An ultrasonic printhead and design method

Abstract

The present application relates to the technical field of 3D printing, and particularly relates to an ultrasonic printing head and a design method. The printing head comprises, in sequence along a longitudinal direction, an ultrasonic transducer, a variable amplitude rod, a heat conduction rod and a nozzle; the resonant frequency corresponding to the first-order longitudinal mode of the variable amplitude rod at normal temperature is higher than the calibration frequency of the ultrasonic transducer, and the resonant frequency corresponding to the first-order longitudinal mode of the heat conduction rod at normal temperature is higher than the resonant frequency of the variable amplitude rod; the variable amplitude rod is provided with a transverse inlet for the wire material to enter, and the variable amplitude rod, the heat conduction rod and the nozzle are all provided with longitudinal through holes for the wire material to pass through; the heat conduction rod is provided with a heating ring outside, the heating ring is used for preheating the wire material passing through the heat conduction rod, and the wire material is subjected to longitudinal ultrasonic action under the nozzle to compensate for the heat gap in printing. The present application effectively excites mechanical vibration, cavitation, acoustic flow and thermal effect, thereby improving the interlayer bonding, reducing the porosity and residual stress.

Description

Technical Field

[0001] This invention relates to the field of 3D printing technology, and more particularly to an ultrasonic printhead and its design method. Background Technology

[0002] In 3D printing technology, fused deposition modeling (FDM) is a commonly used additive manufacturing method. It involves heating and melting thermoplastic filaments and extruding and depositing them layer by layer to form three-dimensional parts. This technology has broad application prospects in aerospace, automotive, wind power and other fields, and is particularly suitable for manufacturing continuous fiber reinforced thermoplastic composite components.

[0003] Currently, in the field of continuous fiber reinforced composite (FDM), some research is attempting to introduce ultrasonic fields to improve print quality. One approach uses an integrated printhead, with an ultrasonic transducer mounted obliquely on the printhead via an L-shaped amplitude transformer. One end of the amplitude transformer is connected to the ultrasonic transducer, and the other end extends to the molten pool area near the nozzle. During operation, the ultrasonic transducer generates high-frequency vibrations, which are transmitted to the molten pool and forming area via the L-shaped amplitude transformer. The aim is to reduce resin viscosity and promote fiber impregnation through ultrasonic vibration.

[0004] However, the L-shaped amplitude transformer has a non-centrally symmetrical design with a smaller top and a larger bottom. This design results in a large lateral vibration component during ultrasonic vibration transmission, with only a portion of the energy acting on the printed part in a longitudinal manner. Furthermore, the oblique mounting of the amplitude transformer makes the entire printhead bulky and lacking in rigidity, causing some of the ultrasonic amplitude to be absorbed by the structural deformation of the printhead itself. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a printhead that can improve the ultrasonic energy transmission efficiency, so as to solve the problems of poor ultrasonic vibration directionality and large energy dissipation in the existing technology.

[0006] To achieve the above objectives, the embodiments of the present invention provide the following technical solutions: In a first aspect, embodiments of the present invention provide an ultrasonic printhead, comprising an ultrasonic transducer, an amplitude transformer, a heat-conducting rod, and a nozzle connected sequentially along the longitudinal direction; the resonant frequency of the first longitudinal mode of the amplitude transformer at room temperature is higher than the calibrated frequency of the ultrasonic transducer, and the resonant frequency of the first longitudinal mode of the heat-conducting rod at room temperature is higher than the resonant frequency of the amplitude transformer; the amplitude transformer has a transverse inlet for filament to enter, and longitudinal through holes for filament to pass through are provided in the amplitude transformer, the heat-conducting rod, and the nozzle; a heating coil is fitted over the heat-conducting rod, the heating coil being used to preheat the filament passing through the heat-conducting rod, and the filament being subjected to longitudinal ultrasonic action under the nozzle to compensate for the heat gap during printing.

[0007] As a further technical solution, an installation assembly is also included, which includes a connecting back plate and a fixing sleeve. The connecting back plate is used to install the entire printhead onto the printer. The fixing sleeve is installed on one side of the connecting back plate. The ultrasonic transducer is installed in the fixing sleeve. The printhead also includes an ultrasonic generator, which is connected to the ultrasonic transducer.

[0008] As a further technical solution, the side of the connecting back plate away from the ultrasonic transducer has a mounting plate, and a cooling fan is mounted on the mounting plate, with the cooling fan facing the amplitude transformer.

[0009] As a further technical solution, a flange is provided at the node position of the ultrasonic transducer, and the flange is installed inside the fixed sleeve.

[0010] As a further technical solution, the bottom of the ultrasonic transducer and the top of the amplitude transformer are connected by a first bolt, and the bottom of the amplitude transformer is connected to the top of the heat-conducting rod by a second bolt, wherein the second bolt is a hollow bolt.

[0011] As a further technical solution, the heat-conducting rod has a flange protruding outward on its lower side, the heating ring is mounted on the flange, the bottom of the nozzle has a planar boss protruding downward, and the intersection of the planar boss and the longitudinal through hole inside the nozzle is provided with a rounded corner.

[0012] Secondly, embodiments of the present invention also provide a method for designing an ultrasonic printhead as described above, comprising: Set printing parameters; Determine the invariable size based on the wire diameter, determine whether there is a heat gap, and if so, deduce the required longitudinal amplitude at the nozzle plane boss, obtain the amplitude threshold for exciting the beneficial effect of ultrasound, and select the larger of the two as the minimum ultrasound amplitude. The axial length is initially determined, and multiple dimensional parameters are set as design variables and given initial values. Finite element analysis is performed on the print head under heating state to solve the contact surface temperature, longitudinal modal frequency, stress distribution and nozzle amplitude. If the deviation of each calculation result from the target value is within the preset range, the variation range of each design variable is determined as the set percentage range of the initial value. A multi-objective optimization model is established with the objectives of maximizing nozzle end face amplitude, minimizing frequency deviation, minimizing contact surface temperature, maximizing modal interval, and maximizing amplitude consistency, and constraints are set. Multiple geometric schemes are generated within the range of variation. The finite element method is used to solve and filter the schemes. The range of variation is iteratively narrowed until the quality of the schemes no longer improves. The schemes that satisfy the constraints are then output.

[0013] As a further technical solution, when deriving the required longitudinal amplitude at the nozzle plane boss, the calculation is based on the relationship between the viscoelastic heat generation per unit volume of wire under a single vibration and the longitudinal amplitude. The relationship is as follows: ; in, The viscoelastic heat generated per unit volume of filament. denoted as the loss modulus of the filament, A1 as the longitudinal amplitude, and h as the layer height.

[0014] As a further technical solution, the design variables include: the length of the contact surface between the amplitude transformer and the ultrasonic transducer in the X-axis direction, the diameter of the heat-conducting rod body, the total length of the amplitude transformer in the Z-axis direction, the length of the large end of the amplitude transformer in the Z-axis direction, the length of the small end of the amplitude transformer in the Z-axis direction, the total length of the heat-conducting rod in the Z-axis direction, the width of the large end of the amplitude transformer in the Y-axis direction, and the width of the small end of the amplitude transformer in the Y-axis direction.

[0015] As a further technical solution, the constraints include: the deviation between the longitudinal modal resonant frequency and the ultrasonic transducer calibration frequency under heating conditions does not exceed a set percentage of the ultrasonic transducer calibration frequency; the frequency interval between the longitudinal modal resonant frequency and the upper and lower order noise frequencies under heating conditions is greater than a set threshold; the temperature of the contact surface between the print head and the ultrasonic transducer is less than the maximum allowable operating temperature of the ultrasonic transducer; the maximum stress of the print head is less than the high-cycle fatigue strength of the metal; and the amplitude at the nozzle plane boss is greater than the minimum ultrasonic amplitude.

