The Army Aviation and Missile Research, Development, and Engineering Center (AMRDEC) Weapons Development and Integration (WDI) Directorate has a program known as PRIntable Materials with Embedded Electronics (PRIME2). PRIME2 will integrate RF and electronics into additive manufacturing processes to reduce size, weight, and overall cost of these components and subsystems. This program will advance the state of the art in printable electronics, and deliver a materials database, process development, modeling, and simulation of 3D-printed objects with embedded conductive elements, passive prototypes, and RF prototypes. PRIME2 will create a new fabrication capability (applied to electronics and RF technology areas), weight reduction, higher reliability, and on-demand (local and immediate) spare components in the field.

Additive Manufacturing

Figure 1. Structures printed using PLA.

Additive manufacturing is a rapidly maturing process by which digital 3D design data are used to build up components in layers by depositing materials, or through the melting and sintering of (powdered) materials to create solid structures. These materials can be conductive (metal) or nonconductive (polymer), and have complex material properties that are dependent on print parameters.

Recently, additive manufacturing has quickly gained adoption and acceptance as a valuable manufacturing technology. There are many different types of printers, including fused filament deposition (FFD), stereolithography (SLA), and laser sintering. The National Aeronautics and Space Administration (NASA) has a FFD machine on the International Space Station (ISS).

Additive manufacturing, known as 3D printing, is rapidly developing to meet the needs of a wide range of commercial and military applications. 3D printing is typically used in prototype development to reduce costs and development time compared to traditional manufacturing. For example, 3D printing enables concept-to-prototype in less than a day at $5 to $8 per cubic inch of material, and it has been used to fabricate prototypes, tooling, fixtures, and forms to test design fit. 3D printing allows free complexity and integration of parts that are too costly or even impossible for traditional manufacturing. In some cases, printing requires no tool adjustments to fabricate hollow and buried structures; therefore, interconnects and connectors are simply printed where they are needed within the volume. This design freedom is particularly relevant to RF antennas where directivity and efficiency are currently limited by manufacturing constraints and losses in conductive feeds.

Figure 2. ABS polymer filaments.

Additive manufacturing brings a new capability that can be explored across all technology areas for benefits and use. The benefits can be many and varied, resulting in components that are not achievable utilizing traditional subtractive machining methods, lower-weight components, low cost, local and immediate prototyping, and component creation.

Traditionally, electronic components and RF components are assembled piecemeal and are not part of the additive manufacturing process. PRIME2 is developing enabling technologies to print an entire printed wiring board with embedded passive components and integrated RF structures in one step.

Materials

A variety of materials are available for additive manufacturing. These include both conductors and dielectrics; however, many of these materials compromise mechanical or electrical performance to enable ease of manufacture. In addition, many of these materials often require incompatible post-processing, such as thermal cures that can disrupt underlying structural elements. The characterizations of FFD (also known as Fused Deposition Modeling (FDM)), SLA, inkjet deposition, and microdispensable dielectric materials are presented here, along with the characterizations of FDM, inkjet deposition, and microdispensable conductive materials.

More than 35 dielectric materials suitable for FDM, SLA, and inkjet were evaluated in an effort to demonstrate a material set that had sufficient process compatibility to be co-fabricated that yielded electronic structures embedded within structural elements, yet also possessed sufficient performance to enable high-frequency RF use.

For dielectrics, relative permittivity and loss tangent are critical for implementing RF systems. In general, most additively manufacturable materials are polymeric, with a dielectric constant that falls within the range of 2 to 6; however, some unique materials are available. In particular, composite materials incorporating metals and ceramics provide enhanced dielectric constants that may be useful in RF design. Some polymer matrix composites can yield low levels of conductivity. These levels are not sufficient for quality RF components, but could be useful for direct current (DC) signals.

The selected conductive materials focused on inkjet and microdispense technologies. These materials demonstrated a wide range of conductivities. Organic conductors were at the low end of the range, and were not suitable for RF applications. Conductive epoxies have desirable features of room-temperature curing. This makes them more readily compatible with other additive manufactured substrates, but their conductivities were an order of magnitude below the nanoparticle inks that are used in aerosol and inkjet techniques. The nanoparticle inks exhibit no better than 50% of the conductivity of solid metal conductors such as electroplated copper. In addition, they require elevated temperatures to sinter the nanoparticles into a conductive sheet. These elevated temperatures can cause incompatibility with certain additively manufactured dielectric materials.

Based on the collected data, a subset of materials was further investigated for co-fabrication and realization of RF structures. Considerations during the selection process included material performance, material compatibility, availability and capability of additive manufacturing tools, and the desired RF component and designs.

Fused Filament Deposition Technology

Fused filament deposition (FFD) uses a continuous filament of a thermoplastic material fed from a spool through a moving, heated, printer extruder head. Molten material is forced out of the printhead’s nozzle, and is deposited on the growing workpiece to form a 3D object.

