**5. Quality requirements and properties**

The coatings are featured by general, operative and technological properties, Table 1. (Svarika 1977, Tomović 1990).


Table 1. Ceramic coating properties

In order to properly understand the role of ceramic coatings for polymer patterns in this process, it is necessary to point out that the polymer pattern degradation is an endothermal process commencing during liquid metal inflow. Kinetics of the pattern degradation is a function of the temperature of liquid metal brought in contact with the pattern. At the inflow stage, while metal is passing through the polymer pattern, 70-90% of pattern degradation products are liquid. Liquid degradation products are pushed toward the upper surface of mould cavity in front of liquid metal front during the process. In case ceramic coatings and mould sand are less permeable, these liquid pattern degradation products are retained in upper parts of castings causing surface, subsurface or volume defects (Figure 4.b). Further degradation of the liquid stage is made by evaporation (formation of the boiling stage); the rest of polymer chain solidifies forming a monomer, as well as benzene and other products of polymer degradation (Aćimović-

Influential factors for the process of pattern degradation and evaporation, apart from the pattern temperature and density, are the type and thickness of the ceramic coating layer lining the expandable pattern, type and size of mould sand grains, i.e. mould sand permeability, casting constructions and inflow systems. Pattern density and permeability of ceramic coating and sand mould determine polymer evaporation speed, Figure 4.c.,d

To obtain the castings with the requested quality, critical parameters of EPC process should be determined for both each individual polymer pattern and the alloy type for casting. It requires a long-term research aimed at optimization of this type of casting process to obtain

The coatings are featured by general, operative and technological properties, Table 1.

General properties Operative properties Technological properties




pattern - thixotropy - drying time - endurance

surface - hygroscopy



Pavlović et al. 2011).

(Aćimović-Pavlović et al. 2003, 2007).

(Svarika 1977, Tomović 1990).




Table 1. Ceramic coating properties


stability



the mouldings with the properties set in advance.

**5. Quality requirements and properties** 

Examinations concerning different physical-chemical properties of foundry coatings showed that there are general conditions which must be met by the coatings, regardless of their type:


The quality of the coating applied depends on its uniformity; it is better if it has a lower precipitation speed. Otherwise, casting surface presents burns from mould or core blend or from the coating itself due to low refractoriness of the filler. During "full mould casting", expendable pattern degradation products created in contact with liquid metal disappear through the refractory coating layer into unbound sand which the mould is made of, if its permeability is satisfactory. It is primarily attained by choosing the suitable coating type, coating preparation procedure, coating suspension density and the coating dry film thickness on the pattern, Figure 5 (Brome 1988).

Ceramic Coating for Cast House Application 271

shows the coating insulation effect on the metal flow length and mould filling time with polymer pattern casting. Taking into account the casting conditions and the character of the EPC method ("full mould casting"), basic requirement in terms of ceramic coating permeability is more expressed than the one of the coatings for sand moulds and cores

Research showed that, depending on use, most of contemporary foundry coatings represent very complex blends consisting of over 15 components. However, four basic components are

Refractory filler is the most important component of ceramic coatings. It determines metal penetration resistance by reducing the permeability of the surface to which it is applied, prevents sand blend erosion and reactions on the metal-mould contact surface. As mentioned above, the choice of the refractory filler depends primarily on the casting alloy type, casting wall thickness and weight, preferable inflow system, i.e. metalostatic pressure in the mould. Main physical-chemical and thermo-physical characteristics of refractory filler

High melting temperature providing for the clean casting surface with no sintered sand

Low heat expand coefficient and its uniform growth reducing risk of coated layer crack;

 Lower hardness of ceramic material enabling eased attainment of suitable filler granularity providing for an eased coating application to the mould or core surface, Non-maceration with liquid metal and low reactivity with metal oxides, i.e. resistance to their activity on elevated temperatures, preventing reactions on the metal-mould

Low density value, since high density value requires more intensive rheology of

 Low heat conductivity coefficient value, since elevated values of this parameter cause insufficient thermal stability, i.e. they decrease heat shock resistance, causing a rapid

 Refractory filler must not develop gases when metal flows in, because it causes gas porosity of the moulding (Aćimović-Pavlović et al. 2007, Brome 1994, Svarika 1977). The choice of the refractory filler highly depends on the metal casting temperature. In the foundry technology, fine grinded mineral raw materials are used for coatings; these mineral raw materials are based on olivine, chromite, zircon, siner magnesite, mica, corundum,

at the same time, higher dimensional casting stability is ensured,

contact surface, providing for the clean and smooth casting surface,

growth of temperature gradient on the metal-mould contact surface,

foundry coatings due to an increased precipitation speed,

cordierite and other refractory materials, Table 2.

(Ballman 1988, Brome 1994).

**6. Composition** 

 Refractory filler, Binding agent,

**6.1 Refractory filler** 

are the following:

appearance,

 Suspension maintaining agent, Liquid carrier or solvent.

the following:

Fig. 5. Gas permeability dependence on the coating dry film thickness

Fig. 6. Coating insulation effect on the metal flow length and mould filling time

Ceramic coatings present an insulation effect influencing liquid metal temperature drop reduction when flowing into a mould; they also influence liquid metal fluidity and the way a mould is filled, certainly affecting the casting quality. Metal fluidity reduction has been noticed while using some types of coatings (based on silica, zircon, graphite); it most frequently resulted in an increment of the coating layer thickness on either a mould or a polymer pattern (Monroe 1994).

With EPC process, when a mould is filled and when polymer pattern degradation and evaporation is carried out, insulation effect of the coating influences liquid metal temperature drop reduction. When the mould is filled up with liquid metal, i.e. when polymer pattern is expended, the coating, due to insulation effect, influences a casting cooling and solidification speed reduction. At the same time, subcooling, created in liquid metal as a consequence of endothermic degradation of polymer pattern in contact with liquid metal, has a significant influence on the casting solidification. If subcooling is huge, fine and tiny-grained casting structure is preferably formed, because such structure provides for the better casting properties. All of that points to the complexity of casting solidification conditions as far as "full mould" is concerned, and to a significant role of ceramic coating in the casting process to form the casting structure and properties. Figure 6 shows the coating insulation effect on the metal flow length and mould filling time with polymer pattern casting. Taking into account the casting conditions and the character of the EPC method ("full mould casting"), basic requirement in terms of ceramic coating permeability is more expressed than the one of the coatings for sand moulds and cores (Ballman 1988, Brome 1994).