[0016] One or more technical solutions provided in the embodiments of the present invention have at least the following technical effects or advantages: In this invention, the ultrasonic transducer, amplitude transformer, heat-conducting rod, and nozzle are connected longitudinally to form a coaxial vibration transmission path. This eliminates the problems of lateral vibration components and insufficient stiffness caused by the oblique installation of the L-shaped amplitude transformer, ensuring pure axial transmission of ultrasonic energy. The room-temperature resonant frequencies of the amplitude transformer and heat-conducting rod are higher than the calibration frequency of the ultrasonic transducer and the resonant frequency of the amplitude transformer, respectively. This frequency gradient design can offset frequency drift under high-temperature conditions, avoid sudden amplitude drops, and solve the problem of temperature rise matching. The amplitude transformer has a lateral inlet, and each component has a longitudinal through-hole. The filament passes through longitudinally and is preheated by the heating coil, but the preheating does not melt the resin. The longitudinal ultrasonic waves below the nozzle act directly on the forming area, using viscoelastic heat generation to compensate for the heat gap during high-speed printing. Compared with existing solutions that confine ultrasonic waves to the molten pool area, this solution allows ultrasonic waves to act normally on the surface of the printed part, effectively stimulating mechanical vibration, cavitation, acoustic flow, and thermal effects, thereby improving interlayer bonding, reducing porosity, and reducing residual stress.

[0017] Advantages of additional aspects of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In addition, the dimensions or spacing between the components are exaggerated to show the position of each component, and the schematic diagrams are for illustrative purposes only.

[0019] Figure 1 This is a schematic diagram of the vibration direction of three different vibration modes; Figure 2 This is a schematic diagram of the overall printhead provided in an embodiment of the present invention; Figure 3 This is a partial cross-sectional view of the printhead provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the nozzle provided in an embodiment of the present invention; Figure 5 This invention provides a simulation cloud map of ultrasonic vibration morphology. Figure 6 This is a flowchart of the printhead design provided in an embodiment of the present invention; Figure 7 This is a schematic diagram of the optimized parameters provided in the embodiments of the present invention; In the diagram: 1. Connecting backplate; 2. Ultrasonic transducer; 3. Fixing sleeve; 4. Amplifier rod; 5. Heating coil; 6. Heat-conducting rod; 7. Mounting plate; 8. Nozzle; 81. Rounded corner; 82. Flat boss; 9. Cooling fan; 10. Wire; 11. Ultrasonic generator; 12. First bolt; 13. Second bolt; Detailed Implementation To more clearly illustrate the technical solutions of the embodiments in this specification, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are merely some examples or embodiments of this specification. For those skilled in the art, these drawings can be applied to other similar scenarios without creative effort. Unless obvious from the linguistic context or otherwise specified, the same reference numerals in the drawings represent the same structures or operations.

[0020] Generally speaking, the terms "comprising" and "including" only indicate that the steps and elements are explicitly identified, and these steps and elements do not constitute an exclusive list. The method or apparatus may also include other steps or elements.

[0021] Example 1 Continuous fiber reinforced composites are widely used in aerospace, automotive, and wind power industries. However, traditional molding processes are heavily reliant on molds and struggle to form complex structures, hindering the rapid iteration of customized components. Fiber deriving (FDM) technology, with its advantages of mold-free manufacturing, design freedom, and high material utilization, is widely used in additive manufacturing of continuous fiber reinforced composite parts. However, traditional FDM processes are limited by the impregnation characteristics of high-viscosity resins and the layer-by-layer stacking principle, generally resulting in defects such as insufficient fiber-resin wetting, weak interlayer bonding, and numerous internal pores. Meanwhile, printing with pure resin filaments also suffers from insufficient interlayer strength, high residual stress, and susceptibility to warping. Furthermore, while increasing printing speed improves efficiency, the short residence time of the filament in the thermal field makes it difficult to fully melt the resin using only an external heat source, similarly limiting the mechanical properties and molding efficiency of the parts.

[0022] To address the aforementioned issues, existing technologies have begun to incorporate ultrasonic fields into the FDM printing process to improve material porosity and interlayer bonding strength, thereby enhancing the mechanical properties of the printed parts. There are currently two types of ultrasonic-assisted printing processes: One type applies ultrasonic energy to the molten pool region, which refers to the area where the resin material becomes liquid upon heating. This molten pool is typically located in the heating area above the nozzle exit, rather than the forming area from the nozzle exit to the printed part, as illustrated in patents CN110328843A and CN113211788A. In this method, ultrasonic energy cannot be directly transmitted to the forming area; it can only reduce resin viscosity within the molten pool, thereby improving the resin-fiber interface bonding. It improves the mechanical properties of the printed part solely by addressing internal defects in the raw material, with little effect on defects generated during the printing process. In this type of printhead, ultrasound is used only as a broad means of improving raw material properties, resulting in significant energy dissipation.

[0023] To fully utilize ultrasonic energy, the amplitude should be maximized and high-frequency vibrations should be effectively utilized. This is mainly reflected in two points: (1) Ultrasonic vibrations need to act normally on the interior of the material rather than on the tangential surface. If the ultrasound acts tangentially along the surface of the wire, it cannot act normally on the interior of the material. (2) Enhancing the output amplitude requires a clear transmission path and precise resonant frequency matching. If the vibration is transmitted to the frame through the connecting rod and spring, the path is messy, the amplitudes cancel each other out, and the spring will consume a lot of amplitude. At the same time, the temperature rise generated by heating will reduce the resonant frequency of the parts. If the resonant frequency matching of the ultrasonic transducer and the print head is not performed under high-temperature conditions, it will lead to a sudden drop in output amplitude or even damage to the equipment. Figure 1 The diagram shows the vibration directions of three different vibration modes.

[0024] Another type, such as in patent CN114536746A, uses an obliquely installed L-shaped amplitude transformer to transmit vibration to the molten pool area and the forming area. Compared with the previous method, this method makes more effective use of ultrasonic amplitude and improves the utilization efficiency of ultrasonic energy to a certain extent. However, the amplitude transformer design in this structure is unreasonable and will lead to the dissipation of ultrasonic energy. The specific reasons are as follows: (1) The L-shaped amplitude transformer cannot make the ultrasonic wave act completely normally inside the printed part. Some energy is converted into transverse vibration and wasted. Moreover, transverse ultrasonic vibration is not conducive to improving the interlayer bonding strength; (2) The rigid connection of the heat sink causes the amplitude transformer to have a non-centrally symmetrical upper and lower design, and the amplitude is reduced rather than amplified; (3) The amplitude transformer needs to be installed obliquely, with a large overall volume and poor rigidity. The amplitude is canceled out by the deformation of the print head itself.

[0025] Therefore, the technical problem to be solved in this embodiment is: 1. Problems with the existing printhead resonant structure design. For example, the asymmetrical L-shaped amplitude transformer design, which is smaller at the top and larger at the bottom, leads to amplitude compression. At the same time, the non-centrosymmetric structure induces unnecessary vibration components, resulting in a significant waste of ultrasonic energy. In addition, the oblique placement of the amplitude transformer results in a large printhead volume and low overall rigidity, and the ultrasonic amplitude is absorbed by the deformation of the printhead itself.

[0026] 2. The existing printhead has poor ultrasonic transmission path directionality. The ultrasonic transducer simultaneously excites the frame and the molten pool to vibrate, resulting in a chaotic vibration transmission path and inconsistent vibration phase angles among the components, which directly leads to mutual cancellation of amplitudes.

[0027] 3. The existing printhead does not clearly define the resonant frequency matching under high-temperature conditions. Temperature changes can cause the component's resonant frequency to drift, and without frequency matching, this can lead to a sudden drop in output amplitude and abnormal heating.

