Investigation into Applicability of 3D-Printed Composite Polymers with Enhanced Mechanical Properties in the Development of Microwave Components

Abstract

Additive manufacturing is currently regarded as one of the enabling technologies for Space Economy since it allows for the reduction of lead time and costs of payloads and platforms. Typically, metal-based additive manufacturing technologies are considered for the development of microwave components for Space applications since they exhibit the best trade-off in radio-frequency performance, benefits, and withstanding adverse environmental conditions. In this view, composite polymers may further increase the benefits arising from the 3D printing of microwave components since lighter parts with the required thermal, mechanical, and RF performances can be placed on board satellites. This paper explores the feasibility of 3D-printed composite polymers, including Ultem and PEEK reinforced with carbon fiber, for the development of microwave waveguide devices intended for Space applications. To this end, three different manufacturing routes were investigated by selecting a specific composite polymer, the corresponding manufacturing system and post-processing, and the necessary metal-plating technique. Hence, relevant radio-frequency test vehicles operating at 10 ÷ 14 GHz were designed, manufactured, and tested. The experimental results prove that waveguide components operating in X and Ku bands can be developed through the material extrusion of PEEK reinforced with carbon fiber, which is subsequently metalized by means of a two-stage electroless/electroplating process.

1. Introduction

Additive Manufacturing (AM), also widely known as 3D printing, encompasses several technologies through which parts are built by adding material layer-by-layer instead of the standard machining techniques. Since their introduction, AM technologies have disrupted many industrial and application sectors, including aerospace and medicine [1]. More recently, AM technologies have also been changing the way Radio Frequency (RF) payloads for space applications are engineered [2].

Traditionally, RF equipment used in Space applications is metallic- or ceramic-based components. These materials are good for the expected RF performance and the extreme conditions of the Space environment. However, in Space applications, weight plays a key role because of the high cost of launching payloads, and hence, the use of polymers can reduce the overall mass of satellites/spacecrafts, thus lowering launch costs and making payload management more efficient. AM technologies, which have already been investigated for the 3D printing of RF components based on polymers, are mainly Material Extrusion (MEX), Stereolithography Apparatus (SLA), and Polymer Powder-Bed Fusion Laser-Based (PBF-LB/P) processes [3]. The state-of-art of RF components 3D-printed in polymer includes filters [3,4,5,6], diplexers [7], ortho-mode transducers [8], and antennas [9,10,11].

Typical polymers used in the 3D printing of RF components are thermoplastic or photo-sensitive materials, such as Acrylonitrile Butadiene Styrene (ABS) and PoLylActide (PLA), which are subsequently metal coated. For Space applications, common polymers may not be suitable because of the harsh environmental conditions. For this reason, composite polymers, such as Ultem, PEEK, and Carbon-Fiber reinforced PEEK (CF-PEEK), are advantageous in terms of mechanical and thermal properties and long-life stability since these materials exhibit superior properties when compared to traditional polymers [12]. Indeed, they are designed to withstand higher heat and mechanical stress. Additionally, they do not release many gases, making them suitable for Space applications where volatile compounds can contaminate sensitive equipment, interfere with sensors, or damage the environment in a spacecraft. RF equipment also undergoes significant ionizing radiation, which can break down its materials over time. However, composite polymers are more resistant against radiation damage compared to other materials. These materials also have an impact on durability; they show high strength, stiffness, and resistance to wear. This helps RF parts survive the physical stress of Space launches, vibrations during use, and strain from temperature shifts in Space. For electromagnetic waveguide applications, polymeric parts must be metal coated to achieve the required low-loss transmission characteristics. To this end, different processes can be used, including silver/copper painting [7] or spraying [9], electroless plating [11], and Physical Vapor Deposition (PVD) [13]. Subsequently, the quality and thickness of the metal coating may be further improved by electroplating the parts [14]. For Space applications, minimization of mass and envelopes is a key requirement, which can be achieved by developing monolithic (i.e., single-part devices) waveguide devices. These parts do not have any flanges and screws, but they exhibit complex internal channels to be metalized.

In this context, the present paper reports on the applicability of 3D-printed composite polymers, including Ultem and CF-PEEK, in the development of microwave waveguide components for Space applications. Three manufacturing routes were first identified by selecting materials, manufacturing systems (including post-processing), and metal coating techniques (Section 2). Hence, relevant RF test vehicles and corresponding measurement setups were designed (Section 3). Finally, the test vehicles were manufactured and tested (Section 4). Assessment of the achieved results and prospects of further developments are discussed in Section 5.

