S. Blackburn

Investigation of paste flow using positron emission particle tracking

Abstract The method of positron emission particle tracking (PEPT) has been adapted for use during extrusion. A particle 2 mm in diameter was tracked through a 45° conical die geometry using ram speeds of 5 and 10 mm min −1 . The particle was placed at different points across the diameter of the barrel in order to determine the dependence of the speed and strain rate on the initial position. Analysis of the speed of the particle during extrusion showed that the clay based paste used in the experiment exhibited plug flow in the barrel and the die land regions. The maximum extensional strain rate was shown to be independent of the ram speed, whereas the shear strain rate at the wall was highly dependent on the ram speed.

Agglomerate breakdown in fine alumina powder by multiple extrusion

Repeated extrusion through dies of various diameters and die entry angles was used to determine the rate of agglomerate breakdown in a paste consisting of a fine alumina powder, carbon black and a binder of hydroxyl propyl methyl cellulose in water. It was found that three extrusion passes were enough to break up all but ∼0.4% of the agglomerates. Dies with orifices of approximately 1 mm diameter and die entry angles of 45 to 90‡ (where the elongational strain and the deformation rates were highest) were the most efficient for disrupting and dispersing agglomerates and distributing the moisture evenly. This process of deagglomeration was studied by monitoring the load required to extrude and moisture distribution during five repeated extrusion passes of each test paste. The density, agglomerate area fraction and agglomerate circularity of dried extradates were quantified and plotted.

Design and characterisation of a co-extruder to produce trilayer ceramic tubes semi-continuously

A co-extruder with three separate barrels operated by a single ram has been designed to produce trilayer tubes semi-continuously. A vital step in the design was to predict the pressure required to generate a sufficiently high extrudate velocity while being able to retain the extrudate integrity. A physically based model was used to predict the pressure drops in the co-extrusion process at three different extrusion velocities for five pastes with different rheological characteristics. In general, predicted and measured values were in good agreement. Other important aspects in the co-extruder design, such as velocity and pressure matching of different flow streams are also highlighted. Trilayer ceramic tubes were successfully produced from the designed co-extruder.

Zirconia toughened aluminas by hydro-thermal processing

Abstract Zirconia toughened alumina materials have been produced by a hydro-thermal powder synthesis route. The resultant sols were concentrated using filtration to form extrudable pastes. The extruded products when dried and sintered had uniform microstructures with improved mechanical properties compared to pure mono-phase rods produced by a similar route. The zirconia particles produced by hydro-thermal synthesis were too small to give the optimum toughening effect, as the zirconia phase is undoped. Two alternative methods were investigated in order to increase the zirconia grain size in the finished body. The hydro-thermally synthesized zirconia was freeze-dried to increase the flock size before mixing with the alumina sol and a commercial monoclinic zirconia powder was added to the alumina sol prior to concentration. The improvements in mechanical performance correlated well with theoretical predictions.

A water-based system for ceramic injection moulding

Water-based binders for injection moulding of ceramics use low moulding temperatures allowing moulding and die temperatures to be similar. Most of the binder system can be driven off as water vapour, cutting process time, reducing burn-out problems and causing less environmental pollution. The difficulties with removing moulded articles from the die must be overcome. To address this problem, gel strength was measured on solutions up to 8 wt% of ten commercially available cellulose derivatives at four temperatures between the gel temperature and the boiling point of water. A paste with rheology that permits mould filling was made from the binder that showed highest gel strength. This was used in moulding trials. A similar formulation based on a low-strength binder was also studied for comparison. The paste based on the highest gel strength binder showed significantly less deformation on release from the die, allowing the processing advantages of the system to be realised.

The effects of liquid-phase rheology on the extrusion behaviour of paste

A description is given of experiments carried out to determine the effect of liquid-phase rheology on the extrusion behaviour of particulate pastes. These were prepared by mixing alumina powder with chosen amounts of four liquids. Measurements show that the properties of the pastes are directly related to those of the liquids from which they were made. Rheological properties of the pastes and the liquid phases are related by factors which are independent of liquid type, liquid content and extrusion rate. An additional set of experiments used pastes made from ballotini particles and one of the liquids.