[0028] 4. Existing ultrasonic vibrations in printheads cannot effectively act on the forming area. Most existing solutions confine ultrasonic energy to the molten pool area and fail to directly transfer it to the forming area between the nozzle exit and the printed part. Therefore, the thermal effect of ultrasound cannot be utilized, and the improvement of defects generated during the printing process, such as interlayer bonding strength, porosity, and residual stress, is limited. It cannot make up for the heat gap during high-speed printing.

[0029] The printhead provided in this embodiment introduces an ultrasonic field into the traditional fused deposition thermoplastic resin 3D printing process. By precisely controlling the direction of ultrasonic wave transmission, it comprehensively utilizes the mechanical vibration effect, cavitation effect, acoustic flow effect, and thermal effect to effectively improve the resin-fiber impregnation effect, reduce internal porosity and residual stress, strengthen the interlayer interface bonding strength, improve energy utilization efficiency, and significantly increase printing speed.

[0030] This printhead is primarily used for additive manufacturing of pre-impregnated continuous fiber reinforced thermoplastic composites, but it is also compatible with 3D printing of pure thermoplastic resin materials.

[0031] like Figure 2 , Figure 3 As shown, an ultrasonic printhead includes an ultrasonic transducer 2, an amplitude transformer 4, a heat-conducting rod 6, and a nozzle 8 connected in sequence along the longitudinal direction. The resonant frequency of the first longitudinal mode of the amplitude transformer 4 at room temperature is higher than the calibrated frequency of the ultrasonic transducer 2, and the resonant frequency of the first longitudinal mode of the heat-conducting rod 6 at room temperature is higher than the resonant frequency of the amplitude transformer 4. The amplitude transformer 4 has a transverse inlet for the filament 10 to enter, and the amplitude transformer 4, the heat-conducting rod 6, and the nozzle 8 all have longitudinal through holes for the filament 10 to pass through. The heat-conducting rod 6 is covered with a heating coil 5, which is used to preheat the filament 10 passing through the heat-conducting rod 6. The filament 10 is subjected to longitudinal ultrasonic action under the nozzle 8 to compensate for the heat gap during printing.

[0032] The ultrasonic transducer 2, amplitude transformer 4, heat-conducting rod 6, and nozzle 8 are connected longitudinally to form a longitudinally extending overall vibration transmission path. Here, longitudinal refers to the direction along the printhead axis, which is the main direction of ultrasonic wave propagation and vibration. The amplitude transformer 4 amplifies the mechanical vibration amplitude output by the ultrasonic transducer 2, and the heat-conducting rod 6 further transmits the amplified ultrasonic vibration to the nozzle 8. A heating coil 5 is fitted around the heat-conducting rod 6 to preheat the filament 10 passing through it. A Teflon coating is embedded in the longitudinal through-hole at the center of the heat-conducting rod 6. It should be noted that the preheating temperature here is usually controlled near the glass transition temperature of the resin, much lower than its melting temperature. This is to soften the hard and brittle continuous fiber pre-impregnated filament 10 at room temperature, preventing damage or breakage due to bending at the nozzle 8 exit, and ensuring smooth filament 10 delivery and printing continuity. Preheating itself does not completely melt the resin; the resin remains in a viscoelastic state.

[0033] The resonant frequency of the first longitudinal mode of the amplitude transformer 4 at room temperature is designed to be higher than the rated frequency of the ultrasonic transducer 2, with a specific difference range of 3%-8%. The resonant frequency of the first longitudinal mode of the heat-conducting rod 6 at room temperature is higher than the resonant frequency of the amplitude transformer 4; specifically, the resonant frequency of the first longitudinal mode of the heat-conducting rod 6 at room temperature is 5-10% higher than the rated frequency of the ultrasonic transducer 2. During the printing process, the heating coil 5 heats the heat-conducting rod 6, causing the temperature of both the amplitude transformer 4 and the heat-conducting rod 6 to rise. This temperature increase leads to a decrease in the elastic modulus of the metal material, resulting in a decrease in the resonant frequency of the components. By pre-setting the resonant frequency at room temperature to be higher than the rated frequency of the ultrasonic transducer 2, the frequency drift caused by temperature rise can be offset. This ensures that the actual resonant frequency of the entire vibration transmission structure can still match the rated frequency of the ultrasonic transducer 2 under high-temperature operating conditions, thereby guaranteeing the output efficiency of the ultrasonic amplitude. Without frequency pre-compensation, frequency mismatch at high temperatures can lead to a sharp drop in output amplitude and even damage to the equipment.

[0034] A transverse inlet on the amplitude transformer 4 is used to introduce the filament 10, while longitudinal through-holes inside the amplitude transformer 4, the heat-conducting rod 6, and the nozzle 8 constitute a continuous conveying channel for the filament 10. After entering the amplitude transformer 4 through the transverse inlet, the filament 10 turns and passes through the amplitude transformer 4, the heat-conducting rod 6, and the nozzle 8 sequentially along the longitudinal through-holes, and is finally extruded from the nozzle 8 outlet. Since the components are arranged coaxially along the longitudinal direction, the filament 10 is preheated by the heating coil 5 when passing through the heat-conducting rod 6, and then subjected to longitudinal ultrasonic action in the forming area below the nozzle 8. The viscoelastic resin molecular chains undergo high-frequency internal friction, generating a viscoelastic heat generation effect, which causes the resin temperature to rise rapidly to the melting temperature, thereby compensating for the heat gap caused by the short residence time of the filament 10 in the heating zone during high-speed printing. At the same time, the direction of longitudinal ultrasonic vibration is perpendicular to the surface of the printed part, which can effectively stimulate the mechanical vibration effect, cavitation effect, acoustic flow effect, and thermal effect of ultrasound, promote fiber wetting, reduce bubbles, reduce viscosity, and strengthen interlayer bonding.

[0035] The longitudinal connection and coaxial arrangement of the components in this printhead ensures pure axial transmission of ultrasonic vibration, avoiding the lateral vibration components caused by the oblique installation and abrupt changes in cross-section of the existing L-shaped amplitude transformer 4. The frequency gradient design between the amplitude transformer 4 and the heat-conducting rod 6 maintains resonance matching under high-temperature conditions, solving the problem of frequency drift caused by temperature rise. The heating coil 5 only performs low-temperature preheating, while the main heat required for melting is supplied by ultrasonic viscoelastic heat generation. This energy supply mode changes the traditional auxiliary method of relying mainly on the thermal field and supplementing with ultrasound, so that the printing process no longer depends on the heating platform and constant temperature chamber, simplifying the equipment structure and reducing energy consumption. In addition, the filament 10 is directly subjected to longitudinal ultrasonic action below the nozzle 8. The ultrasonic energy is precisely applied to the forming area rather than being limited to the molten pool area, thereby effectively improving the interlayer bonding strength and reducing porosity and residual stress.

[0036] The filament 10 includes pre-impregnated continuous fiber filament 10 and pure resin filament 10. The resin matrix in both printing materials needs to possess high toughness near its glass transition temperature to prevent breakage due to high-frequency ultrasonic vibration during printing. Suitable materials include polyetheretherketone (PEEK), polyamide, polylactic acid (PLA), and polyphenylene sulfide (PPS). For the pre-impregnated continuous fiber filament 10, the reinforcing fibers can be continuous carbon fiber, continuous glass fiber, continuous Kevlar fiber, etc. The diameter of the filament 10 is 0.3 mm to 1.75 mm.

[0037] It is worth noting that the viscoelastic properties of thermoplastic resins allow them to rapidly heat up due to intramolecular friction under ultrasonic vibration, resulting in high energy utilization efficiency and fast thermal response, which can effectively reduce residual stress in printed parts. In this embodiment, the filament 10 is subjected to longitudinal ultrasonic action under the planar boss 82 of the nozzle 8, utilizing viscoelastic in-situ heat generation to compensate for the heat gap during high-speed printing. However, in existing structures, because ultrasonic vibration is mostly transverse, sufficient longitudinal strain amplitude cannot be generated within the filament 10, thus failing to fully utilize the thermal effect of ultrasound.