2. Manufacturing Routes

2.1. Materials

Using metal-coated polymeric materials instead of metallic alloys is beneficial in terms of the final mass of the microwave components to be embarked on board the satellites, but it can be problematic because of Space requirements in terms of temperature, outgassing, stiffness, and metal coating adhesion.

Table 1. Physical properties of some relevant polymeric materials used in AM technologies.

Material	AM Technology	Density (g/cm3)	Tensile Strength (MPa)	Tensile Modulus (MPa)	Heat Deflection Temperature at 1.8 MPa (°C)
ABS	MEX	1.06	22	1627	76
ABS	SLA	1.21	42	1950	39
Accura Xtreme	SLA	1.19	40	1900	54
Alumide	PBF-LB/P	1.36	48	3800	144
CF-PEEK	MEX	1.34	87	3900	315
New White	SLA	1.23	55	2600	60
Nylon 12	MEX	1.01	48	1310	82
PA 12	PBF-LB/P	0.95	46	1700	86
PA 12 CF	PBF-LB/P	1.07	60	3654	172
PEEK	MEX	1.32	100	3792	160
PEEK	PBF-LB/P	1.31	90	4250	165
PerForm	SLA	1.61	68	10,500	82
Stonelike	SLA	1.3	52	4500	187
Ultem 9085	MEX	1.34	72	2220	153

lists physical properties, which are relevant for Space applications, of some polymeric materials that can be manufactured through Material EXtrusion (MEX), StereoLithography Apparatus (SLA), or Powder-Bed Fusion Laser Based on Polymer (PBF-LB/P) processes.

With reference to Table 1, density is an important parameter because it defines the capability of reducing the mass of microwave components to be embarked on board satellite platforms, while tensile strength and module are key mechanical parameters. Indeed, tensile strength is the ability of polymers to withstand a maximum amount of tensile stress without failure, while tensile modulus measures material stiffness, that is, the rate of deformation under a specific load. As a measure of the capability of materials to operate in demanding thermal conditions, which are typical of Space, the heat deflection temperature was considered. This parameter measures the polymer resistance to change at high temperatures under a given load (i.e., a 1.8 MPa pressure for the values reported in Table 1).

From the materials’ comparison reported in Table 1, three composite polymers were selected for the development of test vehicles relevant to microwave Space applications, namely aluminum-reinforced polyamide powder Alumide, Ultem 9085 (in the following, referred to as Ultem), and carbon-fiber-reinforced PEEK. These materials were selected as a compromise among their different physical properties while considering the availability of the corresponding AM systems.

2.2. Manufacturing Processes

The MEX process was selected to print Ultem and CF-PEEK, while PBF-LB/P was used to build parts in Alumide. In the following, details of these two AM technologies are reported.

2.2.1. Polymer Powder-Bed Fusion Laser-Based Process

The polymer powder-bed fusion laser-based process, also known as Selective Laser Sintering (SLS), stands as one of the most promising additive manufacturing methods for creating intricate components using polymer materials. The process relies on combining a low-powered CO₂ laser and a thermoplastic powder bed. The steps involved in the process are illustrated in Figure 1 [15]. At the start of the construction process, the building platform is placed in its initial position, and a base layer of polymer powder is spread over it. During the laser sintering process, a nitrogen flow inhibits the oxidation of the powder, while heating components surrounding the building chamber and the platform help to maintain the entire setting near the sintering temperature of the material being processed. The laser beam selectively strikes the cross-section of the components of each layer, allowing the exposed areas to bond with the previously solidified underlying layer. Subsequently, the building platform is lowered by the thickness of one layer, and a new layer of polymer powder is added. The powder is then exposed again, the platform is lowered, and more polymer powder is applied. This cycle continues until the final component is built.

A crucial element of this procedure regarding the sustainability of products is the ability to recycle the powder that is used at an earlier stage. Typically, the reclaimed powder is created by combining 50% of fresh powder, and it reflects the raw materials employed in industrial applications. Within the PBF-LB/P method, merely 10–20% of the powder contained in the part bed chamber contributes to the formation of the components. The remaining powder serves to support the parts throughout the manufacturing process. During the process steps (preheating, sintering, and cooling), powder experiences both physical and chemical degradation, yet it can be recycled and utilized again for additional uses. The preheating phase, alongside the heating systems of the chamber, contributes to an even temperature distribution within the chamber, minimizes thermal gradients, and helps to avoid uneven thermal shrinkages and distortions in the parts. Furthermore, the strength of the sintered powder allows for avoiding the use of support structures, thus providing greater design flexibility. Nevertheless, once the building procedure is completed, the printed components are enveloped in a soft agglomeration of warm sintered powder. A gradual cooling of the entire agglomeration is necessary until it reaches the glass transition and oxidation temperatures within the chamber. After the cooling phase concludes, the entire agglomeration (including parts surrounded by the sintered powder) is extracted from the chamber and cleaned manually to remove any excess powder. Near the surface of the components, the sintering impact is more pronounced, thereby requiring a mechanical process (such as compressed air and then shot peening with glass microsphere) to dislodge the powder adhering to the surface.

