Fabrication
The fundamental processes that turn a 3D-printed component into a high-quality RF waveguide component are lumped together here as parts of the “Fabrication Process.” Unfortunately, this section’s time as our primary focus coincides almost perfectly with the onset of Coronavirus complications, making this section primarily theoretical. It is broken into three major sections: 3D Printing Basics, where a brief overview of the file formats and print requirements will be given; Paint Selection and Application, which defines metrics for the selection and application of metallic coatings to waveguides; and Component Validation which outlines the method to finally
3D Printing Basics:
There are two fields that a user needs to understand when getting started with a 3D printer: the mechanical systems, and the software pipelines. In order to generate high-quality prints one must understand the optimal configuration for areas.
Mechanical
The mechanical control given to a user is very restricted, with users primarily only seeing controllability of the bed-leveling and temperature settings. Having a perfectly flat print bed is fundamental for 3D printing. Printers hold the z-axis constant while they print structures up one layer at time. If the bed is skewed in any direction, layers will not be able to properly adhere to each other, at best causing non-homogenous prints, and at worst causing a print failure. The bed level is controlled by four screws located at each corner of the print bed, the easiest way to guarantee that the bed is leveled is to manually move the extruder between each of the four corners, and ensuring its height remains constant throughout the process. Even then prints should generally be restricted to the center of the print bed whenever possible in order to minimize the effects of any mis-leveling.
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Properly tuning the temperature of the extruder is a less precise process. Most filaments provide optimal temperature ranges, but sometimes there can be variance with regard to the ideal temperature for different rolls. If the temperature is too low there can be low adhesion between layers, or flow issues causing the print to fail. If the temperature is too high, layers might melt into each other, or warp as the print progresses. If any of these issues appear, the temperature might need to be adjusted to resolve them.
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Beyond these two areas, the only real mechanical concern is the integrity of the extruder’s nozzle. It is possible for filament to build up on the nozzle over time, which can cause issues should it begin to come in contact with any prints, so it is good to regularly clean the nozzle.
Software
There are three primary operations in the software pipeline for a 3D printer. First there is model generation, which can be done in any modelling program including HFSS and SOLIDWORKS. It is likely that a modeled RF component, say a waveguide, will be designed in a way that is not conducive to painting. For that reason the second step of the process is to break the full component into printable sub-components. Care will have to be taken to ensure that these sub-pieces are easily assembled, easily painted, and are selected to minimize their effect on our system’s electrical properties. Lastly, once a component has been isolated in a printable form, it can be fed into a “slicing program” such as Cura which will generate files that the printer can interpret.
The first step in this process is very application specific, so it will not be discussed. Rather we begin by looking to methods of decomposing sub-systems. This takes the form of a case study of a standard waveguide pictured on the right.
The design of this system can be broken into two regions: the flange connectors that connect the waveguide to other RF components, and the thinner channel through which the signal actually travels. One fundamental issue in 3D printing is the idea of “floating segments.” Since the printer prints vertically in very short layers, regions with overhangs, such as the unsupported middle section of the waveguide, will not be printable.
This issue can be resolved via the addition of “support structure” to help reinforce these overhangs, but we decided to instead print the channel and the flanges as two separate components as shown on the right. In order to construct a waveguide, one simply paints the interior of two channel pieces, and inserts them into the center of the flange. Use of liquid adhesives such as Locite is recommended to keep the component from falling apart. These sub-systems can be created independently in any modelling tool using measurements of the originally modeled system, or if an object is saved as an STL file, it can be easily imported into SOLIDWORKS, and use of the “Extruded Cut” tool will allow for one to remove sections of the component until only the desired subsection remains.
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Once every sub-component has been defined, they can be imported into a “slicer” program which turns the 3D model into GCODE file for the 3D printer. We chose to use Cura, a program which takes in files in the STL file format. This program allows for the placement and duplication of our components onto the print-bed. First we need to select the base printer used for the model so the system can know the dimensions of our printing platform, then we simply maximize our print quality, arrange our desired components, and press “Slice” to create the print files. Note if there are persisting overhangs in the system, support structures can be automatically generated at this stage. Once it finishes processing, Cura will provide a preview of the printing process, give a time estimate for the print, and provide the file to be transported to the printer!
Paint Selection:
The most critical component of our waveguide creation process is the metallic paint. This coating is what allows signals to be transmitted through our plastic components, so it is important to get as ideal a pain as possible. In order to test the paints, we built a standard testing platform pictured below. This shape consists of a large plane and a sharp corner, allowing us to investigate each paint’s ease of application over the general use case of printed components.
The primary qualities we are looking to optimize in our paint selection are: cost, smoothness, and resistivity. Ideally we would want a system with zero resistivity(infinite conductance), to measure this quantity we use the four-point probe approach. Wherein four equally spaced probes are placed on the surface to be measured, with current flowing between the outer two probes and voltage measured between the two inner. As depicted in the image on the right from https://www.pveducation.org/pvcdrom/characterisation/four-point-probe-resistivity-measurements
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Note in our case with our extremely high frequencies, we are looking for surface resistivity, so we can drop the thickness term from their derivation to arrive at the expression:
where I is the input current, and V is the measured voltage.
Unfortunately we were unable to actually make these measurements before the campus closed, however we were able to paint the test pucks, and some of our tested paints provided datasheet values for their resistivity. This information is related in the following table:
Based on these results, we have decided that MG Chemicals 843AR-340G 843Ar, the spray paint, would be the optimal paint for this process. The spray application is significantly easier than any of the brushed paints, and its resistivity is on par with all of the others, making it an easy choice. One thing to note is that all of these paints would be classified as biohazards, meaning any liquids coming in contact with them, say the the water used to clean paintbrushes, would need to be taken to a hazardous waste removal service.