[0038] Considering that the print head needs to be mounted as a whole on the 3D printer and achieve stable ultrasonic transmission, some further specific examples in this disclosure also include a mounting assembly, which includes a connecting backplate 1 and a fixing sleeve 3. The connecting backplate 1 is used to mount the entire print head to the printer, the fixing sleeve 3 is mounted on one side of the connecting backplate 1, the ultrasonic transducer 2 is mounted in the fixing sleeve 3, and the print head also includes an ultrasonic generator 11, which is connected to the ultrasonic transducer 2.

[0039] The connecting backplate 1 serves as a rigid connector between the print head and the 3D printer's motion mechanism, fixing the entire print head to the mechanism and enabling it to move synchronously with it. A fixing sleeve 3 is installed on one side of the connecting backplate 1 to accommodate and secure the ultrasonic transducer 2. The ultrasonic generator 11 is connected to the ultrasonic transducer 2 via a cable, providing high-frequency AC power. The ultrasonic generator 11 has an automatic frequency tracking function, capable of tracking frequency drift caused by temperature changes in the print head's vibrating components within a certain range, further ensuring resonance matching under high-temperature conditions. Through these mounting components, the overall rigidity of the print head is guaranteed, avoiding the problem of amplitude absorption due to improper installation.

[0040] To prevent the amplitude transformer 4 from overheating during operation and affecting the temperature of its contact surface with the ultrasonic transducer 2, and also to prevent the high temperature from causing a decrease in the material properties of the amplitude transformer 4, in some other specific examples of this disclosure, the side of the connecting back plate 1 away from the ultrasonic transducer 2 has a mounting plate 7, on which a cooling fan 9 is mounted, and the cooling fan 9 faces the amplitude transformer 4.

[0041] A mounting plate 7 extends from the connecting backplate 1, located on the side of the backplate away from the ultrasonic transducer 2, i.e., the side of the backplate facing outwards. A cooling fan 9 is fixed to the mounting plate 7, with its airflow directed towards the amplitude transformer 4. Since the amplitude transformer 4 is close to the ultrasonic transducer 2, and the ultrasonic transducer 2 has certain temperature limitations, and the amplitude transformer 4 itself generates a small amount of heat due to material friction during vibration, coupled with the heat from the heat-conducting rod 6 being conducted upwards along the amplitude transformer 4, active cooling is required. The cooling fan 9 blows cooling airflow onto the amplitude transformer 4, which can remove heat from the surface of the amplitude transformer 4, ensuring that the temperature of the contact surface between the amplitude transformer 4 and the ultrasonic transducer 2 is maintained within the allowable operating temperature range of the ultrasonic transducer 2. This differs from other existing solutions where the rigid connection of the heat sink causes asymmetrical deformation of the amplitude transformer 4. This solution uses an independently installed fan for non-contact cooling, which does not interfere with the vibration mode of the amplitude transformer 4, nor does it alter the geometric symmetry of the amplitude transformer 4.

[0042] In order to avoid unnecessary energy loss of ultrasonic vibration caused by the installation of the fixed structure, in some other specific examples of this disclosure, a flange is provided at the node position of the ultrasonic transducer 2, and the flange is installed in the fixed sleeve 3.

[0043] The wave node position refers to the location where the amplitude along the axial direction of the ultrasonic transducer 2 approaches zero. This position produces almost no vibration displacement; therefore, placing a flange at this location and rigidly connecting it to the fixed sleeve 3 minimizes the absorption of vibration energy by the mounting structure. The longitudinal vibration generated by the ultrasonic transducer 2 propagates from the antinodes to both ends. The wave node, acting as a fixed support point, securely mounts the ultrasonic transducer 2 within the fixed sleeve 3 without affecting the transmission of vibration energy to the amplitude transformer 4. This mounting method ensures that the ultrasonic transducer 2 does not undergo relative displacement during the movement of the entire printhead, while simultaneously ensuring efficient downward transmission of ultrasonic energy. In existing technologies, the ultrasonic transducer 2 is connected to the frame via springs or connecting rods. These elastic or non-centrosymmetric connections consume or cancel out some of the amplitude, while this solution uses a wave node flange for rigid fixation, avoiding these problems.

[0044] In order to ensure the efficient transmission of ultrasonic vibration between the components and to ensure the smooth delivery of the wire 10, in some other specific examples of this disclosure, the bottom of the ultrasonic transducer 2 and the top of the amplitude transformer 4 are connected by a first bolt 12, and the bottom of the amplitude transformer 4 is connected to the top of the heat-conducting rod 6 by a second bolt 13, and the second bolt 13 is a hollow bolt.

[0045] The first bolt 12 connects the bottom of the ultrasonic transducer 2 and the top of the amplitude transformer 4. Sufficient preload is required at the contact surface between them to ensure effective transmission of ultrasonic waves from the ultrasonic transducer 2 to the amplitude transformer 4. The second bolt 13 connects the bottom of the amplitude transformer 4 and the top of the heat-conducting rod 6. Since longitudinal through-holes for the wire 10 need to be formed inside the amplitude transformer 4, the heat-conducting rod 6, and the nozzle 8, the second bolt 13 is designed as a hollow bolt. Its central through-hole serves as part of the wire 10 delivery channel, allowing the wire 10 to pass through the second bolt 13 from inside the amplitude transformer 4 and into the heat-conducting rod 6. The second bolt 13 also requires appropriate preload to ensure a rigid connection between the amplitude transformer 4 and the heat-conducting rod 6 and efficient ultrasonic transmission. Furthermore, a wrench groove is provided at the upper end of the heat-conducting rod 6 to apply sufficient preload.

[0046] In other specific examples of this disclosure, the heat-conducting rod 6 has a flange protruding outward from its lower side, the heating coil 5 is mounted on the flange, and the bottom of the nozzle 8 has a downwardly protruding planar boss 82. A fillet 81 is provided at the junction of the planar boss 82 and the longitudinal through-hole inside the nozzle 8. Figure 4 As shown.

[0047] The flange protruding outward from the lower side of the heat-conducting rod 6 provides a mounting support surface for the heating coil 5. The diameter of the flange is 0.25mm-1mm larger than the diameter of the heat-conducting rod 6. After the heating coil 5 is fitted onto the heat-conducting rod 6, its lower end can rest against this flange to prevent the heating coil 5 from slipping off. The downward-protruding planar boss 82 at the bottom of the nozzle 8 is a key part where ultrasonic vibration directly acts on the filament 10 and the printed part. The difference between the inner and outer diameters of the planar boss 82 is usually set between 1mm and 6mm. During the printing process, the planar boss 82 is close to the surface of the printed part. When the nozzle 8 outputs longitudinal ultrasonic vibration, the planar boss 82 transmits the vibration to the freshly extruded filament 10 and the interface area between the filament 10 and the cured layer. Since the planar boss 82 is a planar structure, its vibration direction is perpendicular to the surface of the printed part, enabling normal action. A fillet 81 is provided at the junction of the planar boss 82 and the longitudinal through hole inside the nozzle 8. The radius of the fillet 81 is usually 0.5mm-2mm. For pre-impregnated continuous fiber filaments 10, if there are sharp edges when the filaments 10 pass through the junction, they are likely to be cut or scratched. The rounded corner 81 structure can provide a smooth transition and avoid fiber breakage. For pure resin filaments 10, since there are no fibers, the rounded corner 81 can be omitted.

[0048] Printhead installation process: The backplate 1 is connected to the fixed sleeve 3 and the cooling fan 9. The ultrasonic transducer 2 is connected to the fixed sleeve 3 via a flange at the wavelet. The remaining vibration transmission components, the amplitude transformer 4, the heat-conducting rod 6, and the nozzle 8, are connected to the ultrasonic transducer 2 via first bolt 12 and second bolt 13, respectively, and require appropriate preload to transmit ultrasonic vibrations. The heating coil 5 is fitted with the heat-conducting rod 6 with a clearance fit.

