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Share basic knowledge of casting technology

1、 What is procrastination

The tape casting method is to add adhesives, solvents, etc. into the powder, conduct vacuum defoaming after ball milling and filtering, and control the viscosity within a certain range. Under constant pressure, this kind of viscous slurry flows and sticks to the film belt through the gap between the slurry scraper and the film belt that is coated with organic silicon and runs at a certain speed. After drying and trimming at intervals, the green belt and the film belt are separated (or never separated) at the end of the tape casting machine, and then rolled in turn to obtain the green belt. Then it goes to the next process again, such as cutting, laminating, printing, sintering, etc. to make the required products, thus forming the tape casting preparation process.

In the tape casting process, the most critical is the slurry preparation and tape casting process.


2、 Application of tape casting


Engineering flow chart of tape casting ceramic sheet

The smooth and uniform tape casting slurry can be obtained by removing the organic or inorganic residues in the slurry through a series of processes such as ball milling. The prepared tape casting slurry can be tape cast on the tape casting machine, and then it can be completed through wetting, cutting, degreasing and sintering.


3、 Introduction to common additives in slurry

1. Solvent

Important factors for solvent selection include: (1) it must be able to dissolve other additives, such as dispersants, adhesives and plasticizers; (2) It has certain chemical stability in the slurry and will not react with the powder; (3) Easy to volatilize and burn off; (4) It is difficult to use safety, medical treatment and environmental pollution.

The most common solvents are divided into organic solvents and water.

The slurry obtained by organic solvent has high viscosity, slow solvent volatilization and long wet time, so organic solvent is more than used in tape casting film. The common ones are ethanol, methyl ethyl ketone, trichloroethylene, toluene, xylene, etc. However, organic solvents have the disadvantages of flammability and toxicity.

Water as solvent also has the advantages of high cost, safe use, medical treatment and easy comprehensive production. Its disadvantages are: (1) less moisture for powder particles, faster volatilization and longer moisture time; (2) The slurry degassing is difficult, and the existence of bubbles will affect the quality of the plain embryo film; (3) The remaining binder used for water-based slurry is emulsion, which is difficult to produce on the market, limiting the selection of binder. The solvent is important to the wettability of the powder due to its surface tension. The greater the surface tension is, the better the wettability of the powder particles is. The surface tension of organic solvent is much higher than that of water, so its humidity performance is better than that of water. The surface tension, dielectric constant and other comprehensive properties of mixed solvent are better than that of single component, and the boiling point is high, and the solubility of dispersant, binder and plasticizer is also poor. In order to ensure volatilization in the wet process, binary azeotrope mixture is often used in casting slurry. The most common are ethanol/methylethanol, ethanol/trichloroethylene, ethanol/water and trichloroethylene/methyl ethyl ketone.


2. Dispersant

The concentration uniformity of powder in tape casting slurry indirectly affects the quality of plain embryo film, and then affects a series of properties of material, such as compactness, porosity and mechanical properties. There are four types of dispersants commonly used in tape casting membranes: non-ionic, anionic, cationic and zwitterionic. Generally speaking, anionic surfactants are mainly used for neutral or weakly alkaline slurry with positive charge on the particle surface, while cationic surfactants are mainly used for neutral or weakly acidic slurry with negative charge on the particle surface. Mikeska et al., through the experimental study on the concentration effect of 70 kinds of commercial dispersants, showed that phosphate, ethoxy compounds and fresh fish oil had the best concentration effect in ceramic powder slurry. The latter two are anionic surfactants, so fresh fish oil is never a surfactant.


