To assess drying of a ceramic shell,


a backup slurry system was used in this trial. That slurry is outlined within Table 1.


The stucco used was Remasil® relative humidity of 48


RG40. The conditions during dipping can be found within Table 2. A


50%


was chosen, as this was felt to be representative of what is used in industry. Three 12-inch oscillating fans were used to aid in drying. One of the fans was used to help ensure consistent temperature and R.H. by aiming it toward the ceiling. The other two were aimed at the drying samples and were placed approximately four to six feet away. For this experiment, it was decided not to have the fans blow on the sample bars 100% of the time, because this rarely would happen in a typical foundry. Instead, the fans were set to have direct air flow on the samples for about 25-30% of the fans’ oscillation cycle. Because of the small size and open configuration of the samples, over drying of the samples was minimised. Obviously, the different drying conditions would shift the end results one way or the other. Five different dry times (between


dips) were chosen. A total of 60 modulus of rupture (MOR) bars were made, and the test was run as follows: • 12 bars each were dipped at one, two, four, eight, and twenty-four hours dry time between dips


• Five dips plus a sealcoat were applied


• At 12 hours after the seal was applied, six bars from each set of 12 were taken for testing


• The edges were sanded off, and half the bars were immediately tested for green strength


• The other half were placed into a small box oven taken from room temperature to 1800°F (980°C) and held there for two hours


• At 24 hours final dry, the same procedure was followed on the remaining six bars of each set


• The fired bars were tested as soon as possible after they were cool


• Testing of the green bars and firing of the other bars was usually accomplished within one half to


®


Material Remasol® DI Water Surfactant Remasil®


LP30™


% Weight 32.13% 3.57% 0.04%


48 325 Mesh Table 1 (above): Slurry Recipe Parameter Room Temperature


Immersion Time in slurry Relative Humidity Air Flow Viscosity


Slurry Solids % Binder Solids Table 2 (above): Build Parameters % Weight


72˚F ± 3 (22˚C ±2) 10 seconds 50% ± 3


50-300 fpm (0.2-1.5 m/s) 14 secs Zahn #4 74% 27%


64.26%


Formula 1


one hour after reaching the final dry time.


The 3-point MOR formula is given


in Formula 1. where PMAX is the fracture load, L, W and H are the length,


width and thickness of sample fracture area respectively.


Results And Discussion The average modulus of rupture results for the samples with 12 hours final dry time are shown in Figure 2.


It can be seen from the green MOR results, that green strength reached a virtual maximum at four hours dry time between dips. The same results were obtained with the fired MOR. Figure 3 shows the average percent difference of


the 12-hour final dry bars using the four hour dry as the control. While there was virtually no difference between four, eight or 24 hours dry with regards to MOR strength, the one and two hour dry samples showed much lower strengths. These samples were obviously not


dry, so their maximum bonding strength was not achieved. Figure 4 helps to show this. Figure 4 basically follows the drying of one of the four hour sample bars on third and fourth dips until a constant weight was reached. It should be noted that reaching a constant weight is not the same as completely drying the mold. Since we are making the assumption that the drying rate was


Continued on pg 14 June 2020 ❘ 23


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