Since former president Obama of America put forward the concept of 3D printing or additive manufacturing, it had been putting into use rapidly and getting acceptance widely. In particular, metal additive manufacturing machines had been successfully applied with pilot demonstration in industry. However, the present metal additive manufacturing machines cannot be directly used in medical fields such as dental restoration because of some different requirements between industry and medical fields. In this case, this paper is aimed for the development of laser fusion printing machine (LFP), also being called as selective laser melting (SLM), for ceramic teeth crown in dental restoration business. Through the reasonable design and development of key components such as machinery unit, optical unit, electrical controlling unit, and software unit, and the integration, debugging, and optimization of the entire system, the laser fusion printing apparatus for dental restoration has been successfully developed. Key technologies such as machine structure design, optical unit design, electrical controlling system design, system software and process software have been overcome, on the basis of which, a lot of process experiments of medical titanium alloy materials were deeply carried out. At last laser fusion printing technology of titanium alloy was mastered, and titanium dental crown by laser fusion printing with relative density up to 97.37% was realized. After post treatment with porcelain, it was found that the laser fusion printed porcelain teeth with titanium alloy has good metal-ceramic bonding strength, which is equivalent to the quality of traditional porcelain teeth, which showed that laser fusion printing can meet the requirements of dental restoration business and has a broad market outlook.
Additive Manufacturing (AM), commonly known as 3D Printing, is the generic terms of all of the rapid prototyping or rapid manufacturing technologies which manufacture physical objects by means of adding materials directly on the basis of three-dimensional computer-aided-designed (CAD) model [
Dental restoration is always an unavoidably realistic or potential problem for anyone because it is possible for teeth to damage in different degrees in our life. So to speak, dental restoration is c-losely related to everyone, which has the obvious characteristics of enormous quantity and wide aspect. So it goes without saying that the market scale of dental restoration is very large. At the same time, dental restoration field also urgently needs real personalized customization technologies because the dental characteristics are in endless variety for everyone. Now in dental restoration fields, the ceramic teeth restoration based on metal crown is still the main method, for which the key process, i.e. the manufacturing of ceramic teeth crown, is mainly made by the traditional process of investment casting. It belongs to lot manufacturing technologies and cannot realize the personalized customization, not only with low precision and long fabrication period but also with very low utilization factor of materials (about 30%) and large environment pollution. Laser fusion printing (LFP), also called as selective laser melting (SLM), is a typical additive manufacturing technology for metal materials with unique advantage of extensive material selection, high accuracy, good part performances and real ability to industrial application [
Based on the characteristics of shape (complex freeform surface), structure (basically thin-walled structure) and dimensions (small overall size of ceramic teeth) and other factors of ceramic teeth, compared with industry-level laser fusion printing machine, three basic principles have been established for developing of laser fusion printing machine of ceramic teeth crown:
1) Single-mode fiber laser with lower laser power (no more than 200 W) are used to achieve smaller focusing accuracy, i.e., smaller focal spot diameter, thereby to improve printing accuracy and surface quality.
2) The use of a smaller printing range, on the one hand, to reduce the difficulty of manufacturing machine body, and on the other hand, also to be convenient to use a miniaturized and high-speed galvanometer scanning system, which ultimately reduces the overall manufacturing costs of machine, and at the same time considerably reduces the machine volume.
3) The appearance and color that match the characteristics of the dental restoration industry are used to make them consistent with the dental industry application scenario.
According to the requirements of laser fusion printing process for dental metal, the laser fusion printing machine to be developed should have the following basic functions:
1) The output function of fiber laser with high quality to provide the continuous fiber laser with the single mode of TEM00 for melting metal powder layer rapidly and adequately.
2) The controlling function of printing atmosphere with high purity, i.e., the real-time dynamic controlling of oxygen content in the printing cabinet, to avoid high temperature oxidation.
3) The fine focusing and precise scanning function to focus the laser within a diameter of 0.1 mm and enable fast and accurate scanning of the specified area as needed.
4) Efficient and accurate powder-spreading function to ensure the smoothness and evenness of powder spreading which makes powder layer density as uniform as possible.
5) The precise lifting function of the building platform including the movement accuracy of the height direction and the levelness of the building platform are considered which require that the reverse gap is as small as possible, and the minimum movement resolution can reach 0.01 mm.
6) Accurate feeding function of feeding platform which can accurately supply powder according to the set powder thickness.
7) Coordinated system control function which need to develop a dedicated system control software to realize the coordinated actions of each unit system.
8) Planning function of scanning path which need to develop a dedicated process software to implement the setting, planning, and optimization of the laser fusion printing process.
