A method to improve crystallinity and manipulate composite infill orientation in 3D printing using in-process heating

Abstract

An additive manufacturing apparatus includes a nozzle configured to deposit a material onto a build surface, a controller programmed to move the nozzle relative to the build surface along a plurality of axes and control deposition of the material from the nozzle onto the build surface along a first track and along a second track adjacent the first track, and a laser directed at an interface between the first track and the second track and configured to heat the first and second tracks at the interface to entangle polymer chains of the first and second tracks together.

Description

Atty. Docket No. 208192-0026-W001A METHOD TO IMPROVE CRYSTALLINITY AND MANIPULATE COMPOSITE INFILL ORIENTATION IN 3D PRINTING USING IN-PROCESS HEATINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63 / 680,961 filed August 8, 2024, the entire contents of which are incorporated herein by reference.BACKGROUND

[0002] Fused filament fabrication (FFF) is a thermal plastic additive manufacturing process having the ability to fabricate complex objects with high accessibility. However, due to an extrusion track-based direct write process mechanism, parts built usingthis method exhibit anisotropic mechanical properties. The rapid cooling process in FFF leads to low crystallinity, resulting in reduced ductility and strength. Additionally, the extrusion process aligns infill materials in a single direction, which weakens mechanical strength alongthe build direction.SUMMARY

[0003] An in-process heating method has been developed to enhance crystallinity and control infill orientation in extrusion-based 3D printing. By utilizing a focused non-contact heating source, such as a near-IR laser, the cooling process can be precisely controlled. This method allows the local heating region to travel with the nozzle, ensuring that all deposited material undergoes heat treatment for a slower cooling rate. This results in higher crystallinity, improving the mechanical strength and ductility of the printed parts.

[0004] This technology also addresses the issue of infill orientation. The local heating, combined with modulation of the material flow through the nozzle, softens the lower layer and restricts the movement of the infill during the 90-degree turn at deposition. This enables the infill to cross the inter-layer interface, achieving better alignment and uniformAtty. Docket No. 208192-0026-W001 properties. The method has been successfully demonstrated with carbon fiber-infused polymers but is applicable to a wide range of materials with various infills.

[0005] The advantages of this technology are significant for both space and Earthbased applications. In-space, it reduces the need for heavy, sturdy configurations by enabling on-orbitfabrication and assembly, drastically lowering maintenance and repair launch costs. On Earth, it offers cost-effective batch production and customization, particularly beneficial for prosthetics, dental implants, and automotive and aerospace prototypes. This method fills a critical gap by providing an in-process solution for improving crystallinity and infill orientation, leadingto superior mechanical properties and durability in 3D printed parts.

[0006] As discussed herein, an in-process laser heating method is introduced to heal interface adhesion between adjacent deposited tracks of the fused filament fabrication process by increasing the interface temperature to promote polymer reptation and enhance bonding strength of the interface of adjacent tracks. With the use of laser heating induced interface healing, the measured flexural strength between adjacent tracks in the same layer increases and exceeds that of the control sample tested along the track direction. The effect of laser on interface healing is also verified by investigating the loaddisplacement curve and morphology analysis of the fractured surface.

[0007] In some aspects, the techniques described herein relate to an additive manufacturing apparatus including: a nozzle configured to deposit a material onto a build surface; a controller programmed to: move the nozzle relative to the build surface along a plurality of axes, and control deposition of the material from the nozzle onto the build surface along a first track and along a second track adjacent the first track; and a laser directed at an interface between the first track and the second track and configured to heat the first and second tracks at the interface to entangle polymer chains of the first and second tracks together.

[0008] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the second track is deposited parallel to the first track.Atty. Docket No. 208192-0026-W001

[0009] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the laser has a power level between 100-250 mW.

[0010] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the laser has a power level of 150 mW.

[0011] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the firsttrack and the second track are within a single layer such that the interface is between adjacent tracks within a single layer.

[0012] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the material is a semicrystalline polymer such that the laser is configured to increase crystallization during cooling of the material.

[0013] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the laser is rotatable relative to the nozzle via a motor and gear set.

