the results were obtained by chance. Any p-value less than 0.05 indicates that there is at least a 95% chance that the design factor is significant. In Table 2, factors that have a significant effect on the design outputs are marked with an X. This information allows us to make some interesting observations: • A four second change of viscosity did not impact any of the design responses. This result is somewhat surprising


as viscosity control in


a foundry is thought to be a key factor in process control. Further testing would need to be conducted to identify how large a change is required to affect the responses.


• No factor interactions were produced, meaning that each factor produced a response that is independent of the results from the other input factors.


• Draining technique and dwell times impact the shell properties tested. One of the primary objectives was to look specifically at this possibility and determine if or how this could be incorporated in the shell building process.


A closer look at the data shows


the effect of each significant factor and allows a better understanding of the impact of draining and dwell times on shell properties. As dwell time of the part in the slurry is increased, the thickness of the shell coats are decreased (Graph 1). For parts where bridging is a concern, increasing dwell time could decrease the thickness in recessed locations. Increasing dwell time could also remove the need to prewet certain parts. A foundry could experiment to see if a longer dwell time could replace any prewetting on their parts. Conversely, to increase shell thickness, the dwell time in the slurry could be reduced. While the draining technique did not


have an effect on flat surface thickness, it did affect the thickness on round permeability samples (Graph 2). A shorter drain time (drain 1) will result in a thicker shell. Adding a stagnate portion, or gush (drain 3) created a thinner shell sample than when constantly manipulated (drain 2), even though the overall drain time was the same for both.


® Graph 2– Effect of Draining Technique on Thickness of Round Parts In practice this indicates that adding


a stagnate drain or gush to each dip cycle before manipulation could reduce the time required to fully drain the parts. This time savings would likely be greater when multiple trees are dipped robotically and there is slurry dripping off one part and on to another. For seal coats, an extended gush could be implemented to allow excess slurry to drain off the parts faster. In summary, foundries should devel-


op individualized draining techniques based on part structure in order to op- timize shell build. While flat parts can have a simplified drain time without con- cern for uneven shell building, complex parts may require a combination of differ- ent draining techniques. For parts where bridging is a concern, the addition of a stagnate drain or gush could benefit the process by creating a thinner coat in re- cessed locations. For foundries suffering


from edge cracking, a reduction in the drain time may prove more beneficial than increasing viscosity in building uni- form edges.


Minimizing dwell and drain times


allows the foundry to improve casting throughput, taking less time per shell dip. Cycle times could be adjusted to improve throughput without a negative impact to shell strength; simply by reducing slurry dwell and drain times. In facilities where shells are handled manually, reducing dwell and drain times may positively impact physical ergonomics for shell room personnel. In robot rooms, speeding


the rotation cycle may be possible. An excerpt from: Berta, Wolfe, Hendricks


(2018) How Process Variables Impact Ceramic Shell Properties and Performance. Maumee, OH: Ransom & Randolph. To request a complete copy of this paper, please email R&R at: RR-Marketing@dentsply.com.


April 2019 ❘ 21


Graph 1: Effect of Dwell Time on Shell Thickness


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