9) Man-machine conversation function with a good user interface which can implement the functions such as file management, process controlling, parameter setting, failure alarm information display and diagnosis.
Generally, laser fusion printing machine for dental application includes two major parts: hardware and software. The hardware includes fiber laser, high-speed galvanometer, atmosphere control system, precision powder-spreading device, feeding cylinder, building cylinder, collecting cylinder, electronic control system, and other auxiliary system components. The software includes 3D modeling software, slice software, process planning software, system control software, and PLC embedded programs, which are shown in
The laser fusion printing machine includes two independent units: the host
machine and the atmosphere control system, which are placed separately. The host machine includes a machine body, a seal box, a working platform, an outer shell, a fiber laser, a high-speed galvanometer system, a powder spreading device, a feeding cylinder, a building cylinder, a collecting cylinder, and an electric control cabinet. The shell of host machine adopts streamlined design, and the material is made of ABS plastic sheet. According to the special requirements in medical fields, the main color of the equipment is sea grey and the decorative color is light blue, as shown in
The external dimension of host machine is approximately 1315 mm (length) × 920 mm (width) × 1780 mm (high). The size of sealed box is approximately 535 mm (length) × 550 mm (width) × 280 mm (height). The internal dimension of the building cylinder and feeding cylinder is approximately 120 mm (length) × 120 mm (width) × 210 mm (height), and the maximum printing range is 105 mm (length) × 105 mm (width) × 80 mm (height).
According to the basic functions and logical structure requirements of laser fusion printing machine to be developed, the design and development of subsystems such as optical unit, machinery unit, electronic control unit and software were carried out.
At present, the most widely used dental metal materials are medical titanium alloys, and their main thermalphysic properties are shown in
According to the material parameters in
Q heat absorption = c ∗ m powder ∗ ( T fusion − T initial ) + m powder ∗ Δ H f (1)
name | properties | value | properties | value |
---|---|---|---|---|
Ti6Al4V | Melting point (˚C) | 1725 | Thermal Conductivity (W/cm.K) | 0.219 |
Density (g/cm3) | 4.5 | Latent heat of fusion (kJ/mol) | 15.45 | |
Specific heat (J/(g.K)) | 0.52 | Relative atomic mass | 47.867 |
m powder = ρ ∗ V = ρ ∗ π ∗ r 2 ∗ h (2)
Q heat absorption = P laser ∗ t = P laser ∗ 4 r / v (3)
Among them, c―material specific heat capacity, J/(g・K); mpowder―mass of powder within the laser spot, g; Tfusion and Tinitial―fusion point of titanium alloy powder and initial temperature, K; ΔHf―latent heat of fusion, kJ/mol; ρ―powder density, g/cm3; r―laser beam radius, um; h―powder bed thickness, mm; Plaser―the average output power of the laser, W; t―the time required to scan a spot size powder; v―scanning speed.
When the thickness of powder layer is h = 0.1 mm, 0.2 mm, and the focal spot diameter is 0.01 mm, for v = 2 m/s, 3m/s and 4m/s respectively, by Equations (1)-(3), the laser power required to fully melt the titanium alloy can be calculated, which were listed in
From
Since the scanning range is small (100 mm × 100 mm), it is proposed to use the scanning first and then focusing (i.e., the pre-objective scanning method) to realize laser’s fine focusing and rapid & accurate scanning. After the laser beam is expanded, it is deflected by the scanning system, and then the laser beam is focused on the working plane (the building platform) via the f-theta focusing mirror, as shown in
Based on the maximum printing range and laser wavelength, a hurry SCANÒ III 14 galvanometer from Scan Lab company was used, with a f-theta focusing lens which focal length is f = 250 mm. In addition, in order to achieve fine focus, the laser beam needs to be expanded (2 - 8 times), and the input end of the beam expander must be able to match the QBH output end of the 200 W laser. For this purpose, the QBH beam expander manufactured by Ray tool company is used which result to the expanded beam diameter is with about 10mm. The minimum
Material | Thickness | Speed | Laser power |
---|---|---|---|
Ti6Al4V | 0.1 mm | 2 m/s | 41.9156 W |
3 m/s | 62.8764 W | ||
4 m/s | 83.8312 W | ||
0.2 mm | 2 m/s | 83.8312 W | |
3 m/s | 125.7468 W | ||
4 m/s | 167.6625 W |
focal spot diameter for laser focusing system can be determined by Equation (4).
