Development of a Rheological Technique to Analyse the Flow of Highly Filled Ceramic Pastes

Development of a rheological technique to analyse the flow of highly filled ceramic pastes

Adam Edwards^ab, Stuart Blackburn^a,, Richard Greenwood^a, Stewart Welch^b, Alex Cendrowicz^c

^aSchool of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT

^bRolls-Royce plc, Precision Casting Facility, Derby, DE24 9BD

^cRoss Ceramics Ltd, Derby Road, Denby, Ripley DE5 8NX

The high pressure turbine blade is an integral component of any modern turbine engine in the aerospace industry. A blade is often manufactured through the investment casting process, where the initial stages of the process are focused on the formulation and production of a ceramic core. The most common method of ceramic core production is through powder injection moulding (PIM), where material is injected via a plunger or screw type mechanism into a mould pattern of the desired geometry.

Understanding the rheological properties of materials is essential in the injection moulding process. The flow of highly filled ceramic paste systems is complex, therefore having knowledge of real material flow properties is advantageous to the operator when trying to produce a defect free product. During the flow cycle, from the barrel through the sprue-runner-gate system and finally into the injection mould, the material is subjected to both high shear and elongational deformation due to the increasingly complex geometry of the desired product. In order to continuously produce a dimensionally accurate and thermally stable part, having the ability to assess any deviations in the fundamental properties of the material under different PVT (pressure-volume-temperature) regimes is key. Measuring rheological characteristics of the ceramic paste can help predict the most efficient operating window in terms of injection conditions, as well as the debinding behaviour of the feedstock. For example, shear viscosity data is a crucial input parameter when performing mold flow analysis to assess the most appropriate fill and hold pressures that are required. Rheological studies also provide a good indication of the homogeneity achieved during mixing, meaning how well the particulate solids have been distributed within the binder matrix.

This study is aimed at assessing the suitability of various conventional rheological techniques, with the aim of developing an industrially relevant analytical tool that operates under the conditions that are representative of the injection moulding process. In essence, the material will have the same thermomechanical history as it would during real processing. The key principle of the novel analytical tool is that it would have the capability of generating more accurate and reliable rheological data in comparison to established techniques. Furthermore, the current lab based quality control measures would be replaced by a more streamlined “inline” process meaning that practical solutions could be implemented within hours as opposed to days.