WO2021126152A1

WO2021126152A1 - Three-dimensional printed polymer objects

Info

Publication number: WO2021126152A1
Application number: PCT/US2019/066545
Authority: WO
Inventors: Krzysztof Nauka; Alvin Post
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2019-12-16
Filing date: 2019-12-16
Publication date: 2021-06-24

Abstract

A three-dimensional (3D) printed object is described. The 3D printed object comprises a polymer. A surface of the 3D printed object has a surface area roughness Sa (arithmetical mean height) of. 5.0 ƒm. A method for preparing a three dimensional (3D) printed object having a smooth surface is also described.

Description

THREE-DIMENSIONAL PRINTED POLYMER OBJECTS

BACKGROUND

[0001] Three-dimensional (3D) printing is an additive printing process, which is often used to make three-dimensional solid parts from a digital model. 3D printing is often used in rapid product prototyping, mold generation, mold master generation, and short run manufacturing. For objects made of polymers, three- dimensional (3D) polymer printing is competing with other manufacturing processes, such as injection molding. Unlike injection molding, the surfaces of parts produced by the 3D printing of polymers are rough because they are covered with partially melted particles fused to the surface.

BRIEF DESCRIPTION OF THE DRAWINGS [0002] Figure 1 is a flow chart showing a method for manufacturing a 3D printed object with a smooth surface according to an example of the present disclosure. [0003] Figure 2 is a schematic side view of an example of a 3D printed object prior to processing with a method of the present disclosure.

[0004] Figure 3 is a schematic side view of an example of a 3D printed object that has been processed with a method of the present disclosure. [0005] Figure 4 is a series of images of a surface of a 3D printed object as it is processed using an example of a method of the present disclosure.

[0006] Figure 5 is a graph showing the evolution of surface roughness as a function of UV irradiation duration for ain example of a 3D printed object containing a white colorant. [0007] Figure 6 is a graph showing the evolution of surface roughness as a function of UV irradiation duration for an example of a 3D printed object containing a black colorant.

[0008] Figure 7 is a graph showing the» evolution of surface roughness as a function of UV irradiation duration for an example of a 3D printed object containing a magenta colorant. [0009] Figure 8 is a graph showing the evolution of surface roughness as a function of UV irradiation duration for an example of a 3D printed object containing a green colorant.

[0010] The figures depict several examples of the present disclosure. However, it should be understood that the present disclosure is not limited to the examples depicted in the figures

DETAILED DESCRIPTION

[0011] As used in the present disclosure, the term “about” is used to provide flexibility to an endpoint of a numerical range. The degree of flexibility of this term can be dictated by the particular variable and is determined based on the associated description herein.

[0012] Amounts and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not just the numerical values explicitly recited as the limits of the range, but also to include individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited.

[0013] As used in the present disclosure, the term “comprises” has an open meaning, which allows other, unspecified features to be present. This term embraces, but is not limited to, the semi-closed term "consisting essentially of' and the closed term “consisting oF. Unless the context indicates otherwise, the term “comprises” may be replaced with either “consisting essentially oF or “consists of’. [0014] It is noted that, as used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

[0015] The present disclosure refers herein to a method for manufacturing a three dimensional (3D) printed object having a smooth surface and to a three- dimensional (3D) printed object.

[0016] The method for manufacturing a three dimensional (3D) printed object having a smooth surface comprises: irradiating a surface of a 3D printed object with UV light to form a melted surface, wherein the surface of the 3D printed object comprises a polymer; and solidifying the melted surface of the 3D printed object. The melted surface of the 3D printed object may be solidified to achieve a surface area roughness Sa (arithmetical mean height) of < 5.0 pm. [0017] A three-dimensional (3D) printed object may be obtained from a method in the present disclosure.

[0018] The three-dimensional (3D) printed object comprises a polymer. A surface of the 3D printed object may have a surface area roughness Sa (arithmetical mean height) of ≤ 5.0 pm. This is the final 3D printed object as described herein below.

[0019] It is to be understood that this disclosure is not limited to the 3D printed objects or methods disclosed herein, lit is also to be understood that the terminology used in this disclosure is used for describing particular examples.

