The importance of Table 2 is in the realization that Baumé, surface tension and Hercules data do not track with % solids, yet density does. The inference here is that dilution to cor- rect for differences in Baumé would be an incorrect practice. This was clearly demonstrated in a previous study, which showed coating solids to impact coat weight and coating thickness.1
This finding reinforces the need for performing multiple control tests. Robotic Dip Coating of Disc-Shaped Specimens
The application of the coating to the specimens was the most critical step in this process. In order to minimize sample to sample variation, a robot (Figure 4) was programmed to dip the specimens. Robotic dipping kept the dipping and drain- ing speeds constant. More importantly, robotic dipping kept the depth of the specimen in the coating container constant as long as the volume of coating in the container was kept constant. The volume of coating in the container was kept constant through continuous monitoring and replenishment.
Materials:
Uncoated disc-shaped PUCB specimens; Coatings: Ct1 (0.35% Surfactant), Ct2 (0.25% Surfactant), and Ct3 (0.15% Surfactant); Teflon tape.
Equipment: Magnetic stirrer, dipping robot, digital balance (± 0.01 g).
Procedure:
Prior to dipping the specimens their sidewalls were wrapped with Teflon tape to prevent the coating from seep- ing during the dipping process. The specimen was secured in the robotic arm ready for dipping. The prepared coat- ing was placed on a magnetic stirrer below the robotic arm and allowed to mix for two minutes at a slow speed to avoid vortexing. The stirrer then was turned off, and the coating was manually mixed with a spoon for one min- ute. The robot then was activated and the dip performed. Upon completion of the dipping cycle, the magnetic stir- rer was restarted and the specimen removed from the ro- bot and placed on a digital balance to collect the wet coat weight. The Teflon tape was removed and the specimen placed onto a metal tray. The specimens were moved to the drying oven and allowed to dry for 24 hours at 105C (221F). The warm coated and dried speci- mens were allowed 10 minutes to cool at room temperature. Specimens then were weighed to determine the dry coat weight prior to being stored in a desiccant cham- ber. It is important to note that all samples dipped in the same coating bath were com- pleted within 30 minutes to avoid moisture losses in the coating. If samples were not completed within 30 minutes, a new solids content test was conducted on the coating bath before the process was resumed.
International Journal of Metalcasting/Spring 11 Materials: Coated disc-shaped PUCB specimens.
Equipment: Nikon Tool Maker’s Microscope (Figure 5). Procedure:
The coated specimens were sectioned and smoothed by lightly brushing them with a fine grit paper in preparation for the coating layer thickness measurement. The measure- ments were taken using a microscope fitted with a cali- brated ocular. The microscope was equipped with a digital readout capable of measurements up to 0.001 mm, which was utilized to obtain coating layer thicknesses. It must be noted, however, that coating suppliers typically measure the coating penetration by counting sand grains. This is not the most precise methodology because sand grain sizes differ from one foundry to the next. Moreover, a batch of foundry sand has a known distribution of a variety of grain sizes within it, which also makes using the sand grain count as a measuring system ill-defined.
Effects of Dip-Time and Surfactant Level Variation
Dip-time is the amount of time the specimen spends im- mersed in the coating bath. Dip-time involves dipping the mold or core in the refractory coating at a predeter- mined depth (20 mm) to permit penetration of the coat- ing below the surface and deposit of additional coating above the surface (Figure 6). For the 0.25% and 0.15% surfactant levels, as dip time increased, coat weight in- creased (Figures 7 and 8). However, for the 0.35% sur- factant level, little change was observed. It is believed that for this coating, the surface tension is low enough to overcome capillary forces, enabling complete wetting
Coating Thickness Measurements
A destructive test was used to measure the coating layer thickness on specimens. To date no reliable non-destructive test measures the consistency of coating layer thickness ap- plied on cores or molds.
Figure 4. Robot dipping
Figure 5. Nikon Tool Maker’s microscope
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