Among the various types of polymers, polyamide 12 (PA12) is one of the materials most used in the MEX process. PA12 is a semi-crystalline substance known for its extended hydro-carbon chain, minimal water absorption, and remarkable impact resistance. The introduction of metallic fillers can improve the strength and thermal characteristics of polyamide, enabling customized properties and increased usability. Aluminum-reinforced polyamide offers satisfactory mechanical attributes and excellent heat resistance, and it is cost-effective in producing powders and parts.

The commercially available aluminum-reinforced polyamide powder Alumide^®, developed by Electro Optical Systems-EOS GmbH (Krailling, Germany), was investigated in the study. The specimens were created with an EOS Formiga P110 Velocis printer. This printer provides a theoretical build space of 200 mm × 250 mm × 330 mm. Nevertheless, a segment of the building volume measuring 40 mm × 30 mm × 330 mm is occupied by the thermocouple that gauges the temperature of the powder bed for every layer and cannot be considered as usable volume.

The process parameters used for hatching are laser power of 25 W, scan speed of 2500 mm/s, and distance of 0.3 mm. The preheating time of the powder bed was set to 10 s. After the production, the total cooling time was 4 h. While cooling, components experience a decrease in volume due to the disparity in polymer density between their liquid and solidified states. High cooling speeds may lead to swift volume reduction, resulting in distortion and dimensional inaccuracies. Distortion primarily encompasses curling and changes in shape that can happen during the building and cooling stages. In the building phase, inconsistencies in the quantity of powder laid down can happen, particularly at the edges, because of excessive cooling of the separate layers within the process chamber. Conversely, in the cooling phase, neglecting the necessary cooling duration after completing the job and consequently removing the container holding the finished parts too quickly or opening the chamber door prematurely leads to swift cooling of the entire assembly. This results in uneven cooling patterns of the workpiece that occur upwards from the base and inwards from the outer surface and may lead to distortions in the lower layers of the workpiece.

Shrinkage can differ depending on the material used and may be non-uniform along different axes. Throughout the operation, the heating systems and the preheating stage help to balance the thermal variations, thus reducing shrinkage. Nevertheless, during the cooling phase, the effects of conduction combined with a rapid cooling rate along the building chamber walls and the poor conductivity of the polymer powder leads to a thermal gradient in the xy-plane and in the build direction (z-axis), creating warmer areas in the center. Typically, cooling happens slowly due to the dependence on conduction through the powder layer and the low thermal conductivity that is typical of polymer powders [16]. To account for shrinkage during cooling, printed objects are intentionally made larger with respect to their final target dimensions.

2.2.2. Material Extrusion Process

In the material extrusion process, also known as Fused Deposition Modeling (FDM), a filament of thermoplastic material is softened and melted with the aid of heat and is extruded, i.e., pushed and forced through a nozzle of reduced diameter and then deposited layer by layer on the building platform or bed [17]. The parts and the supporting structures are built layer by layer in a predetermined path, starting from filaments that are unrolled from the spools and passed through thermally controlled extruders (see Figure 2).

The FDM system used for the printing of the Ultem and CF-PEEK components is Roboze One + 400, which can work with high-temperature polymers in a controlled environment. A dedicated investigation campaign was carried out before sample printing to identify the most appropriate parameters for the selected thermoplastic materials, i.e., CF-PEEK and Ultem, and for geometries that are relevant for the development of microwave waveguide components. As a result, a trade-off analysis between quality and speed led to the setup configurations reported in

Table 2. Parameters of the Roboze One + 400 MEX system used in the printing of Ultem and CK-PEEK components.

Parameters	Ultem	CF-PEEK
Retraction distance (mm)	0.5	1.0
Retraction speed (mm/s)	36.7	36.7
First layer height (mm)	0.1	0.2
Upper solid layers	5	5
Lower solid layers	5	5
Contour/perimeter enclosures	2	2
Infill (%)	20%	40%
Offset	+45/−45	+45/−45
Support infill (%)	20%	30%
Separation offset (mm)	0.7	0.7
Bed Temperature (°C)	150.0	150.0
Extruder Temperature (°C)	380.0	430.0
Speed (mm/s)	36.7	36.7

