BACKGROUND

[0001] Additive manufacturing devices produce three-dimensional (3D) objects by building up layers of material. Some additive manufacturing devices may be referred to as "3D printing devices" because they use inkjet or other printing technology to apply some of the manufacturing materials. 3D printing devices and other additive manufacturing devices make it possible to convert a computer-aided design (CAD) model or other digital representation of an object directly into the physical object.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] The accompanying drawings illustrate various examples of the principles described herein and are part of the specification. The illustrated examples are given merely for illustration, and do not limit the scope of the claims.

[0003] Fig. 1 is a block diagram of a post processing system for an additively manufactured polymer object, according to an example of the principles described herein.

[0004] Fig. 2 is a flow chart of a method for post processing an additively manufactured polymer object, according to an example of the principles described herein.

[0005] Fig. 3 is a diagram of a thin-walled teeth aligner subjected to the method of Fig. 2, according to an example of the principles described herein. [0006] Fig. 4 is a cross-sectional diagram of the thin-walled teeth aligner of Fig. 3, according to an example of the principles described herein.

[0007] Fig. 5 is a flow chart of a method for post processing an additively manufactured polymer object, according to an example of the principles described herein.

[0008] Fig. 6 is a diagram of a post processing system for an additively manufactured polymer object, according to an example of the principles described herein.

[0009] Fig. 7 depicts a non-transitory machine-readable storage medium for post processing an additively manufactured polymer object, according to an example of the principles described herein.

[0010] Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. The figures are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings provide examples and/or implementations consistent with the description; however, the description is not limited to the examples and/or implementations provided in the drawings.

DETAILED DESCRIPTION

[0011] Additive manufacturing devices form a three-dimensional (3D) object through the solidification of layers of a build material. Additive manufacturing devices make objects based on data in a 3D model of the object generated, for example, with a computer-aided drafting (CAD) computer program product. The model data is processed into slices, each slice defining portions of a layer of build material that is to be solidified.

[0012] In one example, to form the 3D printed object, a build material, which may be powder, is deposited on a bed. A fusing agent is then deposited onto portions of the layer of build material that are to be fused to form a layer of the 3D printed object. The system that carries out this type of additive manufacturing may be referred to as a powder and fusing agent-based system. The fusing agent increases the energy absorption of the layer of build material on which the agent is deposited. The build material is then exposed to energy such as electromagnetic radiation. The electromagnetic radiation may include infrared light, laser light, or other suitable electromagnetic radiation. Due to the increased heat absorption properties imparted by the fusing agent, those portions of the build material that have the fusing agent deposited thereon heat to a temperature greater than the fusing temperature for the build material. By comparison, the applied energy is not so great so as to increase the heat of the portions of the build material that are free of the fusing agent to this fusing temperature. This process is repeated in a layer-wise fashion to generate a 3D object. The unfused portions of material can then be separated from the fused portions, and the unfused portions recycled for subsequent 3D formation operations.

[0013] Another way to form 3D printed objects is via fused deposition modelling (FDM). In FDM, a filament of thermoplastic material is moved through a heated extrusion head which melts the thermoplastic material and deposits it on a bed or a previously deposited layer. A controller moves the extrusion head in a two-dimensional pattern to form a layer of the 3D object. The extrusion head may then be raised, or the bed lowered, to deposit another layer of the heated thermoplastic material to form another slice of the 3D object.

[0014] In yet another example, a laser, or other power source is selectively aimed at a powder build material, or a layer of a powder build material, to form a slice of a 3D printed object. Such a process may be referred to as selective laser sintering.

[0015] In another example of additive manufacturing referred to as laser fusion, an array of lasers scans each layer of powdered build material to form a slice of a 3D printed object. In this example, each laser beam is turned on and off dynamically during the scanning process according to the image slice. Similar to a fusing agent-based system, this laser fusion process also forms an object in a layer-by-layer fashion.

[0016] Additive manufacturing has become a respected manufacturing technology for its simplicity, efficacy, and the quality of printed products. In particular, a fusing-agent based system may be able to print a product ten times faster than other additive manufacturing technologies. While such additive manufacturing operations have greatly expanded manufacturing and development possibilities, further developments may make 3D printing a part of even more industries.

