FIELD OF THE INVENTION

The invention relates to a method for manufacturing a 3D item by means of

3D printing. BACKGROUND OF THE INVENTION

Digital fabrication is expected to increasingly transform the nature of global manufacturing. One of the aspects of digital fabrication is additive manufacturing, also known as 3D printing. Many different techniques have been developed in order to produce various 3D items using various materials such as ceramics, metals and polymers.

A widely used additive manufacturing technology is a process known as fused deposition modeling (FDM) or alternatively as fused filament fabrication (FFF) or filament 3D printing (FDP). This is an additive manufacturing technology commonly used for modeling, prototyping, and production applications.

FDM is used for printing items of various shapes using various materials. An FDM printer typically has a build platform and an extrusion head, which includes a liquifier and a dispensing nozzle. In operation, the extrusion head receives 3D-printable material in the form of a filament of a thermoplastic material. Inside the liquifier the filament is heated to a flowable temperature, which typically is a temperature higher than the glass transition temperature of the thermoplastic material. The heated filament is then extruded at a desired flow rate through the nozzle, wherein the extrudate is typically referred to as a "bead". The bead is deposited along a tool path on a building surface, which can be an upper surface of the build platform or of a pre-existing deposited layer. The deposition of a bead creates a layer, and a 3D item is created by stacking multiple layers on top of each other. A controller controls processing variables such as the movement of the extrusion head in a horizontal plane, the movement of the build platform in a vertical direction, and the feeding of the filament into the extrusion head. After the printed layers have solidified the finished 3D item can be removed from the build platform. FDM is currently being further developed for the production of lighting devices, particularly LED luminaires. To enable business in this area there is a drive to increase quality, in terms of both aesthetics and optical functionality.

US-2013/0095302 discloses an additive three-dimensional fabrication process based on extrusion. By varying the deposition rate, an extrusion of a printing material can be obtained that has a surface with periodic protrusions, wherein these protrusions may be staggered from layer to layer so that corresponding ridges are oriented diagonally along a surface of a completed object.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a 3D item having a decorative surface texture on at least one of the inner and outer surfaces of a wall of the item.

According to a first aspect of the invention, the object is achieved by a method of manufacturing a 3D item by means of 3D printing using a 3D printer having a build platform and an extrusion head, wherein the extrusion head comprises a nozzle. The method comprises the steps of (i) providing a 3D-printable material to the extrusion head, (ii) extruding the 3D-printable material through the nozzle at a material flow rate, and (iv) depositing the 3D-printable material along a tool path on a substrate while moving at least one of the extrusion head and the build platform at a tool path speed, thereby forming a stack of layers that have been deposited on top of each other.

For performing the method of the invention, the material flow rate and the tool path speed have to be controllable. According to the method of the invention, the material flow rate is successively varied between a minimum tool path speed and a maximum tool path speed while the material flow rate is kept substantially constant in such a way that each layer of the stack of layers has a layer width that successively varies between a minimum layer width and a maximum layer width, wherein the ratio of the minimum layer width and the maximum layer width is 0.9 or less.

The term "3D-printable material" refers to any material that is suitable for use in a 3D printing method. The 3D-printable material can be provided to the extrusion head in the form of a filament, in the form of granular matter, in the form of a paste, or in any other suitable form. For example, the 3D-printable material may be in the form of a filament of a thermoplastic compound such as acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polyethylene terephthalate (PET), or polycarbonate (PC). For such 3D-printable materials, the extrusion head further comprises a liquifier wherein the 3D-printable material is typically heated to a temperature above the glass transition temperature of the

thermoplastic compound.

In the method of the invention, both the material flow rate and the tool path speed are controllable printing parameters. In the context of the present invention, the term "controllable" is used in relation to a printing parameter to indicate that a user has control over that printing parameter. For example, a printing parameter is controllable if it can be varied over time in a desired way and/or if it can be set to a desired value and kept constant at that value (within the error margin allowed by the equipment). Also, a printing parameter that cannot actually be varied over time in a desired way or set to a desired value by a user, but whose value can be determined (or measured) by a user is considered to be controllable in the context of the present invention.

In the method of the invention, the material flow rate is kept at a constant value (within the error margin allowed by the equipment) while the tool path speed is successively varied between a minimum value and a maximum value.

The purpose of the invention is to provide a 3D item having a decorative surface texture on at least one of the inner and outer surfaces of a wall of the 3D item, wherein dependent on the 3D-printable material the surface texture may also have an additional optical functionality.

The term "wall" refers to a part of the 3D item comprising a stack of layers that have been deposited on top of each other. The stack of layers can constitute the entire wall of the 3D item or just a part of that wall. The aforementioned surface texture is obtained in that each layer of the stack of layers has a successively varying layer width. In the context of the present invention, the term "layer width" refers to the extent of a layer in a direction perpendicular to the tool path of the layer, or in other words, in a direction parallel to the surface normal of the stack (or the wall) at the location of the layer.