. The parameters retraction distance and retraction speed refer to the mechanism that is implemented in FDM systems when the printing head is moved among different areas of the part to be printed. To prevent the flowing of material during the movements and displacements of the extruder, the thermoplastic filament is retracted, thus reducing the pressure inside the nozzle. The first layer height is crucial for ensuring good adhesion to the build platform and overall printing quality. This parameter helps to balance resolution and print speed (i.e., the nozzle speed during the printing job). Instead, upper and lower solid layers refer to the solid sections at the top and bottom of the printed object, ensuring the strength and surface finish of parts. Lower solid layers are directly above the building platform, provide a strong foundation, and help with adhesion to the building plate that is warmed at a determined temperature (i.e., the bed temperature). Additionally, upper solid layers ensure a smooth and solid finish on the top surface of the object. Contour/perimeter enclosures define the outer wall of each layer and provide the object’s external shape and the outer shell as well as the desired strength, durability, and appearance. On the other hand, the object’s internal structure is defined by the infill parameter that leads to weight optimization, material efficiency, strength, and durability. The infill is also driven by the offset parameter that defines the orientation of the material layer by layer (e.g., +45° the first layer and −45° the second one). In terms of material efficiency, the support infill is a crucial parameter in terms of costs and dimensional accuracy. In fact, this structure allows for supporting overhanging parts during the printing process, ensuring that complex geometries are printed correctly without collapsing or warping. Moreover, a small gap (i.e., support offset) is intentionally created between the support and the object to ensure that supports can be easily removed after the printing process. The last crucial parameter is the extruder temperature, which varies depending on the filament type being used.

No specific post-processing is applied to the printed parts, apart from removing the support structures used during printing, followed by sanding of interfaces to smooth out surface roughness.

2.3. Metal Plating Processes

The Direct Metal Plating (DMP) process was selected for the metal coating with either copper or silver of the CF-PEEK and Ultem components manufactured through MEX. As illustrated in Figure 3, the DMP is a double coating process consisting of an electro-less silver substrate and a copper/silver electroplating on top. Indeed, electroless plating is the preferred process for metal coating surfaces that are not directly accessible and are made of non-conductive substrates (i.e., polymers and/or ceramics) [18]. It is a wet chemical process offering several advantages: it is simple and reproducible, it does not need a vacuum, and it allows for the covering of large areas with the possibility to coat substrates having complex geometries; moreover, the process temperatures are low and result in low energy consumption [19]. In electroless plating, the substrate is dipped in an aqueous solution containing salts of the metal to be deposited along with a reducing agent, a complexing agent, a stabilizer, and a buffer system. The metallic ions in the solution will react through a redox reaction with the reducing agent. As a result, the metal ions will migrate to the substrate, nucleating in a thin metallic layer without the application of an external electric voltage [20,21]. Among deposited metals, the most used are copper (Cu) [22], nickel (Ni) [23], and silver (Ag) [18]. The electroless plating process consists of three main steps: (i) surface etching of the substrate; (ii) surface catalytical activation; (iii) electroless plating process where the redox chemical reactions allow for the formation of the coating.

Generally, etchants are strong oxidizing solutions [24,25,26] that can locally remove the polymer from the substrate surface to increase the surface area and change the surface energy and wettability. Indeed, the etching process increases the roughness, generating micro and nanosized holes, which work as the sites for the adsorption of catalytic activation clusters during the activation process. Silver-based activation seems to be one of the best candidates to be used instead of the traditional palladium-based process [27] since Ag catalysts present high activities, ease of preparation, and low costs [28]. The use of Ag as seed layers for the electroless deposition of copper and silver has been investigated [18,25,29,30,31] and has shown good results.

In the present study, samples were stored at 50 °C prior to plating, and a surface chemical etching was first applied to the substrate surface. Two different etching solutions were prepared for PEEK and Ultem samples. For PEEK components, a 12 M solution of sulfuric acid is prepared. For Ultem components, a potassium hydroxide (KOH) hydroalcoholic solution was prepared, dissolving 20 g of KOH (Carlo Erba Reagents, Cornaredo, Italy) in 250 mL of ethanol (Sigma Aldrich, Darmstadt, Germany). Samples were dipped in the etching solution for 2 min, successively washed in an ultrasonic distilled water bath, and dried under a pressurized nitrogen jet. A surface pre-activation process was conducted by dip-coating in a Tin chloride (SnCl₂) bath. For the preparation of the pre-activation solution, hydrochloric acid (HCl, 37%, Carlo Erba Reagents) and Tin (II) chloride dihydrate (SnCl₂·2H₂O Sigma Aldrich assay > 99.99%) were mixed in distilled water in the H₂O:HCl:SnCl₂·2H₂O molar ratio 100:0.5:0.05. The Sn surface pre-activation was performed in a Pyrex glass beaker at Room Temperature (RT), under constant stirring. Samples were immersed for 30 min, rinsed in distilled water, and dried under nitrogen flow. The silver catalytically activation solution was prepared according to the following procedure:

• Distilled water (H₂O), Ammonia solution (NH₃ Carlo Erba Reagents) Silver nitrate (Sigma Aldrich > 99.0%) ammonium sulfate (Sigma Aldrich > 99.0%) were mixed together in the H₂O:NH₃:((NH₄)2SO₄):(AgNO₃) in the molar ratio 14:2:0.1:0.002.
• Mixing was performed in a Pyrex glass beaker at RT under constant magnetic stirring.
• Samples were dipped in the activation solution for 1 min and dried for 24 h at RT.