[0014] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the laser is directed to the interface ahead of the nozzle such that the laser is configured to preheat the interface prior to deposition of the second track adjacent the first track.

[0015] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the controller is programmed to simultaneously control operation of the laser and deposition of the material from the nozzle.

[0016] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the laser is mounted to travel with the nozzle to enable in-process heating.

[0017] In some aspects, the techniques described herein relate to an additive manufacturing apparatus including: a nozzle configured to deposit a material onto a build surface; a controller programmed to: move the nozzle relative to the build surface along aAtty. Docket No. 208192-0026-W001 plurality of axes, and control deposition of the material from the nozzle onto the build surface, and a heat source directed at the deposited material or at the build surface to enhance crystallinity or manipulate infill orientation.

[0018] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the heat source is mounted to travel with the nozzle to enable in-process heating.

[0019] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the heat source is a non-contact heating method capable of heating the material above its glass transition temperature.

[0020] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the heat source if an infrared laser having a wavelength of 0.75 pm to 15 pm and a power level between 0.1 W and 50 W.

[0021] In some aspects, the techniques described herein relate to an additive manufacturing apparatus, wherein the heat source is a thermal radiation heat source or a microwave heat source.

[0022] In some aspects, the techniques described herein relate to a method of operating an additive manufacturing apparatus, the method including: depositing a material onto a build surface via a nozzle; moving the nozzle relative to the build surface to generate a first track within a layer, moving the nozzle relative to the build surface to generate a second track within the layer, the second track being adjacentto the firsttrack, and heating an interface between the first and second tracks to entangle polymer chains of the first and second tracks together.

[0023] In some aspects, the techniques described herein relate to a method, wherein heating the interface and generating the second track occur simultaneously.

[0024] In some aspects, the techniques described herein relate to a method, wherein heating the interface includes heating with a laser mounted to the nozzle.Atty. Docket No. 208192-0026-W001

[0025] In some aspects, the techniques described herein relate to a method, further including rotating the laser relative to the nozzle such that a location of the laser relative to the nozzle remains consistent with a direction of travel of the nozzle.

[0026] In some aspects, the techniques described herein relate to a method, wherein the first track is generated within a layer, and wherein the second track is generated within the layer such that the first track and the second track are generated on the same layer, and wherein the material is a semicrystalline polymer such that heating the interface between the first and second tracks includes increasing crystallization during cooling of the material.BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Fig. 1 a illustrates an orbiting laser preheating apparatus including an orbiting laser preheating apparatus.

[0028] Fig. 1 b is a schematic diagram illustrating operation of the orbiting laser preheating apparatus of Fig. 1 a.

[0029] Fig. 2a illustrates a printed sample illustrating a cutting direction of different flexural bars.

[0030] Fig. 2b illustrates a three-point flexural test of the flexural bars.

[0031] Fig. 3 illustrates a thermal profile of laser preheated spots.

[0032] Fig. 4 illustrates flexural strength of the laser-assisted and control sample flexural bars.

[0033] Fig. 5 illustrates displacement upon flexural failure or ultimate strength of the sample flexural bars.

[0034] Fig. 6 illustrates a load-displacement curve of the sample flexural bars.Atty. Docket No. 208192-0026-W001

[0035] Fig. 7a illustrates a flexural surface of the control sample at low magnification using a scanning electron microscope (SEM).

[0036] Fig. 7b illustrates a flexural surface of a 150mW laser sample at low magnification usingthe SEM.

[0037] Fig. 8a illustrates a flexural surface of the control sample at high magnification usingthe SEM.

[0038] Fig. 8b illustrates a flexural surface of the 150mW laser sample at high magnification usingthe SEM.

[0039] Fig. 8c illustrates a flexural surface of the 100mW sample at high magnification usingthe SEM.

[0040] Fig. 8d illustrates a flexural surface of the 200mW sample at high magnification usingthe SEM.

[0041] Fig. 8e illustrates a flexural surface of the 250mW sample at high magnification usingthe SEM.