PLA is a biodegradable thermoplastic polyester. It is a commonly manufactured from renewable resources such as cornstarch, tapioca roots, and sugarcane. PLA is harder than ABS plastic, has a lower melting temperature (180-220 °C), and a glass transition temperature between 60 and 65 °C. It is dimensionally stable, and can be printed with or without a heated build plate. It adheres easily to borosilicate glass, Lexan, polycarbonate sheets, blue painters’ tape, polyimide (Kapton) tape, and so forth. PLA may be treated with a wide range of post-processing techniques (Figure 1). PLA prints may have slight dimensional variations compared to other materials. Color and brand have some small effects on printing.

ABS is a common thermoplastic. It is less brittle (tougher) than PLA. With a glass transition temperature approximately 105 °C, it requires a higher extruder temperature than PLA — 230 °C ±15 degrees. ABS creates mild fumes when being extruded, and printers should be operated in a well-ventilated area. ABS requires a heated build plate that is heated to approximately 110 °C due to its tendency to warp when printing larger prints. Figure 2 shows example polymer filaments.

The flexibility of the thermoplastic elastomer (TPE) filament makes it quite resilient and sturdy for producing objects with a Shore A hardness of approximately 75-85 A. This filament is easily printed in most printers capable of printing PLA or ABS plastics, although it has a slightly higher melting temperature (240 °C), and is ideal for multi-material applications requiring portions of the design to flex, such as shock absorption devices and hinges. Printing TPE benefits from a build plate that is heated to approximately 60 °C and direct drive extruders.

Stronger than PLA and more durable than ABS, nylon offers the benefit of a material robust enough for functional parts. Nylon’s high melting temperature and low friction coefficient present a versatile printing option that allows flexibility.

Figure 3. PEEK filament.

ULTEM offers high thermal resistance, high strength and stiffness, and broad chemical resistance. ULTEM is available in transparent and opaque custom colors as well as glass-filled grades. Plus, ULTEM copolymers are available for even higher heat, chemical, and elasticity needs. ULTEM 1000 (standard, unfilled PEI) has a high dielectric strength, inherent flame resistance, and extremely low smoke generation. These high mechanical properties perform in continuous use to 340 °F (170 °C), which makes it desirable for many engineering applications.

With its unique mechanical, chemical, and thermal properties, PEEK has many advantages over other polymers, and is able to replace industrial materials such as aluminum and steel. It allows its users to reduce total weight and processing cycles, and increase durability. Compared to metals, the PEEK polymer allows a greater freedom of design and improved performance. PEEK is used to fabricate items used in demanding applications, including bearings, piston parts, pumps, high-performance liquid chromatography (HPLC) columns, compressor plate valves, and electrical cable insulation. It is one of the few plastics compatible with ultra-high vacuum applications. Figure 3 shows an example of a PEEK filament.

Stereolithography Technology

While FFD technology provides a means to rapidly prototype objects, stereolithography (SLA) is often better suited for detail and high-speed production. Parts are constructed in a layer-by-layer fashion using photo-polymerization, a process by which ultraviolet (UV) light causes chains of molecules to link and form polymers that then make up a 3D solid object. The production of these objects relies on materials that are currently available in many forms, including standard and engineering resins.

  • Standard Resins. The material selection for SLA is more limited than FFD, but general-purpose or standard resins have grown to include a variety of colors in varying opacities. Standard resins provide high resolution for applications like visual demonstrations and models.

  • Engineering Resins. Matching the detail provided with standard resins, engineering resins possess additional strength and functionality. The flexible resin variety simulates an 80A durometer rubber, which is often chosen for impact resistance and compression. The tough resin is similar to a finished product formed from ABS plastic. Applications that will undergo high stress and strain are frequently engineered with tough engineering resin, ensuring successful assembly, machining, snap-fits, and living hinge supports. The ceramic resin is UV-curable, with objects often glazed with commercially available coatings after firing.

Conclusion

Based on initial work in the PRIME2 program, a subset of materials was investigated for co-fabrication and realization of RF structures. Standard PLA, standard ABS, PEEK, and ULTEM were selected for further dielectric investigation.

The existence of a suitable solvent for the dielectric material can be helpful in preparing printed substrate surfaces for further additive manufacturing steps. In addition, the melt temperature of the material is important for post-processing steps that may be required when depositing certain conductive materials.

This article was written by Janice C. Booth, Army AMRDEC Weapons Development and Integration Directorate, Redstone Arsenal, AL; and Michael Whitley, Carl Rudd, and Michael Kranz of EngeniusMicro, Huntsville, AL. For more information, visit here .


Aerospace & Defense Technology Magazine

This article first appeared in the February, 2018 issue of Aerospace & Defense Technology Magazine.

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