3. Binders and plasticizers

The tape casting film must have certain strength, toughness and ductility to facilitate separation and handling with the substrate material. For this reason, the binder and plasticizer must be added into the slurry. The factors that should be considered in the selection of adhesive are: (1) the thickness of plain embryo membrane; (2) The type and matching of the selected solvent are conducive to the volatilization of the solvent and no bubbles are generated; (3) It shall be easy to burn off and there shall be no residue; (4) It can stabilize the slurry and restrain the sedimentation of particles; (5) A relatively high plastic transition temperature is required to ensure that no condensation occurs at room temperature; (6) Considering the properties of the substrate materials used, they should never be bonded and easy to separate. Binders can be divided into three categories: non-ionic, anionic and cationic according to the functional group type. Anionic and non-ionic binders are mostly used in the tape casting process, including vinyl and propylene. The common binders in non-aqueous slurry are PVB, polyacrylate methyl ester and ethyl cellulose. Common binders in water-based media are polyvinyl alcohol, acrylic acid emulsion and polyacrylate amine salt. The main function of the plasticizer in the slurry is to reduce the plastic limit temperature Tg of the binder, so that Tg can reach or below room temperature, thus ensuring good fluidity of the binder at room temperature and no condensation. In addition, the plasticizer also plays a role of lubrication and bridging for the powder particles, which is conducive to the concentration and stability of the slurry, but the addition of the plasticizer will reduce the strength of the plain embryo film. The most common plasticizers are polyethylene glycol, phthalate, ethylene glycol and so on, and their effects on the rheology of slurry are not very similar. Phthalate grease can lubricate the powder particles and reduce the viscosity of slurry, so polyglycol can also form an organic bridge in the powder particles, which can increase the viscosity of slurry.


4 Other additives

In addition, during the preparation of tape casting slurry, some functional organic additives are often added to produce some specific slurry properties or perfect dry tape properties.


Defoamer: It is important to use in low molecular solution (PVA) or stable foam water medium where polymer concentration system is not easy to cause harm, especially in the process of stirring (such as using specific wax or using vacuum stirring). Preventing foam is more accurate than eliminating foam. Common defoaming methods are mechanical method plus chemical method. It is to add defoamer into the slurry, and then conduct vacuum stirring for degassing. Common defoamers are mixtures of n-butanol and ethylene glycol.


Wetting agent: it is a surfactant soluble in the liquid phase, which is used to reduce the surface tension of the liquid (especially water) and improve their wettability to the powder and the affected substance. Therefore, surfactants are also used as dispersants. Purging agent: used to increase the mutual solubility of components (such as cyclohexanone), so as to prevent peeling when wet. They also increase the density and tensile strength of the substrate.

Flow control agent: sometimes a small amount is added to prevent the surface of the substrate from being wet very slowly and cracking.

Flocculant: a reagent used to prevent the formation of such low density precipitation in a centralized system.


4、 For tape casting

Tape casting is a very optical process, which requires very strict product quality after tape casting. The following points are for reference:


1. Surface finish of scraper

The casting scraper is usually made of tool steel. It has good wear resistance and short service life. However, care should be taken. After each use, it must be cleaned to prevent hard objects from scratching the surface and keep the scraper smooth and flat. Smooth and flat scraper is the key to obtain uniform thickness and smooth surface film.


2. Highest liquid level of slurry tank

The highest point of the liquid level in the slurry tank is raised, and the pressure in the slurry tank is increased, so that the inflow speed of the slurry through the scraper gap is increased, and the casting film thickness is increased. Therefore, maintaining the same balance of the highest point of the liquid level is critical to control the uniformity of the casting film thickness. Large tape casting equipment generally needs to be equipped with a liquid level sensor to control the slurry supply valve and control the highest point of the liquid level to change to the largest range.

3. Slurry uniformity

The tape casting slurry must be fully and evenly dispersed. If there are hard lumps that have not been dispersed and aggregates that have not been filtered out, scar like defects will occur on the membrane belt, or depression will occur due to different wet firing shrinkage. Therefore, it is necessary to pay attention to the preparation of the slurry. After use, these hard lumps and aggregates must be removed by sieving. If there are bubbles in the slurry, defoaming treatment must be carried out after casting.


4. Casting thickness

The thickness of the scraper gap is never the same as the actual drying and molding thickness. It should be due to the volatilization of solvent during the drying process. When the slurry is stable and other conditions such as flow rate and humidity temperature are constant, there will be a stable ratio. Accurate parameters can usually be obtained by tape casting tests.


5. Develop and implement the best wet process

The slurry film can be peeled off from the substrate after being wet. Therefore, the key factor to obtain low quality film tape is to formulate appropriate wet process. If the wet process is not properly formulated, the cast film will often have bubbles, pinholes, wrinkles, dry cracks, and is easy to fall off from the substrate and other defects. The principle for formulating the wet process is to ensure that the solvent is used slowly, so that the diffusion speed of the solvent in the film and the surface volatilization speed tend to be the same, and to prevent the late stage cracks, blisters, wrinkles and other defects caused by early hardening of the surface.


5、 Advantages of tape casting process

Low degree of automation and low production efficiency. The product quality is good and the qualification rate is low. It is suitable for large-scale substrate production and diversified substrate production.