The terms are not intended to be limiting because the scope of the present disclosure is intended to be limited by the appended claims and equivalents thereof.

[0020] Three-dimensional (3D) printed objects can be printed using, for example, a multi-jet fusion (MJF) process, which is a powder-based technology. 3D printed objects that are printed using an MJF process may have a relatively rough surface caused by poor surface flatness on a microscale level along with partially melted powder particles attached or adhered to the surfaces of the objects. The presence of the extra powder particles may also degrade the optical appearance of the 3D printed object because the rough surface may reflect light away from the surface.

[0021] The method is for the manufacture of a three-dimensional (3D) printed object having a smooth surface. The method involves reducing the surface roughness of a 3D printed object. The term “manufacture” in this context refers to the production of the smooth surface. This term may or may not include the production of the initial 3D printed object. Thus, the method may be a method of producing a smooth surface on a 3D printed object. [0022] The method may be a post-printing method. In the post-printing method, the 3D printed object has already been manufactured. The 3D printed object has been formed from a fused and solidified powder build material.

[0023] The methods disclosed herein provide post-print methods for treating 3D printed objects manufactured by MJF technology or by conventional 3D printing technology.

[0024] In one example, the 3D printed object is obtained from MJF.

[0025] For convenience, the 3D printed object that has initially been manufactured by 3D printing shall be referred to herein as the “initial 3D printed object”. The 3D printed object from a method of the present disclosure shall be referred to herein as the “final 3D printed object".

[0026] The 3D printed object, both the initial 3D printed object and the final 3D printed object, may comprise a polymer A surface of the 3D printed object comprises the polymer. The surface that will irradiated with UV light contains a polymer, which can melt upon irradiation with the UV light.

[0027] Most polymers employed in 3D printing exhibit relatively low absorption in the near IR and visible parts and in a limited UV range (340 nm to 400 nm) below the visible part of the electromagnetic spectrum. At higher photon energies that may correspond to a wavelength below 300 nm to 340 nm light irradiation may lead to the breakage of bonds within the polymer.

[0028] In general, the 3D printed object, including both the initial 3D printed object and the final 3D printed objection, has a body. The body of the 3D printed object may also comprise the polymer. Thus, the polymer may be distributed throughout the bulk, including both the body and the surface(s), of the 3D printed object.

[0029] The body or bulk of the 3D printed object comprises fused polymer particles.

[0030] The 3D printed object may, for example, comprise an amount of at least about 50 wt% of the polymer, such as at least about 75 wt% of the polymer, or at least about 90 wt% of the polymer. Thus, the build material used to manufacture the 3D printed object and the 3D printed object itself are primarily composed of the polymer. The 3D printed object may therefore be referred to herein as a 3D printed polymer object.

[0031] The polymer may be selected from a polyethylene (PE), a polypropylene (PP), a polybutylene terephthalate (PBT), a polyethylene terephthalate (PET), a Nylon (PA), a polyurethane (TPU), a polythalamide (TPA), a polyetherketone (PEEK), a polyphenylene sulphide (PPS) and a combination thereof.

[0032] In one example, the polymer is a crystalline form of a polyethylene (PE), a polypropylene (PP), a polybutylene terephthalate (PBT), a polyethylene terephthalate (PET), a Nylon (PA), a polyurethane (TPU), a polythalamide (TPA), a polyetherketone (PEEK), a polyphenylene sulphide (PPS) or a combination thereof.

[0033] In general, the polymer is, for example, a thermoplastic polymer.

[0034] In one example, the polymer is selected from a nylon, a thermoplastic polyurethane (TPU) and a thermoplastic polyethylene (TPE). [0035] When the polymer is a nylon, then the nylon may, for example, be selected from nylon 12 (PA12); nylon 11 (PA11 ); nylon 6,6; nylon 6 (PA6); nylon 8 (PA8); nylon 9 (PA9); nylon 612 (PA612); nylon 812 (PA812); and nylon 912 (PA912). In one example, the polymer is nylon 12.

[0036] The 3D printed object may comprise the polymer with an additive, such as glass beads or fibers.

