**6.3 Suspension maintaining agent**

Suspension maintaining agent keeps the refractory powder particles in a dispersed state. They are divided into two groups: aqueous coating stabilizers and non-aqueous coating stabilizers. These agents prevent precipitation of refractory powder particles and have an important influence on the coating quality. Small amount of carriers causes a quick precipitation of the filler particles and other solid components contained in the coating composition. If there is an excess of carriers, these will cause an increment of coating density causing difficulties for the coating application on the mould or core surfaces; there will also be a crack risk for the thicker coating layers being dried. It is very important that solid matters used for suspension maintenance have the same mass as the liquid coating stage. In that case, suspension is maintained longer, the coating becomes more efficient and metal penetration is prevented. Aqueous coating stabilizers are the following: bentonite, carboxymethylcellulose, alghynate, and polyacrylamide. Alcohol coating stabilizers are the following: polyvinyl-butyral, polysobutylene, organic types of bentonite. Table 4 shows some suspension maintaining agents (Clegg1978, Svarika 1977, Tomović 1990).

Ceramic Coating for Cast House Application 275

The role of a liquid carrier or solvent is to dissolve and transport the refractory powder to the sand surface in the form of film. Generally, three types are used: water, alcohol

The solvent is chosen depending on several factors, some of which are the following: the type of sand blend, application method, ecological factor, type of production cycle dictating the time allowed for drying and application. Mostly applied solvent is water, for it is the cheapest, but also non-toxic and safe for use. Increment of application costs for this type of coating is due to energy consumption for the coating drying process and for water

Solvents like alcohol mixtures (methyl, ethyl, isopropyl) with pure water are often flammable liquids. The advantage of alcohol over water is faster drying, which is important for the sand moulds; but, since these coatings are flammable, they require more attention to

Isopropyl-alcohol represents technically most acceptable solvent for the flammable foundry coatings, primarily for its combustion properties being close to perfect. This type of combustion has a reduced erosion risk for the surface observed. Other alcohol solvents are less frequently

Choloric hydrocarbon is air-dried, eliminating a fire risk when flammable liquids are used. Air-dried coatings are convenient, like flammable liquids: they are self-extinguishable, they have a high steam pressure, they evaporate easily without a usage of heat, but they are also

Table 5 shows some recipes of the ceramic coatings used for different casting methods and

As far as the effect of ceramic coatings on the quality of the castings obtained is concerned, it is necessary to discuss preparation of the components contained in the coating, coating production and its application rheology which is tightly bound to a proper choice and use of

Generally, an ideal rheology suitable for coating application by immersion and pouring is an achievement of the properties of pseudoplastic solution, i.e. an achievement of such rheology enabling an instant viscosity drop during application and an immediate viscosity recovery as soon as the coating application stops. On the other hand, the rheology required for coating application by either spraying or brush is an achievement of thixotropic solution, i.e. an achievement of a light viscosity drop during application and a bit faster recovery to the initial

applied as pure, but mostly together, even when mixed with water (up to 12,0 vol %).

**6.4 Liquid carrier or solvent** 

elimination from the layers applied.

expensive and toxic matters.

**8. Application rheology** 

metal materials for casting production.

a suitable stabilizer. Generally, it may be defined as follows:

**7. Recipes** 

 immersion, spraying, pouring,

brush application.

(flammable liquids) and chloric hydrocarbons.

be paid during production, use and stock-keeping.


Table 3. Type and properties of binding agents most frequently used


Table 4. Suspension maintaining agents