[0017] For example, fusing-agent based additive manufacturing systems melt plastic particles to form the object. In some cases, un-melted powder particles from the powder bed in which the object is formed may embed themselves in the surface of the fused object increasing the surface roughness of the printed polymer object. These un-melted powder particles, which may have nominal diameters of 100 /im, may increase the surface roughness value, Ra, in amounts of 26 /im or more. While this may be suitable for some products, for other products such as certain health care products like teeth aligners, an Ra value of less than 5 /im may be desirable to meet comfort demands of users. Other products may have similar surface roughness targets which some additive manufacturing processes may not be able to comply with. [0018] Some attempts have been made to increase the smoothness, i.e., reduce the surface roughness, of additively manufactured polymer objects using a variety of methods including chemical processing where a chemical solvent is used to smooth the surface and physical processing such as tumbling and polishing. However, existing methods may alter the geometrical and/or aesthetic features of the polymer object. That is, the existing methods for smoothing a polymer object may alter the geometry and/or color of the additively manufactured polymer object. Such methods are particularly ill-suited for thinwalled objects that are more susceptible to geometric deformation. Moreover, mechanical polishing and tumbling may take hours of manual labor and may therefore be prohibitively inefficient and may not even achieve the desired target smoothness, especially for objects having complex geometries.

[0019] Accordingly, the present specification describes systems, methods, and computer program products, that resolve these and other issues. Specifically, the present specification describes a system and method for processing a 3D printed object (which may be a thin-walled object such as a teeth aligner) to provide a smooth surface (lower surface roughness). In some examples, the 3D printed object may be translucent. The methods, systems, and computer program products of the present specification maintain the clarity /translucency of the 3D printed objects and in some examples even increase the clarity/translucency.

[0020] According to the present specification, the surface of a 3D printed object, which may either be thin-walled or thick-walled, is reflowed without deforming the geometry of the 3D printed object. Doing so results in a low Ra value, in some examples less than 5 /im. This is accomplished by pre-cooling the 3D printed object to below room temperature. A short blast of heat is applied to the 3D printed object to increase the temperature of just the surface of the 3D printed object to be above the melting temperature of the polymer powder while the interior portion of the 3D printed object remains below the melting temperature. In this example, the unfused build material particles on the surface melt and form part of the object, thus reducing the surface roughness of the 3D printed object. That is, the present system raises a temperature of a surface of the 3D printed object. Because the 3D printed object is pre-cooled, the interior portions of the 3D printed object do not raise in temperature as quickly as the surface portions due to the time it takes the heat to penetrate to the interior portions. As a result, the heating is controlled such that the surface rises above the melting temperature of the polymer, but the interior portion does not rise above the melting temperature of the polymer. As the interior portion of the polymer object does not melt, the polymer object does not geometrically deform. Accordingly, smoothness is increased without sacrificing geometrical or aesthetic properties of the 3D printed object. Additional iterations of heating and cooling may be performed as desired to achieve a target surface roughness.

[0021 ] As a particular example, a fusing-agent based printer fuses each layer of a 3D object. In one particular example, the fusing agent is clear and enables printing transparent (clear) and translucent parts (without any color tints). While particular reference is made to post processing transparent and translucent objects, the post processing methods and systems described herein may similarly be applied to opaque polymer objects. The post processing system then pre-cools the 3D printed object to below room temperature and applies a short blast of heat to the 3D printed object. Multiple iterations of heating and cooling may be performed to reduce thermal conduction throughout the 3D printed object, which thermal conduction may give rise to geometric deformities. That is, the 3D printed object may be thermally cycled, which enables the majority of the 3D printed object to stay cool. As there is a time constant to heat conduction, the post processing system re-melts the surface of the 3D printed object, without re-melting the entire 3D printed object and deforming the geometry.

[0022] Specifically, the present specification describes a post processing system. The post processing system includes a cooling system to cool the polymer object and a heating device to raise the temperature of the polymer object. The post processing system also includes a controller. The controller determines parameters for heating and cooling. The controller also controls operation of the cooling system to pre-cool the polymer object. The controller also controls operation of the heating device to raise the temperature of the polymer object such that a surface of the polymer object rises above the melting temperature of the polymer while an interior portion of the polymer object remains below the melting temperature of the polymer. The controller also controls operation of the cooling system to, following heating, cool the polymer object such that the surface of the polymer object falls below the recrystallization temperature of the polymer.

[0023] The present specification also describes a method. According to the method, a polymer object is pre-cooled. Particles embedded in a surface of the polymer object are melted by heating the polymer object such that a temperature of the surface rises above a melting temperature of the polymer while an interior portion of the polymer object remains below the melting temperature of the polymer. Following melting of the embedded particles, the polymer object is cooled such that the surface falls below the recrystallization temperature of the polymer.

[0024] The present specification also describes a non-transitory machine- readable storage medium encoded with instructions executable by a processor of a controller. The machine-readable storage medium includes instructions to, when executed by the processor, cause the processor to determine 2) characteristics associated with a polymer object to undergo post-processing and 2) heating and cooling parameters for post processing the polymer object based on determined characteristics. The machine-readable storage medium also includes instructions to, when executed by the processor, cause the processor to melt particles embedded in a surface of the polymer object. This may be accomplished by 1) controlling operation of a cooling system of a postprocessing system based on determined cooling parameters to pre-cool the polymer-based object, 2) controlling operation of a heating device of the postprocessing system based on determined heating parameters such that a depth of a surface that is a fraction of a diameter of a polymer particle is heated past a melting temperature for the polymer, and 3) controlling operation of the cooling system based on determined cooling parameters to cool the polymer-based object following heating.

[0025] Such systems and methods 1) reduce the surface roughness for certain additively manufactured 3D printed objects; 2) maintain the geometrical features of the treated 3D printed object; 3) do not alter the coloration of the 3D printed object; and 4) are particularly suited for thin-walled 3D printed objects. However, it is contemplated that the systems and methods disclosed herein may address other matters and deficiencies in a number of technical areas. [0026] As used in the present specification and in the appended claims, the surface roughness measurements are represented as Ra, which is calculated as the arithmetic average of surface heights measured across a surface. The unit of measurement of Ra in the present specification is micrometers. For a two-dimensional surface, Ra = 1/n * SUM(ABS[Zi-Zmean] from i = 1 to n.

[0027] Further, as used in the present specification and in the appended claims, the term “controller” may refer to an electronic component which may include a processor and memory. The processor may include the hardware architecture to retrieve executable code from the memory and execute the executable code. As specific examples, the controller as described herein may include computer readable storage medium, computer readable storage medium and a processor, an application specific integrated circuit (ASIC), a semiconductor-based microprocessor, a central processing unit (CPU), and a field-programmable gate array (FPGA), and/or other hardware device.

[0028] The memory may include a computer-readable storage medium, which computer-readable storage medium may contain, or store computer usable program code for use by or in connection with an instruction execution system, apparatus, or device. The memory may take many types of memory including volatile and non-volatile memory. For example, the memory may include Random Access Memory (RAM), Read Only Memory (ROM), optical memory disks, and magnetic disks, among others. The executable code may, when executed by the controller cause the controller to implement at least the functionality of post processing a 3D polymer object to reduce object surface roughness as described below.

[0029] Turning now to the figures, Fig. 1 is a block diagram of a post processing system (100) for an additively manufactured polymer object (108), according to an example of the principles described herein. As described above, the post processing system (100) is operable on various types of additively manufactured polymer objects (108) and melts the particles that are embedded in the surface of the polymer object (108) such that they smoothen out and become part of the polymer object (108).

[0030] As a particular example, the polymer object (108) may be a thinwalled object that is also translucent. As used in the present specification and in the appended claims, the term “thin-walled” refers to a polymer object (108) that is between 0.1 millimeters (mm) and 5 mm thick. While particular reference is made to thin-walled translucent polymer objects (108), the post processing system (100) may operate on any type of polymer object (108), i.e. , those polymer objects (108) that are not thin-walled and that are not translucent.

[0031] Reducing the surface roughness resulting from embedded particles is accomplished by pre-cooling the polymer object (108) such that the subsequent heating of the polymer object (108) does not cause the interior portions of the polymer object (108) to rise above the melting temperature of the polymer. Were the interior portions allowed to rise above the melting temperature, the body or matrix of the polymer object (108) may become more fluid and may alter the geometric properties of the polymer object (108). The pre-cooling however, reduces the initial temperature of the interior portions of the polymer object (108) such that the surface of the polymer object (108) where the embedded particles reside may rise above the melting temperature while other portions do not.

[0032] The post processing system (100) may include a cooling system (102) to 1 ) pre-cool the polymer object (108) prior to heating and 2) cool the polymer object (108) in anticipation of a subsequent heating cycle. That is, the post processing system (100) may subject the polymer object (108) to one or multiple cycles of cooling and heating. Determining whether or not to subject the polymer object (108) to multiple cycles is based on a variety of factors. A specific example is presented below in connection with Fig. 5.

[0033] The cooling system (102) may include multiple cooling devices. For example, a first cooling device may be used to pre-cool the polymer object (108) while a second cooling device may be used to cool the polymer object (108) following heating. This second cooling device may be different from the first cooling device and may cool the polymer object (108) to a different temperature. For example, a liquid nitrogen cooling device may cool the polymer object (108) to -196 degrees Celsius, a frozen water-based cooling device may cool the polymer object (108) to 0 degrees Celsius, and a freezer may cool the polymer object (108) to -18 degrees Celsius. In an example where the cooling devices of the cooling system (102) are different from one another, a liquid nitrogen-based cooling device may pre-cool the polymer object (108) to - 196 degrees Celsius. Following heating, the second cooling device may cool the polymer object (108) to room temperature using a room temperature convective cooling device such as a fan.

[0034] An example of a multi-device cooling system (102) is depicted in Fig. 6, where the polymer object (108) is moved along a conveyance system. In other examples, the cooling system (102) includes a single cooling device that performs both the pre-cooling and the cooling following heating. [0035] The cooling devices of the cooling system (102) may take a variety of forms. For example, the cooling devices may be convective cooling devices that expose the polymer object (108) to a cooling fluid. In this example, the cooling devices use convective heat transfer by exposing the polymer object (108) to the cooling fluid such as, for example, the air in a freezer or liquid nitrogen. As specific examples, the polymer object (108) may be placed in a freezer or a bath of liquid nitrogen.

[0036] In another example, the cooling system (102) may include conductive cooling devices that include heat sinks to contact the polymer object (108) surface. As an example, a heat sink such as copper may contact the polymer object (108) to cool the polymer object (108). In another example, the polymer object (108) may be filled with a freezing media such as ice. In this example, the freezing media may maintain the shape of the polymer object (108) during heating. In other examples, the polymer object (108) may be placed in shaved ice, filled with a cold moldable putty, or be placed in contact with a Peltier cooler. As with ice, the putty (or any material with a high heat capacity) is used to fill the negative space within the polymer object (108) to 1) draw heat from the areas that do not require it, and 2) maintain the structural integrity of the polymer object (108). While particular reference is made to certain types of cooling systems (102), other types of cooling systems (102) may be implemented in accordance with the principles described herein.

[0037] As described above, the cooling system (102) may alter the thermal profile such that as the polymer object (108) is heated, a temperature gradient forms across a thickness of the polymer object (108) due to a heat transfer time constant. As such, when heat is applied, the surface of the polymer object (108) may raise to the melting temperature, but due to the heat transfer time constant, the interior portion of the polymer object (108) does not reach the melting temperature before heat is removed.

[0038] In some examples, the cooling system (102) may cool the polymer object, either during a pre-cooling stage or during a cooling stage that follows heating, at a rate of between 80 degrees Celsius per second (C/s) and 800 C/s. While particular reference has been made to particular cooling temperatures and rates, those temperatures and rates actually implemented may be selected based on thermal calculations of the material of the polymer object (108).

[0039] The post processing system (100) also includes a heating device (104) to raise the temperature of the polymer object (108). Specifically, the heating device (104) may operate to raise the temperature of the polymer object (108) such that a surface of the polymer object (108) is greater than the melting temperature of the polymer while the interior portions of the polymer object (108) do not rise above the melting temperature of the polymer.

[0040] The temperature to which the surface of the polymer object (108) is raised varies depending upon a number of criteria. For example, the temperature to which the surface is raised may depend on the build material and may range between 120 and 400 degrees Celsius. As a particular example, if the build material is nylon 12, which has a melting temperature of 187 degrees Celsius, the heating device (104) may be activated for a period of time such that the surface of the polymer object (108) rises above 187 degrees Celsius, while the interior portion of the polymer object (108) does not. As another example, if the material is polypropylene (PP) with a melting temperature of 160 degrees Celsius, the heating device (104) may be activated for a period of time such that the surface of the polymer object (108) rises above 160 degrees Celsius, while the interior portion of the polymer object (108) does not. As yet another example, if the material is polyamide 11 (PA11 ) with a melting temperature of 200 degrees Celsius, the heating device (104) may be activated for a period of time such that the surface of the polymer object (108) rises above 200 degrees Celsius, while the interior portion of the polymer object (108) does not. Similar to the cooling system (102), the heating device (104) may raise the temperature to the polymer object (108) at a rate of between 80 C/s and 800 C/s and those temperatures and rates actually implemented may be selected based on thermal calculations of the material of the polymer object (108).

[0041] The heating device (104) may take a variety of forms. For example, the heating device (104) may include one or multiple tungsten lamps, light emitting diodes (LEDs), tungsten halogen fusing lamps, heat flames, infrared heating devices, and an oven. While particular reference is made to particular heating devices (104), other types of heating devices (104) may be implemented to raise the temperature of the polymer object (108).

[0042] The post processing system (100) may include a controller (106) to carry out a variety of functions. For example, the controller (106) may determine the parameters for the heating and cooling. As described above, there are a variety of parameters that may be adjusted to effectuate a particular heating and cooling of the polymer object (108). Examples of parameters that may be determined include a pre-cooling temperature, a heating temperature, a heating rate, a cooling temperature, a cooling rate, or combinations thereof. As described above, different materials have different melting temperatures and recrystallization temperatures. If the heating temperature is too great, or the pre-cooling temperature and/or cooling temperature is not low enough, more than the surface of the polymer object (108) may rise above the melting temperature. In so doing, the geometrical accuracy of the polymer object (108) may be compromised. Moreover, if the polymer object (108) is heated too quickly or cooled too quickly, cracks may initiate in the polymer object (108) which may negatively impact the strength of the polymer object (108). As such, these rates as well may be controlled to ensure proper smoothing of the surface of the polymer object (108).

[0043] These parameters may be determined based on any number of characteristics. For example, the parameters (cooling temperatures/rates, heating temperatures/rates) may be determined based on the polymer of the object, a thickness of the polymer object, and a depth of the surface that is to be elevated above the melting temperature of the polymer. For example, different polymers have different characteristics such as thermal conductivity. Accordingly, the melting temperature around which the polymer object (108) is heated and cooled may be based on the melting temperature of a particular polymer.

[0044] Moreover, the overall thickness of the polymer object (108) impacts the thermal conductivity of heat through the polymer object (108). For example, it may take longer for heat to propagate through a thicker polymer object (108). Accordingly, for a polymer object (108) that is thicker, heating over a longer period of time may be desired. Similarly, a depth of the surface, that is a depth to which the polymer material is to be re-melted, affects the rate of heat transfer. Accordingly, if a greater surface depth is desired to be re-melted, a lower heat may be applied for a longer period of time.

[0045] The depth of the surface that is heated may be dependent upon the average diameter of the polymer particles. For example, if a polymer has particles with average diameters of 100 /im, the depth to which the polymer object (108) is to be re-melted may be a fraction of this amount. For example, the depth may be 50 /im.

[0046] Other examples of characteristics upon which the parameters are determined include the object complexity. Another example includes the manufacturing characteristics. That is, polymer objects (108) made by different additive manufacturing processes may respond differently to thermal cycles. Accordingly, the additive manufacturing process used may determine how the polymer object (108) is to be heated and cooled.

[0047] Other examples include the heating device (104) characteristics and the cooling system (102) characteristics. For example, liquid nitrogen may have a cooler initial temperature such that the cooling rate may be less than a cooling system (102) that relies on cold air to drop the temperature of the polymer object (108). Similarly, different heating devices (104) may have different characteristics that affect the heat transfer process. Accordingly, these characteristics may be accounted for by the controller (106) in determining how the polymer object (108) is to be heated and cooled.

[0048] In some examples, the controller (106) may determine the parameters based on user input. For example, a user may input the parameters through a user interface of the post processing system (100). In another example, the user may input characteristics of the polymer object (108) and/or the manufacturing characteristics of the polymer object (108). In this example, the controller (106) may determine the parameters based on the input characteristics. [0049] In another example, the controller (106) may determine the parameters based on an object file. For example, the object file may include metadata describing the polymer object (108). In this example, the controller (106) may determine the parameters based on the metadata included in the object file.