[0037] In one example, the 3D printed object, including both the initial 3D printed object and the final 3D printed object, comprises a colorant.

[0038] The body and the surface of the 3D printed object may comprise the colorant. [0039] Generally, the colorant is distributed throughout the bulk of the 3D printed object. The colorant may be present in the build material used to manufacture the 3D printed object.

[0040] The presence of the colorant can affect the absorption of UV light at the surface of the 3D printed object. Absorption of near-UV radiation increases when a colorant, such as an organic colorant, is present in the 3D printed object. As demonstrated in Example 2 of the present disclosure, the presence of a colorant may change melting process at the surface of the 3D printed object. It should be noted that near UV absorption can be modified not only by “visible” colorants but also by colorants that are white or transparent in the visible part of the electromagnetic spectrum, but strongly absorbing in the near-UV region.

[0041] The colorant may, for example, be a metal oxide, an organic compound or a combination thereof. The combination of a metal oxide and an organic compound may be a mixture.

[0042] The metal oxide may be selected from TiO₂, ZnO and CeO₂. In one example, the metal oxide is TiO₂ or ZnO, such as TiO₂. TiO₂ and ZnO are common compounds, which are low cost, commercially available products that are qualified as being non-toxic and en vironmentally friendly. TiO₂ absorption is negligible in the visible and rapidly increases in the UV range. ZnO and CeO₂ exhibit similar absorption spectra.

[0043] The organic compound may be a colorless or colored organic compound. [0044] The organic compound, including the colorless or colored organic compound, but particularly the colored organic compound, may have an absorption value in the UV range of 300 nm to 400 nm that is at least five times higher than the maximum absorption value in the visible range of 405 nm to 720 nm. These parameters can be measured using a UV-Vis spectrophotometer using the same concentration of the organic compound for the measurements in the UV and visible ranges.

[0045] The organic compound may be selected from methyl-4-hydroxybenzoate, 2-hydroxy-4-methoxybenzophenone, salicylic acid, 1 ,4-bis(5-phenyl-2-oxazolyl) benzene. 2-(2-hydroxyphenol)-benzotriazole, 2,2'-dihydroxy-4- methoxybenzophenone, 3-hydroxyacetophene, quinine sulfate, vitamin K1 (phylloquinone), 8-anilino-1-napthalenesulfonic acid, DARI (4',6-diamidino-2- phenylindole), perylene, anthracene, 9,10-bis(phenylethynyl)anthracene, 1,4- naphthoquinone, TCNQ (7,7,8,8-tetracyanoquinodimethane), 1,4- naphthoquinone-2-sulfonic acid potassium salt, carbon black, quinacridone, phthalocyanine green G (Cu phthalo green) and a combination thereof. In one example, the organic compound is methyl-4-hydroxybenzoate, 2-hydroxy-4- methoxybenzophenone, salicylic acid. [0046] There is a large number of organic compounds with negligible absorption in the visible region of the electromagnetic spectrum and rapidly increasing absorption in the near UV, which can be incorporated into the build material. [0047] In one example, the colorant is a metal oxide or a combination of a metal oxide and an organic compound.

[0048] In another example, the colorant is an organic compound.

[0049] The method of the present disclosure involves irradiating a surface of a 3D printed object with UV light to form a melted surface.

[0050] The surface of the 3D printed object is typically a surface of the initial 3D printed object, such as an untreated or non-processed surface of the initial 3D printed object. Thus, the surface that is irradiated with UV light is a rough surface.

[0051] The surface may be an outer surface or an inner surface of the 3D printed object. The surface must, however, be accessible to UV light to perform the method.

[0052] Generally, the surface of the in itial 3D printed object has a surface area roughness Sa (arithmetical mean height) of > 5.0 pm, such as, for example, about 5.5 pm to about 15.0 pm or about 6.0 pm to about 10.0 pm. The surface area roughness Sa (arithmetical mean height) may be > 10.0 pm or > 15.0 pm. The initial 3D printed object has a rough surface, which is caused by the presence of partially fused powder particles and adhered powder particles from the build material at the surface.

