DEGREE PROJECT IN MECHANICAL ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Additive manufacturing and radio frequency filters A case study on 3D-printing processes, post- processing and silver coating methods
AMANDA HERRERO MARTIN
ANA GARCIA VERDUGO ZUIL
KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT
Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Master's Programme, Production Engineering and Management (TPRMM), 120 cr
School Industrial Engineering and Management
KTH - Kungliga Tekniska Högskolan
18 - August - 2020
Ana García-Verdugo Zuil [email protected] Ericsson Supervisor: Göran Poshman
Amanda Herrero Martín [email protected] KTH Supervisor: Jonny Gustafsson
Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Abstract
Additive manufacturing (AM) is an attractive way to shorten development time, reduce product weight and allow the manufacturing of more complex products than by conventional manufacturing processes. The problem arises when the previous traditional manufacturing requirements need to be fulfilled by AM as well as the volume production capability.
This investigation is done together with Ericsson to evaluate the possibilities of the different AM technologies, post-processing methods and silver coating processes to guarantee the specifications of radiofrequency (RF) filters. Here, minimal RF signal insertion losses are targeted. Since insertion losses are dependent on surface roughness, surface smoothness is sought as well. Ericsson simulation software uses correction factors to account for surface roughness, however there are some inconsistencies between the simulated and actual surface roughness that is allowed in the parts.
In AM parts, surface roughness is not easy to control since it depends on parameters related to feedstock, process and machine properties. Commonly, most AM components do not comply with requirements of lower surface roughness values. Therefore, parts need to be smoothened before silver plated; this step is necessary to ensure the electrical conductivity in this specific application. These finishing processes add costs to the final product and increase time to market.
Firstly, a comprehensive study was carried out to better understand the landscape of AM technologies, post- processing and silver coating methods. Secondly, the different processes are assessed with the help of selection matrices, considering the products requirements. The components to print are two RF filters with different shapes and dimensions but similar requirements. The CAD design is modified depending on each AM process and directly affects the results.
Afterwards, the design of an experimental plan is carried out; the number of samples of each part comparing AM technologies, feedstock, different suppliers (3D printing and post-processing) is obtained. Due to budget and time restrictions, the parts were printed using Multi Jet Fusion and Selective Laser Melting processes. After printing, tolerances and surface roughness were measured.
This thesis results in the selection of suitable AM technologies and post-processing methods for RF filters. For MJF printed cavities at 0˚, 30˚ and 90˚ orientation, the best results for this application are obtained at 30˚ providing a good balance between sharp detail and smooth surfaces. In the case of SLM, waveguides are printed at 0˚ and 30˚. 30˚ waveguides present lower surface roughness values than the 0˚ ones as inner support material is needed at 0˚ orientation. SLM cavities were printed at 30˚ in seek of asymmetry between faces, resulting in higher surface roughness in the downfacing face.
Keywords: Additive manufacturing, radio-frequency filters, post-processing, silver coating, surface roughness, Multi Jet Fusion (MJF), Selective Laser Melting (SLM)
2 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Sammanfattning
Additiv tillverkning (AM) är ett attraktivt sätt att förkorta utvecklingstiden, minska produktvikten och tillåta tillverkning av mer komplexa produkter än vad som är möjligt med konventionella tillverkningsprocesser. Problem uppstår när traditionella tillverkningskrav och volymproduktionskapacitet också måste uppfyllas.
Denna undersökning utfördes tillsammans med Ericsson för att utvärdera möjligheterna med olika AM- tekniker, efterbehandlingsmetoder och silverbeläggningsprocesser för att säkerställa specifikationerna för radiofrekvensfilter. Här är minimerade införingsförluster för RF-signaler måltavlan. Eftersom införingsförluster är beroende av ytojämnhet, eftersträvas även släta ytor. Ericssons simuleringsprogramvara använder korrigeringsfaktorer för att bestämma ytojämnhetens inverkan, men man finner inkonsekvenser mellan den simulerade och faktiska ytojämnheten som tillåts i delarna.
I AM-delar är ytojämnhet inte lätt att kontrollera eftersom den beror på parametrar relaterade till råmaterial, process- och maskinegenskaper. Vanligtvis uppfyller de flesta AM-komponenter inte kraven på låga ytojämnhetsvärden. Därför måste delar jämnas till innan de silverpläteras; detta steg är nödvändigt för att säkerställa den elektriska konduktiviteten i denna specifika applikation. Dessa efterbehandlingsprocesser ökar kostnaderna för slutprodukten och ökar också tid till marknad.
Först genomfördes en omfattande studie för att bättre förstå AM-teknikens landskap, efterbehandling och silverbeläggningsmetoder. Sedan bedömdes de olika processerna med hjälp av urvalsmatriser baserade på produktkraven. Komponenterna som ska skrivas ut är två RF-filter med olika former och dimensioner, men med liknande krav. CAD-designen ändrades för varje AM-process och påverkade resultatet direkt.
Därefter utformades en experimentell plan; antalet prover av varje del som jämför AM-teknik, råmaterial, olika leverantörer (3D-utskrift och efterbehandling) erhölls. På grund av budget- och tidsbegränsningar tillverkades delarna med Multi Jet Fusion och Selective Laser Melting. Efter tillverkningen mättes toleranser och ytojämnhet.
Detta arbete resulterar en rekommendation av lämpliga AM-tekniker och efterbehandlingsmetoder för RF- filter. För MJF-utskrivna håligheter vid 0˚, 30˚ och 90˚ orientering erhölls de bästa resultaten för denna applikation vid 30˚ vilket ger en god balans mellan skarpa detaljer och släta ytor. För SLM skrevs vågledarna ut i 0˚ och 30 ˚. Vågledarna i 30 uppvisar lägre ytjämnhetsvärden än de i 0, som behöver inre stödmaterial. SLM-håligheter tillverkades vid 30 ° för att få en asymmetri mellan ytorna, vilket resulterade i högre ytojämnhet i nedfasningsytan.
Nyckelord: Tillsatsstillverkning, radiofrekvensfilter, efterbehandling, silverbeläggning, ytojämnhet, Multi Jet Fusion (MJF), Selective Laser Melting (SLM)
3 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Acknowledgements
This master thesis project has been carried out in Ericsson under their AM initiative as part of our master studies in Production Engineering and Management at KTH. We would like to thank both parts, Ericsson and KTH, for awakening our interest in the additive manufacturing world and giving us the opportunity of being part of this project.
We would like to especially thank our supervisor at Ericsson, Göran Poshman for guiding us during the project. His valuable inputs and knowledge helped us to approach the thesis in a good way. Also, special thanks to the Prototyping and Production Department at Ericsson, whose help during the whole process was extremely appreciated for us. We would also like to thank Jonny Gustafsson, our KTH supervisor, for the consideration he showed answering to all our enquiries promptly and for his advice.
Finally. thanks to our families and close friends, who have been supporting us during this time, without you this would have never been possible.
4 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Table of contents
1. Introduction ...... 9 1.1. Background ...... 9 1.2. Problem statement ...... 9 1.3. Research questions ...... 10 1.4. Working method ...... 10 1.5. Delimitations and Limitations ...... 10
2. Literature and state‐of‐the‐art study ...... 10 2.1. Radio-frequency filters ...... 11 2.2. Previous studies ...... 12 2.3. Additive manufacturing technologies ...... 13 2.4. Post-processing ...... 21 2.5. Silver coating processes ...... 25 2.6. 3D printed parts orientation ...... 26 2.7. Surface roughness ...... 27 2.8. Experimental setups ...... 29
3. Experimental approach ...... 31 3.1. 3D-Printed components ...... 31 3.2. Selection Matrix for AM processes ...... 33 3.3. Selection Matrix for post-processing methods ...... 35 3.4. Chosen Silver coating method ...... 38 3.5. Design of experiments ...... 39
4. Analysis of Results ...... 44 4.1. Dimensional measurements of MJF parts ...... 44 4.2. Surface Roughness Measurements ...... 44
5. Conclusions and discussion ...... 50
6. Appendices ...... 51 6.1. Appendix 1: Indexing of MJF parts ...... 51
7. References ...... 52
5 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
List of figures
Figure 1: Example of waveguide filter WR15 ...... 11 Figure 2: Example of cavity filter ...... 11 Figure 3: Ku/K-band low pass filters manufactured by AM...... 12 Figure 4: Fifth-order Ku/K-band low-pass filter...... 13 Figure 5: Top-down system and bottom-up system ...... 14 Figure 6: Comparison of SL and CLIP process steps ...... 15 Figure 7: MJF process steps with emphasis on the agents ...... 16 Figure 8: SLM part before support removal manufactured by Concept Laser ...... 17 Figure 9: Schematic electrolytic cell for ECP ...... 23 Figure 10: SLM part before and after support removal processed by Hirtisation ...... 24 Figure 11: Example of an electrochemical unit cell ...... 25 Figure 12: Types of planes, being slicing direction the building direction ...... 27 Figure 13: Ra - Arithmetic mean deviation ...... 27 Figure 14: Illustration showing the slicing process and the related staircase effect ...... 28 Figure 15: Probe of perthometer ...... 29 Figure 16: Common display of perthometer readings ...... 29 Figure 17: Scanner measurements ...... 30 Figure 18: Basic concept of a VNA ...... 30 Figure 19: Experimental set up where the device under test is the circular waveguide ...... 31 Figure 20: Circular Waveguide ...... 31 Figure 21: Small Cavity Filter ...... 31 Figure 22: β = orientation angle of the parts in the build area ...... 32 Figure 23: Small cavity filter orientations (0°, 90° and 30°) ...... 32 Figure 24: Waveguides orientations (0° and 30°) ...... 32 Figure 25: Simplified rectangular magnetic field in a circular waveguide ...... 32 Figure 26: SLM waveguides ...... 38 Figure 27: SLM batches ...... 41 Figure 28: MJF batch displayed in HP software ...... 42 Figure 29: Flow chart of the experimentation steps ...... 42 Figure 30: Steps of Quality check of the parts by means of 3D scanner (a-e) ...... 43 Figure 31: Cavity measurements...... 43 Figure 32: Waveguide measurements ...... 44 Figure 33: Inspected 30° printed part ...... 44 Figure 34: Inspected 0° printed part ...... 44 Figure 35: Inspected 90° printed part ...... 44 Figure 36: Waveguides printed at 0 with supports in the inner surface ...... 45 Figure 37: SLM parts printed by GKN ...... 46 Figure 38: Total average Ra comparison between orientations (Waveguides by GKN) ...... 46 Figure 39: Detail to see how the faces were oriented during printing ...... 48 Figure 40: MJF graphs ...... 48 Figure 41: Total average Ra comparison between orientations -MJF Cavities ...... 49 Figure 42. Silver plated Cavities ...... 49
6 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
List of tables
Table 1. Common geometrical requirements for 3D printed parts ...... 19 Table 2. AM machines requirements and machine manufacturers ...... 20 Table 3. Scoring system for AM technologies ...... 33 Table 4. Selection matrix for AM processes with polymer feedstock ...... 34 Table 5. Selection matrix for AM processes with metal feedstock ...... 35 Table 6. Scoring system for post-processing methods ...... 35 Table 7. Selection matrix for finishing processes for both metal & polymer parts ...... 36 Table 8. Selection matrix for finishing processes for only metal parts ...... 37 Table 9. Input and output variables according to the Pasternack´s skin depth calculator ...... 38 Table 10. Indexing of AM processes ...... 39 Table 11. SLM batches printed by Materialise ...... 40 Table 12. SLM batches printed by GKN ...... 41 Table 13. Surface roughness results for SLM printed parts at GKN ...... 45 Table 14. Surface roughness results for MJF printed parts ...... 47
7 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
List of abbreviations
3DP: Three-Dimensional Printing LMD: Laser-based Metal Deposition
AM: Additive manufacturing LOM: Laminated Object Manufacturing
BJ: Binder Jetting ME: Material Extrusion
CDLP: Continuous Digital Light Processing MJ: Material Jetting
CLIP: Continuous Liquid Interface Production MJF: Multi Jet Fusion
CVD: Chemical vapor deposition NPJ: Nanoparticles Jetting
DED: Directed Energy Deposition PBF: Powder Bed Fusion
DLP: Digital Light Processing PeP: Plasma electrolytic polishing
DLS: Direct Light Sintering PJ: Polyjet
DMLS: Direct Metal Laser Sintering PVD: Physical vapor deposition
DOD: Drop on Demand RF: radio frequency
ECP: Electrochemical polishing SL: Sheet Lamination
EBAM: Electron Beam Additive Manufacturing SLA: Stereolithography
EBM: Electron Beam Melting SLS: Selective Laser Sintering
FFF: Fused Filament Fabrication SLM: Selective Laser Melting
FDM: Fusion Deposition Modeling UAM: Ultrasonic Additive Manufacturing
HP: Hewlett-Packard UV: Ultraviolet
IL: Insertion Loss VP: Vat Polymerization
LENS: Laser Engineering Net Shape
8 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
1. Introduction
In this section, the thesis background is included, and the current problem is stated. Afterwards, the research questions are presented as well as the working method used. Finally, the delimitations and limitations that arose during the project are depicted.