[0042] Fig. 9 illustrates a differential scanning calorimetry (DSC) heat cycle for the control sample and the 150mW laser sample.DETAILED DESCRIPTION

[0043] Additive manufacturing, known as 3D printing, is gaining prominence in the small batch production market due to the elimination of tooling costs and reduced lead times. Amongthe various plastic additive manufacturingtechnologies, Fused Filament Fabrication (FFF) stands out as it uses solid continuous filament, making it suitable for low gravity environments. In recent years, there has been increasing attention on carbon fiber reinforced (or other fiber-shaped infills) polymer materials for their strength enhancement. However, FFF, being an extrusion-based process, extrudes material along one direction. The shear flow during extrusion aligns infills alongthe same direction, causing a 90-degreeAtty. Docket No. 208192-0026-W001 turn from vertical in the nozzle to horizontal upon deposition. This alignment means that carbon fiber reinforced materials are stronger in the in-plane direction where fibers align, butthis does not improve mechanical strength alongthe build direction and weaken it instead. Strength isotropy is crucial for heavy-duty applications to significantly reduce failure. Similarly, other infills like silver nanowire or piezoelectric materials are affected in the same manner.

[0044] To address the issue of infill orientation, a local heating method is developed that softens the interface, allowing infills to cross through the inter-layer interface and align vertically, leading to uniform properties. This method ensures that materials such as carbon fiber reinforced polymers have improved mechanical strength in all directions, enhancing their applicability in various demanding fields.

[0045] Additionally, the extrusion-based FFF process often results in low crystallinity due to the fast-cooling process, which in turn causes low ductility and strength in most materials, as well as diminished piezoelectric properties in smart materials. There has been no in-process method to improve crystallinity until now. We have developed a method to enhance crystallinity in semi-crystalline polymers through local heating. This approach significantly improves the ductility of printed parts, making them 3 to 4 times more ductile, which helps in identifying materials earlier before failure to avoid incidents. Many smart polymer materials, such as PVDF, are semi-crystalline, and improving their crystallinity can enhance their piezoelectric properties, which require polymer alignment

[0046] A new method is developed that uses focused non-contact heating to control the cooling process for crystallinity improvement and infill orientation manipulation in extrusion-based 3D printing. In this work, we use a near-IR laser as the heat source for the local heating. Other local heating methods such as induction heating, conduction heating, electron beam, microwave can also work. The local heating region travels with the nozzle to allow in-process local heating. This is achieved by positioning the laser source on the printhead with the ability to move the laser incident point around the nozzle whileAtty. Docket No. 208192-0026-W001 materials are being extruded and deposited. Therefore, all deposited material will be heat treated for a slower cooling rate.

[0047] For Crystallinity improvement, this method heats up the material for a slower cooling process, therefore resulting in a longer time at higher temperature for the motion of polymer chains to achieve a more stable microstructure (higher crystallinity). While normal 3D printing cools down faster, and there is not enough time and temperature for polymer chains to align (for semi-crystallization).

[0048] For infill orientation manipulation, the conjunction of local heatingwith modulating the material flow through nozzle diameter is used to simultaneously soften the lower layer as well as limit the freedom of infill during the 90-degree turn at deposition to have the infills cross the interlayer interface. The material we used for the demonstration is carbon fiber infilled polymer, but the process can be applied to other materials with different infills as well.

[0049] Additive manufacturing is a layer-by-layer fabrication process of 3D objects from a digital 3D model. Fused filament fabrication (FFF) is an additive manufacturing method used for thermal plastic materials due to its accessibility, and process and material flexibility. In the FFF process, a filament feedstock of thermoplastic polymer is forced through a heated nozzle that operates above the glass transition temperature of the feedstock material. The softened filament flows through the nozzle orifice to form a cylindrical track of reduced diameter. The cylindrical track of thermoplastic material is then deposited and bonded to a build plate surface or previously deposited material layer to form tracks, layers, and eventually 3D objects.

[0050] To control the deposition process, a slicing software is used to slice and convert the 3D model into motion control commands to control movement of the nozzle and the build plate, extrusion speed, temperature, and other process parameters for a 3D printer to operate continuously. FFF has numerous advantages, including a wide range of printable material, an ability to print multiple materials in the same build space, an abilityAtty. Docket No. 208192-0026-W001 to produce an object of a wide-dimensional range, and an ability to process biomedical materials. However, parts fabricated usin this method are anisotropic.

