Powder-binder mixtures are the basis for everyday products such as toothpaste, milk chocolate, and ice cream. Similar ideas using a solid particle (metals, alloys, ceramics, and composites) in a binder (typically organic or organometallic) matrix are applicable to industrial component fabrication. From an engineering viewpoint, the technical specifications for powder-binder mixtures reflect each specialized situation which is a combination of plastic forming technologies and sintered materials. To successfully implement these powder shaping concepts requires an understanding of different disciplines. Core scientific concepts and basic technological principles are generally missing in an undergraduate engineering curriculum, yet the field of powder-binder shaping is growing to reach levels of great commercial significance and the basic technology is discussed in this book.
Shaping technologies based on powder-binder formulations apply to a broad array of engineering components and even reach into medical implants and aerospace components. Powder-binder formulations have also evolved in the form of additive manufacturing, allowing component production without tooling. Additive manufacturing production levels can reach thousands of components per day. Most of the shaping technologies discussed in this book are built on ideas taken from plastics; especially extrusion and injection molding.
The spaces between particles are filled with binder (either completely or partially), such that when the binder melts the particles are easily shaped. Other ideas on powder shaping come from long-established ceramics processes where a sacrificial solvent phase is added for easy shaping. The solvents are water, alcohol, or hydrocarbons that evaporate after shaping. After solvent removal, residual polymer holds the particles in position for subsequent sintering. Common features are use of a small powder, transient binder, and high-temperature sintering, but a wide range of shaping options are possible.
Advances in engineered powder synthesis have enabled these forming ideas. Powders are now available in the small sizes required for sintering in all common metals and alloys, ceramics, and composites. Small particles are needed for sintering densification. These particles are mixed with binders, consisting of multiple ingredients, giving feedstock. The bulk of the binder is a filler phase, such as wax, water, cellulose, or glue (such as polyvinyl alcohol). Strength is derived from a backbone phase, such as polyethylene or polypropylene. Adhesion of the binder to the powder avoids separation during forming. This requires a surface-active agent. Soap-type molecules such as stearates are favorite surfactants. Feedstock formulations often are proprietary, but several are given in this book. Besides feedstock composition, success in this field involves decisions on mixing procedures, mixer design, and testing. In practice, the feedstock formulation is customized to the forming process.
Knowledge of the final component shape is required to select the appropriate tooling and shaping route. The forming equipment capabilities identify the required viscosity as a function of forming parameters, such as shear rate, temperature, and pressure. When properly performed, these shaping options provide sintered property combinations similar to those published in reference material, such as the ASM Handbook series. In other words, the adaption of new forming technologies provides tremendous shaping advantage without a performance penalty. Engineers love the design flexibility, component complexity, and attractive production cost, all of which are far superior to that attained using traditional material forming options. With the advancements in additive manufacturing techniques, shapes are emerging with features not attainable by any other fabrication route (including plastic shapes).
This book focuses on the basic principles but touches on the practical as well. It shows the decisions needed to properly employ powder-binder-based processing. The book is written to embrace a variety of options because fundamentals are similar. Both scientific and technological principles are discussed in this book, leading the reader to use this platform to develop future innovations.
The book idea emerged from a long-running collaboration that involved several materials, powders, binders, and shaping techniques. Further, we both worked with many talented individuals who helped push novel ideas into significant industries. We are most appreciative of the many students and friends who provided so much of the data, images, and relations organized into this book.