The silver electroless solution was composed of two different solutions, namely solution 1 and solution 2.

Solution 1 was prepared by mixing distilled water, ammonia (NH₃), and silver nitrate (AgNO₃) in a Pyrex flask at RT, in the H₂O:NH₃:AgNO₃ molar ratio 12:1:0.1.

Solution 2 was prepared by dissolving Potassium sodium tartrate tetrahydrate (KNaC₄H₄O₆·4H₂O, Rochelle salt, Sigma Aldrich 99%) and magnesium sulfate eptahydrate (MgSO₄·7H₂O Epsom salt, Sigma Aldrich > 99.5%) in distilled water in the following H₂O:KNaC₄H₄O₆·4H₂O:MgSO₄·7H₂O molar ratio 22:0.5:0.1.

Solution 2 was then mixed with solution 1, and the electroless plating solution was magnetically stirred for 30 s. The samples were dipped in the Ag electroless bath for 10 min. After electroless plating, Ag-coated samples were rinsed in distilled water and dried under nitrogen flow.

The produced electrically conductive samples were subsequently electroplated either with copper or silver layers.

The silver electroplating bath was prepared by dissolving 2 g of AgNO₃, 4 g of Succinimide (Sigma Aldrich), 1.5 g of potassium nitrate (KNO₃, Sigma Aldrich), and 0.03 mL of surfactant (Triton X-114 Sigma Aldrich) in 250 mL dH₂O. The electrodeposition process was carried out with a galvanic machine “Galvano mod. L2 412AS v2 Digit” imposing a current density of 0.6 A/cm² and a temperature of the galvanic bath of 50° for 2 h. A pure silver lamina (Ag = 99.9%) was used as the anode.

The copper electroplating solution was prepared by dissolving 60 g of copper sulfate (CuSO₄ Carlo Erba Reagents) in 0.5 M sulfuric acid solution. The electrodeposition process was carried out with a galvanic machine “Galvano mod. L2 412AS v2 Digit” imposing a current density of 1 A/cm² for 2 h and a temperature of galvanic bath of 50 °C. A pure copper lamina (Cu = 99.9%) was used as anode.

After the deposition process, samples were washed in distilled water, dried with a nitrogen jet, and stored at 60 °C to remove moisture. Figure 4 shows some representative microwave samples 3D printed in Ultem and CF-PEEK after different steps of the direct metal plating process, namely after the following: (a) surface activation; (b) silver catalytically activation; (c) silver electroless plating; (d) copper electroplating; (e) silver electroplating.

To test a different metal coating process, the Alumide components manufactured through PBF-LB/P were metalized through Physical Vapor Deposition (PVD), which is an ultra-low-vacuum thermal evaporation process where a wire of ultra-pure aluminum (99.9%) is heated in an ultra-low vapor pressure environment to its sublimation in front of the samples. Due to the low vapor pressure, the mean path of aluminum atoms is long enough to potentially reach the sample surfaces in the internal channels where atoms condense and accumulate in a continuous film.

3. Definition of Microwave Test Vehicles and Measurement Setups

To experimentally assess the applicability of the selected manufacturing processes to the development of microwave components for Space applications, two types of functional microwave test vehicles operating in the 9.5–13.5 GHz band were defined, namely components with low Standing-Wave Ratio (SWR) and single-cavity resonators. The CAD of the WR75-waveguide lines and 90-deg twists considered as test vehicles with low SWR are shown in Figure 5a and Figure 5b, respectively. These components provide standard UBR-120 flanges at the input ports and allow for a broad-band characterization of the RF properties of 3D-printed components. Conversely, the single-cavity resonator shown in Figure 5c exhibits a narrow-band response with a frequency reflection zero and two transmission zeros, which allows for a robust de-embedding of the manufacturing dimensional errors. Indeed, as illustrated in Figure 6b–f, the frequency of each transmission zero mainly depends on the height of the corresponding stub (h₁ or h₂), whereas the cavity length l affects only the position of the reflection zero. Finally, a change in the broadside a of the waveguide causes a frequency shift of the entire response. The nominal geometrical dimensions of the single-cavity resonator are a = 19.050 mm, b = 9.525 mm, l = 13.3 mm, w₁ = 15.973 mm, w₂ = 10.179 mm, h₁ = 13.109 mm, and h₂ = 10.155 mm.