The orientation of binders and particles during ceramic injection moulding

By annealing sections of thick, injection-moulded ceramics, the orientation of the organic binder was studied. A modified calculation method for X-ray diffraction analysis was used for evaluation of particle orientation. Using a finite difference method, the temperature distribution in a mould system was related to the structure found. Fine plate-like particles became highly oriented during injection moulding and the oriented particles restricted the binder orientation and subsequent relaxation of the extended chains.

Characterising paste extrusion behaviour by signal processing of pressure sensor data

Abstract Methods for control of the extrusion of concentrated solid–liquid pastes are limited by the lack of reliable process sensors. A previous study showed that quantitative indicators of the homogeneity of pastes and extrusion defect phenomena could be obtained from statistical analysis of die pressure data. This paper describes the application of more recent statistical techniques to the analysis of such data, including wavelet and Bayesian methods. Methods for calculating fractal properties (specifically, the Hurst parameter) were compared, and those which were not affected significantly by the presence of periodic components were applied to real data. Wavelet analysis proved very effective for de-noising pressure signal data, which were collected from ram extrusion tests using three materials: a starch-based snack food dough, a detergent and a ceramic paste, and analysed for measures of outliers, coefficients of variation, the Hurst parameter (a fractal property) and periodic behaviour linked to regular surface fracture. Multivariate cluster analysis methods were found to be more reliable in distinguishing between pastes and identifying suspect data sets. Different fracture behaviours were found to exhibit different pressure signal characteristics. The extension of these methods to continuously operating machines is discussed.

The influence of powder packing on the rheology of fibre-loaded pastes

The effects of fibre addition on the extrusion rheology of ceramic particulate pastes has been investigated using model pastes containing chopped glass fibre and alumina powders. In these pastes where the fibre diameter was much larger than the alumina particle diameter, there was a decrease in the pressure required to extrude the paste as the powder component was replaced by fibre, up to a ratio of 0.4 powder to 0.6 fibre by volume. When the pastes were characterized using physically based models this behaviour was reflected in lower values of the derived rheological parameters. It is shown that this behaviour can largely be attributed to the packing behaviour of the fibre and powder-phase components. The results also suggest that the presence of fibres increases the die entry pressure drop relative to the die land pressure drop. It is shown that while the models proposed by Milewski in combination with the rheological models can be used to illustrate the expected behaviour of composite pastes, the observed behaviour was better predicted by using modified models proposed by Karlsson and Spring. At high fibre loadings (>60 vol%), extrudable materials could not be formed; this was attributed to packing and mixing problems leading to increased fibre-fibre interaction preventing flow. This is highlighted by materials in which the ratio of powder to fibre was 1 to 4 by volume, where the paste-like body was compressible and exhibited some elastic springback. For pastes where the solid phase contains <50 vol% fibre the rheological behaviour is predictable if the packing behaviour of the fibre and the rheological behaviour of the pastes containing only the powder are known. This can be used as a tool to aid the design of composite pastes and for the development of suitable process equipment.

Slip flow in concentrated alumina suspensions

A model aqueous injection moulding paste was investigated by capillary rheology. The experimental work consisted of 26 pressure measurement on each of three capillaries to provide an extended shear rate from 0.5 to 150 s?. A power law model was found to be adequate for general prediction of the low curve. However, slip flow had to be taken into account to map the shape of the flow curve more accurately. The Hatzikiriakos method, which does not require the use of different diameter capillaries, was applied to measure slip flow and gave rational solutions. The flow curve was modelled using rheological parameters for shear flow and slip flow over the region of the flow curve for which they were valid. The adjusted model incorporating slip flow gave a better correlation with the experimental results.