In order for the successively varying layer width to have an observable effect at all, the width variation must be between a minimum layer width and a maximum layer width, wherein the ratio of the minimum layer width and the maximum layer width is 0.9 or less. For example, the maximum layer width may be twice the minimum layer width

(corresponding to a ratio of 0.5), three times the minimum layer width (corresponding to a ratio of about 0.3), or five times the minimum layer width (corresponding to a ratio of 0.2).

In the method of the invention, the smallest possible layer width and the largest possible layer width are determined by the 3D-printable material and by the nozzle of the extruder head. The smallest possible layer width is equal to the diameter of the nozzle opening, so in the method of the invention the minimum layer width is at least equal to the diameter of the nozzle opening.

The largest possible layer width depends on the geometry of the "shoulder" of the nozzle, being the surface surrounding the nozzle opening, which is typically a flat surface that can be used to flatten the 3D-printable material after extrusion. The larger this shoulder is, the larger the layer width can be.

The method of the invention can be applied with any layer height. The inventors found that, both from a functional and a decorative perspective, optimal refractive optical effects, without requiring a post-processing step, are obtained when the ratio of the layer height and the minimal layer width, which is related to the diameter of the nozzle opening, is in a range between 0.25 and 0.8. Ratios higher than 0.8 are difficult to print, while for ratios lower than 0.25 the optical effects will predominantly be diffusive effects instead of refractive effects, because at such ratios there will only be a very narrow range of angles of incidence at which a light beam will pass through a single layer.

In the method of the invention, the tool path speed is successively varied between a minimum tool path speed and a maximum tool path speed while the material flow rate is kept substantially constant. This has the advantage that while performing the method the 3D-printable material can easily be maintained at a constant temperature, which in turn results in a better control over quality of the 3D item in terms of mechanical stability and/or visual appearance.

A 3D item obtainable by the method according to the first aspect comprises a stack of layers that have been 3D-printed on top of each other. Each layer of the stack has a layer width that successively varies between a minimum layer width and a maximum layer width, wherein the ratio of the minimum layer width and the maximum layer width is 0.9 or less.

The 3D-printable material can be an opaque material, a translucent material, or a transparent material. When the 3D-printable material is a translucent or transparent material, each layer of the stack of layers comprises a translucent or transparent material, which enables the 3D item to provide a decorative light effect when the stack of layers is illuminated with visible light.

The variation in layer width may be chosen such that a surface texture is obtained wherein each layer of the stack of layers comprises an array of lenses, wherein the term "lens" refers to any optical element that is arranged to focus or disperse a light beam by means of refraction. Upon interaction with visible light such surface textures result in light refraction patterns that are considered to be appealing in decorative lighting applications.

Each lens of the array of lenses may be a biconvex or biconcave lens, and the variation in layer width may even be chosen such that each such lens approximates a spherical lens. This allows not only decorative light effects to be obtained, but also more functional optical effects.

Each layer of the stack of layers may have a layer width that varies according to a periodic function, and the periodic functions of adjacent layers in the stack may be displaced relative to each other by half a period of the periodic function.

The method of the invention allows for the formation of lens arrays in layers of a 3D-printed stack of layers, wherein each lens is a biconvex or biconcave lens. FDM inherently already results in the formation of layer stacks with a "rippled" surface pattern. However, this characteristic rippled surface pattern is only present in the direction wherein the stack has been built up, and not in a direction along the tool path.

An example of a 3D item obtainable by the method according to the first aspect is a fixture that may be comprised in a lighting device that further comprises a light source arranged to emit light, wherein the fixture covers the light source to transmit the light emitted by the light source. In order to provide an optical effect, each layer of the stack of layers comprises a translucent or transparent material.

According to a second aspect of the invention, the object is achieved by a computer program comprising instructions which, when the program is executed by a 3D printer, cause the 3D printer to carry out the steps of the method according to the first aspect of the invention.

According to a third aspect of the invention, the object is achieved by a computer-readable data carrier having stored thereon the computer program according to the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically shows a 3D printing process that can be used for performing the method of the invention;

Fig. 2 shows planar cross sections of layers that have been 3D-printed;

Fig. 3 shows planar cross sections of layers that have been 3D-printed with a method of the invention; Fig. 4 shows planar cross sections of layers that have been 3D-printed with a method of the invention;

Fig. 5 shows planar cross sections of layers that have been 3D-printed with a method of the invention;

Fig. 6 shows a 3D item that is obtainable with a method according to the invention;

Fig. 7 shows a 3D item that is obtainable with a method according to the invention, and a lighting device comprising that 3D item;

Fig. 8 shows a planar cross section of a layer that has been 3D-printed with a method of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In order to perform the method of invention, the tool path and the material flow rate can be described by generating machine code with a computer program arranged to output G-Code. For example, one can use a commercially available software tool such as Slic3r, Skeinforge or Cura to slice a CAD model into layers, and to output the G-Code required for each layer.

Figure 1 schematically shows the FDM process, which is a 3D printing process that can be used for performing the method of the invention. In this process, a filament 130 is extruded through the nozzle 120 of an extrusion head 110, at a constant material flow rate and with a constant speed along a tool path 160. The extruded material is deposited onto a layer 140 that has been previously deposited in a similar way, thereby creating a stack 150 of three layers, wherein the stack 150 forms part of a wall of a 3D item.

