Powders are complex systems consisting of solids or particulates, a continuous gaseous phase (usually air) and, almost always, a liquid component. Their properties are a function of composition and an array of particle parameters that includes size and distribution, surface roughness and hardness. Vibration, compaction, attrition, segregation and many other factors influence behavior.
This makes measuring powders in a reproducible and meaningful way difficult; predicting flow properties from basic parameters, such as particle size, is well beyond current capabilities. So, powder processors have learned to rely on experience rather than fundamental knowledge. They tend to solve problems using a trial-and-error approach — and often can’t pin down the underlying cause or even why a solution works.
Such a trial-and-error approach can succeed but is time consuming and expensive. Furthermore, the experience gained, unless it extends underlying knowledge, has limited applicability.
Drivers for Process Improvement
As margins tighten any processing inefficiencies become less and less tolerable — many plants already have solved the easiest problems. Further progress requires manufacturers to develop in-depth understanding to push units to their efficiency limits and maximize return on investment. Typical goals may be:
• raising throughput by decreasing downtime or increasing flow rates;
• reducing waste by consistently maintaining product specification and cutting rework;
• switching to lower cost feeds; and
• automating process control.
To achieve these targets the manufacturing team must know what changes to make — and understand the impact that any change will have on the process and product. The starting point has to be the existing experience base but this, in its raw state, only can be used to improve operation within a well-mapped window.
If aspects of processing experience can be correlated with specific powder properties then a better understanding can be developed. Knowing which variables determine how a powder behaves in a given situation is the first step toward more-effective control. Using such knowledge to extend operation outside the established working envelope is the way to meet more-exacting performance standards.
Figure 1. Powder Rheometer: This device can
A powder rheometer (Figure 1) can serve as an excellent starting point to investigate process behavior. It determines the energy required to make a sample flow by measuring the force/torque acting on a blade rotating through the material . Conditioning, which is a gentle agitation step that produces a homogeneous, loosely packed bed prior to measurement, ensures excellent reproducibility. This makes powder rheometry highly differentiating. It can measure powders in a compacted, conditioned, aerated and even fluidized state, to more accurately simulate process conditions.
Modern rheometers not only provide dynamic measurement but also shear and bulk property testing capabilities. It’s therefore possible, using just a single instrument, to gather a comprehensive set of data in a reasonably short time. This encourages the development of databases of powder properties that can include:
• flowability parameters such as Basic Flow Energy (BFE) and Specific Energy (SE) ;
• shear properties, e.g., yield locus, unconfined yield stress (compression strength), cohesion and internal angle of friction; and
• bulk properties like bulk density, compressibility and permeability.
In addition, investigating specific aspects of powder behavior such as de-aeration, segregation, caking and the effect of moisture and attrition can improve understanding.
Using this database of properties, it’s possible to relate process behavior to the characteristics of the powder. It also becomes feasible to identify those variables that critically impact performance, as the following case study illustrates.
A manufacturer wants to consider other suppliers for a powder feed. The operational team strongly resists, though, because feedstock has been switched before and productivity was severely compromised. The powder flow through the plant, particularly from the feed hoppers, now is easily maintained; it was erratic with the alternative material and blockages were common.