[0050] With the parameters received or determined, the controller (106) may control operation of the cooling system (102) and the heating device (104). First, the controller (106) controls the cooling system (102) to pre-cool the polymer object (108). The controller (106) then controls the heating device (104) to raise the temperature of the polymer object (108) such that a surface of the polymer object (108) rises above the melting temperature of the polymer while an interior portion of the polymer object (108) remains below the melting temperature. The controller (106) then operates the cooling system (102) to, following heating, cool the polymer object (108) such that the surface of the polymer object (108) falls below the recrystallization temperature of the polymer. [0051 ] As described above, the amount of time and settings for the heating device (104) and cooling system (102) to heat and cool as described above, may be calculated or determined based on the parameters described. For example, through experimentation a user may determine the temperature and duration for heating the polymer object (108) to re-melt the polymer object (108) such that the surface re-melts but the interior portion remains below the melting temperature for the polymer. These values may be input to the post processing system (100) such that the controller (106) may activate the cooling system (102) devices and heating device (104) accordingly.

[0052] As described above, the post processing described herein may be particularly suited to thin-walled geometries. For example, thin-walled geometries have a large surface area to volume ratio, which results in a quicker thermal conduction through the polymer object (108). Accordingly, by precooling the polymer object (108) prior to heating, the geometric deformation and discoloration that may otherwise beset a thin-walled polymer object (108) may be avoided. That being said, the post processing described herein may work on non-thin-walled geometries. [0053] Fig. 2 is a flow chart of a method (200) for post processing an additively manufactured polymer object (108), according to an example of the principles described herein. Implementing the method (200), the post processing system (100) re-flows the surface of a 3D printed polymer object (108) without deforming the geometry of the polymer object (108). This is accomplished by pre-cooling the polymer object (108) and then flash heating the surface of the polymer object (108) in one or multiple cycles.

[0054] Accordingly, the method (200) includes pre-cooling (block 201 ) the polymer object (108). As described above, the applications in which some polymer objects (108) may be used may dictate a particular surface roughness that may be unattainable by some additive manufacturing processes alone. Accordingly, such a polymer object (108) may be selected for post processing. The polymer object (108) may be pre-cooled (block 201 ) any number of ways including placing the polymer object (108) in a freezer or dipping the polymer object (108) in a liquid nitrogen bath. While particular reference is made to particular mechanisms for pre-cooling (block 201 ) the polymer object (108), the polymer object (108) may be pre-cooled (block 201 ) in any number of ways. [0055] Following pre-cooling (block 201), the post processing system (100) may melt (block 202) particles embedded in a surface of the polymer object (108). That is, the present method (200) targets a fractional depth, i.e., the surface, of the polymer object (108) for re-melting. This fractional depth may be defined based on the existing surface roughness or the size of the particles that make up the polymer object (108). For example, if the polymer object (108) is made up of particles having a nominal diameter of 100 /im, the depth to which the polymer object (108) is re-melted may be a fractional amount of this nominal diameter. In another example, the depth may be dependent upon an existing surface roughness. For example, if an initial surface roughness of a polymer object (108) is 26 /im, and a target surface roughness is 5 /im, the depth may be more than if the initial surface roughness measurement was 12 /im and the target surface roughness is 5 /im.

[0056] Accordingly, the controller (106) may activate the heating device (104) such that a temperature of the surface of the polymer object (108) rises above a melting temperature of the polymer while an interior portion of the polymer object (108) remains below the melting temperature of the polymer. As described the parameters, i.e., temperature and duration of heating to effectuate such a temperature rise in the polymer object (108) may be calculated by the controller (106) based on the aforementioned criteria or parameters input by a user.

[0057] As a particular example, the heating device (104) may heat the polymer object (108) to a temperature of between 120 and 400 C. While particular reference is made to a particular target temperature, the heating device (104) may be activated to heat the polymer object (108) to any variety of target temperatures. In an example, the heating device (104) heats the polymer object (108) to the target temperature in 0.5 to 10 seconds. As described above, cracking may initiate if the polymer object (108) is heated too quickly. [0058] Following heating, the polymer object (108) is cooled (block 203) such that the surface of the polymer object (108) falls below the recrystallization temperature of the polymer. Note that the recrystallization temperature of a polymer is different than the melting temperature of the polymer. For example, a melting temperature of polypropylene may be 160 degrees Celsius whereas a recrystallization temperature for the polypropylene may be 120 degrees Celsius. Accordingly, the polymer object (108) is cooled (block 203) to fall below this recrystallization temperature so that the melted particles recrystallize to form part of the hardened polymer object (108).

