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Technical Article


How Process Variables Impact Ceramic Shell Properties and Performance


by Dave Berta, Product & Application Specialist; Michael Hendricks, Applications Engineering Director; Casey Wolfe, Technology Manager, Ransom & Randolph


T


he impact of shell material selection on


shell properties has been explored extensively. Investigating


how process variables, like dipping/ draining techniques, impact ceramic shell properties and performance is unexplored territory. Utilizing a Design of Experiment


(DOE) process, the effects that certain controllable shell building process variables have on shell properties were explored. These input variables include; backup slurry viscosity, dwell time on the part in the slurry and the draining technique used. Viscosity is a measure of the thinness


or thickness of a fluid as measured by a flow cup. Thinner materials have a lower viscosity and thicker materials have a higher viscosity, by definition. The actual viscosity a foundry uses is likely determined by the parts being made, specifically the size, geometry and shape. Typically, primary slurries are thicker than backup slurries; although, with the influx in fiber-based slurries in the industry, backup slurry viscosities are approaching those of primary slurries. Dwell time is the time that a ceramic


shell assembly is submerged in the slurry. Slurry dwell time is often overlooked, but it is an important variable. A dried shell that is dipped into a slurry is, to some degree, a ceramic sponge, as it absorbs liquid. While the addition of polymers to a backup slurry can minimize the amount of liquid absorbed, all shells absorb liquid.


If a shell is dipped into a slurry for


a short dwell time and is removed prior to the ceramic sponge absorbing its full capacity, the shell continues to absorb liquid from the slurry coat applied after it is removed from the slurry. The rheology of the slurry layer can change as it continues to absorb liquid, raising the viscosity of the slurry in that layer. A longer slurry dwell time should


20 ❘ April 2019 ® Factor Viscosity


(seconds Zahn 5) Dwell Time


(seconds in slurry)


Draining Technique (gush seconds/


manipulation seconds) Table 1 – Input Variable Definition Factor Viscosity


Dwell Time


Draining Technique


result in the ceramic sponge absorbing its full capacity from the slurry and not from the shell coat; therefore, the rheology of the draining slurry coat from the part will be consistent and noticeably thinner. Essentially, a longer dwell time in a slurry acts similarly to a prewet in many cases. In the shell building process, draining


removes excess slurry from the freshly dipped cluster, ensures all sections of the cluster are covered in wet slurry and provides a uniform coat that can accept adequate stucco on all edges and surfaces. The actual draining technique is a combination of manipulation and time. Wet clusters are manipulated at vertical angles and rotated to keep the slurry uniform, as excess slurry drips off and back into the slurry tank. The time that it takes to achieve the uniform coat is varied as required.


As the cluster exits the slurry tank, more than half of the wet slurry on the


X X X X Table 2 – Significant Factors Affecting Design Output


cluster is estimated to drain off and back into the tank as excess. Recognizing this, it is important to understand what can be done to speed up the time it takes for this excess slurry to drain. If a cluster is constantly being manipulated and rotated, the drain time is extended. However, if the cluster is held motionless for a short time immediately after it exits the slurry, more of the excess slurry drains off. The remaining slurry can then be spread evenly through manipulation and rotation. We refer to this concept of motionless draining of excess slurry before manipulation as a gush. The input variables were tested at


three levels (Table 1) to determine their impact on standard shell properties like strength, thickness and permeability. The DOE software utilized


determines which factors are significant based on the calculated p-value. The p-value measures the


probability that X X MOR (green) AFL (green)


Thickness (flat)


Thickness (round)


Permeability (hot)


Edge build (top)


Edge build


(bottom) Level 1


10 3


Drain 1 - 0/10 Level 2


14 10


Drain 2 - 0/20 Level 3


N/A 20


Gush + Drain 1 - 10/10


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