The electromagnetic characterization of these components was carried out through the measurement setups shown in Figure 7a,b. In the non-resonant setup of Figure 7a, the DUT is connected to the input ports of a network analyzer through a pair of WR75-to-coaxial transitions and anti-cocking flanges that improve the electrical contact at the DUT interfaces. Once the NA is calibrated at the DUT interfaces by means of the thru-reflection-line technique, this setup allows for the direct measurement of the reflection and transmission scattering coefficients of the DUT. By applying a best-fitting procedure to the measured scattering coefficients, the geometrical dimensions and the surface electrical resistivity of the DUT are de-embedded. The latter parameter depends on both the conductivity of the metal coating applied on the polymer surfaces and its roughness.

Figure 8a reports the comparison between measured and simulated scattering transmission coefficients of one of the 3D-printed WR75-waveguide single-cavity resonators. The simulated performance refers to the de-embedded structure with ρ = 5 μΩcm and it was computed through both an in-house Method of Moments (MoM) code and CST Microwave Studio (CST) software (vers. 2024). To highlight the de-embedding accuracy, the MoM responses for ρ = 10 μΩcm and for deviations δ_a = ±0.02 mm in the waveguide broadside are reported in Figure 8b. As can be noticed, this response stays rather outside the grey area, indicating the measurement of the Root Sum of Squares (RSS) confidence interval [32].

The resonant setup of Figure 7b is used to accurately characterize the surface electrical resistivity of waveguide lines and twists that exhibit low SWR. Indeed, in this setup, the DUT is inserted in between two waveguide irises that increase the standing-wave ratio inside the DUT and, hence, the sensitivity of the measurement setup to the surface electrical resistivity. Figure 9a shows the predicted performances of the resonant setup when the DUT is a WR 75-waveguide line with length = 70 mm. The three resonances at 10.37 GHz, 11.83 GHz, and 13.47 GHz correspond to the electrical-field distributions in the E-plane x = 0 that are reported in Figure 9b–d, respectively. The maximum electric field occurs inside the irises because of the smaller dimensions of the waveguide gap in this area and the presence of the edges. Nevertheless, high-standing waves arise inside the DUT at resonance frequencies. This results in higher insertion losses and, thus, in higher accuracy in the de-embedding of the electrical surface resistivity. Indeed, Figure 10a reports the comparison between the predicted insertion loss for the non-resonant setup at 10.5 GHz and for the resonant setup at the second resonance (11.83 GHz) as a function of the electrical surface resistivity ρ. Since the uncertainty ${\epsilon }_{\rho }$ in the estimation of ρ is connected to the uncertainty ${\epsilon }_{IL}$ in the measurement of the insertion loss (in the order of 0.03 dB) through the formula

$${\epsilon }_{\rho }={\epsilon }_{IL}/{\displaystyle \frac{dIL}{d\rho }}$$

(1)

The resonant setup provides much better resistivity accuracy than the non-resonant setup, as illustrated in Figure 10b.

4. Breadboarding and Experimental Results

The test vehicles described in Section 3 were 3D-printed according to the manufacturing processes identified in Section 2. Then, they were electromagnetically characterized through the setups shown in Figure 7. Figure 11a shows a WR75-waveguide line under test, while Figure 11b,c report detailed views of the anti-cocking waveguide flanges and of the E-plane waveguide irises used in the non-resonant and resonant setups, respectively.

The WR75-waveguide line and single-cavity resonator manufactured in Alumide through PBF-LB/P technology are shown in Figure 12, along with the corresponding scattering parameters after the aluminum-plating of the components by means of the PVD process. For this material, a monolithic configuration (i.e., consisting of a single mechanical part) was considered for both test vehicles. Since very low transmission coefficients (in the order of −15/−20 dB) were measured, it can be inferred that the metalization of the internal surfaces of both components was not successful. To better understand this result, the components were cut, and some of the internal surfaces were subjected to Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS). This analysis highlighted the presence of aluminum in an atomic concentration of approximately 60–70% on the surfaces, as shown in the example of Figure 13.

The WR75-waveguide lines in Ultem and CF-PEEK were manufactured through MEX technology and copper-plated by means of the DMP process. The length of the lines is equal to 70 mm. These test vehicles were 3D-printed according to both the monolithic and the split-block configurations shown in Figure 14 and Figure 15. These figures also report the comparison between the measurements carried out through the resonant setup and the simulated performances for the de-embedded geometries that were defined by best-fitting the measurements around the second resonance at approximately 11.83 GHz.

