Fiber Encapsulation Additive Manufacturing: Technology and Applications Update

Introduction

The progression of three-dimensional (3D) printing is directly correlated to the creation of new materials and novel printing processes that expand traditional additive manufacturing capabilities. Each year researchers are expanding the variety of materials that can be printed. Some of these materials include both soft thermoplastic elastomers^1,2 and many types of functional polymers.^3–6 There are a number of new technologies that promise to take 3D printing to the next level by enabling faster printing, ⁷ printing of metal alloys,^8,9 printing of reactionware for chemical synthesis,^10,11 and printing of structures on large^12,13 and small scales.^14,15

Printing multiple material types in a single printer presents added challenges that must be addressed. We are developing Fiber Encapsulation Additive Manufacturing (FEAM), an innovative additive manufacturing process, in which a fiber and a matrix material are simultaneously deposited. FEAM allows fibers, such as metal conductors to be integrated into 3D prints, enabling many new application possibilities. ¹⁶ FEAM permits designers to create complex 3D printed devices with integrated fibers that enhance the functionality of a part. A more specific use of FEAM is the construction of electronic circuits and electromechanical devices incorporating coils such as voice coil actuators, inductive sensors, ¹⁵ and eventually motors and solenoid valves, all of which can be distributed throughout a 3D printed part. To make the FEAM process successful, there are three main challenges to overcome: (1) controlled encapsulation of fiber by matrix material, (2) the ability to start, stop, and cut the fiber in arbitrary locations, and (3) the ability to create intralayer and interlayer junctions between metal fibers with adequate conductivity. The first challenge was addressed in the previous work. ¹⁷ Presently, we are reporting the progress of the last two challenges.

Materials and Methods

FEAM system

In FEAM, both the fiber (e.g., copper wire) and the matrix (e.g., a thermoplastic polymer such as acrylonitrile butadiene styrene (ABS)) are deposited using a single print head. The FEAM printer, as shown in Figure 1a, has a print head comprising of an extruder with a nozzle similar to those used in other material extrusion 3D printing processes and a guide located to the side of the nozzle orifice to deliver fibers. The exit of the guide is located in close proximity to the nozzle's orifice. As the print head moves, the proximity allows fibers to be introduced into a polymer, while it is being extruded. To successfully print fibers, the fiber must always be within the extrudate and be reasonably centered. To keep the wire centered on curved toolpaths a rotational or “theta stage” is used in conjunction with the traditional X-Y stages. All axes are controlled using DMC4183 and DMC2143 motion controllers (Galil Motion Control, Rocklin, CA). Basic operation and capabilities of this process have been described previously. ¹⁷

OPEN IN VIEWER

FIG. 1.

(a) Photo of the FEAM printer. (b) CAD model of the wire feeder/cutter. (c) Schematic bottom view of the wire feeder cutter. FEAM, Fiber Encapsulation Additive Manufacturing.

Wire junctions

Wire junctions were created by using an Electrically Conductive Polymer Composite (ECPC) ¹⁸ consisting of Kraton D1161P ¹⁹ (Kraton Performance Polymers, Inc., Houston, TX) thermoplastic elastomer mixed with silver-coated nickel particles. Both intralayer and interlayer junctions were demonstrated as seen in Figure 2c. Interlayer junctions were formed by laying wire segments on top of one another on different print layers. A cavity was left surrounding the wires, where the ECPC was deposited. Intralayer junctions were formed by laying wire segments side by side on the same print layer with a cavity surrounding them for the deposition of ECPC. ECPC was extruded from a separate, custom-made heated miniature screw extruder that was previously described. ¹ The ECPC filled the cavity creating an electrical junction between the wires. The best measured junctions had resistances of 110 and 250 mΩ for interlayer and intralayer junctions, respectively. ¹⁸

OPEN IN VIEWER

FIG. 2.

(a) Three-dimensional printed stacked pancake coil with 811 turns of copper wire; (b) printed wire segments demonstrating cutting, stopping, and starting of wire; (c) photos and schematic of intralayer (top) and interlayer (bottom) wire junctions; (d) (top) Printed stretchable wire interconnect, (bottom) interconnect elongated 5 cm (e) printed flashlight with battery, resistor, and light-emitting diode inserted into printed TPE circuit board; (f) schematic showing how a surface mount resistor was connected to printed wires.

Results

Using the FEAM system we were able to demonstrate the codeposition of copper wire with several different thermoplastics. Any arbitrary planar shape can be produced within the resolution of the extrudate width (∼0.45 mm). The fiber will remain encapsulated with a shape that directly depends on the preprogrammed stage trajectories and the solidification of the extrudate. Between successive layers at least one fiber can be continuous allowing for vertical circuitry to be produced. When more than one interlayer fiber trace is required for the desired part, in general, an interlayer junction must be used.

Figure 2a shows a stacked pancake coil of height ∼21 mm and outside diameter of ∼20 mm that was created by printing wire continuously in a spiral pattern, inward to outward and outward to inward on alternating layers. The resulting coil had 811 total turns and produced a force of 32 gf on a steel plunger when energized with 0.5 A of current. Figure 2b shows fed and cut segments of 127 μm diameter and 6 mm length copper wire encapsulated in BendLay, a semitransparent ABS-based thermoplastic (Orbi-Tech GmbH, Leichlingen, Germany). ²⁰ The smallest wire segment length tested and printed reliably was 6 mm. The smaller fiber segments are pushed by the feeder into the extrudate flow using subsequent segments as a ram. The smallest bend we have been able to achieve with 127 μm diameter bare copper wire has a radius of 100 μm through a right-angle turn. However, the smallest continuous turn produced so far is a 3 mm diameter continuous helical coil. Wire segments can be produced with or without BendLay encapsulation. Wire segments without encapsulation were used where wire junctions were desired. Figure 2d shows a stretchable wire interconnect that was created by codepositing wire in a serpentine pattern. The device was elongated 5 cm without any observable change in resistance. Figure 2e shows a simple flashlight that was printed with wire for electrical connections and a soft thermoplastic elastomer “circuit board.” The flashlight consisted of a coin cell battery, surface mount resistor, an light-emitting diode, and copper wires. To connect and support the flashlight components, cavities were formed that had copper wire protruding within them. When an electrical component was inserted into the cavity, the wire bent downward to create an electrical connection with the component. A schematic representation of how the surface mount resistor was connected to the wires is shown in Figure 2f.

Conclusion

We are developing FEAM, an innovative multimaterial AM process. FEAM can be used with a variety of materials. Our current focus is using thermoplastic polymers as the matrix material and metal wires as the encapsulated fiber, with the goal of enabling the direct fabrication of circuit boards and active electromechanical devices. While there is still work needed to optimize the process, we have nevertheless addressed the three main challenges of FEAM, including achieving good centering of the wire within the extruded polymer; having the ability to controllably start and stop the wire to produce short wire segments; and demonstration of intralayer and interlayer junctions between wires. We have created a number of simple devices ranging from multilayer stacked pancake coils, a stretchable wire interconnect, and a printed flash light circuit.