[0049] Install the print head onto the motion mechanism of the 3D printer, connect the ultrasonic transducer 2 to the ultrasonic generator 11, and simultaneously turn on the heating coil 5 and the cooling fan 9. After the internal temperature of the heat conduction rod 6 stabilizes, perform a frequency tracking test. If the ultrasonic generator 11 can scan the resonant frequency and output it normally, proceed to the next step.

[0050] Subsequently, the filament 10 is introduced into the heat-conducting rod 6 and printing begins. During printing, the filament 10 is heated inside the heat-conducting rod 6, at which point the resin in the material melts due to the heat, preventing it from breaking when passing through the lower rounded corner 81 of the nozzle 8. During printing, the filament 10 passes under the planar boss 82 and is directly subjected to the beneficial effects of high-frequency ultrasonic vibration. Among these effects, the mechanical effect promotes fiber wetting and interfacial bonding; the cavitation effect induces bubble collapse and promotes molecular cross-linking; the acoustic flow effect reduces resin viscosity, reduces defects, and strengthens interlayer bonding; and the thermal effect compensates for the heat gap in high-speed printing, reduces residual stress, and increases printing speed.

[0051] The printhead in this embodiment achieves precise matching between the resonant frequency and the calibration frequency of the ultrasonic transducer 2 under thermal conditions, eliminating energy losses caused by oblique vibration transmission, asymmetric deformation, and unclear ultrasonic propagation paths in existing inventions. This ensures that ultrasonic vibration is efficiently and purely transmitted along the printhead axis to the molten pool and nozzle 8, and acts perpendicularly on the printed part, effectively stimulating the various beneficial effects of ultrasound. Figure 5 As shown, the amplitude and direction of the printhead at the resonant frequency are illustrated, with blue areas representing zero amplitude and red areas representing maximum amplitude. Special attention should be paid to the amplitude and direction at nozzle 8, which vibrates longitudinally with virtually no lateral component; therefore, this design maximizes the effective amplitude at nozzle 8.

[0052] Traditional printheads require heating the entire nozzle, heating platform, and chamber to above the resin's melting temperature. This structure preheats the filament 10 only at the heat-conducting rod 6 to near its glass transition temperature, far below the melting temperature. This softens the hard, brittle continuous fiber filament 10, preventing bending damage, while the resin remains in a viscoelastic state. Subsequently, ultrasonic vibration is applied directly to the nozzle 8 outlet and the printing interface, rapidly raising the resin to its melting temperature through the viscoelastic heat generation effect. Heat is highly concentrated in the forming area, achieving precise localized melting. The molten material continues to be subjected to ultrasonic action during the laying process, further enhancing compaction and cavitation effects. This printhead eliminates the need for a heating platform and a constant-temperature chamber, relying solely on low-temperature preheating and ultrasonic internal heat generation to complete the melting process, simplifying the equipment structure and reducing energy consumption. Unlike existing models that primarily rely on thermal fields with ultrasound as a secondary element, this solution uses ultrasound to simultaneously provide the main heat supply and interface strengthening function, fundamentally changing the energy supply mode.

[0053] By utilizing the various beneficial effects of ultrasound, the mechanical properties of the samples and the printing speed are improved. Experimental results show that this printhead can reduce the porosity of continuous fiber reinforced composite materials from 7.2% to 3.4%, increase the interlaminar shear strength from 22.7 MPa to 31.8 MPa, reduce residual stress by 24%, and increase printing speed by 30%, significantly improving mechanical properties and printing efficiency.

[0054] Example 2 The method provided in this embodiment is used to design the aforementioned ultrasonic printhead. Based on preset printing parameters and the properties of the printing material itself, key dimensional parameters such as nozzle structure and internal flow channels are first determined, and the minimum amplitude that can excite the beneficial effects of ultrasound is set. On this basis, the printhead is subjected to adaptive multi-objective iterative optimization to ensure that ultrasonic energy can be efficiently and stably transmitted longitudinally to the forming area under thermal conditions.

[0055] The design method includes: setting printing parameters; determining the invariable dimensions based on the filament diameter, judging whether there is a heat gap, and if so, deriving the required longitudinal amplitude at the nozzle plane boss, obtaining the amplitude threshold for exciting the beneficial ultrasonic effect, and selecting the larger of the two as the minimum ultrasonic amplitude; initially determining the axial length, setting multiple dimensional parameters as design variables and assigning initial values, performing finite element analysis on the printhead under heating state, solving for contact surface temperature, longitudinal modal frequency, stress distribution, and nozzle amplitude, and if the deviation of each calculation result from the target value is within a preset range, determining the variation range of each design variable as the set percentage interval of the initial value; establishing a multi-objective optimization model with the objectives of maximizing nozzle end face amplitude, minimizing frequency deviation, minimizing contact surface temperature, maximizing modal interval, and maximizing amplitude consistency, and setting constraints; generating multiple geometric schemes within the variation range, solving and filtering them using the finite element method, iteratively narrowing the variation range until the scheme quality no longer improves, and outputting the scheme that meets the constraints.

[0056] First, printing parameters need to be set, including printing speed, printing temperature, filament diameter, and layer height. These parameters are the basic inputs for subsequent design. Then, the invariable dimensions are determined based on the filament diameter. For example, the diameter of the central through-hole of the heat-conducting rod is usually the filament diameter plus 0.1mm-0.2mm. The nozzle outlet diameter is determined according to different specifications based on the type of filament. The difference between the inner and outer diameters of the nozzle bottom plane boss is also determined in this step. Next, the thermal field distribution during printing is simulated using finite element simulation to determine whether the heating coil alone can fully melt the filament. If a heat gap exists, the required longitudinal amplitude at the nozzle plane boss is deduced based on the filament's specific heat capacity, density, and the required heat compensation, combined with the viscoelastic heat generation mechanism. At the same time, the amplitude threshold that can effectively excite beneficial ultrasonic effects on this material is obtained by consulting literature or through experiments, and the larger of the two values ​​is taken as the minimum ultrasonic amplitude A.

[0057] Then, based on the half-wavelength principle, the overall axial length of the amplitude transformer, heat-conducting rod, and nozzle was initially determined. This was based on estimates of the sound velocity and the frequency of the ultrasonic transducer. At least eight geometric dimensions were set as design variables and assigned initial values. Finite element analysis was performed on the printhead under heating conditions, including steady-state thermal analysis, thermal modal analysis, and harmonic response analysis, to solve for four key parameters: the temperature of the contact surface between the amplitude transformer and the ultrasonic transducer, the frequency corresponding to the longitudinal vibration mode, the stress distribution of the printhead under resonant conditions, and the amplitude at the nozzle plane boss. If the deviation of the calculated results of these parameters from their respective design target values ​​does not exceed the preset range, the initial values ​​are considered reasonable, and the variation range of each design variable is determined as the set percentage range of the initial values.

[0058] Then, a multi-objective optimization model is established. There are five objective functions. The constraints include: the deviation between the longitudinal modal frequency and the ultrasonic transducer calibration frequency under heating conditions does not exceed a set percentage; the interval between the longitudinal mode and the upper and lower noise frequencies is greater than a set threshold; for example, the contact surface temperature is less than the maximum allowable operating temperature of the ultrasonic transducer; the maximum stress is less than the high-cycle fatigue strength of the metal; and the amplitude at the nozzle plane boss is greater than the aforementioned minimum amplitude A.

[0059] Finally, a large number of geometric schemes are generated within the given range of design variable variations. The objective function parameters of each scheme are solved using the finite element method, and the schemes are then selected based on the constraints. During the selection process, the modal confidence criterion is used to identify the mode closest to pure longitudinal vibration, ensuring that the optimization always targets the correct vibration mode. During the iteration process, the program automatically narrows the range of design variable variations, iterating repeatedly until the quality of the scheme no longer improves, and then outputting the scheme that satisfies all constraints.

