Structural electronics, or structronics, seeks to build wiring and electrical interconnects directly into component structures to diminish the need for structural fasteners. A MakerBot Cupcake Computer Numerical Control (CNC) was used to investigate the fabrication of plastic structures by printing, and a Fujifilm Dimatix Materials Printer (DMP) was used to create flexible Printed Circuit Boards (PCBs). After developing processes to make these machines easier to use, others will be able to utilize them to develop structures with integrated electronics.

The DMP-2831 Materials Printer deposits fluidic materials on flexible substrates by using a piezoelectric inkjet cartridge.
The MakerBot Cupcake CNC is a 3D printer used to print plastic models of objects drawn using a 3D CAD program. This machine is of interest to the structronics project because it provides the ability to prototype a design very cheaply without leaving the lab. The processes developed for using the MakerBot are essential to the continuation of developing in-house prototypes with integrated electronics.

The printer works by slicing each model into layers, working from the top of the object to be printed to the bottom of the object to be printed. Then, it uses the layers to generate GCode, a machine code that tells the machine where it needs to deposit plastic on the build platform to print the object layer-bylayer. After generating GCode, the build process can be executed directly from the computer used to slice the model, or by saving the file to a Secure Digital (SD) card, it can be executed with no computer connected to the machine. During the build process, Acrylonitrile Butadiene Styrene (ABS) filament is pushed through a nozzle heated to 220 °C in much the same way as a hot glue gun is used to extrude glue. The extruder travels along the path defined by the GCode, leaving behind plastic to build the desired model.

The models used were chosen to investigate four areas of capability with the MakerBot: maximum size of models, minimum feature size of models, hollow object printing, and precision.

The first test sought to determine the maximum size of a model printed with the MakerBot. The test used a standard 3D printing test print called the Stanford bunny, a model from a repository of 3D scans made by Stanford University that measures 89.42 millimeters tall by 75.95 millimeters wide with a depth of 49.66 millimeters. Height was a limiting factor when printing this model because any attempt to make the object taller than 95 millimeters would be unsuccessful because the extrusion tool would collide with the top of the machine. The width of the object was also on the verge of being too wide for the machine to handle. All of the 3D models are printed on a surface called a raft, which makes objects easier to remove from the printer and serves to prevent failure of an entire print if Z-axis misaligns during printing. While the object size is limited for each print, it is possible to divide objects into several smaller prints and put them together with adhesive or design the parts to snap together.

The second test was to show the minimum feature size achievable on a plastic model made on the MakerBot. The test used a small figure of a NASA space shuttle orbiter that measures 52.45 × 54.48 × 16.11 millimeters. The model shows the minimum feature size attainable on the MakerBot with its three thin wings. The wings measured only 2.57 millimeters at their thickest point, and the smallest of them is only 5.85 millimeters tall.

The third test is the ability to print objects with open spaces in their center. Hollow objects are difficult with 3D printers because of the nature of an additive building process. It is impossible to deposit melted plastic into the air and have it stay in place, but objects can be made that are hollow given proper planning. One model that shows this is the hollow pyramid model that measures 36.78 × 36.74 × 32.65 mm. By making the sides less than a 45-degree angle, the overhangs made by the printer assures that the upper parts of the model do not fall, which makes it possible to print models with open spaces.

The fourth capability of the MakerBot to be tested is precision printing. With the MakerBot, a product can be created that is broken down into several smaller parts for easier printing or to allow for moving parts, such as hinged arms. Two designs of very similar models were created to test this capability. The iris box model is a cylindrical box designed to open by twisting the top of the box, which should allow the shutters that seal the box to rotate outwards and reveal the contents of the box. While the MakerBot is unable to print small objects like pegs that fit exactly where they are supposed to fit, it is capable of printing parts that fit together.

With more testing, the integration of electronics into structural components is achievable with the current equipment. The MakerBot Cupcake CNC can print prototypes, as long as they are designed such that no single print is larger than the area the extruder can travel. Metallic traces deposited on substrates by the Dimatix printer are viable for use as flexible PCBs, and could be used to make a circuit board that goes around or inside of a prototype printed on the MakerBot.

This work was done by Tracy D. Hudson and Carrie D. Hill of the Army Research, Development, and Engineering Command. ARL-0148

This Brief includes a Technical Support Package (TSP).
Three-Dimensional Plastic Part Extrusion and Conductive Ink Printing for Flexible Electronics

(reference ARL-0148) is currently available for download from the TSP library.

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This article first appeared in the June, 2013 issue of Defense Tech Briefs Magazine.

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