Optimising Ceramic Powder Specification
Dry powder pressing techniques are the most cost-effective, flexible choice for the manufacture of a wide range of ceramic components. Identifying powder blends and granulates that are best suited to such applications is an essential element of process development but brings its own challenges. One critical factor is to select an appropriate method for characterising ceramic powders, the importance of which is illustrated in the case study below using experimental data from a study of die filling.
A first step towards the successful manufacture of a ceramic component using a powder pressing technique is to develop an appropriate feed material. As ceramic powders are often supplied as submicron or nanosized particles it is usual to include an initial granulation step. The ideal outcome from granulating to a suitable endpoint a well-specified mix of ceramic powder, binders, lubricants and dispersants, will be a feed of consistent composition that delivers high levels of productivity and meets product quality targets. Material must flow easily from a hopper, fill a die adequately and compress to form a stable green body that can be successfully ejected from the mould before sintering and final machining. A good match between feed and plant will result in a robust and stable process that maximizes throughput and quality.
Developing granulates with properties that are suitable for a defined application requires careful manipulation of a variety of parameters. These include particle size and distribution, degree of agglomeration, composition (binder content for example), and particle shape and texture. It is common, for example, to select relatively fine granules for the production of ceramics that have very demanding specifications, such as those used for medical applications. Finer particles tend to form more homogeneous green bodies and ultimately deliver a final product with few defects.
One of the major challenges implicit in this development process is how to characterise granulates in ways that relate to in-process performance and finished product quality. Without appropriate measurement tools the performance of a potential feed material can be assessed only through plant trials, either at lab or pilot scale - a costly and time-consuming option. Characterising powder properties effectively can both accelerate and optimise the development cycle.
Characterising Powders for Ceramic Processing
Although ceramics processors have a wide selection of powder characterisation techniques from which to choose, the applications within the industry exert some quite specific (though not unique) demands. Quantifying flowability is essential because the material needs to flow freely, in a controlled way, out of a hopper into the feed shoe and from there to the die. Understanding the response of the powder to air is also critical.
Air exerts a major influence on powder flow properties and is especially important in die filling by gravity. Here, the air content of the powder in the feed shoe, while often uncontrolled, affects flowability during discharge. As powder leaves the shoe it entrains air which lubricates the filling process, simultaneously reducing bulk density. If this air is not easily released as the powder settles in the die, then bulk density remains low, compromising fill weight. De-aeration behaviour is therefore also highly pertinent. Sensitive differentiation in all these areas is a vital attribute of a truly useful characterisation tool.
Many conventional powder measurement methods are limited by their lack of reproducibility and questionable relevance to the manufacturing process. For example, zirconia has excellent flow characteristics when tested using the Hall flowmeter, but can flow poorly during die filling operations. Furthermore, many techniques record just a single parameter for a given powder, failing to capture the behavioural complexity that ceramics processors need to assess. Powder characterisation technology has, however, developed significantly over the past decade. The maturation of dynamic measurement techniques and the emergence of universal powder testers that offer multi-faceted characterisation are both worthy of note.
The FT4 Powder Rheometer®, a universal powder tester, combines conventional shear and bulk testing with more modern dynamic methodologies, in a single instrument. Dynamic measurements of the powder in motion not only quantify flowability in an intuitively relevant way but also permit investigation of the impact of air. Samples can be measured in a compacted, conditioned, aerated or even fluidised state. Closely defined test protocols and sample conditioning give such instruments the excellent reproducibility that makes them highly differentiating. They are therefore valuable to those charged with developing ceramic granulates that process well.
See the Application Note in Featured Downloads (below) to read more about::
- Investigating die filling performance
- Rationalising process performance