1.1. Background
Ericsson is a Swedish multinational networking and telecommunications company headquartered in Stockholm, that works to enable the full value of connectivity for service providers. This master thesis took place at the “Mechanical Design BB and Materials Technologies” department under the Mechanical, Interconnection and Thermal subunit and the Common Hardware Design unit. This department is included in the Networks business area associated with the development of networks.
As an innovative and leading company, Ericsson wants to be ahead of the state-of-the-art technology. For that reason, their R&D department has started an additive manufacturing (AM) initiative. AM is an attractive way to shorten development time, reduce product weight and allow for the manufacturing of more complex products than the ones manufactured by conventional processes. The problem arises when the previous traditional manufacturing requirements need to be fulfilled by AM as well as the volume production capability.
The objective of this project is to evaluate the possibilities of the different AM technologies, post- processing methods and silver coating processes to guarantee the specifications of radio-frequency (RF) filters.
1.2. Problem statement
In the telecom industry, when manufacturing RF filters, minimal RF signal insertion losses are targeted. Since insertion losses are dependent on surface roughness, surface smoothness is sought too. Simulation software make use of correction factors to account for surface roughness. However, there are some inconsistencies between the simulated and actual surface roughness that is allowed in the parts. Insertion losses are often underestimated in simulations due to unconsidered or insufficiently accounted surface roughness (Gold & Helmreich, 2017).
According to RF engineers, the surface must be very fine, however this investigation wants to show that the insertion loss (IL) is overestimated in simulation software and, components could be manufactured with not so fine surface.
In AM parts, surface roughness is not easy to control since it depends on parameters related to feedstock, process and machine properties. Commonly, most AM components do not comply with lower surface roughness values. Therefore, they need to be smoothened before plating with silver; this step is necessary to ensure the electrical conductivity in this specific application. These finishing processes add costs to the final product and increase time to market.
9 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
1.3. Research questions
The objective of this master thesis is to answer the following research questions:
● How does the printing orientation affect the insertion loss of an RF signal?
● How does the post-processing time affect the insertion loss of an RF signal? ○ Could these processes be skipped to save time, weight and money?
● Is the dependence between insertion loss and surface roughness as strong as the simulation shows or can Ericsson handle more surface roughness than is anticipated by the software?
1.4. Working method
In order to deal with the problem statement, a better understanding of the AM technologies, post-processing and silver coating methods landscape is needed. From the different options available in the market, optimal and real solutions are selected and defined. This analysis is carried out with the help of selection or decision matrices. Thanks to the selection matrices, the different processes are assessed versus the products requirements.
The parts to print are two RF filters with different shapes and dimensions but similar requirements. The CAD design file is directly responsible for the results and it is adapted according to each AM process. Afterwards, the design of an experimental plan is carried out. The number of samples of each part comparing AM technologies, the feedstock, different suppliers (3D printing, post-processing and silver coating services) is obtained. As a further step, suppliers are contacted and selected based on capabilities, location, lead time, among others. After printing, tolerances and surface roughness are measured, and insertion losses are tested at Ericsson facilities.
1.5. Delimitations and Limitations
To evaluate the properties of different AM processes, a waveguide filter and a small RF-cavity filter are printed. These components have a specific resonance frequency with a specific RF-mode used. By this, the current distribution is well known and can be correlated to the surface roughness in different directions. This work is dependent on the availability and lead time of the suppliers (Swedish and Europeans). Materials are delimited to metals and polymers. Both must withstand 100°C in-service and can be silver coated.
Some limitations occurred during the project. Among others, suppliers' lead time was longer than usual due to the current Covid-19 situation, difficulties when finding the needed adapters for the insertion loss test, to connect waveguide with the testing machine, and the number of printed parts were reduced due to budget restrictions too. Under these reasons, the result could not answer the problem statement completely.
2. Literature and state‐of‐the‐art study
This section describes the literature reviewed during the thesis project. Firstly, RF frequency filters technology is illustrated as well as the components to print, later similar projects developed in this field with similar objectives are shown. Afterwards, AM technologies are presented as well as the post- processing and silver coating methods available in the industry. Finally, the surface roughness in AM and the experimental set ups are depicted.
10 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
2.1. Radio-frequency filters
RF filters are electronic devices used to limit signals depending on their frequency bands. In a receiver, the signals received from the antenna that could interfere with the performance of signal processing are limited. In a transmitter, the signal is transmitted to the antenna together with some unwanted signals that are limited by the RF filter previously (OneSDR. n.d.).
There are several RF filter technologies available in the industry, but this investigation aims at waveguides filters and cavity filters. Waveguides are hollow metallic structures with commonly rectangular cross- sections where the signal is transmitted through in different RF-modes. The cross-section of the waveguide will determine the frequency, the presence of resonators enables the undesired signals rejection, however the insertion loss increases as the signal has to travel a longer path (Hunter, 2001). Figure 1 (adapted from Wikipedia contributors, 2012) shows an example of a waveguide filter with resonators. Cavity filters are complex filters whose bandwidth covers 400 MHz to 40 GHz. They can handle high power with low insertion losses (API Technologies, n.d.). Multiple signals come into the cavity that suppresses noise or lower the noise level, so the output of the filtering is the desired signal (Göran Poshman, personal communication). Figure 2 (adapted from API Technologies, n.d.) shows an example of a cavity filter.
Figure 1: Example of waveguide filter WR15 (Wikipedia Figure 2: Example of cavity filter (API Technologies, n.d.) contributors, 2012)
Some essential filter parameters are bandwidth and insertion loss (IL). The former represents the range of wanted signal frequencies and the latter depicts how well the filter rejects the undesired signals as well as passing wanted signals. The IL depends on the filter technology, filter design, application, number of poles/resonators; for many applications 1.5 dB would be the maximum. In commercial filters, the IL ranges between 1 and 5 dB (OneSDR. n.d.). IL increases with the frequency (Gold & Helmreich, 2017). The lower the losses, the better is the signal coming out from the cavity (Göran Poshman, personal communication).
Signal attenuation (훼) or transmission losses can be obtained from the reduction of the output signal and can be calculated according to the following equation (Tischer, 1979): 훼 = (푅푠/푍표) ∗ 퐺 (dB/m); where Rs: internal surface resistance related to the skin effect, the high frequencies signals travel through the internal surfaces of the conductive cross-section. In turn, surface roughness contributes to IL. The higher the frequency, the lower RF signal penetration on the material (skin depth); Zo: free-space wave impedance; and G: geometrical factor that depends on the cross-section width and height, and the free space wavelength.
11 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
In Tischer´s experiment (1979), for waveguides machined by the drawing process and silver plated with frequency of 75 GHz, 2 dB/m of insertion loss was obtained. Insertion loss (- values) can be expressed as (Electronic notes, n.d.): G=20log(Vtransmitted/ Vincident), where Vtransmitted is the voltage of the transmitted wave and Vincident is the voltage of the forward or incident wave.
Nowadays, Ericsson RF filters are machined for small production volumes; for larger volumes, high pressure die casting (HPDC) is used. The die casting tooling is trimmed so that no post machining of RF- surfaces is necessary. Only the lids surface is machined as well as holes drilling and threading. After that, the filters are silver plated. The surface roughness achieved is around 0.5-1.2 µm in most cases. The die casting tool lifetime for HPDC is expected to be around 50 000 parts. Thus, for high volumes, Ericsson has several sub vendors using 5 -10 tooling per year.
2.2. Previous studies
Additive manufacturing is a game changer as new products can be introduced in the market in less time, and this is the common objective of most of the previous studies consulted (Peverini et al. 2017). In most of the cases, AM processes and postprocessing were compared, looking for the optimization of the results by varying parameters like the orientation. There are several institutions developing microwave devices by AM like millimeter-waveguides, conical horn antennas and other filters like Ku/K-Band waveguides as seen in figure 3 (Peverini et al. 2017). In most of the cases they decided to experiment with stereolithography (SLA) and selective laser melting (SLM) technologies.
Figure 3: Ku/K-band low pass filters manufactured by AM. a) SLA prototype with copper plating, b) SLM in Maraging steel 1.2709 prototype with silver-plating, c) SLM prototype in titanium alloy with silver-plating, d) SLM prototype in aluminum alloy AlSi10Mg without silver-plating (Peverini et al. 2017)
In SLA, post-processing is always required to remove supports and remaining resin (Peverini et al. 2017). After metallization, parts reach good surface finish, however, for RF waveguides internal channels can be critical in terms of adhesion, thickness and uniformity. The main drawback printing microwave components in SLA are the discrepancies between simulation and measurements in the resonance frequency and gain (Tak et al. 2018), this may be due to the slight bending of the structure caused by the printing direction and resin sinking to the bottom. Fabrication and metal coating imperfections affect directly the measured return losses, insertion losses and insertion phases. With SLA printed parts, the insertion losses are higher than the theoretical values, so the gain values are lower than predicted (Dimitriadis et al. 2017).
12 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Some parts printed by SLM, for instance, Aluminum printed parts, avoid electroplating as they are printed from metal powder. The optimum angles to print are 47.7° on the xz-plane and 14.4° on the yz-plane as seen in figure 4, according to Peverini et al. (2017). Compared with the traditional metallic waveguides fabricated by injection molding and micromachining (Zhang & Zirath, 2016), the metallic 3-D printed waveguide has advantages of short fabrication time, low cost, and small carbon footprint. In the case of SLM, differences between simulated and measured frequency are caused by dissipation losses and due to the mismatch losses. The latter losses at the flange are originated by loose dimensional control of the waveguide, misalignment between the vector network analyzer (VNA) extender waveguide and the device under test.
The immaturity of the process can lead also to defects on the surfaces, like bumps. Ultrasonic treatments help to get rid of most of the left-over powder. Dimensions of the waveguide aperture and the length of the straight waveguides can reach ±5% tolerance and 6µm surface roughness (Shen & Ricketts, 2019).
Figure 4: Fifth-order Ku/K-band low-pass filter. a) Front view, b) Longitudinal cross-section in the E-plane, c) Perspective view, d) STL model with supporting structures (Peverini et al. 2017)
When printing sharp features, poor metal adhesion can cause plated metal to crack along the corners which affect insertion loss. To prevent this problem, the design can be modified to include curved surfaces in place of sharp edges and other fine geometric features (Shen & Ricketts, 2019).
After printing, post-processing is needed if these components are to accomplish precise requirements. Post- processing methods include silver-spraying, which achieves good conductivity and ensures compatibility with the SLA process and surface roughness between 3-5µm. SLM parts get Ra <5µm after shot peeing or chemical polishing post processing (Peverini et al. 2017).
2.3. Additive manufacturing technologies
The AM categories described in this section are based on the standard for defining terms used in AM technology established by the International Organization for Standardization (ISO, 2015). This standard specifies seven AM categories according to the processes involved, namely Material Extrusion (ME), Vat Polymerization (VP), Material Jetting (MJ), Binder Jetting (BJ), Powder Bed Fusion (PBF), Sheet Lamination (SL) and Directed Energy Deposition (DED).
