In a high wall pressure set, varying capillary wall surface roughness gives average pressure changes with little change in slip behavior. The addition of 0.8% total fatty acid reduced the wall pressure to a level where varying capillary wall surface roughness results in changes in wall slip behavior.
Understanding Rheological Behavior
In Figure 3, the yield stress of a non-Newtonian shear-thinning material is represented by the y-intercept of the flow curve.10 As the shear rate increases in pseudoplastic materials, the shear-thinning behavior occurs nonlinearly. This demonstrates the shear-thinning property of ceramic materials, making it easier to achieve higher shear rates with very little addition of shear stress.9.
Evolution of Rheological Models
Extracting Wall Shear Stress from Ram Extrusion Data
The pressure drop at the entrance of the mold describes the pressure resulting from the deformation of the batch as it passes from the vessel to the mold. The mold land pressure, also called wall pressure in this body of work, is measured via the resulting pressure drop as the batch moves along the mold surface. The last piece needed to understand the wall behavior of a batch is the shear stress at the batch-wall interface.
The pressure drop in the die land (Pl) generates a net force on the paste which is counteracted by the wall shear force given by the product of the wall stress (τω) and the wetted perimeter of the die land. Using the experimental wall pressure, we obtain the expression for τω as in equation 11 for a constant velocity and temperature:3.
Adjusting Waviness Through Capillary Hole Manufacturing Techniques
Going a step further, making capillary arrays with varying bore and surface treatment, on the same L/D; the relationship between surface finish on batch/matrix surface interactions can be evaluated and is the focus of the work described in this thesis. Tuning of waviness through capillary cavity fabrication techniques Forms of surface finish roughness can be grouped into basic categories viz. This creates a uniform surface finish and in this case can be fine-tuned to give extremely smooth underlying surfaces.
This process can be employed in a way that allows for varying long wavelength, also called "wavy" surface finishes prior to coating treatment. By adapting the feed velocity algorithm for a specific turbulator shape, waviness can be imparted to the metal hole which can be measured by the waviness (Wa) parameter of surface finish.14,19.
Adjusting Roughness Through Capillary Wear Resistant Coating Techniques
Peters16, the idea of adding turbulence, or periodic changes in the inside diameter, of the duct was introduced to increase the heat transfer rate and to add turbulent flow to the cool air passed through the blades. This same process would add a sufficient degree of hole surface finish to the capillary dies used in this study. Adjusting roughness via capillary wear-resistant coating techniques While drilling techniques are capable of creating large-scale surface finish features.
For more than a decade, nitride-based protective coatings such as TiN have been the industry standard in the tool industry due to their excellent wear resistance. The application of these wear-resistant coatings is the final step in determining the surface treatment of the capillaries used in this study.
Experiment Purpose, Scope, and Hypothesis
The desired extrusion batch had to be a base composition at a single bundle stiffness value and coarse to fine alumina ratio that, when tested across two levels of fatty acid lubricant, created high wall pressure and low wall pressure conditions with enough difference to generate confidence that the results were outside the variability of the test was. Although the use of one batch with both high and low wall pressure regions was possible, the temperature dependent transition region between the two was not consistent and therefore unpredictable. The dry batch components of the composition were added up to 100% by weight with water and fatty acid as super additive to the total dry batch weight.
The batch composition workspace includes two levels of fatty acid, water, and a ratio of coarse to fine alumina.
Tests Used in Experimentation Procedure
The first of two tests in this device, a temperature sweep, aims to look for the behavior of the wall pressure as a function of temperature. The duration time of the speed sweep program phase can be modified to include sufficient time for stabilization. To allow for sufficient stabilization time and to maximize the speed sweep test data collection time, the phase length was extended to two minutes per phase.
Analysis of the velocity sweep test yields coefficients characterizing the effects of surface finish, velocity and composition on wall behavior. Log10 transformation of wall shear stress plotted against log of extrusion velocity shows the accuracy of the trend fit for predicting β and n.
Capillary Die Preparation
Based on the images and grain lengths in Figure 16 and the correlation in Figure 17, the range of coating thickness to create a micro-roughness response while not covering the capillary holes used for this experiment was between 10 and 18 microns. The tools were coated with TiCN and TiBCN using chemical vapor deposition (CVD) inside a commercial model BPX-pro 750 S CVD reactor manufactured by Ionbond of Olten, Switzerland to create differences in the micro-roughness levels of the surface finish. Fifty optical surface profilometer measurements were taken from each capillary via a Zygo New View 6300 manufactured by Zygo Corporation headquartered in Middlefield CT.