[0053] The arithmetical mean height surface area roughness, Sa, as used herein may be measured in accordance with ISO 25178-2:2012 using a laser profiler. [0054] The purpose of irradiating the surface of the 3D object with UV light is to cause melting of the polymer at the surface. When the polymer is exposed to UV light, it is electronically excited and produces heat to cause melting of the surface. Once the polymer at the surface has melted, it is allowed to reflow and level off, thereby forming a smooth surface once; it has re-solidified. [0055] For the avoidance of doubt, this is not a fusing step, such as used during the 3D printing process. [0056] In general, before irradiation with UV light, the surface of the 3D printed object is a solid surface. The surface is not coated with, for example, an agent for smoothing the surface, such as a solid or a liquid agent.

[0057] The surface of the 3D printed object may be irradiated with UV light having a wavelength of from about 300 nm to about 405 nm, such as from about 340 nm to about 400 nm. In one example, the UV light has a wavelength of from about 360 nm to about 405 nm, such as about 370 nm to 400 nm.

[0058] UV light having a power of about 5 W/cm² to about 10 W/cm² may be used to irradiate the surface. In one example, the UV light has a power of about 6 W/cm² to about 10 W/cm².

[0059] The surface of the 3D printed object may be irradiated with UV light for about 0.2 to about 4.0 seconds, such as about 1.0 to 2.0 seconds.

[0060] The UV light may be provided by an array of UV LEDs. High power near- UV LEDs arrays are commercially available at low cost. The UV LEDs can be obtained in a large number of array geometries, in high emission power, and in a variety of emission wavelengths. There is a large overlap between the absorption spectra for polymers commonly used iri 3D printing, such as described above, and the emission spectra of commercially available UV LEDs. This overlap indicates that the UV emission from UV LEDs is highly effective for the heating of polymers at the surface of a 3D printed object.

[0061] In general, the surface of the 3D printed object is uniformly irradiated with the UV light. This is to ensure that the UV light, and the resulting heat that is generated, is evenly distributed over the surface.

[0062] To ensure efficient surface smoothing by UV light, every part of the irradiated surface should receive the same irradiation dose. Since 3D printed objects can have complex shape, the irradiation source may need to provide illumination conditions that are identical at each point of irradiated surface.

[0063] The UV light may be provided by a plurality of UV light sources arranged on a surface of a device, which is arranged to surround the surface of the 3D printed object.

[0064] For example, the surface of the 3D printed object may be uniformly irradiated with UV light using an array of UV LEDs arranged to surround or partially surround the surface. The UV LEDs may be arranged on a surface of, for example, an ovoid shaped cover for surrounding or partially surrounding the surface of the 3D printed object.

[0065] Uniform irradiation of the surface of the 3D printed object with UV light may also be achieved by moving the UV light source, such as the array of UV LEDs, relative to the surface of the 3D printed object. The 3D printed object may, for example, be mounted on a turntable; with the surface of the 3D printed object located under the UV light source.

[0066] Irradiation of the surface with UV light does not heat the body of the 3D printed object because the exposure time is usually very short. If a surface or the

3D printed object itself is to be irradiated with UV light multiple times, then it is unnecessary to include rest periods between successive irradiations to allow the body of the 3D printed object to cool.

[0067] The surface of 3D printed object may be irradiated with UV light to partially or completely melt the surface.. When the surface is partially melted, then parts of the surface may remain solid.

[0068] Generally, the surface is melted to cause the polymer to reflow over the surface. When the polymer reflows over the surface, it may flatten or level off. [0069] Irradiation with UV light may melt the surface of the 3D polymer object to a depth of from about 1 pm to about 100 μm, such as about 5 pm to about 50 pm or from about 10 pm to about 40 pm.

[0070] In the present disclosure, the method involves solidifying the melted surface of the 3D printed object. The final 3D printed object is then obtained. [0071] When the surface is partially melted, then the partially melted surface of the 3D printed object is solidified.

[0072] The melted surface of the 3D printed object may be solidified by allowing the surface to cool. The surface may be allowed to solidify without being disturbed.