The dimensional errors and the surface electrical resistivity of the de-embedded geometries are listed in

Table 3. Dimensional errors and surface electrical resistivity ρ of the copper-plated WR75-waveguide lines manufactured in Ultem and CK-PEEK through MEX technology.

Material	Configuration	Δa (mm)	Δl (mm)	ρ (μΩcm)
Ultem	monolithic	−0.18	+0.02	1000
Ultem	split-block	+0.25	−0.30	310
CK-PEEK	monolithic	−0.40	−0.05	130
CK-PEEK	split-block	+0.25	+0.35	8

. From these results, it can be inferred that the dimensional errors for all the lines are in the order of 0.2 ÷ 0.3 mm. For each material, the split-block layout provides the lowest value of surface electrical resistivity, which is 8 μΩcm for the CF-PEEK material. This value is also lower than those exhibited by both a commercial WR-75 waveguide line and a WR75-waveguide line 3D-printed in aluminum AlSi10Mg. Indeed, values of 12 μΩcm and 23 μΩcm were measured for these lines, respectively. Conversely, the monolithic waveguide lines exhibit high surface electrical resistivity that is as high as 1000 μΩcm for the Ultem prototype. Moreover, it can be noticed that the simulated responses of the monolithic lines do not perfectly match the measurements around the first and third resonances. This implies that the copper layer is not homogeneous along the waveguide lines since the E-field pattern changes with the resonance frequency (see Figure 9).

The WR75-waveguide single-cavity resonators manufactured in Ultem and CF-PEEK are shown in Figure 16 and Figure 17, along with the corresponding scattering coefficients that were measured after copper DMP. For this test vehicle, monolithic and split-block layouts were also considered.

The best-fitting procedure necessary to de-embed the 3D-printed geometry was not applied to the split-block prototype in Ultem since its measured performances were affected by the imperfect flatness of the input flanges. The de-embedded geometrical errors and surface electrical resistivity for the other prototypes are listed in

Table 4. Dimensional errors and surface electrical resistivity ρ of the copper-plated WR75-waveguide single-cavity resonators manufactured in Ultem and CK-PEEK through MEX technology.

Material	Configuration	Δa (mm)	Δl (mm)	Δh1 (mm)	Δh1 (mm)	ρ (μΩcm)
Ultem	monolithic	−0.04	+0.02	−0.07	0.06	42
Ultem	split-block	Not de-embedded
CK-PEEK	monolithic	−0.30	+0.06	−0.12	−0.15	440
CK-PEEK	split-block	+0.03	+0.10	+0.18	−0.10	5

. In accordance with the outcomes from the breadboarding of the WR75-waveguide lines, the split-block configuration in CF-PEEK is the prototype that exhibits the best IL performance corresponding to a surface electrical resistivity of 5 μΩcm, which is a very promising result. The geometrical errors in the manufacturing of the single-cavity resonators are in the order of 0.1 ÷ 0.2 mm.

Considering these results and the higher thermal-mechanical properties of CF-PEEK with respect to Alumide and Ultem (see Table 1), CF-PEEK was regarded as the most promising material for investigation of the DMP process based on silver electroplating that is the preferable coating material for Space applications. To this end, three WR75-waveguide lines and three WR75-waveguide 90-degree twists were manufactured in CF-PEEK and subsequently silver coated.

Figure 18a,b show two prototypes and the corresponding comparison between measured and simulated transmission coefficients for the resonant setup. The simulated performances refer to the geometries de-embedded by means of the best-fitting procedure applied around the second resonance at approximately 11.2 GHz. It can be inferred that the de-embedded structures provide a good correlation between experimental and theoretical performances at the other resonances as well. The dimensional errors and surface electric resistivity values of the six prototypes are listed in

Table 5. Dimensional errors and surface electrical resistivity ρ of the silver-plated WR75-waveguide lines and twists manufactured in CK-PEEK through MEX technology.

Component	Prototype Number	Δa (mm)	Δl (mm)	ρ (μΩcm)
Line	1	−0.17	+0.15	50
Line	2	−0.21	+0.02	10
Line	3	−0.17	−0.3	12
Twist	1	−0.34	−0.15	150
Twist	2	−0.36	−0.05	20
Twist	3	−0.33	−0.05	16

. The corresponding mean values are approximately 0.2 mm and 40 μΩcm, the surface electric resistivity being as low as 10–20 μΩcm for most of the prototypes

5. Conclusions and Discussion

The experimental results achieved in this study prove that 3D-printed metal-coated composite polymers can be used in the development of microwave components operating in the X/Ku band intended for Space applications. Specifically, a comparative investigation was carried out among three different composite materials (Alumide, Ultem, and CF-PEEK), two manufacturing technologies (MEX and PBF-LB/P), and two metal coating techniques (PVD and DMP).