[0051] The anisotropic property of an FFF-built object originates from the nature of the extrusion-based process. In this process, filament is extruded from a nozzle and undergoes a diameter reduction usually from 1 .75 to 0.4 mm, while it is deposited steadily on an existing surface. The mechanics and rheological behavior of the extrudate in this process are described by the following two models: the “nozzle flow” and the “90° turn.” These models provide insight into this process with a thermomechanical explanation. As described by these two models, polymer chains are highly stretched and disentangled in deposited tracks, especially within the region in proximity of the track surface. In order for the printed part to be isotropic, full reptation and relaxation are utilized for the polymer chains at the surface of the deposited track and at the interface between tracks. Both relaxation and reptation are time- and temperature- dependent.

[0052] If enough time at sufficiently high temperature is allowed, polymer chains are capable of fully relaxing and entangling to form microstructures similar to those of parts fabricated using traditional methods such as injection molding. However, during the actual deposition process, polymer extruded from a nozzle and deposited on or next to a previously deposited track is at a lower temperature due to the time it takes for the nozzle to travel along its prescribed toolpath. The resulting lower temperatures on interfaces do not allow reptation and relaxation to fully occur. As a result, the mechanical strength of parts fabricated with FFF is anisotropic, primarily in the directions normal to track-track and layer-layer interfaces being the weakest.

[0053] Various efforts are made to improve the mechanical strength uniformity of FFF- printed parts. Efforts include improving the interlayer bonding strength by optimizing printing parameters, such as nozzle temperature, build plate temperature, print speed, layer thickness, and raster strategy. However, the range of effectiveness of optimizing printing parameters is limited due to the process physics of FFF method (e.g., stretch and disentanglement from extrusion, and low interface temperature between tracks and layersAtty. Docket No. 208192-0026-W001 for reptation and relaxation). Additional efforts include postprocesses such as annealing used to increase bonding strength. In-process approaches using laser heating and ultrasonic vibrations also achieve increased adhesion at inter-layer interfaces.

[0054] However, the efforts described above focus either on (1 ) the bonding strength at the interlayer interface or (2) mechanical strength along the in-plane direction. Work focusing on (3) the interface healing process between adjacent tracks in the same, single layer is novel and described in greater detail below. In particular, the effect of in-process laser-assisted method on healing the interface between adjacent tracks in the same layer is discussed herein. In some embodiments, the process described herein may also be utilized between tracks of two different layers. An in-process laser preheating apparatus is designed and implemented. The interfaces between adjacent tracks of control and laser- assisted samples are characterized and compared. Mechanical testing is used to investigate strength as well as flexural behavior of laser-assisted samples of different process inputs.

[0055] In a non-limiting example, a commercial 3D printer (Type A Machine Series 1 , San Francisco) is used as the platform for the laser preheating apparatus. The controller of the printer is replaced with a Duet 2 Wifi (Duet3D, United Kingdom) for laser input rotation control. A laser (808 nm) preheating apparatus 110 is implemented (Fig. 1 a). To rotate the heat source 110 (e.g., laser) about the nozzle and filament axis, a stepper motor 150 is used to drive a gear set 140, which defines the angular position of the nozzle-laser mount assembly. The stepper motor 150 is controlled as an independent axis whose position is defined in the same g-code file used for printing. A custom postprocessor is developed to add rotational commands to an existing g-code file generated by a commercial slicer software. The final g-code file contains commands that allow the laser illuminated spot to stay on the boundary of a pre-deposited adjacent track 160 and the track being deposited 130. The nozzle is movable relative to a build surface 160 such that the tracks 130, 160 are deposited onto the build surface 170. In some embodiments, the build surface 160 may be a flat build plate or an earlier layer of the deposited material. Operations of the laser 110, nozzle 120, and motor 150 as described herein are controlled by a controller 190 thatAtty. Docket No. 208192-0026-W001 is programmed to simultaneously operate and control the laser 110, nozzle 120, and motor 150.