[0073] The melted surface is solidified to achieve a flat or a smooth surface. The smooth surface may have a surface area roughness Sa (arithmetical mean height) of < 5.0 pm, such as from about 0.5 pm to about 5.0 pm or from 1.0 pm to 4.0 pm. [0074] The method of the present disclosure may include forming the 3D printed object by multi-jet fusion (MJF), before irradiating a surface of the 3D printed object with UV light.

[0075] The 3D printed object of the present disclosure, which may be obtained from the method, has a surface area roughness Sa (arithmetical mean height) of ≤ 5.0 pm, such as from about 0.5 pm to about 5.0 pm or from 1.0 pm to 4.0 pm. [0076] Turning more specifically to the figures, Figure 1 shows a flow chart with an example of the method. Figure 2 shows a schematic side view of an initial 3D printed object (102) in a first state (200) after being printed and prior to performing a method of the present disclosure.

[0077] As shown in Figure 2, the 3D printed object (102) may be formed through, for example, an MJF process iin which powder particles are fused together through application of a fusing agent and heat. Powder particles (206) at a surface (208) of the 3D printed object (102) may not have fully coalesced with the main body (204) of the 3D printed object (102). Excess powder particles (112, 202) may have also adhered or fused to the surface (208) of the 3D printed object (102). Both the partially fused powder particles (206) and the excess powder particles (112, 202) adhered to the surface (208) cause the surface (208) of the 3D printed object (102) to have a higher surface roughness than desired. [0078] Figure 3 shows a schematic side view of a final 3D printed object (102) at a second state (300) after processing using a post-print method of the present disclosure. The surface (208) of the 3D printed object (102) is irradiated with UV light (108) to decrease a surface roughness of the surface (208) in the region (302). In Figure 3, for example, the UV light (108) is used to melt a portion (302) of the surface (208). The exposure to UV light (108) causes the partially fused powder particles (206) and the excess adhered powder particles (112, 202) to melt and reflow. When the melted particles have solidified, a smoother surface is formed.

[0079] The surface (208) of the 3D printed object (102) is exposed to UV light (108) at an intensity (e.g. at an energy level, at a speed, and/or for a duration of time) that is sufficient to cause the partially fused powder particles (206) and the excess adhered powder particles (112, 202) to melt and flow without causing the surface (208) of the main body (204) of the 3D printed object (102) to melt. The excess adhered powder particles (112, 202) in region (302) will, for example, start to flow and fill surface voids and/or depressions. As a result, the surface roughness in selected region (302) of the surface (208) will be reduced. [0080] Unlike methods that remove material from a surface of a 3D printed object, such as using chemical etching to remove unwanted surface particles, the surface (208) of the 3D printed object (102) is treated by re-melting the surface (208).

[0081] Only a relatively small amount of energy from the source of UV light (108) is needed to melt the partially fused powder particles (206) and the excess adhered powder particles (112, 202). This means that the main body (204) of the 3D printed object (102) is not heated, which avoids distorting the shape and/or the dimensional profile of the 3D printed object (102).

[0082] When the main body of the 3D printed object (102) is heated, then the polymer in the main body (204) may become soft and can sag. Also, air bubbles trapped within the main body (204) may expand. These distortive effects can be avoided by the method of the present disclosure because the main body (204) is not heated.

[0083] Figure 3 shows the irradiation of a region (302) of the surface (208) with UV light (108), whereas other regions (304) have not been irradiated. It should be understood that both an irradiated region (302) and non-irradiated regions (304) are shown in Figure 3 to schematically show the difference between the surface characteristics of these regions. An entire surface of the 3D printed object (102) may, however, be irradiated with UV light (108), instead of only a region (302), to ensure that a smooth surface is obtained.

[0084] Figures 2 and 3 show the position of the 3D printed object (102) with respect to the source of UV light (108). By way of example, after irradiating the region (302) with UV light, the orientation of the 3D printed object (102) with respect to the source of UV light (108) may be changed, so that a different surface of the 3D printed object (102) may be irradiated with the UV light (108). EXAMPLES

[0085] The present disclosure will now be illustrated by the following non-limiting examples. Example 1

[0086] An object was 3D printed by MJF using a PA12 polymer powder as the build material.