13 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Material Extrusion (ME)
In this process droplets of the build material are selectively deposited (ISO, 2015). Parts are built layer by layer by extruding a thermoplastic onto a build plate through a heated nozzle. The best example of ME is Fused Filament Fabrication (FFF) which is more known by its commercial name Fusion Deposition Modeling (FDM). This AM technology is one of the most popular due to the low-cost prototyping, functionalities and easy access. The extrusion nozzle determines the shape and size of the extruded filament. Since the material is extruded, the AM machine must be capable of scanning in a horizontal plane as well as starting and stopping the material flow. Once a layer is completed, the machine must move upwards, or move the part downwards, to continue printing. Materials employed are limited to thermoplastics and fiber- reinforced thermoplastics like PLA, nylon or ABS. The main drawbacks of this process are the slow building rate and the limited dimensional accuracy (Gibson et al. 2015).
Vat Polymerization (VP)
“Process in which liquid photopolymer in a vat is selectively cured by light-activated polymerization” (ISO, 2015). This technology outperforms other AM processes in terms of part accuracy and surface finish. However, the printable materials are limited to acrylates and epoxies that age, meaning that their mechanical properties degrade over time (Gibson et al. 2015). In the three processes mentioned below, supports from the same resin are needed in the areas with overhangs or undercuts. After the easy support removal, further curing of the resin in a UV chamber could be required (Schmidleithner et al. 2018).
In Stereolithography (SLA), each layer of photopolymer resin is cured by being exposed to Ultraviolet (UV) laser. This laser scans the required cross-section point by point. New resin layers are applied by means of a mechanical sweeper. Two types of systems can be found; first, the top-down system where the laser comes from the top of the vat, the platform moves vertically down, and the part grows upwards. Second, the bottom-up system where the laser comes from the bottom of the vat, the platform moves vertically up and the part grows downwards (Schmidleithner et al. 2018). The aforementioned systems and the part with its supports are depicted in figure 5 (adapted from Schmidleithner et al. 2018:5).
Figure 5: Top-down system and bottom-up system (adapted from Schmidleithner et al. 2018:5)
For Digital Light Processing (DLP), each layer of photopolymer resin is cured by being exposed to UV light coming from a projector screen. The whole pixelated image is projected onto the required XY layer. Compared with SLA, this process is faster but shows lower accuracy (Schmidleithner et al. 2018).
Continuous Liquid Interface Production (CLIP) process is also known as Digital Light Synthesis (DLS) or Continuous Digital Light Processing (CDLP). It is carried out by a bottom up system similar to DLP. However, in this case, the window at the vat bottom is oxygen-permeable that makes the thin layer between
14 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
this window and the building platform (dead zone) remain liquid allowing the platform to move continuously. This enables faster speeds and improvement in surface roughness (Schmidleithner et al. 2018). This surface quality outperforms the other VP processes (Hart, 2018b). The main advantage of this non-stop process is that isotropic properties are achieved, no matter the orientation of the part that its strength will remain the same (Xometry, n.d). The differences between CLIP and SL processes (SLA and DLP) are illustrated in figure 6 (adapted from Düzgün & Nadolny, 2018:6),
Figure 6: Comparison of SL and CLIP process steps adapted from Düzgün & Nadolny (2018:6)
Material Jetting (MJ)
MJ translates two-dimensional inkjet printing into a printed part, this AM technology deposits selectively droplets of build material (ISO, 2015). There are three main methods presented below: material jetting/PolyJet, Nanoparticles Jetting and Drop on Demand. The main division between them are the materials employed.
The best example of MJ is PolyJet (PJ). This technology deposits droplets from a liquid photopolymer using a print head with multiple nozzles onto a build tray by layers and is later cured instantly using UV light. The main advantages of this method are its high speed, precision, smooth surface, accuracy and multiple colors and materials. Nevertheless, this method faces some disadvantages like the complex formulation of liquid material and droplet formation, the lack of accuracy for large parts and the degradation of the material over time with exposure to sunlight (Gibson et al. 2015). PolyJet is helped by support material to make possible more complex architectures which is easy to remove as it is soluble in water (3D HUBS, n.d.-a). All overhanging features require support material as the droplets for the next layer require a surface for impact, spreading and curing (Hart, 2018c).
In the case that metal nanoparticles are desired to be printed, MJ has a technology called Nanoparticles Jetting (NPJ). Metal solid nanoparticles in a liquid suspension are jetted from thousands of nozzles. Inside the system’s build envelope, high temperatures cause the liquid around the nanoparticles to evaporate, leaving dense ultrafine layers of the build material. Parts produced need to be sintered to remove the support
15 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
material. Comparing NPJ with other laser AM technologies, this method is five times faster and easier to manipulate as there are no hard-to-handle hazardous powders. The main drawback is the limited number of materials employed, restricted to stainless steel and zirconia (XJet 3D, 2018-a).
Another principle of MJ but less common is Drop on Demand (DOD), that uses pressure pulses -thermal or piezoelectric- to produce individual droplets from the nozzle (Gibson et al. 2015). DOD is characterized by smaller drop size and higher placement accuracy than other MJ processes. Mainly waxes are employed in this technology but also polymer solutions and metals. Also, post processing is needed to remove support material.
Binder Jetting (BJ)
BJ is a “process in which a liquid bonding agent is selectively deposited to join powder materials” (ISO, 2015). This process is also known as Three-Dimensional Printing (3DP). This liquid binder is deposited by a printed head on usually metallic powders. Sand, ceramic and polymer powders are available too. Since unbound powders act as support, the parts can be stacked in the build volume to achieve a high throughput rate (Gibson et al. 2015). Some of its limitations are the high porosity of the as-printed part and the shrinkage that occurs during sintering, that can cause part inaccuracy (Hart, 2018d). After printing, metal parts are loaded into a curing oven where the debinding process takes place. Afterwards, parts are de- powdered and ready to be further sintered or commonly bronze infiltrated to get almost full density (ExOne Metal 3D Printing Process, 2015; Doyle et al. 2015)
Powder Bed Fusion (PBF)
“Processes in which thermal energy selectively fuses regions of a powder bed” (ISO, 2015). There are six main methods presented below: Multi Jet Fusion, HP Metal Jet, Selective Laser Sintering, Direct Metal Laser Sintering, Selective Laser Melting and Electron Beam Melting. The main differences among them are the feedstock and the source of energy.
Starting with Multi Jet Fusion (MJF), after polymer powders are coated in a layer into the bed, functional agents are printed into them. These agents carry out two functions, to selective fuse the powders (fusing agent) and to decrease the heat in the surrounding area (detailing agent) (Caron et al. 2019; HP, 2017). The previous process is illustrated in figure 7 (HP, 2017:4). The fusing energy comes from UV light. Currently, this method is limited to Polyamide family (PA-11, PA-12 and reinforced-PA). Moreover, no supports are needed since the underlying powder will support the component (HP, 2017).
Figure 7: MJF process steps with emphasis on the agents (HP, 2017:4)
16 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
HP Metal Jet is an extension of HP MJF, in such a case, the polymer binding agents are deposited to a metal powder bed. Later, when the part is sintered the binding polymer will finalize its decomposition and the part will achieve more than 90% of density. Compared to other metal PBF processes, HP Metal Jet does not need a build plate and can create isotropic finished components (HP, 2018a). Stefan Hundrieser from GKN Sinter Metals Components (email communication, 13th March 2020) shared that the common feedstock for this process are 316L and 17-4 stainless steel powders. This process is not widely used in the industry since it is still in a development phase.
In Selective Laser Sintering (SLS), thermoplastic powders are sintered in cross sections by a high-power laser in a nitrogen-filled chamber. First, powders are distributed by a recoater and pre-heated by infrared heaters to a lower temperature than the melting temperature. After that, the laser sinters the powder to build a layer of the component (Swift & Booker, 2013). Loose powder is enough to withstand SLS parts during printing, enabling internal cooling channels and complex features creation (Gibson et al. 2015).
In the case of Selective Laser Melting (SLM), also known as Direct Metal Laser Sintering (DMLS) a laser beam melts the powder particles and forms a melt pool which solidifies as a fused layer of material (Kumar et al. 2019). The most common metal powders used are iron, titanium and nickel (Hart, 2018f). This process, as the other PBF technologies is also capable of getting parts nearly full density (>99,5%) with fine pores. However, it is slow and expensive and it has limited surface finishes (Hart, 2018f), all features that overhang require support material like shown in figure 8 (adapted from 3D HUBS, n.d.-d) and post-processing might be required to improve surface tolerances (Additively, 2020a). Supports are also used to withstand deformation owing to residual stress, to dissipate heat, and to remove the printed part from the build plate more easily (Hart, 2018f).
Figure 8: SLM part before support removal manufactured by Concept Laser (3D HUBS, n.d.-d)
In Electron Beam Melting (EBM), metal powder is melted layer by layer with an electron beam in a high vacuum (Additively, 2020b) where metal conductors like titanium and stainless steel are commonly used. Compared to the rest of powder bed technologies, this process is faster than SLM and it is also capable of obtaining high densities around 99%. The main advantages are the good mechanical properties, the high powder efficiency and the very high scan speeds (Gibson et al. 2015). Due to the poor surface finish, a lot of post-processing is required (Additively, 2020b), not only to remove the support material.
17 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Sheet Lamination (SL)
“Process in which sheets of material are bonded to form a part” (ISO, 2015). This layer bonding can be achieved by adhesive, thermal, clamping or ultrasonic welding. Afterwards, the layer is cut to produce the cross-sectional shape using a CO2 laser, hot wire or CNC machining (Gibson et al. 2015).
Laminated Object Manufacturing (LOM) is one of the most common SL processes, where sheet material coated with an adhesive is supplied in a roll to the build platform. In the platform, a heated roller applies pressure to bond it to the previous layer below. Afterwards, a laser creates the required cross-section and crosshatches the non-necessary material. The most used material is paper, but this process can be used for metal foil and some thermoplastics too. Little shrinkage and distortion occur during printing; however, hollow parts cannot be created, and stair-step effect can appear on sloping surfaces leading to poor surface finish (Swift & Booker, 2013).
Ultrasonic Additive Manufacturing (UAM) is considered a hybrid process where metal sheets are bonded by ultrasonic vibration. CNC milling is needed to remove the unbonded metal. The positive aspects of this process are the low energy usage and thermal residual stresses, and the possibility of internal geometries creation and embedding electronics or fibers. On the other hand, the UAM parts are anisotropic due to the rolling used to manufacture the metal foils (Gibson et al. 2015).
Directed Energy Deposition (DED)
DED is one of the most complex processes within AM, by this reason is mostly used to add material or repair existing pieces. This technology focuses thermal energy to fuse materials by melting as they are being deposited (ISO, 2015). Energy is directed from the source (laser, electron beam or plasma arc) to the materials to be melted. Feedstock can be in form of powder or a wire.
A well-known DED technology is Laser Engineering Net Shape (LENS). Lasers are used to build parts layer by layer directly from the powder in a hermetically sealed chamber with low levels of oxygen. The metal powder material is directly delivered to the material deposition head. Metal prototypes near-net- shaped can be fabricated accomplishing high quality and larger dimensions than other methods. LENS is limited to metal parts and support material is not required during the printing process (Liu et al. 2016).
Similar to LENS but disposing of a pool, there is the process known as Laser-based Metal Deposition (LMD). A laser beam forms a pool of melted metal on the surface of the metallic substrate into which metal powder is injected using a gas stream. The absorbed metal powder produces a deposit on the surface. Metals like steels and titanium alloys are used. The main advantage of this process is the high level of control over the laser, the exact amount of energy is applied to the highly defined regions of the substrate. As the energy is controlled, the process creates less distortion and heat damage to the part (SPI Lasers, 2020).
Another technology in the DED category, but more oriented to repair parts from previously machined by traditional methods, than to create them, is Electron Beam Additive Manufacturing (EBAM). Electron beam gun deposits metal via a wire feedstock, layer by layer, until the part reaches near-net shape and is ready for finish machining. Like LMD, it employs similar feedstock like titanium or nickel (Make 3D Experience: 3DS, 2020).
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AM process capabilities The capabilities of the different AM processes are investigated for subsequent decision-making. Table 1 depicts some of the common geometrical requirements or characteristics of AM components.
