Tape casting doctor blade

The thickness of the tape is controUed by the sHp characteristics, the height of the doctor blade, the casting rate, and the pressure head of the sHp reservoir behind the doctor blade. SHp viscosities in the range of 1 5 Pa-s (10—50 P) are used to cast tapes at 5—100 cm/s. To achieve the desired strength and flexibUity in the green tape, tape casting slurries contain more binder than those used in sHp casting, as weU as a plasticizer to ensure flexibUity. 

The coatings were applied to one side of a steel substrate by means of a spray gun for the lower viscosity coatings, or by doctor blading with an adjustable Gardner knife for the higher viscosity materials. A casting technique was also used in which a known volume of the coating material was poured into a known area defined by heavy tape and was allowed to spread while on a level surface. 

Fig. 3.6 Schematic diagram of the doctor-blade tape-casting process.

Figure 7.26 Tape-casting process. Slip is contained in a reservoir, one side of which has a stationary doctor blade. A moving carrier travels under the doctor blade, carrying with it a uniform layer of slip, which dries to a green tape.

FIGURE 13.8 (a) Tape casting apparatus with (b) detail of the flow of fluid under the doctor blade. Note opposite flow directions in (a) and (b). 

All of these rheological expressions (equations 13.16, 13.17, and 13.18) can be used to analyze the flow under the doctor blade in tape casting. Using the momentum balance equation 13.14 and one of the preceding equations for the shear stress, the differential equation which governs the velocity V,., can be determined. For Newtonian fluids, the solution is given by... 

Because the object of this work is to describe the thickness of the cast tape far from the doctor blade, it is necessary to calculate the... 

For these reasons, operation of a nearly dry second doctor blade may lead to tape cast thickness fluctuations. Much more stable operation can be obtained from operating with only one doctor blade with a full doctor box and control of tape speed and applied pressure. 

Using a suspension of hard spheres immersed in water at a volume fraction of 45% determine the cast thickness if the suspension is tape cast with a doctor blade 10 cm long and 1 cm wide onto a moving piece of mylar which is 0.2 cm from the doctor blade. The mylar is moving with a velocity of 5 cm/sec and the heif t of the suspension in the doctor box is 5 cm. No gas pressure is applied to the doctor box. 

Tape casting results from the relative movement between the doctor blade and the support or carrier (Fig. 5.14). There are two possibilities the doctor blade moves and the carrier is fixed, or the doctor blade is fixed and the Ccirrier moves [24]. 

In laboratory operations, batch casting with a mobile doctor blade and stationary carrier are used. Plate glciss is usually used as a carrier. Tape drying can take place on the support by electric heaters placed imder the support. If only partial drying occurs, the tape, with or without the support, is placed in an oven to complete the dr5dng. Tapes of 0.5 m wide, 2 m long and 1-2 mm thick can be obtained. 

Large productions are based on continuous casting machines where the doctor blade is stationary. A moving carrier, such as an endless stainless-steel or plastic-film belt, moves under the fixed doctor blade. In this case, the carrier width is 1 m and the length can reach 40 m. The tape is dried by passing through a tunnel where solvents are evaporated. Dried tape can be removed from the carrier and rolled for storage at the end of the carrier. 

Fig. 6 Schematic of a tape casting process used to form thin sheets of ceramic material. The slurry of ceramic particles and carrier fluid flows beneath the doctor blade onto a moving carrier film. As the tape moves down the bed, it dries and is then spooled on a take up reel.

Doctor blading, also known as tape casting or knife coating, uses a scraping blade, known as the doctor for the removal of excess substances from a moving surface being coated. 

The thickness of the deposited layer is determined by the height of the doctor blade above the polymer sheet. Multilayer films are possible using the approach illustrated in Figure 27.2. The special feature of the doctor blade is that it produces a very uniform thickness. A typical doctor blade assembly is shown in Figure 27.3. The cast tape is dried, peeled from the polymer sheet, and reeled up to wait for further processing. At this stage of the process the tape is flexible because it still contains a... 

As illustrated in Fig. 4.34, the key component of the tape casting facility is the assembly of the doctor blade. It consists of an adjustable doctor blade mounted in a frame with a reservoir to hold the slurry before it is cast under the blade to form the... 

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. 

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