[0060] This method solves the problems of lack of system design process and inability to achieve frequency matching and amplitude maximization under high temperature conditions in the existing technology through adaptive multi-objective iterative optimization, thus ensuring the performance of the printhead in actual operation.

[0061] In some further specific examples of this disclosure, when deducing the required longitudinal amplitude at the nozzle plane boss, the calculation is performed based on the relationship between the viscoelastic heat generation per unit volume of filament under a single vibration and the longitudinal amplitude. The relationship is as follows: ; in, The viscoelastic heat generated per unit volume of filament. denoted as the loss modulus of the filament, A1 as the longitudinal amplitude, and h as the layer height.

[0062] This formula originates from the mechanical loss theory of viscoelastic materials. When a filament is subjected to high-frequency longitudinal vibration, its internal molecular chains undergo periodic deformation. Due to viscous damping, some mechanical energy is converted into heat energy. The loss modulus reflects the energy dissipated by the material per cycle under unit strain. The strain amplitude can be approximated as A1 / h, which is the ratio of longitudinal amplitude to layer height. The heat generated per unit volume per cycle is proportional to the loss modulus and the square of the strain amplitude. Using this relationship, the required longitudinal amplitude can be deduced from the required heat compensation. In specific calculations, since E'' is a function of temperature, and temperature depends on the accumulated heat generation, an iterative method is usually used to approximate A1. This formula provides designers with a theoretical basis for quantitatively calculating the minimum amplitude, avoiding blindly choosing values.

[0063] In other specific examples of this disclosure, such as Figure 7 As shown, the design variables include: the length of the contact surface between the amplitude transformer and the ultrasonic transducer in the X-axis direction, the diameter of the heat-conducting rod body, the total length of the amplitude transformer in the Z-axis direction, the length of the large end of the amplitude transformer in the Z-axis direction, the length of the small end of the amplitude transformer in the Z-axis direction, the total length of the heat-conducting rod in the Z-axis direction, the width of the large end of the amplitude transformer in the Y-axis direction, and the width of the small end of the amplitude transformer in the Y-axis direction.

[0064] These variables correspond to the main adjustable geometric parameters of the printhead. The length L1 of the contact surface between the amplitude transformer and the ultrasonic transducer in the X-axis direction affects the area and stress distribution of the interface. The diameter L2 of the heat-conducting rod affects its lateral dimensions and vibration characteristics. The total length H1, the length of the large end H2, and the length of the small end H3 of the amplitude transformer in the Z-axis direction together determine its amplification factor and frequency characteristics. The total length H4 of the heat-conducting rod in the Z-axis direction affects its frequency and heat transfer effect. The widths W1 and W2 of the large end and small end of the amplitude transformer in the Y-axis direction, together with the dimensions in the X-axis direction, control the change in the cross-sectional area of ​​the amplitude transformer. By adjusting these eight variables, the objective function can be optimized while satisfying the constraints. A rectangular hole for the filament to pass through is located at the center of the amplitude transformer; the size of this hole is usually fixed to simplify the design and is not considered a variable.

[0065] In other specific examples of this disclosure, the constraints include: the deviation between the longitudinal modal resonant frequency and the ultrasonic transducer calibration frequency under heating conditions does not exceed a set percentage of the ultrasonic transducer calibration frequency; the frequency interval between the longitudinal modal resonant frequency and the upper and lower order noise frequencies under heating conditions is greater than a set threshold; the temperature of the contact surface between the printhead and the ultrasonic transducer is less than the maximum allowable operating temperature of the ultrasonic transducer; the maximum stress of the printhead is less than the high-cycle fatigue strength of the metal; and the amplitude at the nozzle plane boss is greater than the minimum ultrasonic amplitude.

[0066] In the above constraints, the percentage setting for frequency deviation is typically within 0.2% to ensure resonance matching. The threshold setting for frequency interval is typically above 1.5kHz to avoid modal coupling caused by temperature drift or other interference during operation, which could lead to aliasing of longitudinal vibration modes with adjacent bending or torsional modes. The upper limit for contact surface temperature is determined by the allowable operating temperature of the ultrasonic transducer itself, with a common value of 75℃. The lower limit for amplitude is the previously determined minimum ultrasonic amplitude A. These constraints collectively ensure that the printhead can operate safely, stably, and efficiently under high-temperature conditions. If a scheme fails to meet any of these constraints during the optimization process, it is discarded. By combining the constraints with the objective function for multi-objective optimization, the final output scheme achieves a balance between amplitude output, frequency matching, temperature control, structural strength, and modal purity.

[0067] The specific design methodology and process are as follows: Figure 6 As shown: 1. Set printing parameters.

[0068] The following printing parameters need to be defined at the initial design stage: printing speed, printing temperature, filament diameter and layer height, which are used to determine the non-variable size parameters of the print head and to calculate the minimum ultrasonic amplitude A.

[0069] 2. Establish a constitutive model for the material.

[0070] Using simulation software or experimental methods, the elastic modulus, thermal conductivity, density, Poisson's ratio, and specific heat capacity of the metal material used in the printhead within the operating temperature range were obtained, and a material model was established. Dynamic thermomechanical analysis (DMA) and the time-temperature equivalence principle were used to obtain the storage modulus, loss modulus, and loss tangent of the filament at different temperatures under high-frequency vibration. Simultaneously, the specific heat capacity and density of the filament were measured for subsequent calculations of the temperature rise of the printing material under ultrasonic waves.

[0071] 3. Determine the invariable dimensions and minimum ultrasonic amplitude.

[0072] (1) Determine the invariable dimensions based on the selected wire material: According to the wire diameter Determine the nozzle through-hole size and set the diameter of the center through-hole of the heat-conducting rod. Generally speaking, This design ensures smooth passage of the filament while absorbing some ultrasonic vibrations to enhance the material. Simultaneously, the nozzle outlet diameter is adjusted according to the type of filament. If the filament is a pre-impregnated continuous fiber filament, the nozzle outlet diameter And make a corresponding fillet R at the outlet to smoothly transition the through hole and the outlet; if the filament is pure resin filament, adjust the nozzle outlet diameter. Simply set the values ​​to the preset values ​​in the slicing software. Finally, set the difference Δd between the inner and outer diameters of the nozzle bottom plane boss. At this point, the geometry of the 8 nozzles is completely determined.

[0073] (2) Determine the minimum ultrasonic amplitude A at the nozzle end.

[0074] The minimum ultrasonic amplitude is determined by the following procedure: The constitutive models of the printhead metal material and the filament established in step 2 are imported into the finite element simulation program, and the nozzle and filament are assigned corresponding material properties. The thermal field distribution of the printhead and filament during printing is simulated using finite element simulation to determine whether the heating device alone can fully melt the filament. If the simulation results show a heat gap, the required heat compensation, combined with the mechanism of viscoelastic heat generation, is used to deduce the longitudinal amplitude A1 that needs to be output at the nozzle plane boss. The general process is as follows: Based on the specific heat capacity, density, and the difference between the actual and target temperature rise of the filament, the total heat Q that needs to be compensated per unit volume of filament can be calculated. For a given ultrasonic vibration frequency f, the total number of vibration cycles experienced by the filament within the ultrasonic wave's effective range is constant at N. That is, under ideal amplitude, after N ultrasonic vibrations, the filament generates Q Joules of heat due to viscoelasticity, causing its temperature to rise to the preset printing temperature. The viscoelastic heat generation per unit volume of filament in a single vibration is... It can be expressed by the following formula: ; In the formula, h is the floor height; Let be the loss modulus of the wire, which is a function of the wire temperature T. This is calculated using... The temperature rise ΔT of the wire under a single vibration can be deduced using specific heat capacity and density. The larger the value of ΔT, the higher the final wire temperature T. According to the above formula, it is clear that A1 exhibits a monotonically increasing relationship with ΔQ and T. Therefore, iterative methods such as the bisection method and Newton's method can be used to quickly approximate A1.