Table 1. Common geometrical requirements for 3D printed parts Minimum Minimum Horizontal Vertical Tolerances Maximum part wall thickness layer Surface Surface [mm] dimensions [mm] [mm] thickness roughness roughness [mm] [Ra µm] [Ra µm]
FDM 0.8 - 1.0f 0.1-0.4f 0.1-1x layer 1 x layer ± 0.2f 900 x 600 x 900f thicknessg thicknessg
SLA 0.1a 0.025–0.15a 25-250c 0.1-1x layer ±0.03-0.14i 600 x 500 x 500a thicknessc
a b i i i b DLP 0.1 0.025–0.15 < Ra SLA < Ra SLA ±0.01-0.1 190 x 142 x 230
k l l l k k CLIP 0.2 0.1 < Ra SLA < Ra SLA ±0.05-0.13 94 x 58 x 16
PJ 0.1j 0.016 - 0.032b 5-25j 10-100j ±0.005-0.2j 500 x 280 x 200j
NPJ 0.1s 0.01s 3.2-6.3s - ±0.025-50s 500 x 280 x 200s
BJ 0.2a 0,05-0.5b 10 - 100d 10 - 100d ±0.1-0.5a 600 x 500 x 400a
MJF 0.3-1m 0.08m - 9-12n ± 0.2m 380 x 285 x 380m
p q p p p HP Metal Jet 1,5 0,05-0.1 - 45 (Rz) ± 0.6-1,15 100 x 100 x 80
SLS 0.5-1e 0.075–0.15a 10 - 100e 8 - 12e ±0.05-0.25a 450 x 375 x 325a
SLM 0.04-0.2t 0.03 3-12v 10-32v ± 0.100f 600x400x500t
EBM 0.1u 0-05u 20.3-25.4u - ± 0.200u 350 x 350 x 380u
LOM 0.2a 0.05–0.2a 30–40a 30–40a ±0.1-0.25a 800 x 550 x 500a
UAM - 0.05- 0.38r - - ±0.15r 600 x 910r
LENS 1.5 mmy 0.025w - - ±0.25 mmw 900 x 1500 x 900w
LMD 0.8-1.6 mmz 0.3-1.5x 25.4 µmz - <0,5x -
EBAM - 0.1w - - ±0.05w 2794 x 1575 x 1575w aSwift & Booker (2013); bGibson et al. (2015); c Hart (2018b); dHart (2018d); eHart (2018e); f3D HUBS (n. d.-b); gHart (2018a); hKazmer (2017); i3D HUBS (n. d.-c); jHart (2018c); kXometry (n.d); lSculpteo (n.d.); mCaron et al. (2019); nHP (2018b); pStefan Hundrieser (email communication, 13th March 2020); qHP (2018a); rFabrisonic (n.d.); sXJet 3D (2018-b); tAdditively (2020a); uAdditively (2020b); vHart (2018f); wEDA (2018); xCandel-Ruiz et al. (2015); yMudge & Wald (2007); zThe Welding Institute (2007). Note: Horizontal surfaces are parallel to the layer plane and vertical surfaces are perpendicular to the layer plane.
19 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Table 2 shows some machine requirements together with some machine type and machine manufacturers.
Table 2. AM machines requirements and machine manufacturers Printing rate Machine Machine manufacturers [cm3/h]
FDM 15a Fortus 250mc- Stratasysa Stratasys, Ultimaker, Prusa, Markforged
SLA 42a Projet 7000 HD- 3D systema 3D systems, Formlabs, DWS
DLP 120-400b B9 Core 550- B9 Creatorb EnvisionTEC, B9 Creator
CLIP 800-1400c Envision One Mechanical- Carbon3D, EnvisionTEC EnvisionTECc
PJ 6 a Eden 260v- Stratasysa 3D systems, Solidscape, Stratasys
NPJ 200s XJet Carmel 1400 XJET
BJ 3600d X1 25Pro- ExOned Metal: ExOne and Sand: 3D systems, Voxeljet
MJF Up to 5058e HP Jet Fusion 5200- HPe HP
HP Metal Jet - HP Metal Jet 3D printer- HP HP
SLS 60a Formiga P110- EOSa EOS, 3D systems, Sinterit, Sintratec
SLM 170i SLM 280-500-800m SLM solutions, EOS, Renishaw, 3D systems...
EBM 80l Arcam EBMl Arcam AB
LOM - Solidimension SD30e Helisys, now Cubic Technologies
UAM 240-490f SonicLayer seriesf Fabrisonic
LENS 3.7j Lens Systems Optomec
LMD 76j TruPrint 2000 Trumpf
EBAM 660-2400k Sciaky Machinek Sciaky aKazmer (2017); bJackson (2017); cEnvisionTEC.(n.d.); dExOne (n.d.); e3Dnatives (n.d.); fFabrisonic (n.d.); gHart (2018c); iHart (2018f); jLiu et al., (2018); kSciaky (2020); lGE Additive (2020); m(SLM Solutions Group AG, 2020)
20 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
2.4. Post-processing
After printing, components must be either cut off from the build table or removed out of the excess build material that envelops them. Some AM processes need support structures to help keep the part from collapsing or warping during the build process or posterior steps as sintering (Gibson et al. 2015).
Metal components after printing using BJ processes are known to be porous (30-75% powder, 10% binder and the rest empty space) and brittle. Therefore, after the curing and the depowering stage, the most common post-processing steps are sintering and metal infiltration. Both processes aim to densify the parts, but sintering yields larger and anisotropic shrinkage. Metal infiltration consists of a molten metal commonly bronze flowing into the porous part thanks to capillary forces. After infiltration, the part has less porosity and is even 4 times stronger than a sintered part (Garzon et al. 2017; Cordero et al. 2017).
Another optional but commonly used post-process is surface finishing. This is crucial for applications where surface roughness from as-printed parts must be minimized. In the industry, several processes are available depending on material to treat. They are divided according to the materials they can treat. Firstly, for polymers and metals: mass finishing processes, micro machining processes, as well as chemical etching, abrasive blasting and ultrasonic abrasive finishing. Secondly, only for metals: shot peening, abrasive flow machining, electrochemical polishing processes and plasma electrolytic polishing; and lastly; only for polymers: vapor smoothing.
Mass finishing processes can be divided into tumbling or barrel finishing and vibratory finishing. Generally, tumbling can provide good finish for external surfaces and treat multiple parts at once, but its efficiency depends on the number of parts in the barrel; the fewer parts, the better the finish (Simpson, 2018). Three sub-processes can be distinguished: barrel, centrifugal barrel and spindle finishing. SLM parts can be smoothened when rotated in an octagonal rotating barrel together with media and water (Boschetto et al. 2020). SS316L SLM parts can be held by a fixture to be immersed in the rotating media to be spindle finished (Whip et al. 2019). Metal BJ parts can get a ~90% surface roughness reduction (from 15 µm after sintering to 1.25 µm) after barrel finishing (ExOne, 2019). 316L steel SLM parts were spindle finished with ceramic media achieving a Ra reduction rate of 0.03-0.04 µm/min (Kaynak & Kitay, 2019).
Vibratory finishing machines make the parts and media to grind against each other in a vibratory bowl or tub (Innovatec Machinery, 2018). However, this process loses efficiency with time (HP, 2018b). Also, processing internal surfaces and channels can be a challenge (Simpson, 2018). The surface roughness of PA12 MJF parts can be homogenized with this method using ceramic or plastic media, as presented by HP (2018b). In this experimentation, ceramic media was stated to be more effective than plastic. With plastic, more time is needed to get the same surface roughness than with ceramic. However, plastic media implies lower material removal rate so more small features can remain intact. Ra reductions of 7 µm and 5,5 µm can be achieved after 4h using ceramic and plastic media, respectively. SLS parts and metals can be also treated with this process (Kumbhar & Mulay, 2016).
Parts thinner than 0.6mm can easily be damaged during both tumbling and vibratory finishing (Products finishing, 2004). Uneven edges and corners could be damaged during the process (Innovatec Machinery, 2018). According to barrel and bowl sizes in the market, 3D parts aimed in thís investigation can fit inside (Innovatec Machinery, n.d.). Barrel finishing is a slow process, followed by vibratory finishing ( approx.
21 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
2-8 parts/h) followed by centrifugal barrel finishing ( approx. 20-40 parts/h), ending with spindle finishing that is the fastest process ( approx. 160 parts/h) (Davidson, 2004; Jamal & Morgan, 2017)
Under the traditional categories, the micro-machining process plays an important role. It is a mechanical and physical and mechanical process aided by a catalyst that activates the engineered microtool technology that removes micro-roughness. Both polymers and metals are post-processed by this technology, mostly after FDM and SLM processes. This process has a high level of reproducibility and is capable of reaching good homogeneity in the results. The main drawback is the difficulty to implement for large structures, as it is driven by microtools it is more restrictive for bigger parts. This technology can remove more than 50 µm acquiring technical finishes between 1-10 µm, or even obtaining a superfinishing of 0.01 µm. Tolerances are kept during the procedure and generally gives values between 10% up to 20% (Micro Machining Process Technology, 2018).
In the case of laser micromachining, it is similar to the previous process, so depending on the manuals it is included as part of micro-machining. By means of the laser, the material at micron level to be processed is transferred from solid to gaseous in a very short pulse duration (ReithLaser, 2020). Polymer, ceramics and metals can be treated, being the last group the most common, DMLS parts can be treated as well. This main advantage of laser micro machining is its flexibility as it is a contactless machining process and it has the possibility of high automation (Kumbhar & Mulay, 2016). Also, this technology requires higher investment (Bhattacharyya & Doloi, 2020). In contrast, laser micro-machining achieves good surface results and it is capable of machine up to 1000 parts/h nearing surface roughness values between 0.8 and 6.3 µm (Fox et al. 2020).
In chemical etching, the specimen is submerged in a chemical solution to smoothen the part surface, the chemical reacts with the outer surface removing material and leaving a smooth outer surface (Beamler, 2020). This process is used mostly in polymers and metals (like steel, stainless steel, nickel and different alloys). In spite of the big improvements in surface roughness that this technology shows, the main disadvantage is the lack of control during the process is carried out. As a result of the uncontrolled behavior, some features that should remain can be etched off. Chemical etching handles low-production runs and leaves surface roughness between 0.8 and 6.3 µm. This process is also known as photochemical milling (Kumar et al. 2019). The advantage of this process is its ability to reach into closed areas.
In abrasive blasting, particles are accelerated with compressed air to provide a stream of high velocity particles used to clean objects mainly on external surfaces. The media commonly used as abrasive varies between sand, glass beads, steel shots and grits; they depend on the result or the material to treat. Glass beads are excellent for stainless steel applications. Some vendors like HP recommend this process for cleaning parts after printing (Kramer, 2020). Steel abrasives are cost-effective alternatives to prepare a surface for final coatings (Finishing Systems, 2020). Metals from SLM or BJ parts from 316L steel can be post-processed by abrasive blasting (Löber et al. 2013), being effective and inexpensive and also used in complex geometries. The main drawbacks are the labor-intense and difficulties of achieving a uniform and precise finish on the entire part (Simpson, 2018). Looking at the results, the surface roughness values are higher than in other processes, between 50-100 µm achieved with glass which can reduce ⅔ of the initial roughness (Löber et al. 2013). Components manufactured by BJ can reach 50% reduction after being abrasive blasted and show values around 7.5 µm (ExOne, 2019). The disadvantage of this process is that closed areas cannot be reached.
22 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Shot peening is a cold working process where a stream of material (steel, ceramic or glass beads) is shot using high pressure against a metallic surface. It aims to increase the surface hardening and fatigue strength. Also, it can effectively reduce surface roughness (Maamoun et al. 2018). SLM parts can be shot peened to eliminate surface defects on the as-printed parts (Kumbhar & Mulay, 2016; Maamoun et al. 2018). A Ra reduction of 6 µm in surface roughness of SLM parts after shot peening from a single Ø19 mm nozzle was achieved (Maamoun et al. 2018). It works well for external surfaces, but internal surfaces or channels can be challenging (Simpson, 2018). Thin workpieces are susceptible to deformation; dimensions could be changed, i.e. holes could turn into oval (Institute of Metal Finishing, n.d.).
