallel to the normal direction to the surface on which material is deposited.

In other embodiments, the heating apparatus may be configured so that the optical guides are angled with respect to the normal direction of the deposition surface, so as to form elliptical spots on the deposited droplets of material. This configuration may be particularly advantageous when the apparatus is designed to operate at high deposition speeds, such that the relative velocity between the deposition unit and the underlying substrate causes the as-deposited droplets to be elongated in the direction of the x axis in Figure 1. In such embodiments, by configuring the heating apparatus so as to form elliptical spots, the shape of the spots can be matched to the shape of the deposited droplets.

The process of assembling the melting unit 30 will now be described with reference to Figures 8 and 9. In a first step 800, optical guides 50 are inserted into grooves 304 formed in the body 302. The pitch of the grooves 304 can be predetermined and calculated based on the pitch of the nozzles 12 of the print head 10.

In step 802, the relative position of the optical guides 50 along the respective grooves is adjusted so as to align the optical guides to focus electromagnetic radiation onto a common focal plane. Figure 9 illustrates apparatus for aligning the optical guides in the respective grooves, according to an embodiment of the present invention. In this embodiment, the heating apparatus 30 is disposed opposite a plate 901 in which a plurality of openings are formed, each opening being aligned with one of the optical guides. The plate 901 is positioned at the desired focal plane. The openings maybe formed to have a diameter that is less than the Gaussian beam width of the incident laser beam. The openings maybe formed using any suitable fabrication method, for example laser or e-beam lithography, laser drilling, e-beam erosion, or ion beam milling. For each of the openings, the maximum amount of light will be transmitted through the opening when the respective optical guide is aligned so as to focus the spot on the opening.

A plurality of optical detectors 902 are disposed behind the plate 901, that is, on the opposite side of the plate 901 to the heating apparatus 30. Each optical detector 902 can be used to measure the amount of light transmitted through a respective one of the openings in the plate 901.

In step S804, it is checked whether the optical guides are correctly aligned. If further alignment is necessary, the process returns to step S802. In the present embodiment, the optical guides are correctly aligned when the maximum transmitted intensity is obtained for each channel. The absolute value measured by each optical detector 902 may vary from channel to channel, for example due to variations in the emission and detection characteristics of the light sources and the detectors 902, and variations in size/ shape among the openings in the plate 901. During steps S802 and S804, each optical guide may be gradually moved towards the optimum focus, as indicated by a progressively increasing transmitted intensity measured by the corresponding detector 902. Once a fall in transmitted intensity is observed, signalling the passing of the optimum point, the maximum signal observed on that particular channel is recorded, and the optical guide is then returned to the position at which the known maximum signal is again registered. This position is thus determined as the optimum focussing distance for that particular channel.

Once it is determined that all of the optical guides 50 are correctly aligned in their respective grooves, the optical guides 50 are clamped in position using the clamping member 306 in step 806. In this way the optical guides 50 are secured in their respective grooves and prevented from moving, retaining the alignment between the optical guides 50. This method provides a robust heating apparatus which, once assembled, can easily be handled and installed in an additive manufacturing device without disturbing the positions of the optical guides 50.

Advantageously, the compact form of the optical guide 50 permits the stacking of a large number of optical guides 50 in a small space. Additionally, the use of grooves 304 arranged in a body 302 reduces the complexity of manufacturing the melting unit 30, as aligning the optical guides 50 is made easier. The use of the GRIN lens 28 also reduces the complexity of manufacturing the melting unit 30, as the desired spot size is easily configurable by adjusting the lens strength and focal length. The arrangement allows the spot size to be adjusted using the same melting unit 30 by replacing the GRIN lens 28, or by adjusting the distance of the GRIN lens 28 from the substrate 46 onto which material is deposited. For example, the spot size can be selected to be greater than or equal to a size of a droplet deposited by one of the plurality of nozzles.

It would be readily understood by the skilled person that the optical guide 50 described with reference to the melting unit 30 could equally be applied in the same form to the drying unit 22. In other words, the heating apparatus described above with reference to Figures 3 to 8 can be used in at least one of the drying unit 22 and the melting unit 30. The process for manufacturing the drying unit 22 may also take the same form as the process for manufacturing the melting unit 30. To avoid repetition, this will not be described. Although a few exemplary embodiments have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these exemplary embodiments without departing from the principles of the invention, the range of which is defined in the appended claims.