[0087] A Keyence VK-X200 laser profiler was used to measure the roughness of a surface of the 3D printed polymer object when performing an example of the method disclosed herein. The images in Figure 4 were obtained from the laser profiler and show the surface roughness in terms of an arithmetical mean height as represented by S_a = - Aʃʃ AǀZ (x,y) | dxdy. This parameter is the mean of the absolute value of the height of points within the defined area.

[0088] The measurements of surface roughness, including the measurement of the surface area roughness Sa (arithmetical mean height), were performed in accordance with ISO 25178-2:2012.

[0089] Figure 4(A) shows the surface roughness of an as-printed surface of a 3D printed polymer object. This is the surface of the initial 3D printed object. It can be seen from the image that there are high frequency height variations due to individual particles or particle agglomerates fused to the surface. The surface roughness was measured, and the Sa was determined to be about 6 pm.

[0090] The surface was irradiated with UV light (395 nm) using an array of UV- LEDs (model NVSU233B, T = 25°C) mavufactured by Nichia Corp. The LEDs were operated as continuous wave devices with rapid turn on/of, which allowed the duration of emission to be precisely controlled.

[0091] As shown in Figure 4(B), in the early stage of UV irradiation the surface is heated to a point where surface particles start melting and coarsening occurs. [0092] Further UV irradiation heating melts the remaining large particles and creates a very smooth liquified surface, as shown in Figure 4(D). By stopping UV irradiation and the associated heating at this point, and allowing the melted surface to re-solidify, a 3D printed polymer object having a smooth surface. The surface roughness was meas red and the Sa was determined to be 2.8 μm. [0093] Continued heating may initiate the formation of an even thicker liquified layer, in which local instabilities may lead to formation of convection currents growing with extended irradiation. If the surface is allowed to re-solidify at this stage, then it will be rough and have a macroscopic valleys-and-hills appearance, which reflect the presences of convection currents.

Example 2

[0094] Several objects were 3D printed by MJF using a PA12 polymer powder as the build material, as in Example 1 above. In contrast to Example 1, the build material used to manufacture each object included either a white, black, magenta or green colorant mixed with the polymer powder.

[0095] The white colorant contained only a metal oxide, specifically TiO₂. The black, magenta and green colorants each contain a metal oxide (the same amount as the white colorant) and an organic compound to provide the visible color (i.e. black, magenta or, green). The organic compound in the black colorant was carbon black. The magenta colorant may have contained quinacridone as the organic compound. The green colorant may have contained Cu phthalo green. The black, magenta and green colorants are absorbing in the UV because they each contain two UV absorbers. [0096] The surface roughness of each of the as-printed 3D printed polymer objects was measured before irradiation with UV light, as in Example 1.

[0097] To determine the UV exposure conditions that should be used to obtain a smooth surface, for each colorant four sample surfaces were irradiated with UV light (395 nm) at a specific power for varying exposure durations. The results are shown in Figures 5 to 8. In the caption, “100% UV” indicates the maximum irradiation that was permitted by the UV LED source that was used. This was about 9 to 10 W/cm². The system with the UV LED source allows the precise control of the irradiation energy. The expression “90% UV” refers to 90% of the maximum value, which 90% of 9 to 10 W/cm². [0098] The difference in the results obtained for the colorants is because the amounts of UV absorbed by black, magenta or green differ from each other. [0099] For the 3D printed polymer object containing the white colorant, the results are shown in Figure 5. The initial 3D printed polymer object had an Sa of about 15 pm.

[0100] It can be seen from the results for 100% UV that moderate surface smoothing (Sa of about 10 pm) could be obtained after a short exposure duration of 1.0 seconds. Further exposure to 100% UV resulted in the surface becoming rougher for the reasons explained in Example 1 above.

[0101] An improvement in surface smoothness was observed when the surface was exposed to 70% UV for 2.0 seconds and 2.5 seconds, and also when exposed to 60% UV for 4.0 seconds. However, optimum surface smoothness (Sa of about 2 pm) was obtained when the surface was exposed to 80% UV for 2.0 seconds.