Abrasive flow machining is a process where a viscous fluid containing abrasive particles smooth internal surfaces and passageways from metal components (Simpson, 2018). This process creates uniform polished surfaces on difficult-to-access cavities; however, tooling is needed to hold the part for external and internal processing. Its processing time is around 100 parts per hour (Rhoades, 1991). Starting with Ra of 0.7-7 µm, this process can reduce the surface roughness up to 10 times (Singh et al. 2017).
Ultrasonic abrasive finishing is an advanced finishing process that finishes workpiece surfaces in an effective way compared to other traditional methods. It removes material from the surface of a part through high frequency, low amplitude vibrations of a tool against the material surface in the presence of fine abrasive particles. Materials treated by this process are metal-based and non-metal such as glass and non- conductive metals, that cannot be machined by alternative methods (they are hard and brittle) (Kalpakjian, 2008). Parts from SLS and EBM can be finished with this method. Internal and external surfaces of metal parts are treated as well. The main drawback of this process is that it requires careful handling of the abrasive material and the waste products (Guzzo et al. 2020). This process can reach a reduction of surface roughness of 129 µm/min being capable of removing between 3 and 4 µm in 2 seconds. Finished parts have a final roughness between 0.3 and 0.8 µm (Spencer et al. 1993).
In electrochemical polishing (ECP), an electrical current is used to stimulate a chemical etchant, treating internal and external surfaces of a metal component. Figure 9 from Yang et al. (2016) illustrates an electrolytic cell for electropolishing. It shows a good performance for irregular and complex shapes (Simpson, 2018). SS316L parts printed by SLM were electrochemically polished, achieving a 40% reduction of Ra and 1.4 µm/min of removal rate (Löber et al. 2013). Parts with dimensions 30 x 30 x 30 cm could be treated by this method as stated by Electron Tube Store (n.d). By automating this process, multiple pieces can be done in one cycle, leading to a processing volume of 300 parts/h; for parts with similar dimensions (Ø5-15 cm) to the ones of this thesis (Taylor & Inman, 2014). Depending on the electro- chemistry used, this process can more or less target sharp edges.
Figure 9: Schematic electrolytic cell for ECP (Yang et al. 2016:4)
23 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Hirtisation® by Hirtenberger Engineered Surfaces, is a chemical-electrochemical process that uses liquid and particles media for a 3-step process for metal AM components. First, powder cake and supports are removed, second the surface is smoothed (Ra< 2 µm) and third, optional part polishing (Ra 0.5 µm) (Lehmann Additive, 2019; Scott, 2018). Difficult-to-access and cavities areas can be reached deeply. Surface is levelled while the edge sharpness is retained (Hirtenberger, 2020). The supports must be printed in a brittle way for the process to be able to remove them (Figure 10)
Hirtisation can treat steel alloys and aluminum alloys like AlSi10Mg, among others (Hirtenberger, n.d.). The process is capable of meeting requirements of series production. The H12000 machine can finish 500 parts/h (Lehmann Additive, 2019). Depending on part size and quality requirements, process time ranges between 30 min to 5 h (Hirtenberger, n.d.). H6000 machines can process parts up to 50 x 50 x 30 cm (Scott, 2018).
Figure 10: SLM part before and after support removal processed by Hirtisation (Lehmann Additive, 2019:1)
Drylyte, known as DLyte®, was developed by GPAINNOVA. This process is considered the first dry electropolishing system and is used for metal parts that require superior finish. As the process extracts only the material from the peaks of the roughness, it does not round the edges but penetrates the internal cavities which cannot be polished mechanically. Drylyte is a careful process and it can process complex geometries without leaving micro scratches and keeping the tolerances of the piece. The main benefits of this process are the easy processing through channels and cavities, and the homogeneous finish results across the surfaces. At the end of the process, surface roughness can achieve 0.09 µm Ra. The main drawbacks of this post-process are that the edges are damaged, and abrasives do not penetrate all places. In parts with very narrow channels, particles can get stuck and cannot be removed afterwards. This technology allows automation of the process, ensuring high scalability, which makes it attractive for AM (Gpainnova, 2019).
In plasma electrolytic polishing (PeP), metal surface peaks are removed by plasma that was created by applying current to an electrolytic cell. This process has a well controllable removal rate (5 µm/min) and low processing times (60-120 parts/h). It can achieve Ra<0.02 µm. Processing materials other than stainless steel and titanium could be challenging (Nestler et al. 2016). SS316L parts printed by SLM were plasma polished, achieving a 35 % reduction of Ra (Löber et al. 2013). Parts bigger than 300 mm could be treated by PeP (Cornelsen et al. 2017).
Vapor smoothing is the only process considered for polymers in this review. Here, the plastic component is exposed to a chemical vapor which reacts with the surface improving the surface finishing. The main benefit of vapor smoothing is the excellent results in the internal features of components, as it works pretty well inside compared to other technologies (Carville Services, 2020). ABS and PLA are the most common materials post-processed by vapor smoothing. In this technology, the control over the chemical solvent
24 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
increases the feature detail and accuracy of the parts, although there is less control of the temperature (Stratasys, 2020). The time each part takes vary generally between 15 minutes up to 1 hour (Flynt, 2020).
2.5. Silver coating processes
Metal coating processes, among them silver coating, are used to improve mechanical, electrical and protective properties of AM components as well as reducing their surface roughness. The quality of the coating is dependent on the surface of the as-printed parts, the cleaning of the parts and the preprocessing of the printed parts before the silver deposition (Dresler et al. 2019). Various silver coating processes can be found in the industry, such as electroplating, electroless plating, physical vapor deposition, chemical vapor deposition and cold spray.
During the electroplating process, a difference of potential is applied between anode (silver) and cathode (substrate), this makes the silver ions move to the substrate through the working electrolyte as illustrated in figure 11. The substrate must be conductive hence metals can be plated with this method (Fotovvati et al. 2019). However, companies like Galvotec (n.d.) can electroplate polymer parts printed with SLA and SLS processes.
This electrochemical process is used in RF waveguides, provided that their inner cross section is large enough (bigger than 6 × 13 mm and 3.6 × 7 mm). Normally some auxiliary anodes are needed to go through the intricate channels of the waveguide (Shukla et al. 2014). The material deposition rates of electroplating (0,1 µm/min, (Fotovvati et al. 2019)) are quicker with lower costs, compared to electroless plating. Also, this process can coat thicker thicknesses than electroless plating. The metal coats achieved with this process can exceed 75 µm (Skelly, 2008).
Figure 11: Example of an electrochemical unit cell
Electroless plating is also known as autocatalytic plating. Simple chemical process that coates thin silver film layers onto conductive or non-conductive surfaces without an external current source. Metals and polymers can be treated. In the literature, there are examples of 3D printed parts treated with this technique, namely, AlSi10Mg parts that were SLM printed and ABS components FFF printed. This process operates at low temperature and it is suitable for coating deep cavities (Dresler et al. 2019).
Electroless silver coating for metal parts is thickness-limited, there are no commercial baths that can coat more than 1 µm (Shukla et al. 2014). There are processes that can coat more than 1 micron. Most work is carried out on the activation of the underlying surface and thus naturally stops as the layer gets thicker. Plating acts fast in the beginning and then drops off asymptotically to a limit. There is one water-based process that is not dependent on the part plated but rather on the water balance. By evaporating water from the bath, silver is released onto a parts surface. This can continue for as long as one likes as long as the silver ion - water balance is controlled (Göran Poshman, personal communication). Electroless silver
25 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
coating provides a more uniform finish than electroplating and can coat up to 12.5 µm thicknesses in plastic substrates. To plate plastic parts, some preprocessing steps are needed, such as immersion of the parts into chemical etch tanks to increase their surface roughness, and activation of the surface. Followed by a deposition of nickel to create an undercoat (Skelly, 2008). A silver electroless plating deposition rate of 0.3-0.5 µm/min onto SLM AlSiMg10 parts was achieved (Dresler et al. 2019). Silver was deposited onto aluminum rf-waveguides with a rate of 0.0075 µm/min (Shukla et al. 2014).
During Physical vapor deposition (PVD), solid or liquid silver material is heated up to its evaporation point in a high vacuum chamber, the vaporized atoms will travel in the vacuum and deposit onto the substrate (Fotovvati et al. 2019). Metals and polymers manufactured in AM have been used as with this method (Additive Manufacturing Media, 2020). A drawback of this process is that it is normally requires that parts to be plated must withstand temperatures of 145° C for a period of time. (Göran, personal communication)
In Chemical vapor deposition (CVD), the substrate is exposed to vaporized chemicals that will be deposited onto that substrate after a chemical reaction. Polymers and metals can be silver plated using this method, however, since the substrate must be heated up to very high temperatures, it is limited to heat resistant materials. There are 3 types: Atmospheric pressure CVD, low-pressure CVD and ultra-high vacuum CVD (Fotovvati et al. 2019).
Cold spray process is also known as cold gas dynamic spraying. This process is carried out at low temperatures that result in low efficiency and low reliability (Fotovvati et al. 2019). Metal powder is not melted, but only accelerated to high velocities through a nozzle, when the particles impact with the substrate, their high energy results in plastic deformation and the creation of a deposition layer (Lupoi & O'Neill, 2010). Silver showed almost of the bulk conductivity in the as-deposited state after cold spray (Chavan et al. 2011). Another similar process is plasma spraying, fast and cheap but less controllable. Here, metal powder or wire is passed through a high-speed plasma. The plasma spray is directed onto a part and the metal melts to the part. Wood, paper, plastics, metals and ceramics can be treated. The surface roughness is normally high as the surface is built up by splashing droplets (Göran Poshman, personal communication).
2.6. 3D printed parts orientation
3D-printed parts accuracy is highly affected by the part orientation in the build volume. For any AM process, a cylinder printed vertically is expected to have more accuracy and less surface roughness than the same cylinder printed horizontally. The latter could show a stair-step effect (Gibson et al. 2015).
AM components are generally complex; therefore, it is not clear which orientation would be optimal. When making this decision, other parameters may come into play such as building time, need of support, required surface roughness, accuracy, desired part strength in a defined direction or building cost. In some processes like MJF, parts can be stacked to achieve more pieces per build volume, thus, reducing printing costs (Gibson et al. 2015; Romero, 2020)
Cheng et al. (1995) developed an algorithm for determining the optimal orientation of SLA parts using a multi-objective approach (taking into account the specified accuracy and the building time). In 3D printing, the XY plane is located at the part base and the building or slicing direction corresponds to the Z axis.
26 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Figure 12 adapted from Cheng et al. (1995:16) illustrates the different planes that may be present in a 3D- printed part.
Figure 12: Types of planes, being slicing direction the building direction adapted from Cheng et al. (1995:16)
Industries such as Sculpteo have developed software that find the best part orientation. In Sculpteo's case, their Agile Manufacturing Technology software focuses on automatically orienting metallic parts using a parametric algorithm that minimizes supports. The user can weigh some of the algorithm´s parameters such as supported surface area, height in the building direction, etc. Moreover, orientation can be done manually for SLS polymer parts that do not need supports (Richardot, 2017).
2.7. Surface roughness
Surface roughness is defined as “the deviation from the geometrical ideal shape, where the ratio of feature length to height is small”. In a roughness profile, position is plotted on the X axis versus profile height in the Y axis (Gold & Helmreich, 2017) as seen in figure 13.
There are seven different standard height-based surface roughness metrics that are used to describe a surface: arithmetic mean (Sa/Ra), root mean square average (Sq/Rq), distribution skewness (Ssk/Rsk), distribution kurtosis (Sku/Rku), maximum peak height (Sp/Rp), maximum valley depth (Sv/Rv) and maximum peak to valley (Sz/Rz). R accounts for linear roughness that is measured along a length while S accounts for areal roughness that is measured along a surface (Tomkowski, 2019).
Ra parameter is paramount for this project. “Ra represents the arithmetic mean of the absolute ordinate Z(x) within the sampling length l” (Tomkowski, 2019), exemplified in Figure X (Tomkowski, 2019:43).
