Patent Application
20130340507
The present invention relates to measuring viscosity of ceramic slurries with usage for quality control of ceramic casting processes and myriad other usages in research and development and/or quality control.
Ceramic slurries in flowing conduits and static tanks have many usages including provision of material for “casting” by coating substrate parts immersed in the slurry. Such coating can be done in single or multiple stages, the latter often including a first encapsulating stage with high precision control of coating thickness and density. Later coating stages also require such control. The substrate parts can be permanent or removable after coating and solidification to leave the coating as a free standing part (investment casting). Complex substrate surface geometries and internal reentrant surfaces can be controllably coated. Ceramic casting presents special challenges of achieving reliable slurry characteristics and measuring them reliably, including measuring viscosity for its own sake and as a measure of other characteristics. After coating the ceramic parts are fired to attain ultimate strength, dimensions and morphology as finished products. Yields of usable products are dependent on reliable coating.
Viscosity and other characteristics relatable to it (e.g. shear rate and weight percent of solids) vary with depth in a tank containing substantial amounts of ceramic slurry, conditions of movement of slurry (generally induced to maintain homogeneity), capture by cast parts and replenishment or adjustment. There are several approaches to measuring viscosity—sometimes by extracting samples for viscosity measurements off-line and sometimes in situ through instrument probes inserted into the casting bath to various depths. The immersed probes have been vulnerable to drift and error over the course of casting operations and some may need frequent removal, cleaning or replacement, down time and recalibration of the probes and instruments. Some probes can be cleaned in situ without removal from the tank but are subject to undesireable and variable coating of transducer elements.
It is the object of the invention to provide an effective adjustment of the process and apparatus for viscosity measurement substantially mitigating such problems and optimizing thickness, increasing yields and enabling longer durations of casting process with minimal instrument related interruptions with reliable and predictable response to viscosity variations.
The objects of the invention are met by a system that operates in a linear or rotational smooth flow of reduced turbulence of ceramic slurry, preferably through a cylindrical portion of an otherwise turbulent mixing regimen doing so in a cylindrical or toroidal chamber. An immersed probe extends vertically into the slurry with a turn of the probe end essentially orthogonal to the vertical probe, i.e. horizontal. The horizontal end of the probe faces the flow of slurry but is protected from it by a baffle or other barrier in front of the probe end, but spaced from it, to let the slurry work around the barrier and contact an active horizontal probe end, usually of rod or wire form. The probe rod is part of a vibrational transducer system for viscosity measurement, but the transducer can be of other mechanical, electrical, magnetic or acoustic formats.
As used herein, “horizontal” and “vertical” are defined as within plus or minus 45 degrees variation from true horizontal or vertical but preferably within plus or minus ten degrees or even five degrees for many applications. The horizontal probe assembly is movable over a significant depth range to take a series of measurements at different depths since viscosity of the slurry can vary substantially with depth end means for raising or lowering the probe as a whole, or at least its turned end, are provided. Means are also provided to rotate the probe or at least at its end within a horizontal plane or nearly so, i.e. with a plus or minus 45 degree range, preferably plus or minus 10 degrees or less to achieve substantially linear alignment of a transducer portion with the local slurry flow direction.
Typically the slurry is in a rotating tank that has internal fixed paddles to create a general mixing but with a rotational flow components Alternatively, rotatable paddles can be used within a fixed or rotating tank.
The baffle or other barrier in front of the probe end assembly may have holes or edge designs to optimize bringing the flow to the transducer within the probe end while mitigating against heavy probe coating or erratic flow.
The baffle or other barrier is spaced from the end of the probe end by rods or other means allowing a substantial flow horizontally and/or vertically through the spaces therebetween, but these will provide a protective function for the probe as well as stand-off supports for the baffle.
The transducer can be an ultrasonic rod form electrode mounted within a cage of several electronically/acoustically neutral rods for stand-off spacing the barrier from the instrument and forming a protective cage.
Ceramic slurry easily passes to/from the electrode without solid build up on the active part or surrounding structure of the vibrating probe.
The vibrating electrode probe instrument may be substantially as in the state of the art presented by the Brookfield Advanced Sensor Technology (AST-100) product. Vibrating at resonant frequency (typically 1-5 kilohertz) (preferably tuned to under 1 kilohert for slurry casting), it provides cyclic torsional drive control, then cyclically paused to measure a relaxation response which is electromagnetically measured and correlated to a relative or absolute viscosity and tuneable to a range of 2 to 12,500 centipoise (but which can be displayed in other scales, e.g. milliPascals), operating in a temperature range of −20° C. to +200° C.
Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which:
The ceramic slurry in the tank is subject to partly predictable, partly unpredictable changes with height in the tank and other factors (e.g. sedimentation or agglomeration affected by ceramic compositions), particle average size and size distribution and morphology. A viscometer probe 10 is inserted and can be lowered and raised manually or by an actuator A to take readings at various heights and transmit data to an instrument for reading at a visual display D or digital recorder and the signal provided to a process controller PC to modify tank conditions and part P lowering/raising.
The probe has a baffle 22 supported ahead of the end of the sensor rod 12 by multiple rods 24 (typically stainless steel). The rod 12 is oriented essentially parallel to flow in the tank region of the probe end (typically circumferential flow in a cylindrical tank), within plus or minus 45 degrees, preferably in the plus or minus ten degrees range and the shield, which can be flat, conical or domed or other shapes is essentially orthogonal to the flow with plus or minus 45 degrees preferably within plus or minus ten degrees.
We have discovered that this orientation can provide a remarkable improvement in stability of the measuring process and make it effective over long periods of process operation operating at a single depth or over a range of depths in the tank.
It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.