Given the fine micro-roughness of the coating, a 2000x magnification setting was applied to the exposed surface to capture both waviness and roughness and confirm that the goal of creating different surface finishes was achieved. The finishing of the drilled and coated blanks into cylinders with a nominal diameter (mm) and a nominal length to diameter ratio (L/D) of 28.5 was achieved using a combination of wire electrical discharge machining (wEDM) to form the cylindrical shape. bases and drilled holes to achieve 28.5 L/D on each die.
Test Procedure
The key requirements of the two groups selected for investigation were to have stable pressure at two temperatures. This meant that the high and low wall pressure behavior should be within Region 1 limits (high wall pressure) and Region 3 limits (low wall pressure) from Figure 11, and the upper temperature limits of the bundle should be high enough to prevent the occurrence of beam stiffening as seen in Region 4 of Figure 11 during testing. The two batch compositions used in this process were determined from the exploration of the batch composition space described in Figure 10.
The inlet pressures (0.5 mm length) from this test determined the relative stiffness of the group entering the screed to determine the water level, while the wall pressure behavior would determine the effect of the varying fatty acid level on a composition of special. The final set of tests was the first time that the experimental surfaces would be used in this experiment, so to test the effect of the manufactured capillary surface on the rheology of the set, low temperature cleaning tests were first performed on each type of capillary for proven wall pressure. surface to surface differences.
Selection of Ceramic Paste Extrusion Material
Reduction of wall pressure due to an increase in fatty acids relative to water and changes in the ratio of coarse to fine alumina. Through these tests, space exploration resulted in a single compound that could be used at two fatty acid levels that would elicit a significant wall pressure response while maintaining the same stiffness. The hypothesis tested was that the wall pressure response could be modified by varying the surface finish of capillary dies.
If true, the surface finishes created on the capillary dies used will change the gap between the high and low wall pressure compositions. Furthermore, the strong wall pressure response between the high and low fatty acid compositions would be sufficient to ensure statistical significance of the final data.
Temperature Dependence of Batch Composition
Establishment of equivalent stiffness was another requirement prior to using the experimental die surfaces to assess compressive behavior. The corresponding nozzle inlet pressure difference between mode 1 and mode 5 in Figure 19 was investigated to measure the shear force needed to deform the batch moving from the cylinder of the capillary rheometer into the square inlet of the feed hole at two fatty acid levels. From previous work, the inlet pressure within a +/- 0.5 MPa range was considered the same, so to create an equivalent inlet pressure response, the water level was adjusted to compensate for the difference in inlet pressure due to fatty acid levels.
Additional testing performed on both fatty acid compositions confirmed the required water content change to reach equivalent inlet pressure. The temperature at which the nozzle inlet pressure exceeds the mean pressure between 25°C and 35°C by 15% is considered the point at which critical stiffening has occurred.
Effect of Waviness and Roughness on Temperature Driven Pressure Behavior
Effect of waviness and roughness on temperature-driven compressive behavior Temperature sweep tests were performed on each surface at the two fatty acid levels in . The rate of wall pressure drop with respect to temperature stratifies according to the overall waviness and roughness of each surface. Holes with low overall waviness, combined with a low roughness coating produced the lowest overall wall pressure and slope with respect to temperature.
Increasing waviness and roughness lead to higher wall pressure drop with respect to temperature, indicating significant surface finish-dependent shear behavior. Effect of waviness and roughness on velocity-driven wall pressure behavior It is assumed that the wall pressure behavior coefficients, β and n, can be.
Effect of Waviness and Roughness on Velocity Driven Wall Pressure Behavior
As can be seen in Figure 22, increasing the fatty acid lubricant resulted in higher n-values; significantly lowers the pressure in the wall. At the same temperature and fatty acid level, surfaces with low waviness had lower β values compared to specimens with high waviness. Compared to the smooth surface baseline, the frictional forces generated by the rough coating resulted in lower n values at low temperature at both fatty acid levels.
Effect of combined roughness (Ra) and waviness (Wa) on n and β at low and high temperature and percentage of fatty acids. The soil velocity factor β is affected by surface waviness and roughness, fatty acid level, and temperature. In addition to the combination of surface finish and fatty acid percentage, a strong correlation with temperature has been identified.
Coupled with multiple extrusion speeds, a range of wall pressures can be applied for each surface roughness and fatty acid level combination.