[0102] For the 3D printed polymer object containing the black colorant, the results are shown in Figure 6. The Sa of the initial 3D printed polymer object was about 21 pm.

[0103] It can be seen from the results for 100% UV that optimum surface smoothing (Sa of about 5 pm) was obtained after a 2.0 second exposure of 100% UV. Improvements in the surface smoothness were also for the other UV exposures, the improvements resulting from a balance between the UV power and the exposure duration.

[0104] The results are shown in Figure 7 for the 3D printed polymer object containing the magenta colorant. The Sa of the initial 3D printed polymer object was about 20 pm.

[0105] Optimum surface smoothing (Sa of about 5 pm) was obtained when the surface was exposed for a 2.0 duration to 80% UV. At the 2.0 second exposure duration, an improvement in the surface smoothness was also observed for 70% UV and 60% UV.

[0106] Finally, for the 3D printed polymer object containing the green colorant, the results are shown in Figure 8. The Sa of the initial 3D printed polymer object was about 15 pm.

[0107] Excellent surface smoothness (Sa of about 5 pm) was obtained after a 2.0 second exposure of 80% UV. [0108] The example containing the black colorant and the results shown in Figure 5 relate to a 3D printed object where the colorant is only a metal oxide (TiO₂). The other examples and results (see Figures 6 to 8) relate to a colorant that is a combination of a metal oxide (TiO₂) and a UV absorbing organic compound.

[0109] From these results, it can be seen that the presence of a colorant in the 3D printed polymer object can affect the UV irradiation conditions that should be used to achieve a smooth surface.

[0110] With the exception of the black colorant, optimum surface smoothness was obtained when the surface was exposed for 2.0 seconds to 80% U V.

[0111] For the black colorant, optimum surface smoothness was also obtained when the surface was exposed for 2.0 seconds. However, 100% UV had to be used to achieve this result. This may be due to the black colorant being a better UV absorber than the other colorants.

Claims

1. A three-dimensional (3D) printed object comprising a polymer, wherein a surface of the 3D printed object has a surface area roughness Sa (arithmetical mean height) of ≤ 5.0 pm.

2. The 3D printed object of claim 1 , wherein the surface of the 3D printed object comprises the polymer.

3. The 3D printed object of claim 2 comprising a colorant.

4. The 3D printed object of claim 3, wherein the colorant is a metal oxide, an organic compound or a combination thereof.

5. The 3D printed object of claim 4, wherein the colorant is a metal oxide selected from TiO₂, ZnO and CeO₂.

6. The 3D printed object of claim 4, wherein the organic compound is a colorless or colored organic compound having an absorption value in the UV range of 300 nm to 400 nm that is at least five times higher than the maximum absorption value in the visible range of 405 nm to 720 nm.

7. The 3D printed object of claim 1 , wherein the polymer is a thermoplastic polymer.

8. A method for manufacturing a three dimensional (3D) printed object having a smooth surface, the method comprising: irradiating a surface of a 3D printed object with UV light to form a melted surface, wherein the surface of the 3D printed object comprises a polymer; and solidifying the melted surface of the 3D printed object to achieve a surface area roughness Sa (arithmetical mean height) of < 5.0 pm.

9. The method of claim 8, wherein the surface of the 3D printed object is irradiated with UV light having a wavelength of from about 300 nm to about 405 nm.

10. The method of claim 8, wherein the surface of the 3D printed object is irradiated with UV light having a power of about 5 to about 10 W/cm².

11. The method of claim 8, wherein the surface of the 3D printed object is irradiated with UV light for about 0.2 to about 4.0 seconds.

12. The method of claim 8, wherein the UV light is provided by an array of UV LEDs.

13. The method of claim 8, wherein the surface of the 3D printed object is uniformly irradiated with UV light.

14. The method of claim 13, wherein the UV light is provided by a plurality of UV light sources arranged on a surface of a device arranged to surround the surface of the 3D printed object.

15. The method of claim 8, wherein the polymer is a thermoplastic polymer.