Figures 2(a) and 2(b) show two different planar cross sections passing through the center of layers 211 and 221, respectively. Each of these planar cross sections is that of a layer that forms part of a stack of layers, which stack has been manufactured by means of the FDM process shown in Figure 1.

The planar cross section shown in Figure 2(a) is that of layer 211 that has been deposited along a straight tool path 212. The layer width 213 is the extent of the layer 211 in a direction perpendicular to the tool path 212, or in other words parallel to the surface normal 214 of the stack that contains the layer 211.

The planar cross section shown in Figure 2(b) is those of layer 221 that has been deposited along a non- straight tool path, being curved tool path 222. Each planar cross section shown in Figure 2 belongs to a layer that has a substantially constant layer width.

Figure 3 shows two different planar cross sections passing through the center of layers 311 and 32, respectively. Each of these planar cross sections is that of a layer that forms part of a stack of layers, which stack has been manufactured by the method of the present invention, using the FDM process shown in Figure 1.

The planar cross sections shown in Figures 3(a) and 3(b) are both that of a layer that has been deposited along a straight tool path 312 and 322, respectively. The layer width is the extent of the layer in a direction perpendicular to the tool path, or in other words parallel to the surface normal (315 and 325, respectively) of the stack that contains the layer.

Each planar cross section shown in Figure 3 belongs to a layer that has a layer width that successively varies between a minimum layer width 313 and 323, respectively, and a maximum layer width 314 and 324, respectively, according to a repetitive pattern, wherein the repetitive pattern is a regular repetitive pattern. In other words, the layer width varies according to a periodic function. A periodic function is a function that repeats its values in regular intervals or periods.

Figure 4(a) shows the planar cross section passing through the center of layer 411, which is similar to layer 321 of Figure 3(b). In this cross section, the period P of the periodic function according to which the layer width varies has been indicated. A periodic function with a period P will repeat on intervals of length P.

Figure 4(b) shows the same layer 411, but now also layer 412 (indicated with dashed lines) that is present on top of layer 411, forming part of the same stack of layers. Layers 411 and 412 both have a layer width that varies according to the same periodic function with period P, wherein the periodic functions are displaced relative to each other by half a period P.

Figure 5 shows a planar cross section of a stack of layers, comprising layer 510, layer 520 that is present on top of layer 510, and layer 530 that is present on top of layer 520. Each of the layers 510, 520 and 530 has a layer width that varies according to a periodic function, wherein the periods of the periodic functions are the same. Each layer has the same minimum layer width 540, but different maximum layer widths: layer 510 has maximum layer width 511, layer 520 has maximum layer width 521 (being smaller than maximum layer width 511 of underlying layer 510), and layer 530 has maximum layer width 531 (being smaller than maximum layer width 521 of underlying layer 520). Figures 6(a) and 6(b) show 3D item 610 that has been manufactured with a method according to the invention. The 3D item 610 has a surface texture that is formed by stacks of layers, each stack comprising ten layers on top of each other (one such stack is located between dashed lines 611 and 612). Each of these stacks comprises layers that are similar as those illustrated in Figure 5.

Figures 7(a) and 7(b) show 3D item 710 that has been manufactured with a method according to the invention. The surface texture of 3D item 710 is formed by layers that have been stacked on top of each other, wherein each such layer is shaped as illustrated in Figure 3(b).

The 3D items 610 and 710 can be manufactured while using as 3D-printable material a filament comprising an opaque white material such as polyethylene terephthalate (PET) and a white diffusing additive. The 3D items 610 and 710 can also be manufactured using a transparent material as 3D-printable material, optionally with a colored and/or light- diffusive additive.

In Figure 7(b), 3D item 710 has been manufactured with a transparent material as 3D-printable material. The 3D item 710 is used as a fixture that covers light source 720 to transmit the light emitted by the light source 720, the 3D item 710 and the light source 720 being part of lighting device 700.

Alternatively, 3D items 710 can be manufactured using a translucent material, so that the amount of light that can pass through the stack of layers dependent on the layer width. The larger the layer width, the more light that is emitted by light source 720 will be blocked by the 3D item 710. This allows the creation of bright and dark areas in the 3D item 710. Such bright and dark areas can be used to make patterns, logos and other decorative effects.

In the examples described above, a 3D item is obtained that has a surface texture on both of the inner and outer surfaces of a stack of layers. It is noted that with the method of the invention, such surface textures can also be provided in only one of the inner and outer surfaces of a stack of layers. This is illustrated in Figure 8, showing a planar cross section passing through the center of layer 810. Layer 810 has a minimum layer width 811 and a maximum layer width 812. Only side 814 of layer 810 has a surface texture; the opposite side 813 being flat. This is achieved by means of the non-straight tool path 820, which is chosen to "counteract" the surface texture on side 814.

Other variations to the examples disclosed herein can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope