New 3D-printing and manufacturing techniques grant more control over energetic material behavior, improving safety

Much like baking the perfect cake involves following a list of ingredients and instructions, manufacturing energetic materials — explosives, pyrotechnics and propellants — requires precise formulations, conditions and procedures to ensure they are safe and perform as intended.

Because any small tweaks or environmental changes can dramatically alter how energetic materials function, Purdue University engineer Monique McClain is developing state-of-the-art tools and methods to control these materials’ behavior throughout the manufacturing process and down to the particle level.

McClain, a Purdue assistant professor of mechanical engineering, specializes in the “upstream” or earlier manufacturing stages, such as selecting binders with unique properties to hold energetic particles together and determining how they are mixed to create the final formulation. She focuses on how manufacturing alters the structure and mechanical properties of an energetic material and, in turn, how those changes affect performance and sensitivity.

“An energetic material’s manufacturing history, from beginning to end, strongly determines how it behaves during combustion or detonation,” McClain says. “We want to ensure that each step is catered toward the material and its intended use so that we’re getting a final product that functions in the way we expect.”

Much of McClain’s body of work focuses on additive manufacturing or 3D printing of energetics. Traditionally, energetic materials have been manufactured using processes such as casting or milling, which prioritize efficiency and scalability. But while these methods are ideal for large batch production, customization is difficult, thereby limiting innovation and compromising on optimal performance.

Additive manufacturing, on the other hand, gives researchers the freedom to experiment with complex geometries and tune specific properties such as burn rate and blast shape.

For instance, McClain and her research team design intentional defects — referred to as pores — to either increase or decrease the likelihood of ignition when materials are subjected to various conditions such as friction, impact or extreme temperatures. Additive manufacturing makes this possible because researchers can customize a 3D printer’s nozzles and program it to print specific shapes and patterns.

“Pores and defects are often inevitable, but we can control how and where they show up,” McClain says. “When we focus on the microstructure of these materials, we can deliberately select particle sizes or compaction schemes to produce preferred pore distributions that enable the behaviors that we want to see.”

Additive manufacturing also makes it easier to experiment with multiple types of materials. In addition to her work with pores, McClain explores how energetic particles adhere to various binders through the 3D-printing process and how to print propellant materials made of multiple materials with disparate characteristics.

In a study published last spring, McClain and her team looked at adhesion between two polymers with different mechanical properties — a stiff thermoplastic and a soft elastomer — that have been combined into one structure. They found that the 3D-printed surface texture and type of thermoplastic greatly affected how well the two materials blended and held together.

“This study provides a framework and method for studying adhesion of dissimilar materials. This is important because no such guide — and, in turn, little data — on this topic previously existed,” McClain says. “The ability to print energetics made of multiple materials gives us even more options for controlling behavior and improving safety.”

Although 3D printing is a major part of McClain’s work, she also explores how to improve more traditional manufacturing methods.

McClain developed a patent-pending method for manufacturing a polymer-bonded explosive (PBX) molding powder that saves time, eliminates potential hazards and reduces manufacturing waste. McClain disclosed this technology to the Purdue Innovates Office of Technology Commercialization, which has been applied for a patent through the U.S. Patent and Trademark Office to protect the intellectual property.

The method involves mixing energetics with a binder that’s been partially cured or hardened prior to compaction within a mold. McClain found a sweet spot of time — approximately eight hours — where the binder becomes solid enough to prevent leakage during compaction, but not so hardened that it becomes brittle and prone to cracking.

“We aimed to provide a repeatable, tunable method for fabricating solid energetic composites, like PBX, with uniform mechanical and chemical properties,” McClain says. “We succeeded in developing a streamlined set of steps where researchers can make slight adjustments based on their goals for the final product.”

While following the proper steps of the energetics manufacturing process helps to ensure a safe, well-designed final product, external conditions also influence its performance.

According to McClain, factors such as room temperature can dramatically affect a material’s printability and behavior.

“Environmental control matters much more than many people expect,” McClain says. “You could print the same mixture twice in the same day and they might behave in completely different ways if something like the temperature or humidity level in the room changed.”

Ultimately, McClain advocates for a holistic approach to manufacturing rather than one driven by technology advancement. She wants researchers to avoid “force fitting” materials into a particular machine or method and instead consider which geometries and properties are required to create the desired effect.

“If you need a complex internal structure, additive wins. If you need a highly dense mixture or a large batch of material, conventional methods are often more appropriate. To get the best of both worlds, we can also incorporate molding and milling alongside 3D printing,” McClain says. “As long as we understand every aspect of the process, from selecting material all the way to packaging and storing them, we’ll be able to make the best and safest choice.”

McClain’s research is sponsored by the U.S. Air Force Office of Scientific Research’s Young Investigator Research Program and the Army Research Office Energetic Materials Basic Research Center. McClain conducts her research at Maurice J. Zucrow Laboratories, the largest academic propulsion lab in the world.