S = ( λ ∗ f ∗ M 2 ∗ k ) / d (4)
Among them, S―Focal spot diameter (mm); λ―Laser wavelength (193 nm - 10.6 μm); f―Focal length (30 mm - 2000 mm); M2―Laser quality factor (Related to laser); k―Correction factor (The ideal value is 1.27, generally between 1.5 and 2.0); d―Beam diameter before focusing (generally 6 - 70 mm); For the laser scanning and focusing system described above, M2 = 1.05, the laser wave length λ is 1070 nm, and the focal length f is 250 mm, the minimum focal spot diameter in theory is about 40 um, by Equation (4).
In general, the entire machine tool is mainly divided into upper and lower parts. The upper part is a building cabinet, whit which subsystems such as a seal box, an optical fiber, a beam expanding mirror, a galvanometer, a field lens, and a powder-spreading device are installed. The lower part is a body part, which mainly includes a machine body, a working platform, a feeding platform, a building platform, laser, collecting cylinder and so on. The machine body is an inverted T-shaped structure and is an integral casting made of HT250. The top surface of machine body is used for supporting the working platform, and the vertical surface is the installation surface of two vertical axes (feeding cylinder and building cylinder). The hollow part of machine body is used to install the balancing weights, as shown in
The seal box provides a closed isolation environment for the laser fusion printing process, and is connected with the high-speed galvanometer, atmosphere control system, working platform, building cylinder, feeding cylinder, and collecting cylinder, as shown in
There are three square holes in the middle of the work platform, as shown in
ensure the airtightness of the connection with the seal box. A rectangular hole is designed in front of the building cylinder to drain off the metal dust during processing.
The laser fusion printing machine includes three mechanical motion units, namely the horizontal movement of the powder-spreading device (X axis) and the vertical movement of the feeding cylinder (Z1 axis) and the building cylinder (Z2 axis). According to the printing range and accuracy, the main performance of the three motion units is designed, as shown in
The overall control platform based on the IPC + open motion controller was designed to realize the coordination control of subsystem such as light, machine, electricity and gas for the entire equipment, as shown in
name | index | tolerance |
---|---|---|
Z1 axis | Displacement | 100 mm |
Straightness accuracy | 0.01 mm | |
Positioning accuracy | 0.02 mm | |
Repeated positioning accuracy | 0.01 mm | |
Load | 30 kg | |
Z2 axis | Displacement | 100 mm |
Straightness accuracy | 0.01 mm | |
Positioning accuracy | 0.02 mm | |
Load | 30 kg | |
X axis | Displacement | 300 mm |
Straightness accuracy | 0.01 mm | |
Load | 30 kg | |
Z1, Z2 axis | Parallelism | 0.02 mm |
X, Z1 axis | Verticality | 0.02 mm |
X, Z2 axis | Verticality | 0.02 mm |
The industrial personal computer (IPC) is used as the integrated control platform. Using the PCI slots, the motion control of the galvanometer mirror and the output control of the laser are realized through the RTC5 galvanometer control card. The control of the mechanical motion axes are realized by connecting the open Clipper motion control card and the actuators through the Ethernet port (Respectively, corresponding to the feeding cylinder, building cylinder and powder-spreading device), all of which are point to point control and use the digital drive mode, pulse + direction (Pulse + Dir), in order to improve anti-jamming capability. It uses the digital I/O interface to realize the start and stop function of the laser and the printing atmosphere control device. The power setting of the laser is realized through an industrial computer RS232 interface or an analog output port, as shown in
The control system software is divided into four subsystems according to the modular design concept: human-computer interaction subsystem, scanning galvanometer control subsystem, powder-spreading mechanism subsystem and laser control subsystem, as shown in
From a system point of view, on the basis of based on the selected hardware system, a hierarchical software architecture is established, which is divided into four layers: a hardware interface layer, a control logic layer and a human-machine interface layer, as shown in
Development of human-machine interface programs, built-in process programs and PLC programs was carried out. The man-machine interface program mainly includes a startup interface, an automatic printing interface (
galvanometer adjustment interface, a powder-spreading interface, and other auxiliary operation interface. The built-in process program is designed and developed using the motion program language of the clipper controller. It is invoked through human-computer interaction software to realize the motion control of each moving axis (building cylinder, feeding cylinder, and powder-spreading mechanism), mainly including automation powder-spreading program, and quick reset program and limited position controlling program of the powder spreading mechanism. The PLC program has been set in the clipper controller to implement functions such as emergency stop and reset of the machine.