Claims

1. A heating apparatus for heating material deposited in an additive

manufacturing device, comprising:

a body comprising a plurality of grooves formed in a surface of the body; and a plurality of optical guides disposed in the plurality of grooves, each of the optical guides comprising an optical fibre and a gradient index GRIN lens arranged to receive a beam of electromagnetic radiation from the optical fibre and focus the beam of electromagnetic radiation onto the material, so as to heat the material.

2. The heating apparatus according to claim l, wherein the at least one optical guide further comprises a spacer disposed between the optical fibre and the GRIN lens.

3. The heating apparatus according to claim 2, wherein a first interface between the optical fibre and the spacer is configured to lie at an angle to a plane perpendicular to an optical axis of the optical guide, so as to suppress back-reflections along the optical axis.

4. The heating apparatus according to claim 2 or 3, wherein a second interface between the spacer and the GRIN lens is configured to lie at an angle to a plane perpendicular to an optical axis of the optical guide, so as to suppress back-reflections along the optical axis.

5. The heating apparatus according to claim 3 or 4, further comprising a first ferrule arranged to secure the optical fibre to the spacer at the first interface, and/or a second ferrule arranged to secure the spacer to the GRIN lens at the second interface.

6. The heating apparatus according to any one of the preceding claims, further comprising a clamping member arranged to clamp the at least one optical guide against the body so as to secure the at least one optical guide in the at least one groove.

7. The heating apparatus according to claim 6, wherein the clamping member comprises at least one groove facing the at least one groove of the body.

8. The heating apparatus according to claim 6 or claim 7, wherein the clamping member comprises at least one second groove formed in a surface facing away from the at least one optical guide, and wherein the at least one second groove is configured to receive at least one second optical guide.

9. An additive manufacturing device comprising:

at least one reservoir for storing a colloidal suspension of material and a liquid carrier;

at least one print head comprising a plurality of nozzles in fluid communication with the reservoir, each nozzle configured to deposit a droplet of the colloidal suspension onto a substrate;

a drying unit disposed adjacent the at least one print head, configured to selectively supply a first energy pulse to a deposited droplet in order to evaporate the liquid from the deposited droplet; and

a melting unit disposed adjacent the drying unit, and is configured to selectively supply a second energy pulse for melting the material in a droplet dried by the drying means

wherein the drying unit and/or the melting unit comprises heating apparatus according to any one of the preceding claims, respectively arranged to supply the first or second energy pulse from one of the at least one optical guides.

10. The additive manufacturing device of claim 9, wherein the heating apparatus comprises a plurality of grooves spaced apart by a distance equal to a pitch of the nozzles of the print head.

11. A method of aligning a plurality of optical guides in a heating apparatus for heating material deposited in an additive manufacturing device, the method

comprising:

inserting a plurality of optical guides into a plurality of grooves formed in a surface of a body, each of the at least one optical guides comprising an optical fibre and a gradient index GRIN lens configured to receive a beam of electromagnetic radiation from the optical fibre and focus the beam of electromagnetic radiation onto the material, so as to heat the material;

adjusting the relative positions of the optical guides along the respective grooves so as to align the optical guides to focus electromagnetic radiation onto a common focal plane; and

clamping the plurality of optical guides against the body to secure the plurality of optical guides in the plurality of grooves.

12. The method according to claim 11, wherein adjusting the relative positions of the optical guides comprises:

transmitting electromagnetic radiation through the plurality of optical guides and onto a surface disposed on the intended common focal plane; and

adjusting the relative positions of the optical guides so that the electromagnetic radiation emitted by each of the optical guides forms a predetermined spot size on said surface.

13. The method according to claim 12, wherein the plurality of optical guides are configured to heat material deposited in an additive manufacturing device comprising a plurality of nozzles configured to deposit droplets of a colloidal suspension, and the predetermined spot size is selected to be greater than or equal to a size of a droplet deposited by one of the plurality of nozzles.

14. The method according to claim 11, 12 or 13, wherein each of the optical guides further comprises a spacer disposed between the optical fibre and the GRIN lens.