Figure 13: Ra - Arithmetic mean deviation (Tomkowski, 2019:43)
27 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Surface Roughness in AM
Staircase effect is one of the causes of the poor surface finish during 3D printing. Accuracy is reduced when slicing the 3D CAD model into 2D layers as shown on figure 14 (adapted from Kumbhar & Mulay, 2016:3). It occurs along inclined faces. The steeper the slope, the worse the stair step is. The thinner the layer thickness is, the smaller the stair step, thus, the surface roughness will be lower (Kumbhar & Mulay, 2016; Cheng et al. 1995).
Figure 14: Illustration showing the slicing process and the related staircase effect (Kumbhar & Mulay, 2016:3)
In RF filters, the roughness is not considered only as a surface grated perpendicular to the direction of the signal propagation but also, an isotropically corrugated surface. This isotropy will depend on the AM process (Gold & Helmreich, 2017).
As previously mentioned, surface roughness is very dependent on the build orientation of that surface. For the chosen AM processes, the relationship between part orientation and surface roughness is analyzed. Starting with the Metal BJ process, similar Ra is achieved in horizontal and vertical faces of the printed parts (Hart, 2018d). SS316L parts were printed with orientations of 0˚, 22.5˚ and 45˚ from the horizontal, concluding that the best Ra is achieved in horizontal surfaces (0˚) and that Ra increases with increased part orientation angle (Myers et al. 2019).
Some SS316L parts printed using SLM showed less surface roughness in their horizontal (0˚) surfaces. Ra decreased with angle approaching the vertical and more homogeneous finishing occurred in downwards surfaces (Strano et al. 2013). Both sides of Ti6Al4V components (upward facing surface and underside of the component) showed more uniform Ra when the SLM component was oriented in the range 0˚-30˚ from the horizontal (Triantaphyllou et al. 2015). When printing Inconel 718 parts, vertical build direction produced the lowest internal surface roughness of the cylindrical channels of all tested parts. Also, the maximum inscribed diameter was closest to the CAD drawing (Snyder et al. 2015).
As already mentioned, MJF parts do not need supports during printing, hence, the part orientation in the build volume can be flexible. Orienting PA12 parts at 30˚ provides a good balance between sharp detail and smooth surfaces. Surfaces located in the XY plane show better dimensional accuracy but slightly rougher surface finishing in upfacing surface. Moreover, downfacing surfaces show very good finishing (Romero, 2020; Caron et al. 2019)
In the SLS process, also the parts have more freedom of orientation since no supports are needed. The orientation in Duraform polyamide parts played a more significant effect than the layer thickness in the average surface roughness. Higher surface roughness appeared at angles of 0˚and 90˚ from the horizontal. Better finishing occurred in downward sloping surfaces due to filleting effect, this is the flow of molten
28 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
polymer filling the stair steps caused by staircase effect. Also, Ra is minimal along the angle range of 45˚- 90˚ (Strano et al. 2011).
In SLA, parts should be oriented to minimize the projected area; this means that the contact between the printed layer and the tank is reduced. Hence, the print withstands less force every time the build platform goes up with each layer (Hart, 2018b; Formlabs, n.d.). Printing large and flat surfaces at 10-20˚ inclination is recommended by Formlabs (n.d.). Printing SLA parts with orientations of 95˚, 125˚ and 135˚ resulted in less roughness (<10 µm) in downward sloping surfaces (90˚ - 150˚) than upfacing surfaces (Campbell et al. 2002). Higher surface roughness occurs at 0˚ (Hart, 2018b). CLIP printing providers like Xometry (n.d.) suggest orienting large broad flat sections at a min 15-20˚ from the platform to minimize cross-section per layer position.
2.8. Experimental setups
Surface roughness measurement equipment
To measure the surface roughness different measurement methods, such as stylus, optical, electrical and pneumatic methods can be found (Tomkowski, 2019). These methods measure the raw profile to then perform a low-pass filter in the extracted profile using a cutoff wavelength to separate roughness from waviness (Gold & Helmreich, 2017; Tomkowski, 2019).
A perthometer by Mahr (Figure 15) is a stylus probe contact measurement device located at Ericsson facilities that is used to obtain Ra values from the surfaces of the small cavities and waveguides. The probe covers 5.6 mm of the surface but only 4 mm are considered for the gathering of parameters, 5 measures in that length. A common display is shown in figure 16.
Figure 15: Probe of perthometer Figure 16: Common display of perthometer readings
29 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Dimensions & Tolerances measurement equipment
ATOS non-contact scanner by GOM Optical Measurement Techniques is available at Ericsson facilities to quality-check the dimensions of the parts as well as nominal actual comparisons. After attaching some reference points on the part (see figure 17), it is placed in a rotating table that automatically will turn while the camera is scanning the component. After it, a 3D model is generated and GOM software can be used to compare that model with the CAD. By setting the admitted tolerances, the software can show a color gradient in the part depending on that tolerance.
Figure 17: Scanner measurements. a) Part with reference dots, b) Experimental set up, c) Detail of the scanning camera
Insertion losses measurement equipment or waveguide test equipment
Radio frequency Vector Network Analyzer (VNA) is a test instrument that measures the transmitted and reflected waves as a signal passes through a device under test (see figure 18 from Electronics Notes (n.d.). By measuring these signals across a desired frequency band, the device characteristics can be determined such as transmission and reflection parameters. Another way of expressing transmission coefficient (S21) is the insertion loss, similarly, reflection coefficient (S11) can be expressed as return loss (Electronics Notes, n.d.).
Figure 18: Basic concept of a VNA according to Electronic notes (n.d.)
Transition connectors (see figure 19) are needed to connect the circular waveguide to the VNA. VNA provides a file .s1p or .s2p, an ASCII text file that contains information about n-port network parameter data and noise data of linear active devices, passive filters or interconnect networks. This file can be exported (2D Matrix) and read as .txt. Images/graphs can be edited later using software such as MATLAB or Python.
30 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Figure 19: Experimental set up where the device under test is the circular waveguide
A metal lid manufactured by metal sheet is needed to connect the small cavity to the VNA.
3. Experimental approach
This section describes the printed components and their requirements; subsequently, the selection matrices for AM processes and for finishing methods are illustrated, together with the description of the chosen silver coating method. At the end, the design of experiments subsection includes the definition of the batches as well as the measurements and tests that were performed.
3.1. 3D-Printed components
The pieces to be printed are a circular waveguide in figure 20 and a small cavity filter i figure 21. Small cavity dimensions are 38,5 x 38,5 x 37 mm with a thickness of 2,5 mm and waveguide length is 150 mm with a thickness of 2,5 mm and inner diameter is 25 mm. The design of the waveguides changes depending on the AM process. Metal and polymer were used as feedstock. Both filters work similarly to a rectangular waveguide WR90, its recommended frequency band ranges from 8.20 to 12.40 GHz. Small cavity filters work with frequencies around 5 GHz and waveguides around 10 GHz.
Figure 20: Circular Waveguide Figure 21: Small Cavity Filter
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The part orientation with respect to the 3D printing build table was considered.
Figure 22: β = orientation angle of the parts in the build area
β=0°, the waveguide is placed horizontally (figure 24) and the small cavity is placed on its bottom face (figure 23) β=30°, the waveguide is placed according to figure 24 and the small cavity is placed according to figure 23 β=90°, the small cavity is placed on its side face (figure 23)
Figure 23: Small cavity filter orientations (0°, 90° and 30°) Figure 24: Waveguides orientations (0° and 30°)
The 30。orientation was chosen since the surface roughness of the inner faces of the parts was aimed to be asymmetric. Thanks to that, 2 testing setups can be obtained while testing only one piece in different positions. The magnetic field interacts with the internal surfaces of the circular waveguide when the signal passes through. For instance, in one setup as shown in figure 25, the area of interaction is the two red surfaces. By turning the waveguide 90 degrees, the second possible setup is obtained where the area of interaction is represented in yellow.
Figure 25: Simplified rectangular magnetic field in a circular waveguide
32 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
3.2. Selection Matrix for AM processes
The information gathered from literature review showed in section AM process capabilities was used to assess the AM processes versus the products requirements. The chosen scoring system ranges from 0 to 3, as shown in table C, being 0: no; 1: low; 2: medium; 3: yes, no support material and high; and -: information not available. Table 3. Scoring system for AM technologies Scoring system Ra (µm) Build rate (cm3/h)
1 Low 25 - 30 <20
2 Medium 20 - 25 20 - 150
3 High 10 - 20 >150
For smooth surface, surface roughness with Ra<30 µm is targeted. When these selection matrices were developed the length of the waveguide was supposed to be 250 mm, hence, the maximum part dimensions were selected to be <300 mm so the parts can be fitted in the build volume. In the conventionally machined waveguides, the wall thickness is 3-4 mm but redesign considerations for future parts were considered when aiming for a wall thickness of 1 mm. The dimensional accuracy of the parts after being printed should comply with ±0.2 mm of tolerances.
Starting with AM processes that use polymer feedstock, polymers with heat deflection temperature (HDT) larger than 100 °C are targeted since in-service filters must withstand temperatures of 100°C. Some polymers that can be printed could be PA12, PerFORM and PA-AF (Aluminum filled).
Looking at the results of the selection matrix shown in table 3, it is seen that SLS, MJF, SLA and CLIP are scored higher. This is due to these processes accomplishing most of the company's needs and the requirements of the parts printed. The most restrictive requirements are those related to the material properties, as the polymers must withstand more than 100°C and not all processes are able to print with materials like PerFORM and PA-AF. Regarding dimensions, not all processes allow pieces longer than 300 mm, being also a high restriction when classifying.
SLS is the highest score as it can print in a wide variety of materials and the material support can be removed easily, however parts were not printed using this method due to time and cost constraints. MJF collects most of the requirements with a full score and it is convenient as it is less expensive than SLS, as the laser beam cost requires high amounts of energy. In the case of SLA, commonly the support material removal is less easy than compared to the previous mentioned but, for some prints support material is very fine and almost just barely hangs onto the material. SLA can print materials like PerForm. CLIP is selected because it has better part accuracy and surface finish than other AM processes.
According to budget and time limitations, only MJF was used to print parts.
33 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Table 4. Selection matrix for AM processes with polymer feedstock AM Technologies FDM SLA DLP CLIP MJ DOD BJ MJF SLS LOM Requirements
Print polymers (HDT>100°C) 3 3 3 3 0 0 0 3 3 0
Print PA12 3 3 3 3 0 0 0 3 3 0
Print PerFORM 0 3 0 0 0 0 0 0 0 0
Print PA-AF 0 0 0 0 0 0 0 0 3 0
Smooth Surface 2 2 3 3 3 - 3 3 3 0
Part Size (L<300 mm) 3 3 0 3 3 0 3 3 3 3
Wall Thickness: 1 mm 3 3 3 3 3 - 3 3 3 3
Ease to remove support material 2 2 2 2 3 3 3 3 3 3
Build rate (cm3/h) 1 2 1 2 1 - 3 3 2 2
Dimensional accuracy: ±0.2 mm 3 3 3 3 3 - 3 3 3 0
TOTAL 17 21 15 19 0 0 0 21 23 0
For metal AM processes, metal feedstock such as AlSi10Mg, Stainless steel 316L and Cu are targeted. From the selection matrix shown in table XI, metal BJ and SLM scored higher since the previous metal feedstock can be printed using these methods. These technologies have high capabilities in terms of large build space, meet the required dimensional tolerances and create relatively smooth surfaces (< 20 µm). Among the three technologies, metal BJ do not need supports while SLM supports are time-consuming to remove after the print. Regarding the build rate, BJ is the fastest process followed by SLM.
According to budget and time limitations, only SLM is used to print parts.