[0059] Such cooling (block 203) may accommodate another cycle of heating the polymer object (108). For example, it may be the case that heating of the polymer object (108) to sufficiently melt the embedded particles is not feasible in a single cycle. For example, a polymer object (108) may be sufficiently thin that a temperature differential where the surface is above the melting temperature long enough for full surface melt of the polymer particles to occur and the interior portion is below the melting temperature is unachievable. In this example, multiple iterations of raising the temperature by a certain amount and cooling the polymer object (108) in cycles may be implemented. That is, surface roughness change may be reduced per cycle, with the target surface roughness reached after a few cycles. In this example, the parameters, i.e. , heating rate, heating temperature, etc., may vary between iterations.

[0060] In any event, cooling (block 203) the polymer object (108) prepares the polymer object (108) to be heated again. That is, the cooling (block 203) operation sets the initial temperature of the polymer object (108) at a level where action of the heating device (104) will raise the temperature of the surface as desired, i.e., above the melting temperature of the polymer, while maintaining the temperature at an interior portion below the melting temperature by leveraging the thermal conduction of the polymer material. Again, to prevent the formation of thermal stress induced cracks, the polymer object (108) may be cooled to the target temperature (i.e., below the recrystallization temperature) in between 0.5 to 10 seconds.

[0061] Fig. 3 is a diagram of a thin-walled teeth aligner (310) subjected to the method (200) of Fig. 1 , according to an example of the principles described herein. As described above, the polymer object (108) may be formed by selectively hardening powdered build material in particular patterns. In some examples, this may be done in a layer-wise fashion, wherein individual slices of the polymer object (108) are formed. The post processing operations described herein may be particularly suited to thin-walled translucent polymer objects (108) such as the teeth aligner (310) depicted in Fig. 3.

[0062] As described above, there are material characteristics of powder polymer material that may cause additively manufactured objects to be rough and non-translucent parts. For example, the powder polymer material may be crystalline where the crystals scatter light, preventing clarity. As another example, powder may become embedded in the polymer object (108) surface. Thin-walled geometries may not be robust against mechanical smoothing operations. For example, if a thin-walled object with a high surface area-to- volume ratio were heated without pre-cooling, the entire body of the polymer object (108) may rise to a temperature sufficient to re-melt and deform the geometry. However, with thin-walled polymer objects (108) such as the teeth aligner (310) depicted in Fig. 3, a pre-cooling post processing system (100) enables smooth, translucent, and thin-walled polymer objects (108) which have been additively manufactured.

[0063] Fig. 4 is a cross-sectional diagram of a thin-walled teeth aligner (310) operated upon by the post processing system (100) of Fig. 1 , according to an example of the principles described herein. Specifically, Fig. 4 depicts a zoomed-in view of the surface of the teeth aligner (310) before post processing and after. Fig. 4 depicts the particles (412) embedded on the surface. As depicted in Fig. 4, the particles (412) melt to form part of the surface of the teeth aligner (310). Note that in Fig. 4, the particles (412) that are embedded in the surface and that later melt and harden to form part of the surface are not drawn to scale. Rather, these components are enlarged to highlight the operation of the post processing system (100).

[0064] A test was performed validating the post processing system (100) described herein. In the test, an additive manufacturing system was used that employed two sets of 365 /im ultraviolet (UV) fusing lamps mounted on the left- (leading) and right-hand (trailing) side of a printing carriage. A clear UV fusing agent was used to fuse the powder of each layer of a part to create a translucent 3-dimensional teeth aligner (310) without any color tints. Specific examples of clear UV fusing agents include triazine, benzotriazole, benzophenone, and ecamsule. Different translucent polymer teeth aligners (310) were then subjected to different post processing treatments. A first was sandblasted, a second was tumbled for 8 hours using a straight cut cylindrical ceramic media, and the third was processed with the post processing system (100) and method (200) described herein. The first sample (sandblasted) had a surface roughness Ra value of 26 /im. The second sample (tumbled) had an Ra value of 18/im while the third sample (post processed as described herein) had an Ra value of 3 /im. Accordingly, the test validated that the post processing method (200) described herein resulted in a thin-walled polymer object (108) that maintained desired geometrical and translucency properties and had a surface roughness that met a 5 /im threshold.