[0075] By consulting literature or conducting experiments, the optimal amplitude threshold A2 that can effectively excite beneficial ultrasonic effects on this material was obtained.

[0076] Compare A1 and A2, and select the larger value as the minimum amplitude A at the nozzle end.

[0077] 4. Define design variables and set initial values ​​and ranges.

[0078] First, the printhead is parametrically modeled and preliminarily designed based on the half-wavelength principle, and the axial length of the resonant component is initially determined according to the half-wavelength formula. Since changes in the printhead shape affect its own frequency, this axial length is only an estimated value and will change slightly during subsequent iterations. The calculation formula is as follows: ; In the formula, L is the overall length of the amplitude transformer, heat conduction rod, and nozzle; The speed of sound in metallic materials; The calibration frequency for the ultrasonic transducer.

[0079] like Figure 7 As shown, to accelerate calculations, at least eight dimensions marked in the figure should be set as design variables and initialized. Changing these dimensions allows for efficient control of the overall printhead performance. In the figure, L1 represents the length of the contact surface between the amplitude transformer and the ultrasonic transducer in the X-axis direction; L2 represents the diameter of the heat-conducting rod body; H1 represents the total length of the amplitude transformer in the Z-axis direction; H2 represents the length of the large end of the amplitude transformer in the Z-axis direction; H3 represents the length of the small end of the amplitude transformer in the Z-axis direction; H4 represents the total length of the heat-conducting rod in the Z-axis direction; W1 represents the width of the large end of the amplitude transformer in the Y-axis direction; W2 represents the width of the small end of the amplitude transformer in the Y-axis direction. The large end refers to the side closer to the ultrasonic transducer with a larger cross-sectional area in the XOY plane, i.e., region H2; the small end refers to the side farther from the ultrasonic transducer with a smaller cross-sectional area in the XOY plane, i.e., region H3. Subsequently, using the finite element method, the amplitude transformer, heat-conducting rod, nozzle, and filament were respectively assigned to the material constitutive models established in step 2. Steady-state thermal analysis, thermal modal analysis, and harmonic response analysis were performed on the printhead under heating conditions to solve for the following parameters: the temperature of the contact surface between the amplitude transformer and the ultrasonic transducer, the frequency corresponding to the longitudinal vibration mode, the stress distribution of the printhead under resonant conditions, and the amplitude at the nozzle plane boss. If the deviation of the calculated results of the above four parameters from their respective design target values ​​does not exceed ±20%, the initial values ​​are considered reasonable, and based on this, the variation range of each design variable is determined to be ±20% of the initial values.

[0080] 5. Establish an adaptive multi-objective optimization model.

[0081] After setting the initial values ​​and range of variation of the design variables, the optimization objective function is set as follows: maximize the amplitude of the nozzle end face; minimize the deviation between the longitudinal resonant frequency and the calibrated frequency of the ultrasonic transducer; minimize the temperature of the contact surface between the printhead and the ultrasonic transducer; maximize the frequency interval between the longitudinal vibration mode and adjacent noise frequencies; and maximize the amplitude consistency at the nozzle plane boss.

[0082] Meanwhile, the following constraints are set: the difference between the longitudinal modal resonant frequency of the printhead and the calibrated frequency of the ultrasonic transducer under heating conditions shall not exceed 0.2% of the calibrated frequency of the ultrasonic transducer; the interval between the longitudinal modal resonant frequency of the printhead and the upper and lower order noise frequencies under heating conditions shall be at least greater than 1.5 kHz; the temperature of the contact surface between the printhead and the ultrasonic transducer shall be less than the maximum allowable operating temperature of the ultrasonic transducer; the maximum stress of the printhead shall be less than the high cycle fatigue strength of the metal; and the amplitude at the protrusion on the nozzle plane of the ultrasonic printhead shall be greater than A.

[0083] 6. Perform adaptive multi-objective iterative optimization.

[0084] Within the design variable range given in step 3, a large number of printhead geometric schemes are generated, and the parameters are solved using the finite element method. These schemes are then filtered according to the objective function and constraints set in step 4. By comparing the merits of each scheme, inferior schemes are eliminated, and a batch of candidate schemes with mutual advantages are retained to avoid getting trapped in local optima. Simultaneously, since changes in geometric dimensions may lead to distorted vibration modes, it is necessary to use the modal confidence criterion to identify the actual vibration mode closest to the pure longitudinal vibration mode and ensure that optimization is always performed on the correct longitudinal vibration. This involves calculating the mode shape matching values ​​of each vibration mode with the ideal longitudinal vibration mode, and taking the mode corresponding to the highest MAC (modal confidence criterion) value as the longitudinal mode. During the iteration process, the program automatically narrows the range of changes in the printhead design variables based on the current results, iterating repeatedly until the quality of the scheme no longer significantly improves, and finally outputting multiple schemes that satisfy all constraints.

[0085] The design method provided in this embodiment will be illustrated with two examples: Printhead design process example 1: A continuous carbon fiber reinforced polyetheretherketone pre-impregnated filament with a diameter of 0.4 mm was selected for the print head design. The printing speed was set to 20 mm / s, the layer height to 0.16 mm, and the ultrasonic transducer frequency to 35 kHz.

[0086] The printhead body is made of TC4 titanium alloy, whose elastic modulus E is related to temperature T by E(T) = 110 - 0.042T (GPa), and its density is 4430 kg / m³. 3 The Poisson's ratio is 0.34. Using DMA testing combined with the time-temperature equivalence principle, the storage modulus and loss modulus variation curves of the filament at a 35kHz ultrasonic frequency within the temperature range of 25℃ to 400℃ were obtained. DSC testing yielded a specific heat capacity of 1350 J / (kg / K) and a density of 1.45 g / cm³. 3 .

[0087] Next, the immutable dimensions are determined. Based on the wire diameter of 0.4mm, the immutable dimensions are determined as follows: the central through hole of the heat-conducting rod is 0.5mm, the nozzle outlet diameter is 1.2mm with a 0.5mm radius, and the difference between the inner and outer diameters of the nozzle bottom plane boss is Δd=2mm.

[0088] Finite element simulation revealed that at this printing speed, the filament needs to generate heat through viscoelasticity to make up for the heat gap. The required longitudinal amplitude at the nozzle is 12 μm, which is greater than the minimum amplitude of 4 μm required to excite the beneficial effect of ultrasound. Therefore, the minimum ultrasonic amplitude is determined to be 12 μm.

[0089] Finally, the iterative optimization process is implemented. First, the initial values ​​and optimization ranges of various dimensions of the amplitude transformer are determined, and the print head is parametrically modeled. To simplify the iteration process, a rounded rectangular through-hole with dimensions of 14×23mm is fixed at the center of the amplitude transformer. Subsequently, based on the actual working conditions, the following objective function and constraints are set: Objective function: (1) Maximize the amplitude of the nozzle end face; (2) Minimize the deviation between the longitudinal resonant frequency and the calibrated frequency of the ultrasonic transducer; (3) Minimize the temperature at the contact surface between the printhead and the ultrasonic transducer; (4) Maximize the frequency spacing between the longitudinal mode and adjacent spurious frequencies; (5) Maximize the consistency of nozzle end face amplitude.

[0090] Constraints: (1) The longitudinal frequency deviation from the calibrated frequency under heating condition is ≤70Hz; (2) The interval between the longitudinal mode and the adjacent spurious frequency under heating is >1.5kHz; (3) The temperature of the contact surface between the printhead and the ultrasonic transducer is <75℃; (4) The maximum stress on the print head is less than the high-cycle fatigue strength of metal, which is 350 MPa; (5) The amplitude A at the nozzle plane boss of the ultrasonic printhead is greater than 12μm.

