Wet-tape thickness

The previous section in this chapter discussed a variety of different control techniques for governing the slip height. Reservoir height does not usually have as large an effect on tape thickness as does the blade gap. In theory, this means that the doctor blade gap can be seen as a rough control for wet-tape thickness, while reservoir height can be used to fine-tune the wet thickness. Not many casters use this technique. 

This section has explored the variables that define wet-tape thickness. As the solvents are removed, the tape shrinks in the z-dimen-sion so that the dry-tape thickness (green tape thickness) is less than the wet thickness. The following section explores the drying process. 

The uniformity of the blade gap from cast to cast can also be a source of variation in tape thickness (wet and dry). Many manufacturers use an adjustable mounting system for the doctor blade, which is set to height before each cast. Many of these blades are set by hand and are subject to human error. This human factor can significantly affect repeatability. To avoid this, some manufacturers simply minimize the... 

At this point the slip is ready to cast. Casting is done on a Mylar carrier in a continuous tape casting machine. The doctor blade gap setting, speed of casting, etc. are determined by the tape thickness desired as an end product. The wet dry ratio for casting this slip is approximately 6.5 1. 

Wall board or gypsum- A manufactured panel made out of gypsum plaster and encased in a thin cardboard. Usually 1/2" thick and 4 x 8 or 4 x 12 in size. The panels are nailed or screwed onto the framing and the joints are taped and covered with a joint compound. Green board type drywall has a greater resistance to moisture than regular (white) plasterboard and is used in bathrooms and other "wet areas". 

In a typical tape casting process, the slip or slurry is poured into a puddle or reservoir behind the doctor blade, and the carrier to be cast upon is set in motion. The doctor blade gap between the blade and the carrier defines the wet thickness of the tape being cast. Other variables that come into play include reservoir depth, speed of carrier movement, viscosity of the slip, and shape of the doctor blade (to name just a few). The wet film of slip passes into a drying chamber of some sort, and the solvents are evaporated from the surface, leaving a dry tape on the carrier surface. 

During continuous casting, such as that used in a production mode, the slip is delivered from the filter to the doctor blade reservoir through a pipe or tube that has some sort of valving mechanism in series. This valve controls the flow of slip to the reservoir and helps to maintain a constant "head, or in some cases puddle, of slip behind the doctor blade. The control of the level of slip in a reservoir-type doctor blade assembly is one of the variables that combine to regulate the wet thickness of the tape being cast. This subject will be discussed in detail in Section 4.2 on the physics of tape casting. 

Wet thickness Thickness of the tape casting slurry after passing under the doctor blade. This thickness is related to but rarely ever equal to the blade gap. This term is often used interchangeably with Blade Gap anyway, since true wet thickness is difficult and/or expensive to measure. 

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