Researchers at the University of Oklahoma in the US have developed a method to rapidly produce metallic prototypes for radio and microwave applications. They reproduce a standard rectangular cavity resonator with a high quality factor and centre frequency within the desired range. These results demonstrate that the novel application of Lost-Shell Casting (LSC) techniques to electromagnetic component fabrication is a viable and cost-effective manufacturing method. LSC is the process of creating a metallic object in the space left by a 3D printed part that is burned away. The voided space is filled with metal as the printed part is burned. The “Shell” component of the name refers to the fact that the printed part is made with as little fill as possible, making it ideally just an outer shell, in order to minimise the amount of material used. Lost-Shell Casted cavity resonator immediately following production. Casting sand was used as the prototype moulding medium. LSC can be performed quickly with inexpensive materials, making it an ideal option for quickly fabricating and testing prototypes. While widely used in other fields, it has not been used in electromagnetic applications before this Letter. LSC is a useful technique for fabricating radio frequency (RF) components as it produces a completely metallic prototype in an overall time-span from design to final fabricated and tested prototype in only two or three days, depending on the size and complexity of the design. This process also leverages some of the benefits of 3D printing, allowing for significantly more complicated designs without the fabrication difficulties and limitations of other methods. The LSC process begins with a CAD design, which is then printed using a 3D Polylactic acid (PLA) printer. Next, the printed part is encased in a casting medium (casting sand in this Letter) inside a steel vessel. This entire vessel is then placed in a programmable oven with an aluminum ingot placed on top and heated such that the PLA will completely burn off and the aluminum will completely fill the void left by the burned PLA. After cooling, the casted part is collected and fitted with the appropriate connectors for testing. Other fabrication techniques for rapid prototyping that are based on similar 3D printing techniques will often create a component out of plastic or resin. Following the printing a thin conductive layer, such as copper plating or conductive paint, is applied. However, such coated components lack the structural integrity of metal and are not representative as prototypes of a finalised design. LSC, on the other hand, creates an entirely metallic component, but still maintains the design complexity of 3D printed parts. The resonator produced using LSC in this Letter possessed greater durability than those created by other 3D printing methods with high quality factor, and the desired prototype properties were verified in subsequent testing. “Metallurgical issues were the main challenge in this work” explains one of the authors David Mitchell, as “several aspects of the design do not affect the electrical performance of the part but had to be added in order to ensure quality”. The structures on either side of the cavity, shown in the Letter, are necessary to provide the excess liquid aluminum a place to flow as well as to allow the aluminum to shrink as it cools without affecting the electrically important area itself. Surface imperfections on the interior cavity prototype are due to gases released by the molten aluminum being unable to escape, which reduces the quality factor of the team's produced component. A potential solution would be to release these gases, but as this would involve direct handling of the molten metal, the team did not explore this in their Letter and remains a topic for future research. LSC could be widely used to develop more durable and more complex filter and antenna designs as a novel method for rapidly iterating on new designs. As the technique is refined and developed it has potential to become a widespread standard for creating complex, fully metallic components. The team have begun investigating other methods of rapid prototyping using 3D printing, and Mitchell has high hopes for the field, stating: “We have only just cracked the surface. Each method has design types for which it is best suited, but as these fabrication methods are combined and adapted previously unimaginable designs become possible.”
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Nov 12, 2018
Ineu Widaningsih Sosodoro + 2
Open Access