[0056] More specifically, half of the intensity falls on the adjacent track 130 (current layer), while the other half falls on the previously deposited track 160 (one layer lower than the current layer) underneath the current track 130 (directly below the nozzle), as shown in Fig. 1 b. The shape of the laser spot from the laser 110 is rectangular with a length of 1 mm and a width of 0.5 mm (fully focused spot size with the distance between the laser source and the laser-heated spot fixed). The center of the laser spot is positioned at an intertrack interface to avoid side effects (post-heating, this effect can also be increased by increasing laser focal size) on nearby tracks.

[0057] A black polylactic acid (PLA) filament (MakerGear, Beachwood) is used as the feedstock material. The print parameters are shown in Table 1. The printed samples are multiwall rectangular blocks (as shown in Fig. 2a), with dimensions of 80 mm by 50 mm by 5 mm. The printed samples are cut into 4-mm-wide bars (along the dashed lines 210) using a diamond cutter (Preciso-CL; Top Tech Machines Co. Ltd., Taichung, Taiwan) for use as bending test specimens. The same procedure is used for samples with laser powers from 0 to 250 mW. Another control group of samples is fabricated for the flexural behavior along track direction (longitudinal direction, marked with horizontal dash lines 220; Fig. 2a) without laser.Table 1 . Print ParametersAtty. Docket No. 208192-0026-W001

[0058] An MTI-2K tensile testing machine (Measurement Technology, Inc., Marietta) and a set of customized three-point bending tooling are used for the flexural test. For each laser power setting, load-displacement data from four samples are collected. The preload for the flexural test is 30 N, and the tool displacement rate is 5 mm / min. The thickness of all flexural samples is 5 mm. The actual width of each sample is measured before each test and is used for calculation of flexural strength. The distance between the two fixed lower supports in the three-point flexural test is 15.11 mm, as shown in Fig. 2b. Due to the configuration of the bending test, the bottom of the bar experiences tensile stress, while the top side experiences compression.

[0059] Thermal images of samples during printing are collected using a Teledyne FLIR LLC a6753sc thermal camera (FLIR, Wilsonville). The highest temperature 310 detected at the laser spot is considered the temperature of the preheating spot. The heated spot temperature versus input laser power is plotted, as shown in Fig. 3. Due to the size of the thermal camera and configuration limitations of the setup, the thermal images are captured from an angle of 53° with respect to the nozzle axis (where the nozzle is orthogonal to the build plate). Therefore, the temperature profile is used only as a reference but not for calculation. An example of a thermal image taken during print is shown in Fig. 3.

[0060] A scanning electron microscope (SEM) (TESCAN for control and 150 mW laser sample, Zeiss Auriga for other samples) is used to image the fracture surface of bending samples. The bottom tracks of samples that experience tension during a bending test are the focus of the imaging. The microscopic images of two areas — interlayer position and intertrack interface — are analyzed to determine the effect of laser heating on fracture mechanism.

[0061] PLA is a semicrystalline polymer. The energy input from laser heating may result in a slower cooling rate, therefore increasing crystallization during cooling. Differential scanning calorimetry (DSC; TA Instrument, New Castle) is used to characterize the effectAtty. Docket No. 208192-0026-W001 of laser heating in crystallization duringthe cooling process by measuringthe cold crystallization enthalpy. The heating rate used is 10°C / min.

[0062] As shown in Fig.4, the flexural strength of the laser-assisted sample along with two control groups, one for control samples of flexural testing without laser predeposition heating (lower horizontal line) and the other group for strength of printed track along the track direction (upper straight line) is tested. Strength along track direction shows the strongest value obtained in the FFF process as a result of breakage of the deposited track along its length direction. In contrast, the flexural strength of the control sample (that represents adhesion between adjacent tracks) shows 80.3% of strength along track direction. With the laser preheating process, the flexural strength of 150 mW laser sample achieves 106% of that for samples along track direction.

[0063] The 6% increase can be explained by previous research that a 10% increase in mechanical strength along track direction was noted with the usage of a laser, while it was still weaker than the raw material. Hence, the flexural strength of 150 mW still lies in a reasonable range. Therefore, this process demonstrates the effect of resolving strength anisotropy by healing the interface between adjacent deposited tracks within a certain range of laser power. A decrease in flexural strength, however, is observed once laser power exceeded 150 mW. This trend can be attributed to polymer degradation, which has been seen in previous works. At the lower range, no significant effect on flexural strength was detected with the use of 100 mW laser predeposition heating. This cutoff limit may be viewed as a consequence of not reaching the threshold level required for occurrence of thermal diffusion.