[0091] After iterative optimization, the key parameters of the print head are set as follows:

[0092] Printhead design process example 2: In this embodiment, a polyamide 66 pure resin filament with a diameter of 1.75 mm was used as the printing object, the printing speed was 90 mm / s, the layer height was 0.3 mm, and the ultrasonic transducer frequency was 28 kHz.

[0093] The printhead body is made of 7075-T6 aluminum alloy, whose elastic modulus E is related to temperature T by E(T) = 71.7 - 0.034T (GPa), and its density is 2810 kg / m³. 3 The Poisson's ratio is 0.33. Using DMA testing combined with the time-temperature equivalence principle, the storage modulus and loss modulus variation curves of the filament at different temperatures under a 28kHz ultrasonic frequency were obtained. DSC testing revealed a specific heat capacity of 1700 J / (kg / K) and a density of 1.14 g / cm³. 3 .

[0094] The immutable dimensions were then determined. Based on a wire diameter of 1.75mm, the following settings were established: a central through-hole of 1.9mm for the heat-conducting rod, a nozzle outlet diameter of 0.4mm, no fillet radius (R), and a difference in inner and outer diameter of the nozzle bottom plane boss with a diameter Δd = 2mm.

[0095] Finite element simulation revealed that at this printing speed, the resin temperature can meet the printing requirements, eliminating the need for in-situ heating through viscoelastic heat generation. Literature review determined that the minimum ultrasonic amplitude is 4 μm, which is used to excite the various positive effects of ultrasonic vibration.

[0096] Finally, the iterative optimization process is implemented. First, the initial values ​​and optimization ranges of various dimensions of the amplitude transformer are determined, and the print head is parametrically modeled. To simplify the iteration process, a rounded rectangular through-hole with dimensions of 14×23mm is fixed at the center of the amplitude transformer. Subsequently, based on the actual working conditions, the following objective function and constraints are set: Objective function: (1) Maximize the amplitude of the nozzle end face; (2) Minimize the deviation between the longitudinal resonant frequency and the calibrated frequency of the ultrasonic transducer; (3) Minimize the temperature at the contact surface between the printhead and the ultrasonic transducer; (4) Maximize the frequency spacing between the longitudinal mode and adjacent spurious frequencies; (5) Maximize the consistency of nozzle end face amplitude.

[0097] Constraints: (1) The longitudinal frequency deviation from the calibrated frequency under heating condition is ≤56Hz; (2) The interval between the longitudinal mode and the adjacent spurious frequency under heating is >1.5kHz; (3) The temperature of the contact surface between the printhead and the ultrasonic transducer is <75℃; (4) The maximum stress on the print head is less than the high-cycle fatigue strength of metal, which is 350 MPa; (5) The amplitude A at the nozzle plane boss of the ultrasonic printhead is greater than 4μm.

[0098] After iterative optimization, the key parameters of the print head are set as follows:

[0099] While the specific embodiments of the present invention have been described above, they are not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. An ultrasonic printhead, characterized in that, It includes an ultrasonic transducer, an amplitude transformer, a heat-conducting rod, and a nozzle connected in sequence along the longitudinal direction; The resonant frequency of the first longitudinal mode of the amplitude transformer at room temperature is higher than the calibrated frequency of the ultrasonic transducer, and the resonant frequency of the first longitudinal mode of the heat-conducting rod at room temperature is higher than the resonant frequency of the amplitude transformer. The amplitude transformer has a transverse inlet for the wire to enter, and the amplitude transformer, the heat-conducting rod and the nozzle all have longitudinal through holes for the wire to pass through. The heat-conducting rod is fitted with a heating ring, which is used to preheat the filament passing through the heat-conducting rod. The filament is subjected to longitudinal ultrasonic action under the nozzle to compensate for the heat gap during printing.

2. The ultrasonic printhead as described in claim 1, characterized in that, It also includes a mounting assembly comprising a connecting backplate and a retaining sleeve. The connecting backplate is used to mount the entire printhead to the printer. The retaining sleeve is mounted on one side of the connecting backplate. The ultrasonic transducer is mounted in the retaining sleeve. The printhead also includes an ultrasonic generator connected to the ultrasonic transducer.

3. The ultrasonic printhead as described in claim 2, characterized in that, The side of the connecting backplate away from the ultrasonic transducer has a mounting plate, on which a cooling fan is mounted, the cooling fan facing the amplitude transformer.

4. The ultrasonic printhead as described in claim 2, characterized in that, The ultrasonic transducer has a flange at the node position, and the flange is installed inside the fixed sleeve.

5. The ultrasonic printhead as described in claim 1, characterized in that, The bottom of the ultrasonic transducer and the top of the amplitude transformer are connected by a first bolt, and the bottom of the amplitude transformer and the top of the heat-conducting rod are connected by a second bolt, wherein the second bolt is a hollow bolt.

6. The ultrasonic printhead as described in claim 1, characterized in that, The heat-conducting rod has a flange protruding outward on its lower side, the heating ring is mounted on the flange, and the bottom of the nozzle has a planar boss protruding downward. The planar boss and the longitudinal through hole inside the nozzle are provided with a rounded corner.

7. A method for designing an ultrasonic printhead as described in any one of claims 1-6, characterized in that, include: Set printing parameters; Determine the invariable size based on the wire diameter, determine whether there is a heat gap, and if so, deduce the required longitudinal amplitude at the nozzle plane boss, obtain the amplitude threshold for exciting the beneficial effect of ultrasound, and select the larger of the two as the minimum ultrasound amplitude. The axial length is initially determined, and multiple dimensional parameters are set as design variables and given initial values. Finite element analysis is performed on the print head under heating state to solve the contact surface temperature, longitudinal modal frequency, stress distribution and nozzle amplitude. If the deviation of each calculation result from the target value is within the preset range, the variation range of each design variable is determined as the set percentage range of the initial value. A multi-objective optimization model is established with the objectives of maximizing nozzle end face amplitude, minimizing frequency deviation, minimizing contact surface temperature, maximizing modal interval, and maximizing amplitude consistency, and constraints are set. Multiple geometric schemes are generated within the range of variation. The finite element method is used to solve and filter the schemes. The range of variation is iteratively narrowed until the quality of the schemes no longer improves. The schemes that satisfy the constraints are then output.

8. The design method of the ultrasonic printhead as described in claim 7, characterized in that, When reversing the longitudinal amplitude required at the nozzle plane boss, the calculation is based on the relationship between the viscoelastic heat generation per unit volume of wire under a single vibration and the longitudinal amplitude. The relationship is as follows: ； in, The viscoelastic heat generated per unit volume of filament. denoted as the loss modulus of the filament, A1 as the longitudinal amplitude, and h as the layer height.

9. The design method of the ultrasonic printhead as described in claim 7, characterized in that, The design variables include: the length of the contact surface between the amplitude transformer and the ultrasonic transducer in the X-axis direction, the diameter of the heat-conducting rod body, the total length of the amplitude transformer in the Z-axis direction, the length of the large end of the amplitude transformer in the Z-axis direction, the length of the small end of the amplitude transformer in the Z-axis direction, the total length of the heat-conducting rod in the Z-axis direction, the width of the large end of the amplitude transformer in the Y-axis direction, and the width of the small end of the amplitude transformer in the Y-axis direction.

10. The design method of the ultrasonic printhead as described in claim 7, characterized in that, The constraints include: the deviation between the longitudinal modal resonant frequency and the ultrasonic transducer calibration frequency under heating conditions does not exceed a set percentage of the ultrasonic transducer calibration frequency; the frequency interval between the longitudinal modal resonant frequency and the upper and lower order noise frequencies under heating conditions is greater than a set threshold; the temperature of the contact surface between the printhead and the ultrasonic transducer is less than the maximum allowable operating temperature of the ultrasonic transducer; the maximum stress of the printhead is less than the high-cycle fatigue strength of the metal; and the amplitude at the nozzle plane boss is greater than the minimum ultrasonic amplitude.

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