[0065] While this particular test was conducted on a UV-based additive manufacturing process, the systems (100) and methods (200) described herein may be implemented for all thermoplastic parts including those made with infrared (IR) or any other powder-based polymer additive manufacturing technology.

[0066] Fig. 5 is a flow chart of a method (500) for post processing an additively manufactured polymer object (108), according to an example of the principles described herein. In the example depicted Fig. 5, the method (500) includes iteratively heating and cooling the polymer object (108) until the surface roughness is less than a target value.

[0067] In some examples, there may be an upper limit on the surface roughness reduction that can be achieved in a single cycle of heating. For example, a highly conductive material may inhibit achieving a surface temperature that is different than the internal temperature for a period of time to complete re-melting of the surface particles. In another example, the polymer material may be particularly sensitive to deformation. Accordingly, rather than melting the entire surface in one iteration, the post processing system (100) melts a portion of the desired thickness of the surface over each of multiple iterations until the desired surface roughness is met.

[0068] Accordingly, the method (500) includes pre-cooling (block 501 ) the polymer object (108), melting (block 502) particles embedded in a surface of the polymer object (108), and cooling (block 503) the polymer object (108) as described above in connection with Fig. 2. In this example, the method (500) includes determining (block 504) if the target surface roughness is reached. If so, (block 504, determination YES), the method (500) ends. If not (block 504, determination NO), the post processing system (100) thermally cycles the polymer object (108) by again melting (block 502) the particles and cooling (block 503) the polymer object (108).

[0069] Fig. 6 is a diagram of a post processing system (100) for an additively manufactured polymer object (108), according to an example of the principles described herein. As described above, the polymer object (108) may be pre-cooled within the cooling system (102). In the example depicted in Fig.

6, the cooling system (102) includes a liquid nitrogen bath (614) that the polymer object (108) is dipped into. In Fig. 6, a pre-cooled polymer object (108) is depicted in dashed fill.

[0070] In some examples, the post processing operations may be automated. That is, the post processing system (100) may include a conveyor system (616) to transport the polymer object (108) between the cooling system (102) and the heating device (104). While Fig. 6 depicts particular transport devices of the post processing system (100) other transport devices may be implemented in accordance with the principles described herein. In this example, the polymer object (108) is hung in a basket (618) and transported through a heating device (104) which may be an oven. As described above, once through the oven, the polymer object (108) remains translucent and the embedded particles have been re-melted to form part of the surface of the polymer object (108).

[0071] Fig. 7 depicts a non-transitory machine-readable storage medium (720) for post processing an additively manufactured polymer object (108), according to an example of the principles described herein. To achieve its desired functionality, the post processing system (100) includes various hardware components. Specifically, the post processing system (100) includes a processor and a machine-readable storage medium (720). The machine- readable storage medium (720) is communicatively coupled to the processor. The machine-readable storage medium (720) includes a number of instructions (722, 724, 726, 728) for performing a designated function. The machine-readable storage medium (720) causes the processor to execute the designated function of the instructions (722, 724, 726, 728).

[0072] Referring to Fig. 7, determine properties instructions (722), when executed by the processor, cause the processor to determine characteristics associated with a polymer object (108). Determine parameters instructions (724), when executed by the processor, may cause the processor to, determine, based on determined characteristics, heating and cooling parameters for post processing the polymer object (108). Control cooling instructions (726), when executed by the processor, may cause the processor to control operation of a cooling system (102) of the post processing system (100) based on determined cooling parameters to pre-cool the polymer object (108). The control cooling instructions (726), when executed by the processor, may also cause the processor to control operation of a cooling system (102) of the post processing system (100) based on determined cooling parameters to cool the polymer object (108) following heating. Control heating instructions (728), when executed by the processor, may cause the processor to control operation of a heating device (104) of the post processing system (100) based on determined heating parameters such that a depth of a surface that is a fraction of a diameter of a polymer particle is heated past a melting temperature for the polymer.

[0073] Such systems and methods 1) reduce the surface roughness for certain additively manufactured 3D printed objects; 2) maintain the geometrical features of the treated 3D printed object; 3) do not alter the coloration of the 3D printed object; and 4) are particularly suited for thin-walled 3D printed objects. However, it is contemplated that the systems and methods disclosed herein may address other matters and deficiencies in a number of technical areas.