[0064] The effect of laser preheating on flexural strength between adjacent deposited tracks can be explained using the following equation proposed by Ezekoye.Atty. Docket No. 208192-0026-W001

[0065] where <Jt, amaxare the strength of the interface and the strength of the bulk material, respectively, tweldis the healingtime of the interface (ortime during which the interface stays above glass transition or melting temperature), rrepis the reptation time (time needed for polymer chains to reptate as far as Rs), Dsis the center of mass diffusivity of polymer chains (a function of temperature), and Rgis the radius of gyration of polymer chains.

[0066] Both Dsand tweidincrease with increasing temperature, and the mechanical strength atis dominated by these two values. Here, the mechanical strength is only controlled by the temperature at the interface and its duration if the material (radius of gyration) is set constant. With the application of laser preheating, in effect, the interface temperature increases, allowing more mass transport to happen across the interface to heal the interface, thereby increasing mechanical strength.

[0067] In addition to the mechanical strength, the interface healing from the laser preheating process also has observable effects on maximum displacement at fracture (ductility). As shown in Fig. 5, the displacement of the sample at fracture corresponds to data points shown in Fig.4 for each sample. Alllasersamples show a similar displacement value upon fracture, and all stay higher than those of the control sample. The displacement of the 150 mW laser samples shows a slightly higher value than those of all other samples and reaches 68.8% of samples tested along the track direction. Displacement for the control sample is only 50.1 % compared with that of samples along track direction.

[0068] To better understand the mechanical behavior of each sample group before fracture, the load-displacement curves of one sample in each group (control samples 630, 150mWlaser samples 620, and samples along track direction 610) are investigated, as shown in Fig. 6. The flat region from 0 to ~0.6 mm is not counted as displacement (due to sliding). Both of the control sample and the 150 mW laser sample show stiffer behavior than the sample along track direction. The highest load and slop data for laser samples are observed at 150 mW.Atty. Docket No. 208192-0026-W001

[0069] With contined reference to Fig. 6, the curve 620 for the 150 mW laser sample is slightly steeper than the curve 630 of the control sample, though the flexural strength of the 150 mW laser sample is significantly higher. The difference is attributed to the interface healing induced by the laser predeposition heating process. To be more specific, with the laser predeposition heating process, a higher interface temperature is achieved and results in longer time for mass transfer across the interface (reptation) and relaxation. Relaxation is defined when stress in a linear polymer chain relaxes through a curvilinear diffusion path and finally confines into a tube region for reptation. Hence, the fracture mechanism of control sample contains a higher percentage of pulling disentangled polymer chains out, while in that of the 150 mW laser sample, there is a higher amount of breaking entangled polymer chains.

[0070] In the sample along the “track” direction where no laser heating process is involved, a higher fraction of polymer chains are orientated along the same direction with a lower amount of entanglement due to the nozzle flow and 90° turn. The fracture mechanism becomes elongating and breaking / pulling out polymer chains.Furthermore, necking behavior has been observed for the lower Layer during flexural test. Therefore, the lower flexural strength of sample along the track direction can be explained by a Lesser amount of entanglement and reduction of width during test.

[0071] The microimages of the fracture surfaces (between adjacent tracks) shown in Fig. 7a and 7b are taken using SEM. The flexural failure surface 710 of the control is shown in Fig. 7a and the flexural failure surface 750 of the 150 mW Laser sample is shown in Fig. 7b. The area shown in Figs. 7a and 7b represents the bottom of the respective sample during the flexural test shown in Fig. 2b. In the flexural test, the bottom of the sample experiences tension, while the top of the sample experiences compression. The SEM image focused only on the layers at the bottom since it is the Location where the fracture is initiated. The photograph that enlargesthe local region (Fig. 7a) shows the interface of adjacent tracks clearly in three Layers. Gaps 720, 730 are observed between tracks from the neighboring layers.Atty. Docket No. 208192-0026-W001

[0072] Clearly, in the 150 mW laser sample shown in Fig. 7b, no gap between tracks in the same region is observed. The gaps between tracks represent the weakest region in FFF 3D-printed parts. It is a straight region located at the intersection of two planes: interlayer interface and the interface between adjacent tracks. Fracture under load typically initiates in this region. The laser preheating process irradiated heat directly into this weakest region (edge of tracks), and therefore promotes mass transfer of polymer chains in that region and reduces the anisotropic behavior.

