High-performance ceramic materials are characterized by high strength, high hardness, wear and corrosion resistance, and are widely used in energy, information, aerospace, electronics, biology and medicine. Ceramic molding method is an important factor in determining the performance of ceramics, and cast molding is one of the common methods used to produce thin electronic ceramic sheets. Today we will introduce the flow casting process and its classification.

I. Flow Molding Process and Its Flow
1. Cast molding
Casting (Tape casting, also known as Doctor blading or Knife coating), refers to the ceramic powder in the addition of solvents, dispersants, binders, plasticizers and other components, to obtain a uniformly dispersed stable slurry, in the cast machine to produce the required thickness of the film of a molding method.

Compared with other molding processes, cast molding has many advantages.
1) Simple equipment process and continuous production.

2) the possibility of preparing single-phase or complex-phase ceramic sheet materials

3) small defects in the product, uniform properties, high production efficiency, and continuous operation

4) large and small batches are possible, suitable for industrial production.

5) Very suitable for the preparation of large thin ceramic parts, which is the most important feature of cast molding and is difficult to achieve by pressing or extrusion molding process.

However, there are some problems with cast molding, mainly the high content of binder, so the shrinkage of the blank is larger in the process of dispensing, and the thin plate is prone to defects such as cracking, curling, and uneven thickness.

2. Casting process
Cast molding was first used in the fields of paper, paint and plastic, etc. It is a long-established molding technology, which mainly includes preparation of slurry, ball mill mixing, molding, drying, gluing and sintering processes.

Firstly, ceramic powder and dispersant are added into solvent (water or organic solvent), and the particle agglomeration is opened by ball milling or ultrasonic oscillation, and the solvent wets the powder, then binder and plasticizer are added, and a stable and homogeneous slurry is obtained by secondary ball milling; then the slurry is molded on the calender to obtain the plain billet; then drying is carried out to make the solvent evaporate and the binder forms a mesh structure between the ceramic powder to obtain The blank film; then the blank film is machined to get the specific shape required; finally, the finished product is obtained by gluing and sintering.

Cast molding process classification
According to the type of solvent, cast molding can be divided into two categories: water-based cast system and organic cast system. With the continuous improvement and perfection of cast molding technology, a series of special cast molding methods have been researched and developed in recent years, such as gel cast molding, UV-initiated polymerization cast molding and isostatic pressure cast molding.

1. Organic cast molding
Organic solvent cast molding technology has been more mature in the application of ceramic preparation. Because of the low boiling point of organic solutions, which is conducive to the drying of the blank, while organic solutions have good solubility for polymer binders, plasticizers, etc., and will not react with the ceramic particles hydrolysis, which can ensure the stability of the slurry, so the organic cast molding of ceramic raw blanks, uniform internal structure, high strength, good flexibility, easy to cut and process, is now widely used in industrial production and laboratory.

The common solvents used in organic cast molding are methyl ethyl ketone, toluene, xylene, ethanol and trichloroethylene, etc. In practice, binary azeotropic solvents such as ethanol/toluene and ethanol/trichloroethylene are commonly used.

Advantages.
Due to the compatibility of organic solvents, easy volatility, low latent heat of evaporation, low surface tension and the ability to prevent the hydration of ceramic powders, the organic cast molding system has a wide range of additive choices, fast solvent volatilization, short drying time, and easy to obtain a uniform structure, small size of the blank defects, high strength and flexibility of ceramic sheet.

Disadvantages.
Organic solvents have certain toxicity, which inevitably bring harm to human and ecological environment, and higher production cost, higher organic content of finished products, lower density, easy to crack in the process of glue discharge, etc.

2. Water-based cast molding
Water-based cast molding uses water as solvent and water-soluble polymer as binder, which overcomes the disadvantages of organic cast system such as environmental hazards, high cost and low density of finished products, and is suitable for mass production.

The common binders for water-based cast molding are polyvinyl alcohol (PVA), cellulose (MC) and polyacrylic acid (PAA), etc. The common plasticizers are dibutyl phthalate (DBP), propylene glycol (glycerol), polyethylene glycol (PEG), etc.

Advantages.
1) Non-polluting, green and environmentally friendly.

2) Low cost, easy for mass production.

Disadvantages.
1) Few types of dispersants and binders soluble in water, so the range of options available is narrow and the effect is poor.

2) low solvent evaporation rate, the film is not easy to dry, especially not suitable for thicker blanks of the flow calendering.

(3) high binder content in the row of glue when the plain blank shrinkage rate is large.

4) multiple agglomeration of powders due to the presence of hydrogen bonds.

5) sensitivity to changes in process parameters and low quality of the plain billet film.

6) The brittleness of plain billets is large, and they are easy to crack when drying, etc.

3. Gel cast molding
Gel cast molding is a new process combining gel injection curing mechanism and cast molding method, which is using the principle of organic monomer polymerization for cast molding.

The method is to disperse ceramic powder in an aqueous solution containing organic monomer cross-linking agent to prepare a concentrated suspension with low viscosity and high solid phase volume fraction (volume fraction greater than 50%), and then add initiator and catalyst to trigger organic monomer polymerization under certain temperature conditions to increase the viscosity of the suspension, which leads to in-situ solidification and molding to obtain a certain strength and machinable blanks.

The slurry used in water-based gel cast molding is prepared from ceramic powder, organic monomer, crosslinker, solvent, dispersant, plasticizer and other components.

Advantages.
In can greatly reduce the amount of organic matter used in the slurry, improve the solid phase content of the slurry, thereby increasing the density and strength of the plain billet, greatly reduce environmental pollution, and significantly reduce production costs.

Application
Due to its simple equipment, continuous operation, high production efficiency, and uniformity of blank performance, the cast molding method has become an important method for preparing large-area, ultra-thin ceramic substrates, and is widely used in the electronics industry, energy industry and other fields.

It can be used for the production of monolithic capacitor ceramic wafers, Al2O3 substrates for thick and thin film circuits, piezoelectric ceramic diaphragms, structural ceramic flakes, capacitors, thermistors, ferrite and piezoelectric ceramic blanks, hybrid integrated circuit substrates, etc., while it is also used in the field of cell phone ceramic covers.

Article source: Electronic materials circle, powder molding circle, etc.
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