Particle packing and compaction are important in many aspects of materials science and technology, including the flow and storage of slurries and of granular agricultural and fertilizer products; the forming of ceramic components, powder metallurgy (P/M) parts, and pharmaceutical tablets; road and dam construction; landfill and waste-remediation-site design and development; shoreline and earthquake stabilization; predicting and controlling mud slides, rock slides, and avalanches; and mine and oil-well positioning and stability. On the less technical side but perhaps of more practical interest are how packing and compaction produce the “apparently” half-empty packages of cereal and potato chips, and how to make better sand and snow castles. There is a great need for engineers and scientists to understand and control particle packing and powder compaction.

Particle packing is directly controlled by the particle-size distribution of a material being processed. For this reason, particle packing is important to all particulate/fluid systems. After the solids fraction of a body is defined, interparticle chemistry controls how the body will pack and flow. A system of powders can never pack better than the maximum possible level defined by the particle-size distribution alone. Proper control of interparticle chemistry however can help achieve maximum packing, can be used to open the structure, and/or can be used to modify rheological or other process properties.

The main goals of particle-packing research have been to determine how systems of particles pack, to develop algorithms for calculating packing densities and porosities for any distribution of particles (spherical or nonspherical, rough or smooth, wet or dry), and to determine how packing and its properties affect the variety of industrial operations that utilize particulate/fluid systems.

Powder pressing, either uniaxially or isostatically, is the most common method used for high-volume production of ceramic components. The object of a pressing process is to form a net-shaped, homogeneously dense powder compact that is nominally free of defects. A typical pressing operation has three basic steps: (1) filling the mold or die with powder, (2) compacting the powder to a specific size and shape, and (3) ejecting the compact from the die. To optimize a pressing operation, experienced press operators generally understand and control parameters such as die-fill density, die-wall friction, packing density, and expansion on ejection.

Die filling/uniformity influences compaction density, which ultimately determines the size, shape, microstructure, and properties of the final sintered product. To optimize die filling and packing uniformity, free-flowing granulated powders are generally used. Spherical granules (i.e., agglomerates or clusters of finer particles) range in size from ~44 to 400 μm with the average size being ~100–200 μm. They are typically produced from 0.5 to 10-μm median particle-size powders by spray drying a ceramic powder slurry. To produce processable powders, various organic additives are typically added to the slurry prior to spray drying. These include binder(s) for strength, plasticizers that produce deformable granules, and lubricants that mitigate frictional effects. Consistent batching and dispersion of the granulated feed are critical for reproducible and uniform die filling. Granule densities that are 45–55% of the theoretical density (TD), and bulk-powder and die-fill densities of 25–35% TD are typical for ceramic powders.

Powder metallurgy (P/M) involves a multiple-step process to prepare and consolidate granulated metal and composites into near-net-shaped components. The modern P/M method starts with a precisely defined final geometry and a set of properties for a desired component. Decisions are then made about a variety of processing variables, recognizing that many variables strongly interact counter to the desired properties. Finally preproduction fabrication trials are conducted to optimize key variables until he specifications are achieved, and the processing conditions are established. A critical step in the overall process is the compaction of powder into a sufficiently strong, low-porosity, properly shaped part with uniform microstructure that can be sintered, or alternately heated and pressed to the final dimensions and properties.

It has been estimated that more than 80% of all medication doses are administered as tablets—that is, in unit dosage forms prepared by compacting powders in dies. For drugs with acceptable oral-absorption profiles, the compacted tablet is the first choice as a delivery system in drug development. Modern tablets range in size from 25 milligrams (veterinary implants) to several grams (veterinary “boluses”). The most common tablet shape is the double convex-faced disk. Shapes range from capsule to triangular forms, in many instances with bisects embossed or debossed with identification codes and company logos. Tablets may be either film- or sugar-coated, may be designed for immediate or extended release, or may be prepared as multilayer or laminated forms. In addition to oral administration, tablets may be predissolved or designed to be disintegrated in the mouth, or may be administered by injection or as suppositories. Production rates for over-the-counter products such as acetaminophen tablets can exceed three 1,000-kg batches per week, requiring high-speed multiple-station tablet presses for production. In contrast, implants—which have low-volume market requirements—must be made under aseptic conditions on special singlestation presses.

The microstructure and properties of a ceramic component are largely predetermined by the processes and process controls used to manufacture them. The metric for success in manufacturing is often based on gross density. For example, optimizing pressure-density response, maximizing overall density, and minimizing springback and delaminations in powder pressing all focus on characterization and control of the overall (macroscopic) state of a powder compact. Unfortunately this focus on macroscopic effects has contributed to a general neglect of the compact at the microstructural level. Process-control variables in powder compaction have been defined and discussed by many workers, but their quantitative application to predict and control compaction behavior is limited. Advances in characterization technology and computer modeling now allow us to quantitatively characterize and simulate microstructures more easily. These and other tools can help provide the scientific and technological foundation necessary to predict and control microstructure and microstructural evolution during processing.

The compaction process involves stress transmission via rigid or flexible (die) walls and the propagation of stresses within a powder mass. The particles that comprise the powder distribute the stress by a variety of kinematic processes that involve sliding, rotation, particle deformation, and rupture. In practice the “particles” are often agglomerates of finer particles that have a range of properties. All of these factors must be considered in developing a comprehensive predictive model for compaction.

The modeling of powder-compaction processes has a significant history that has been greatly advanced by the relatively recent general availability of powerful computers and their peripherals as well as by appropriate softwares. Compaction modeling may attempt to provide a basis for machine-loading specifications, or it may provide guidelines to help minimize “capping” defects where failure cracks form at the top of the green compact. It may also provide “green-body heterogeneity” through predicted stress and density distributions within a compact. Likewise compaction models may be combined with binder burnout and sintering models to predict internal microstructural features such as grain size and porosity, and the external shape of the sintered product. This article will deal only with the modeling of the compaction process; important elements such as powder flow for die filling and subsequent processing steps such as sintering and net shape predictions are not directly addressed.