34 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Table 5. Selection matrix for AM processes with metal feedstock AM Technologies NPJ Metal HP SLM EBM LOM UAM LENS LMD EBAM Requirements BJ Metal Jet
Print AlSi10Mg 0 0 - 3 0 0 0 0 0 0
Print 316L 3 3 3 3 3 3 3 3 3 3
Print Cu 0 3 0 3 3 3 3 3 - 3
Smooth Surface 3 3 - 3 1 0 - 0 0 0
Part Size (L<300 mm) 3 3 0 3 3 3 3 3 3 3
Wall Thickness: 1 mm 0 3 0 3 3 3 - 0 3 -
Ease to remove support 3 3 3 1 1 3 3 3 3 3 material
Build rate (cm3/h) - 3 - 3 2 2 3 1 3 3
Dimensional accuracy: 3 3 0 3 0 0 0 0 - 3 ±0.2 mm
TOTAL 15 18 6 19 13 14 12 10 15 15
3.3. Selection Matrix for post-processing methods
The information gathered from literature review showed in section 2.3 was used to assess the post- processing methods versus products requirements. The chosen scoring system ranges from 0 to 3, as shown in table 6, being 0: no; 1: low; 2: medium; 3: yes and high; and -: information not available.
Table 6. Scoring system for post-processing methods Scoring system Material removal rate (MRR) Processing rate (parts/h) (µm/min)
1 Low <2 <60
2 Medium 2 - 100 65 - 390
3 High >110 >400
35 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
For smooth surface, surface roughness with Ra<10 µm is targeted. The printed parts must fit the post- processing volume (<300 mm). Parts with wall thickness of >1 mm must be treated without any damage. Their material removal rate as well as processing rate must be reasonable. The smoothing processes must be accurate (with ±0.1 mm of tolerances). It is paramount that the inner surfaces are post-processed since they are functional surfaces in both small cavities and waveguides. To produce a component with high accuracy, the printed parts would be compensated for surface treatment loss and buildup, e.g. the Hirtisation process removes 200-600 µm depending on degree of surface finish. This would be compensated in the CAD geometry.
As shown in table 7, the most suitable finishing processes that can treat both metals and polymers are laser micromachining and media blasting, and vapor smoothing for just polymers. These methods are selected based on the requirements proposed, being outstanding over the others because they allow longer parts than 200 mm, achieve high accuracy and do not cause part nor edge damage.
Vapor smoothing exposes the plastic printed parts to a chemical vapor improving surface roughness and clarity and leave excellent finishes in the internal features of the components. In the case of laser micromachining, the parts are ablated by very short pulses however, the thermal zone is difficult to control, and tool wear may appear. Regarding media blasting, it is quite used to remove support structures as it is inexpensive and achieves effective results even in complex geometries.
Table 7. Selection matrix for finishing processes for both metal & polymer parts Finishing Tumbling Vibratory Media Micro Laser Chemical Vapor Ultrasonic processes blasting machining micro etching smoothing abrasion process machining (only for polymers) Requirements
Part Size 3 3 3 0 3 0 3 0 (L<300 mm)
Smooth Surface 3 3 3 3 3 3 3 3
Treat wall 3 3 3 3 3 3 3 3 thickness: 1 mm
MRR (µm/min) 1 1 2 3 1 2 2 3
Accuracy (±0.1 2 2 2 3 3 2 3 3 mm)
Can treat inner 1 1 2 3 2 3 3 3 surfaces
Processing rate 1 1 2 3 3 2 1 3 (parts/h)
No part/edge 1 1 2 2 2 3 2 3 damage
TOTAL 15 15 19 0 20 0 20 0
36 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
From the selection matrix of methods that can only process metal parts shown in table 8, the Hirtisation process, electro-chemical polishing and plasma polishing scored higher than the other methods. This is due to their high accuracy and ability of not damaging edges of the parts while processing. Regarding the processing rate, Hirtisation can process more parts per hour (until 500 parts using H12000 machine) than the other two methods. Also, it can reduce the surface roughness faster, followed by plasma polishing and lastly, ECP that can reduce the Ra in 1.4 µm per min.
Since Plasma polishing has difficulties to treat inner surfaces and Hirtisation is an improvement from the common ECP methods, parts are post-processed using Hirtisation.
Table 8. Selection matrix for finishing processes for only metal parts Finishing processes Shot AFM Electro- Hirtisation Drylyte Plasma Peening chemical polishing (ECP) (ECP) polishing Requirements (ECP)
Part Size (L<300 mm) 3 3 3 3 3 3
Smooth Surface 3 3 3 3 3 3
Treat wall thickness: 1 mm 3 3 3 3 - 3
MRR (µm/min) 1 1 1 3 - 2
Accuracy (±0.1mm) 1 1 2 3 - 3
Can treat inner surfaces 2 3 2 3 3 1
Processing rate (parts/h) 2 2 2 3 - 2
No part/edge damage 1 2 3 3 3 3
TOTAL 16 18 19 24 12 20
Post-processing stages and Machining
The figure 26a exemplifies how SLM parts look as printed, sometimes they need to be de-powered after being taken out from the printing chamber, others need to be machined out of the bed table. In this case, since the waveguide was printed at 30˚, support material was needed during printing. Figure 26b shows that waveguide after being treated with Hirtisation.
37 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Figure 26: SLM waveguides. a) Waveguides with supports after printing b) Waveguides after Hirtisation
Different stages of post-processing using Hirtisation are studied to visualize and compare their effects to the final output: Stage 1: Support and powder removal (after the minimum treatment time required) to a Ra value Stage 2: General leveling (1/4 T min) to a Ra value lower than Stage 1 Stage 3: General leveling (2/4 T min) to a Ra value lower than Stage 2 Stage 4: High polishing (T min) to a Ra value lower than Stage 3 At Ericsson facilities, holes are drilled, and contact surfaces are machined to comply with flatness tolerances. The griping structures close to each flange in the waveguides (see figure 26b) were designed to allow better gripping during machining, reducing deviations caused by vibrations. They allow the two surfaces to be machined in the same setup which guarantees that both surfaces are parallel. Machining is done after pre-treatment (Hirtisation) but before silver plating.
3.4. Chosen Silver coating method
All parts are silver coated with the same silver thickness, that ranges between 3-6 µm. As the silver coating method was already chosen, a selection matrix was not considered in this case. The silver thickness should be bigger than the skin depth. Pasternack’s skin depth calculator returns skin-depth as a function of a material’s resistivity and permeability and the frequency of the signal. The customized silver material has a resistivity of 1.70 μΩ cm and a relative permeability of 0.9998 are used in the calculation.
Table 9. Input and output variables according to the Pasternack´s skin depth calculator Component Frequency range (GHz) Skin depth (µm)
Small cavity filter 1-10 2.08 - 0.656
Circular waveguide filter 8.2- 12.5 0.725 - 0.587
Ericsson has both electroplating and chemical systems providers. In the case of metals, vendors use technologies such as Candor, Atotech or MacDermid-Enthone; they use electroplating systems as they are much more effective than chemical plating when it comes to thicker layers (2 µm). When plating plastics
38 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
and non-conductive materials, providers most often start with an activation and a chemical layer. Then after that goes an electroplating layer, copper, copper plus silver or just silver.
Vendors
Ericsson Prototyping and Production facilities have a HP Jet Fusion 540 3D printer (MJF).
Materialise is a 3D printing service provider established in Belgium. They offer SLS, SLA Polyjet, SLM, DMLS and MJF, among others. Their lead time is 10 (+2 days) from start to delivery of printed parts. They also offer post-processing services through their network of suppliers.
Hirtenberger Engineered Surfaces is a process provider and technical partner for innovative functional metallic surfaces. They offer smart production processes for electrochemical surface finishing. They have developed the Hirtisation process to post-treat AM metal parts. They are located in Austria.
Metalizz is a service provider of post-processing of printed parts established in France. SLS, SLA FDM, parts can be treated. They also offer chemical metallization processes of plastic parts by spraying at room temperature and pressure with their METALFOG machines. Metalizz uses their own chemical CVD- system, a cold process that operates at 25-40 degrees. Normally the CVD systems are warm.
Bomans Lackering AB is a coating and surface treatment service provider for silver coating they use processes from Candor and MacDermid-Enthone.
3.5. Design of experiments
In this section the indexing of the parts and batches is described per chosen AM technology, as well as how the different measurement and test were carried out.
Indexing of parts and batches
All parts have their unique number or index to enable traceability through the flow steps and keep them traced. PXSY.Z
● X represents the AM process (P) carried out for each part, see table below.
Table 10. Indexing of AM processes AM process Indexing
SLM Process 1 (P1)
MJF Process 3 (P3)
● Y represents the batch (S) in which the piece is included, one vendor can print more than one batch.
● Z represent the part position in each set
39 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
SLM (P1)
Each supplier (Materialise and GKN) printed 10 parts, 6 circular waveguides and 4 small cavity filters in 2 batches (see figure 27).
Materialise parts were indexed as P1.S1.X - P1.S2.X. The experimental conditions of the batches are shown in Table 11. ● Batch 1: 4 small cavity filters at 30° and 2 waveguides at 0° (P1S1.1-.4; P1S2.1-.2) ● Batch 2: 4 waveguides at 30° (P1S1.1-.4)
Table 11. SLM batches printed by Materialise Experimental conditions
Batch Type Part Indexing Orientation Post-processing stage
1 Small cavity P1S1.1 30° As printed, removal of supports and powder residues
Small cavity P1S1.2 30° General leveling I
Small cavity P1S1.3 30° General leveling II
Small cavity P1S1.4 30° High polishing
Waveguide P1S2.1 0° High polishing
Waveguide P1S2.2 0° High polishing
2 Waveguide P1S1.1 30° As printed, removal of supports and powder residues
Waveguide P1S1.2 30° General leveling I
Waveguide P1S1.3 30° General leveling II
Waveguide P1S1.4 30° High polishing
GKN parts were indexed as P1.S3.X - P1.S4.X. The experimental conditions of the batches are shown in Table 12. ● Batch 1: 4 small cavity filters at 30° and 2 waveguides at 0° (P1S3.1-.4; P1S4.5-.6) ● Batch 2: 4 waveguides at 30° (P1S4.1-.4)
40 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Table 12. SLM batches printed by GKN Experimental conditions
Batch Type Part Indexing Orientation Post-processing stage
1 Small cavity P1S3.1 30° As printed, removal of supports and powder residues
Small cavity P1S3.2 30° General leveling I
Small cavity P1S3.3 30° General leveling II
Small cavity P1S3.4 30° High polishing
Waveguide P1S4.5 0° High polishing
Waveguide P1S4.6 0° High polishing
2 Waveguide P1S4.1 30° As printed, removal of supports and powder residues
Waveguide P1S4.2 30° General leveling I
Waveguide P1S4.3 30° General leveling II
Waveguide P1S4.4 30° High polishing
a) b)
Figure 27: SLM batches. a) Batch 1, b) Batch 2
The griping structures close to each flange in the waveguides were designed to allow better gripping during machining, reducing deviations caused by vibrations. They allow the two surfaces to be machined in the same setup which guarantees that both surfaces are parallel. All parts are silver coated.
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MJF (P3)
One batch was printed at Ericsson HP machine, it consisted of 36 small cavity filters, 12 filters per orientation (0°, 90°, 30°) as shown in figure 28. More details can be found in Appendix 1.
Figure 28: MJF batch displayed in HP software
Tests performed
Experimentation was divided into 6 main steps, as shown in the following flow chart displayed in figure 29.
Figure 29: Flow chart of the experimentation steps
42 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
The methodology to dimensional measure and inspect the 3D printed parts is the following (a to e). The parts are scanned individually using the GOM equipment (figure 30a); all the scanned data create a 3D Mess in stl. format that can be opened by GOM Inspect Software (figure 30b); the CAD file of the part is now imported into the software in the same space than the 3D mesh (figure 30c); after, both actual and nominal parts are aligned (figure 30d) to compare dimensions; finally, a color scale can be adjusted according to tolerances (±0.2 mm). The negative and positive deviations appear symmetrically in the legend. The red indicates that the material lies above the CAD model surface and the blue indicates that the material lies below the CAD surface (figure 30e).
a) b) c)
d) e)
Figure 30: Steps of Quality check of the parts by means of 3D scanner (a-e)
The surface roughness of the 4 inner faces of the small cavity filter are measured (see figure 31).
a) b)
Figure 31: Cavity measurements. a) Surfaces measured with perthometer, b) Measurement paths made in a face
The surface roughness of the inner cylinder of the waveguide is measured in 4 measuring paths, 4 measures per hole opening as shown in figure 32.