[0073] At a higher magnification, as shown in Figs. 8a-8e, the control sample (Fig. 8a) appears to have a smoother fracture surface 810, while that of the 150 mW laser sample (Fig. 8b) is rougher withastring-shapeand peeling-offsurfacefeature 820. The 100mW lasersample(Fig.8c) showsafracture feature 830 between that of the control and the 150 mW laser sample with very few peeling-off feature. Therefore, the fracture process between tracks in control sample exhibits less plastic deformation; while in comparison, the 150 mW laser sample shows significant string-shape material pullingout feature. This result further verifies the reptation across the interface between adjacent tracks.

[0074] The fracture surface 840 of the200 mW sample (Fig. 8d) is similarto that of the 150 mW sample. As the laser power increases to 250 mW (Fig. 8e), a decrease in tensile strength is observed (Fig. 4), the part of the fracture surface 850 (right side) is similar to the control sample, such that the amount of reptation that happened across the interface is limited. The left side of the image (Fig. 8e) shows a brittle fracture feature. This indicates that polymer degradation happens at the interface, and therefore weakens the bonding between adjacent tracks. As shown in Fig. 9, the difference between the control 910 and the 150 mW laser sample 920 in crystallization is observed from DSC heating data.

[0075] The first peak 930 and last peak 950 are for glass transition and melting, respectively. The peak 940 in the middle is for cold crystallization. The cold crystallization peak 940 of control sample 910 is slightly higherthan the 150 mW laserAtty. Docket No. 208192-0026-W001 sample 920, which means a higheramount of crystallization is allowed to happen in the control sample during the DSC heating cycle. Therefore, the crystallinity of control sample 910 is lower than that of 150 mW laser sample 920 due to the difference in cooling process during fabrication. The cold crystallization enthalpy for the control and the 150 mW laser sample is 7.94 and 5.57 J / g, respectively. Therefore, this in-process laser heating technique increases polymer crystallinity in FFF-printed parts.

[0076] The effect of laser preheating on interface healing between adjacent tracks of 3D-printed PLA using FFF is investigated, as discussed above. Flexural tests are performed on control and Laser preheating bending samples. The effects of laser preheating on flexural behavior and energy-material property interaction are observed and discussed herein. An increase in flexuralstrength between adjacent tracks up to 106% of that along track direction is found, as well as an increase in displacement upon fracture from 50.1 % to 68.8%. The effects associated with the use of laser preheating are attributed to the thermal-induced increase in polymer reptation and relaxation, hence giving rise entanglement at the interface between adjacent tracks.

[0077] This new technology significantly enhances the uniformity of properties in composites with infills and improves material crystallinity, leading to superior mechanical strength, ductility, and piezoelectric properties. Unlike traditional methods like injection molding, the technique described herein is significantly more affordable for small batch production and is especially beneficial for in-space applications. By enabling the fabrication of spare parts on-orbit, the cost of launches for maintenance or repair purposes can be drastically reduced. Additionally, this technology supports on-orbit joining and assembly, eliminating the need for spacecraft to be assembled and folded on Earth in heavy, sturdy configurations to withstand the extreme acceleration and vibration during launch. The lower gravity in space also means that the strength requirements are less stringent compared to on-Earth applications.

[0078] For Earth-based applications, this technique offers significant advantages in fields requiring customization or cost-effective batch production. It is particularly useful inAtty. Docket No. 208192-0026-W001 the production of prostheses, dental fillings and implants, and prototypes for the automotive and aerospace industries where high strength and quality are paramount. The ability to manipulate infill orientation and enhance crystallinity in materials means that parts produced using this technology can achieve superior mechanical properties and durability.