The laser fusion printing process software system for dental applications is mainly composed of STL model display module, slice processing module, scanning path generation module, process parameter setting module, printing files processing module, command processing module and process simulation module. The software interface of main function module is shown in
This software has designed a composite scanning strategy being composed of contour offset scanning and inner partition scanning. That is, the inner and outer contours of a layer are filled with contour offsets, and the inner areas are block-like scanning symmetrically by laser. In this scanning mode, the advantages of high scan accuracy of contour offset scanning and high efficiency and stability of partition scanning can be fully utilized. The strategy is shown in
The assemblage and adjustment of machinery components were carried out, which mainly including level adjustment of the basic components, adjustment of the movement accuracy of the building cylinder and feeding cylinder, optimization of the movement precision of the powder-spreading device, installation of the galvanometer mirror, installation and adjustment of the fiber laser, adjustment of allaxis’s limit switch, the out shell’s installation and adjustment, and the installation and adjustment of other mechanical components.
The integration and test of all electrical controlling components was conducted, mainly including the improvement and production of electrical control system, weak electric signal processing and electrical safety protection system, the signal connection and test of controlling system with each movement axis, the signal connecting and adjustment of laser system with controlling system, the controlling parameters optimization of every motion axis, the joint debugging of allaxis’s motion and signal test and improvement of all I/O points.
Integration and debugging of system control software with hardware system of the developed machine was carried out, including the logic control of the machine's operation panel, the PID adjustment of three movement axes in the powder-spreading process, the automatic and manual controling of the powder-spreading mechanism, the hardware system integration and debugging of laser fusion printing machine body and the integration of galvanometer mirror’s control software with system software.
After completing the integration and test of all of hardware and software, a test and evaluation of full system was carried out, mainly including axis motion function test, program operation test, laser control test, emergency stop control test, instructor light control tests, and the mechanical axes motion accuracy test. The test results of the main performance of the laser fusion printing machine showed in this paper are shown in
The 3D printing process test of titanium alloy material (TC4) was carried out using the self-developed laser fusion printing machine for ceramic dental application. When the main process parameters are as followed: laser power = 180 W, scanning speed = 200 mm/s, scanning space = 0.1 mm, layer thickness = 0.1 mm, tool compensation = 0.1 mm, 3D printing of titanium alloy samples can be achieved with a relative density of 97.37%.
The laser fusion printing accuracy is closely related to tool compensation parameter and the proportion coefficient (i.e., the X-axis and Y-axis’s magnification factors). By optimizing the tool compensation parameter and the galvanometer ratio coefficient, the accuracy of laser fusion printing is significantly improved. When the tool compensation parameter is 0.1mm, and the proportion coefficient is X = 0.9975062, Y = 0.9950249, the printing accuracy of the titanium alloy material is about ±0.02 mm, which means the additive manufacturing, i.e. customization fabrication of the titanium alloy ceramic teeth crown by laser fusion printing could be realized (
According to the market characteristics of the dental prosthetics industry, through the rational design and development of key components such as mechanical units, optical units, electronic control units, and software units, as well as the integration and optimization of the whole system, the laser fusion printing
technical specification | design | actual |
---|---|---|
maximum printing range | 80 mm × 80 mm × 80 mm | 100 mm × 100 mm × 100 mm |
positional accuracy | repeated: 0.01 mm positioning: 0.02 mm | repeated: 0.0072 mm positioning: 0.0072 mm |
straightness (Z1) | 0.02 mm | 0.009 mm/100 mm |
straightness (X) | 0.02 mm | 0.009 mm/100 mm |
machine for dental restoration was successfully developed. The equipment has overcome key core technologies such as machine body structure design, optical unit design, electrical controlling system design, and the development of system software and process software. Based on this, a number of in-depth laser fusion printing processing experiments for medical titanium alloy materials was conducted. Thus, the laser fusion printing technology forTC4 titanium alloy material have been initially mastered, and the individuation manufacturing of titanium-based ceramic teeth crown by laser fusion printing with a relative density of 97.37% has been realized. After post treatment with porcelain, it was found that the laser fusion printed titanium-based ceramic teeth crown has good metal-ceramic bonding strength, which is equivalent to the quality of traditional casting-based ceramic teeth crown. It is shown that the laser fusion printed ceramic teeth crowns are able to meet the requirements of dental restoration and have a broad market prospect. At the same time, some improvement such as the optimization of building vat an feeding vat, seal ability of building cabinet, controlling software and process software for the developed LFP machine will be carried out in the near future for better acceptance.
Supported by the key R&D program of Sichuan province with the granted number of 2017GZ0076.
Zhang, R.S., Yang, J.L., Wu, Z.Q., Tang, X.H., Yu, W., Ma, S.X. and Ji, F. (2018) Development of Laser Fusion Printing Machine for Ceramic Teeth Crown. Journal of Minerals and Materials Characterization and Engineering, 6, 465-481. https://doi.org/10.4236/jmmce.2018.64033