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Feature Article


Foundry 4.0 – Smart Precision Casting Needs Viscosity Control of Ceramic Slurries


By Manpreet Dash, Dr. Vijoya Sa, Dr. Sunil Kumar, Rheonics


Investment Casting– The Catalysts of Growth nvestment


rapid production rates while ensuring consistent dimensional accuracy. It offers weight reduction, less machining, improved delivery time and lower inventory carrying costs, making it one of the most popular production methods for metal parts today. The variety of parts manufactured by investment casting is ever increasing with growing complexities of requirements from the oil & gas drilling, automotive, aircraft, defence, medical and military industries.


I


Ceramic Slurry for Shell Building – Viscosity is Crucial to Getting Reliable Slurry Ceramic shell, formed by dipping the wax assembly multiple times in the ceramic slurry is critical to achieving precise, reproducible parts. To ensure consistent


shells, it is vital to have


consistent ceramic slurry. Viscosity is one of the key properties of the ceramic slurry which defines the overall thickness and adhesion of the multiple coats during shell formation.


After


coating, the ceramic shell is fired to attain ultimate strength, dimensions and morphology as finished products. Yields of usable products are dependent on reliable shells. The quality of the ceramic shell depends on the slurry composition and the process by which it is built. Typical ingredients in an investment casting slurry include: colloidal silica, water and/or polymers, wetting agents, anti- foaming agents, and refractory flours. The raw materials (refractory, binder, wetting


agent, and antifoam) 28 ❘ January 2021 ® used to make slurry play a major role in casting process, also


known as lost wax process, is relatively simple, which enables


determining the overall final ceramic shell characteristics. Suitable choice and good mixing of ceramic materials can lead to smooth surface finish, and high accuracy of the metal castings.


Slurry Viscosity


A good slurry composition alone cannot guarantee production of smooth and defect free shell if the slurry is prepared in an inadequate manner. Slurry quality control is one of the most important operations of the investment casting process, with viscosity control of the slurry being key to ensure compliance. Key points on how slurry viscosity affects the investment casting process are: • Slurry viscosity depends on the composition (filler to binder ratio) as well as the aging time and is an important indicator of the slurry stability. It is initially high when slurries


are mixed; however, as


mixing continues and the refractory material is wet and air is released, the viscosity decreases and asymptotically approaches a stable value.


• Surface finish will be an important characteristic of the casting, therefore the surface quality of ceramic shell for investment casting of metal alloys must be adequate. The surface finish is highly dependent on the viscosity of the primary slurry and the overall stability of the slurry mixture.


• Viscosity


is an effective indicator of the particle size distribution of the slurry. Particle size distribution variability of ceramic flour (powder) can affect slurry properties including slurry density, rheology, and coating thickness. Shell properties that can be affected are permeability, thermal characteristics, thickness, edge


coverage, and strength. For critical applications like Titanium and DS/ SC casting, control of particle size can mean the difference between success and failure.


• Continuous process control is essential for achieving target bending strength of the investment shell.


Increase in slurry viscosity


increases the bending strength of the investment shell, but decreases beyond a limiting viscosity attributed to larger content of refractory flour to binder content. In order to have an effective shell building process and so as not to waste materials and optimise energy usage, it is highly desirable that the slurry viscosity be regulated automatically to a pre-set value. Real-time, in-line viscosity monitoring and control in the mixing and coating process is essential to improve performance and reduce costs.


Process operators realise the


need of a viscometer that monitors viscosity and temperature, and could use temperature-compensated viscosity as the key process variable (KPI) to ensure consistency and reduce reject rates of the final castings plus avoid seasonal fluctuations affecting the slurry preparation.


Ensuring Top-Quality Products – Viscosity Affects Slurry Coating During Shell Formation There are four significant factors that make viscosity management important in investment casting application: 1. Investment shell and casting quality: Poor coating quality can adversely affect the desirable properties of the investment shells – surface roughness, thermal conductivity, chemical reactivity, permeability and shell strength


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