[0079] This innovation fills a critical gap in the market by providing an in-process method for crystallinity improvement and infill orientation manipulation in 3D printing. By usingfocused non-contact heating to control the cooling process, the method significantly improves the ductility and piezoelectric properties of smart materials compared to conventional Fused Filament Fabrication (FFF) 3D printing. This makes it ideal for batch production applications where cost reduction and high-quality material properties are essential. Applications include the production of prosthetics, prototypes for the automotive, and aerospace parts, which demand exceptional ductility and strength.

Claims

Atty. Docket No. 208192-0026-W001CLAIMSWhat is claimed is:1 . An additive manufacturing apparatus comprising: a nozzle configured to deposit a material onto a build surface; a controller programmed to: move the nozzle relative to the build surface along a plurality of axes, and control deposition of the material from the nozzle onto the build surface along a first track and along a second track adjacent the first track; and a laser directed at an interface between the first track and the second track and configured to heat the first and second tracks at the interface to entangle polymer chains of the first and second tracks together.

2. The additive manufacturing apparatus of claim 1 , wherein the second track is deposited parallel to the first track.

3. The additive manufacturing apparatus of claim 1 , wherein the laser has a power level between 100-250 mW.

4. The additive manufacturing apparatus of claim 1 , wherein the laser has a power level of 150 mW.

5. The additive manufacturing apparatus of claim 1 , wherein the first track and the second track are within a single layer such that the interface is between adjacent tracks within a single layer.

6. The additive manufacturing apparatus of claim 1 , wherein the material is a semicrystalline polymer such that the laser is configured to increase crystallization during cooling of the material.Atty. Docket No. 208192-0026-W0017. The additive manufacturing apparatus of claim 1 , wherein the laser is rotatable relative to the nozzle via a motor and gear set.

8. The additive manufacturing apparatus of claim 1 , wherein the laser is directed to the interface ahead of the nozzle such that the laser is configured to preheat the interface prior to deposition of the second track adjacent the first track.

9. The additive manufacturing apparatus of claim 1 , wherein the controller is programmed to simultaneously control operation of the laser and deposition of the material from the nozzle.

10. The additive manufacturing apparatus of claim 1, wherein the laser is mounted to travel with the nozzle to enable in-process heating.

11. An additive manufacturing apparatus comprising: a nozzle configured to deposit a material onto a build surface; a controller programmed to: move the nozzle relative to the build surface along a plurality of axes, and control deposition of the material from the nozzle onto the build surface, and a heat source directed at the deposited material or at the build surface to enhance crystallinity or manipulate infill orientation.

12. The additive manufacturing apparatus of claim 11 , wherein the heat source is mounted to travel with the nozzle to enable in-process heating.

13. The additive manufacturing apparatus of claim 11 , wherein the heat source is a noncontact heating method capable of heating the material above its glass transition temperature.

14. The additive manufacturing apparatus of claim 11 , wherein the heat source if anAtty. Docket No. 208192-0026-W001 infrared Laser having a wavelength of 0.75 m to 15 pm and a power level between 0.1 Wand 50 W.

15. The additive manufacturing apparatus of claim 11 , wherein the heat source is a thermal radiation heat source or a microwave heat source.

16. A method of operatingan additive manufacturingapparatus, the method comprising: depositing a material onto a build surface via a nozzle; moving the nozzle relative to the build surface to generate a first track, moving the nozzle relative to the build surface to generate a second track, the second track being adjacent to the first track, and heating an interface between the first and second tracks to entangle polymer chains of the first and second tracks together.

17. The method of claim 16, wherein heating the interface and generating the second track occur simultaneously.

18. The method of claim 16, wherein heating the interface includes heating with a laser mounted to the nozzle.

19. The method of claim 18, further comprising rotating the laser relative to the nozzle such that a location of the laser relative to the nozzle remains consistent with a direction of travel of the nozzle.

20. The method of claim 16, wherein the first track is generated within a layer, and wherein the second track is generated within the layer such that the first track and the second track are generated on the same layer, and wherein the material is a semicrystalline polymer such that heating the interface between the first and second tracks includes increasing crystallization during cooling of the material.

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