43 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
a) b)
Figure 32: Waveguide measurements. a) 4 measurement paths in the inner cylindrical surface by perthometer, b) Detail of measurement paths in one hole opening of the waveguide
4. Analysis of Results
This section describes and analyzes the results from the dimensional inspection of the 3D printed parts, from the surface roughness measurement of some of the components.
4.1. Dimensional measurements of MJF parts
The software GOM Inspect 2019 is used to compare the dimensions of the CAD part and every MJF printed part and analyses how it complies with the tolerances (± 0.2 mm). Generally, 30° oriented parts meet the tolerances in a better way than the other orientations, it also shows less negative deviation (less blue color appears in the inspection), this is exemplified in the figure 33. Most of the 0° oriented parts show more negative deviation (blue color) at the bottom part, this is exemplified in the figure 34. Some 90° oriented parts show differences between contiguous faces (out of tolerance in both positive and negative side, this is exemplified in the figure 35.
Figure 33: Inspected 30° printed part Figure 34: Inspected 0° printed part Figure 35: Inspected 90° printed part
4.2. Surface Roughness Measurements
SLM printed from GKN
Table 13 depicts the results of the surface roughness measurements of the cavities per face and of the waveguides per path. Moreover, the uncertainty associated to the average values is shown as the standard error or standard deviation of the mean.
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Table 13. Surface roughness results for SLM printed parts at GKN Type of part SLM Orientation Total Ra Total Ra (µm) with standard (µm) error
Face 1 30° 11,28 11,28 ± 0,4 µm
Face 2 30° 8,29 8,29 ± 0,24 µm Cavities Face 3 30° 8,67 8,67 ± 0,44 µm
Face 4 30° 7,77 7,77 ± 0,19 µm
Path 1 30° 12,25 12,25 ± 0,55 µm
Path 2 30° 9,05 9,05 ± 0,45 µm
Path 3 30° 10,28 10,28 ± 0,43 µm
Path 4 30° 9,16 9,16 ± 0,36 µm Waveguides Path 1 0° 25,94 25,94 ± 1,5 µm
Path 2 0° 5,98 5,98 ± 0,38 µm
Path 3 0° 13,99 13,99 ± 1,96 µm
Path 4 0° 5,70 5,7 ± 0,15 µm
The surface roughness measures in the SLM printed parts showed that at 30° orientation, the Ra ranges between 7,16 - 13,16 µm (see figure 37a and figure 37c). For 0° orientation, the Ra ranges between 5,68 - 26,2 µm (see figure 37b). The waveguides printed at 0° needed support in the inner cavity (Figure 36) to prevent collapse of the structure during printing, this explains the drastic difference between minimum and maximum values of the range. The support is located between surfaces corresponding to paths 1 and 3.
Figure 36: Waveguides printed at 0 with supports in the inner surface
Starting with the cavities, in the 30° orientation (figure 38a), when comparing the total average of the surface roughness in each face, Face 1 is observed to have higher Ra in comparison with faces 2, 3 and 4 that follow a similar trend. Face 1 was printed downwards and face 3 upwards.
Moving to waveguides, the 0° orientation (figure 38b), the directions corresponding to the measurement paths 2 and 4 were printed simultaneously; this explains why they follow a very similar tendency. The direction corresponding to path 1 was printed downfacing, its Ra values are almost 5 times larger than Ra
45 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
from paths 2 and 4. The direction corresponding to path 3 was printed upfacing, its Ra values are between 2-3 times larger than Ra from paths 2 and 4 as seen in figure 38b. In 30° waveguides, direction 1 was printed downwards and direction 3 upwards. According to the graph in figure 38c, direction corresponding to path 3 results in higher surface roughness than the rest.
Figure 38d illustrates the standard deviation (std) of the Ra values per faces, the std corresponding to 30° is almost constant among the faces, in contrast, the std corresponding to 0° shows more variation between faces.
a) b)
c) d)
Figure 37: SLM parts printed by GKN. Orientation vs Ra. a) Ra in SLM cavities printed at 30°, b) Ra in SLM waveguides printed at 0°, c) Ra in SLM waveguides printed at 30°, d) Standard deviation of Ra of SLM waveguides
Comparing the total average of the surface roughness in each path, the lowest Ra is obtained in Path 4 in both orientations (Figure 38). Moreover, both orientations shore very close values of Ra for paths 2 and 4.
Figure 38: Total average Ra comparison between orientations (Waveguides by GKN)
46 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Errors on measurements are originated due to diverse reasons like human errors when measuring, remaining of supports in the surface, lack of cleaning of the parts, scratches, dust, etc.
MJF printed at Ericsson
The MJF parts were sandblasted in order to remove the powder from them, therefore, the as-printed surface roughness. Table 14 depicts the results of the surface roughness measurements per face of the cavity and considering the 3 different orientations (0°, 30° and 90°). Moreover, the uncertainty associated to the average values is shown as the standard error or standard deviation of the mean.
Table 14. Surface roughness results for MJF printed parts MJF Orientation Total averaged Total averaged Ra (µm) with standard Ra (µm) error
Face 1 0° 11,70 11,7 ± 0,31 µm
30° 10,81 10,81 ± 0,20 µm
90° 9,96 9,96 ± 0,21 µm
Face 2 0° 12,74 12,74 ± 0,34 µm
30° 12,47 12,47 ± 0,32 µm
90° 11,81 11,81 ± 0,29 µm
Face 3 0° 12,01 12,01 ± 0,3 µm
30° 13,28 13,28 ± 0,23 µm
90° 13,52 13,52 ± 0,28 µm
Face 4 0° 12,15 12,15 ± 0,27 µm
30° 12,14 12,14 ± 0,23 µm
90° 11,42 11,42 ± 0,20 µm
The surface roughness measures in the MJF printed parts showed that at 0° orientation, the Ra ranges between 8,7 - 15,2 µm (Fig. 41a); at 30° orientation, the Ra ranges between 9,2 - 15,3 µm (Fig. 41b); and at 90° orientation, the Ra ranges between 8,6 - 14,8 µm (Fig. c).
Figure 39 shows how the faces were placed during the MJF printing.
47 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Figure 39: Detail to see how the faces were oriented during printing
In 0°, the 4 faces were printed simultaneously layer by layer. According to the graph in Fig. 40a, every face follows a similar Ra tendency, being observable how the pair Face 1-3 and Face 2-4 follows each other closely. In 30°, face 1 was printed downwards and face 3 upwards. According to the graph in figure 40b, face 3 results in higher surface roughness than face 1. Also, face 2 and 4 follow a similar Ra tendency. In 90°, face 1 was printed downfacing and face 3 upfacing. According to the graph in figure 40c, face 3 results in higher surface roughness than face 1. Also, face 2 and 4 follow a similar Ra tendency.
Figure 40d illustrates the std of the Ra values per faces, the std corresponding to 0° is almost constant among the faces, in contrast, the std corresponding to 90° shows more variation between faces.
Figure 40: MJF graphs. a) Surface roughness in parts printed at 0°, b) Surface roughness in parts printed at 30°, c) Surface roughness in parts printed at 90°, d) Standard deviation of Ra of MJF parts
48 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
Comparing the total average of the surface roughness in each face, it is observed some similarities between faces. As seen in figure 41, faces 2 and 4 are paired and lead to similar surface roughness results. This behavior is expected as they are positioned similarly, specially in 30° and 90° orientation. The opposite case occurs for faces 1 and 3, instead of having similar results, a divergent trend is noticed as orientation is increased. Face 1 is turned from vertical orientation at 0° to downfacing at 90°, which explains its minimum surface roughness value. In the case of the face 3, for 90° degrees it is positioned up faced, giving the maximum value of surface roughness also of the entire results.
Figure 41: Total average Ra comparison between orientations -MJF Cavities
Errors on measurements are originated due to diverse reasons like human errors when measuring, lack of cleaning of the parts, scratches, dust, etc. Also, as it is the first time the machine has been run, some printing inaccuracies can be expected. In addition to that, the HP machine takes the powder that is not used to print and puts it back to the immediate tank, so each time only 20% of the powder is fresh and the rest is recycled; which reduces the total quality of the material.
The 2 parts printed at 30° that showed highest Ra (approx. 12,8 µm) silver coated by Metalizz (P3S3.9C & P3S3.9F) shown in figure 42. One part was only sandblasted before silver plating (left cavity in Figure 43), after the process, the average Ra is reduced to 7,1 µm. The second part was sandblasted and smoothed by a mechanical/electrochemical process, after it, the average Ra is reduced to 1,46 µm.
Figure 42. Silver plated Cavities
49 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
5. Conclusions and discussion
This investigation aimed to discover the insertion loss values in the cavity and waveguide filters that are 3D printed to compare them with the simulation values. However, the last steps of the experimental approach ( silver coating & measurements by VNA equipment) were not finished on time.
After analyzing the different AM technologies with use of selection matrices, SLM and MJF processes seem to highly fulfill the product requirements. The electrochemical smoothing process known as Hirtisation is optimal for treating metal filters. How the Ra is influenced by varying parameters such as printing orientation, stages of postprocessing is examined. Asymmetry between inner surfaces of the part is intended to later test the behavior of the signal in 2 different setups with only one part.
For SLM cavities printed at 30˚, the downfacing face showed higher surface roughness. There is some asymmetry between faces too. When comparing SLM waveguides, Ra values from 30˚ waveguides are lower the 0˚ ones since the latter imply the creation of supports. However, the 0˚ waveguides can be used to validate the ability of Hirtisation post-processing to remove inner supports.
For MJF cavities, printing at 30˚ shows a good balance between asymmetry in the Ra values of the faces and low average Ra value. The printed parts comply with the literature review in terms of what Ra to expect depending on the part orientation, as seen in face 1, a downfacing surface, showed the best finishing. The literature review also recommends printing PA12 parts at 30˚ to get smooth surfaces while obtaining sharp details. Moreover, after the dimensional inspection, the 30˚ printed cavities meet the tolerances in a better way than the ones oriented at 0˚ and 90 ˚.
5.1. Future work
It has not been possible to see how the printing orientation and how the post-processing stages affect the insertion loss and if Ericsson can handle more surface roughness than is anticipated by simulation. Due to this, several corrections and implementations can be developed in the future. Trying different AM technologies and optimizing the results for each process is the most immediate future solution as only a few technologies were studied. Same case occurs in post-processing, also new technologies may be an advantage and give better results. For each AM technology and post-process, parameters like orientation, time or support material can be tested.
In this master thesis project two pieces are developed, a waveguide and a cavity filter, in the next future the same work can be carried out for new pieces, starting from the work already done and taking the results into account. VNA experiments can be tested in a future time, to obtain better insertion losses results. New materials like PerForm or others in development can be printed as new implementations of the project.
The main drawback of additive manufacturing is the low construction time, that is why the applicability of AM in volume production is still a dream. The study of the applicability in volume production is a big topic that depends on many aspects, but a good approach can lead to a large reduction in money and time, being an answer in search of a greener future.
50 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
6. Appendices
6.1. Appendix 1: Indexing of MJF parts
Experimental conditions Experimental conditions
Part Indexing Orientation Part Indexing Orientation
P3S1.1 0° P3S3.9A 0° P3S1.2 90° P3S3.9B 90° P3S1.3 30° P3S3.9C 30° P3S1.4 0° P3S3.9D 0°
P3S1.5 90° P3S3.9E 90° P3S1.6 30° P3S3.9F 30° P3S1.7 0° P3S3.9G 0° P3S1.8 90° P3S3.9H 90° P3S1.9 30° P3S3.9I 30° P3S2.9A 30° P3S4.1 0° P3S2.9B 90° P3S4.2 90° P3S2.9C 30° P3S4.3 30° P3S2.9D 0° P3S4.4A 0° P3S2.9E 90° P3S4.4B 90° P3S2.9F 0° P3S4.4C 30° P3S2.9G 30° P3S4.4D 0° P3S2.9H 90° P3S4.4E 90° P3S2.9I 0° P3S4.4F 30°
51 Additive manufacturing and radio frequency filters: A case study on 3D-printing processes, post-processing and silver coating methods
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