Table 6. Comparison among different manufacturing technologies for Space-borne microwave waveguide components.

	Machining	Electro-Forming	PBF-LB/M	PBF-LB/P	MEX	SLA
Material	Al 6061	Cu	Al alloys	Alumide	CF-PEEK, Ultem	Stonelike
Dimensional accuracy (μm)	5–20	1–5	40–80	40–80	100–200	40–80
Surface roughness Ra (μm)	0.4–1.5	0.4–1	3–6	3–6	10–30	1–3
Electrical resistivity (μΩcm)	4–6	3–5	6–12	metal plating is mandatory
Melting point (metals) Glass transition temperature (polymers) (°C)	650	1083	570	144	145–186	T.B.D.
Density g/cm3	2.7	8.9	2.7	1.3	1.2–1.3	1.3
Tensile strength (MPa)	260–310	200–360	440–480	48	72–87	52
Near-net shapes	No	yes	yes	yes	yes	yes
Mechanical layouts	Split-block Multi-layer	Monolithic	Monolithic	Spilt block layouts may be required for metal plating
Lead time/cost	Medium	High	Low	Low	Low	Low
TRL	9	8–9	8–9	3	3	2

reports the comparison among the investigated manufacturing routes and other technologies already applied to the development of Space-borne microwave waveguide components, among which are standard marching (i.e., milling, turning, electrical discharge machining), PBF-LB process based on metal, and electro-forming. Based on the results of the study, the authors have currently identified the material extrusion of CF-PEEK as the most promising manufacturing route for further research activities aimed at the implementation of complex and integrated microwave devices. The advantages of manufacturing Space-borne RF components in CF-PEEK with respect to common 3D printing polymers are related to their higher thermal and mechanical properties. The suitability of PEEK and CF-PEEK to operate under Space environment conditions is already under study [33,34,35]. Because RF parts do not implement structural functionalities, thermal stability is the most relevant feature to guarantee RF performance in the Space environment. In this view, Figure 19 reports the simulated scattering transmission coefficients of the WR75-waveguide single-cavity resonator at different temperatures for aluminum (CTE = 23 × 10⁻⁶ °C⁻¹), CF-PEEK (CTE = 40 × 10⁻⁶ °C⁻¹ [36]), PLA (CTE = 68 × 10⁻⁶ °C⁻¹ [37]), and ABS (CTE = 90 × 10⁻⁶ °C⁻¹ [37]). The temperature range (−80, +80) °C was considered appropriate for waveguide front-ends shielded inside satellite platforms. As can be inferred, the lower CTE of CF-PEEK leads to better stability of RF performance over temperature compared to ABS and PLA. Although the thermal stability of CF-PEEK is not as good as that of aluminum, this material can be used for the manufacturing of waveguide components exhibiting broad-band and low SWR responses.

Considering Table 6, further investigation on the 3D printing of CF-PEEK/PEEK through PBF-LB/P is planned with the aim of reducing surface roughness and increasing dimensional accuracy, which are key technological aspects for the development of RF components operating at higher frequency bands (such as K/Ka bands). Figure 20 shows a relevant K/Ka-band application of metalized composite-polymer 3D printing, consisting of an array of K/Ka-band smooth-wall feed-horns for multi-beam High Throughput Satellite (HTS) reflector antennas. This RF sub-system is particularly suited to this manufacturing technology since it exhibits broad-band and low SWR response while having all the inner channel surfaces easily accessible for metal plating. Additionally, the achievable mass saving is relevant for the application, and the entire array can be printed monolithically in a single manufacturing run.

The impact of surface roughness on insertion loss performance is illustrated in

Table 7. Equivalent surface electrical resistivity ρ and attenuation of a copper-plated WR75-waveguide line for different values of surface roughness Rq.

Surface Roughness Rq (μm)	Equivalent Surface Electrical Resistivity ρ (μΩcm)	Attenuation (dB/m)
0.5	4	0.25
1	8	0.36
2	17	0.52
3	25	0.63
4	31	0.71
5	37	0.77
6	42	0.82

, which reports the equivalent surface electrical resistivity ρ and the attenuation of a copper-plated WR75-waveguide line for different values of surface roughness Rq. The bulk conductivity of copper is 59.6 MS/m. The influence of the surface roughness was taken into account by adopting the gradient model [38]. With the goal of reducing surface roughness, stereo-lithography 3D printing of Stonelike may be an additional manufacturing route applicable for RF satellite components.