**8. Application rheology**

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


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

Ceramic Coating for Cast House Application 277

When using a coating, it is necessary to mix it constantly (at the speed of 1 rpm) to get a homogenous suspension which, after being applied, provides for an even and uniform coating layer on the surface of mould, core or pattern. Figure 7 shows the influence of suspension mixing process on the applied deposit weight, while Figure 8 shows the

**8,4** constant mixing

non-mixing

**"B"**

**0 40 80 120 160 200 240**

**Time, min**

**Mixing grade**

influence of suspension mixing on the coating viscosity.

**6,0 6,4 6,8 7,2 7,6 8,0**

Fig. 7. Influence of mixing on the deposit weight (Brome 1988)

**"A"**

**"C"**

Fig. 8. Influence of mixing extent on the coating viscosity (Brome 1988)

**Viscosity**

**Deposit weight, g**

value after application. It practically means that the coating flows well when being applied since recovery to the initial value is not instant, as with pseudo-plastic solutions.



value after application. It practically means that the coating flows well when being applied

**maintenance agent** 

dextrin 0,3-0,5%, carboxymethyl cellulose- (CMC), 0,3- 0,5%

dextrin 0,3-0,5%,

dextrin 0,5%,

Na3P3O3 1-3%, carboxymethyl cellulose 0,5%

dextrin 0,5%, carboxymethyl cellulose 0,5%

BentoneSD-3 1.2-1.5%; phenolphormaldehide resins, 1-1.7 %

> Carboxymethyl cellulose 0,3-0,5%

dextrin 0,3-0,5% carboxymethyl cellulose, 0,3-0,5%

Sulphide alkali (1,30

Sulphide alkali (1,30

bentonite 1-1,5%; Dextrin: 1,5-2 % water

kg/m3 ), 1,5% water

kg/m3 ), 1% water

CMC, 0,3-0,5% water

CMC, 0,5% water

**Solvent, density 1,8-2 kg/m3**

water

water

water

alcohol

water

water

since recovery to the initial value is not instant, as with pseudo-plastic solutions.

bentonite 3%; bindal H 7,5%

bentonite 3%; bindal H 5%

bentonite 4%; bindal H 7%

bentonite 1,5-2,5%; bindal H 1,3-2%

> bentonite 3%; Bindal H 6%

colophonium (C20H30O2), 1.2-1.5%; dextrine 0,5-1%

phenolphormaldehide resins, 3-3,5%;

> bentonite 3,5-5%; bindal H 8%

bentonite 2%; melasses (1,4 kg/m3), content 3%

bentonite 1%; melasses (1,4 kg/m3), content 5-6%

**O. No. Content of filler Binding agent, Suspension** 

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

**zircon**, with grain size of 40·10-6 m, 86-89%

**mulite**, with grain size of 40·10-6 m, 91%

**mica**, with grain size 35- 40·10-6 m, 88-90%

**mica**, with grain size <30·10-6 m, 90 - 94%

**alumina**, with grain size 45·10-6 m, 90%

> **cordierite**, with grain size <40·10-6 m, 93-95%

**chromite**, with grain size 40·10-6 m, 95-96%

**talc**, with grain size 40·10-6 m, 85%

**zircon**, with grain size 40·10-6 m, 98%

**chrome-magnesite sand**, with grain size 40- 45·10-6 m, 99%

**olivine,**  with grain size 35- 40·10-6 m, 85,5-90 %

Table 5. Composition of various ceramic coatings

When using a coating, it is necessary to mix it constantly (at the speed of 1 rpm) to get a homogenous suspension which, after being applied, provides for an even and uniform coating layer on the surface of mould, core or pattern. Figure 7 shows the influence of suspension mixing process on the applied deposit weight, while Figure 8 shows the influence of suspension mixing on the coating viscosity.

Fig. 7. Influence of mixing on the deposit weight (Brome 1988)

Fig. 8. Influence of mixing extent on the coating viscosity (Brome 1988)

Ceramic Coating for Cast House Application 279

Coating layers applied must be completely dried either in an oven or in air, Figure10. If the coating is not dry enough, remaining humidity reacts with liquid metal causing porosity

To produce high quality coatings with the properties set in advance, it is necessary to attain a homogenous distribution of refractory filler in the coating suspension. It is also necessary to define the composition of the coating with rheology properties controlled and specially adjusted for a concrete application methods, Figure 11. To attain suspension sedimentation stability, filler particles should be up to 40·10-6 m in size, as it is expected that more tiny filler particles are precipitated slower and that a suspension may homogenize easier and faster. Furthermore, these particles more evenly and fully cover mould and pattern surfaces where the coating is applied. Homogenous coating suspension, Figure11.a, is obtained when the filler particles are rounded and even in size. When larger filler particles are present to a lower extent, with medium grain size of 45·10-6 m, Figure 11.b, it is estimated that the rounded particles with different grain size will also contribute to form a continuous coating layer, due to a better interaction of particles. In case the coating suspension is unevenly stirred during application, coating stratification happens, Figure 11.c (Clegg 1978,

a. homogenous coating suspension b. coating suspension with larger particles

c. non-homogenous coating suspension (drop-like)

One should bear in mind that homogenous distribution of refractory filler in a suspension depends on suspension preparation during coating application. Especially, an important process parameter is the coating suspension density, mostly within the limits from 1,8-2 kg/m3. In order to attain even coating layers on mould or pattern surfaces during immersion (into a coating suspension tank), the way of eliminating the excess of coating

Fig. 11. Microphotos of ceramic coating suspensions

defects on castings.

Trumbulović 2004).

Best results are achieved if certain suspension temperature (usually room temperature) is reached and maintained, apart from suspension stirring when being applied on sand moulds and cores or expendable patterns. Coating suspension temperature influences the quantity of residue and it needs to be constantly controlled (Figure 9). When the temperature rises, coating layer mass drops until certain temperature is reached; afterwards, the mass increases with the temperature growth.

Fig. 9. Temperature influence on deposit weight (Brome 1988)

Fig. 10. Influence of drying time on humidity elimination from the coating (Brome 1988)

Best results are achieved if certain suspension temperature (usually room temperature) is reached and maintained, apart from suspension stirring when being applied on sand moulds and cores or expendable patterns. Coating suspension temperature influences the quantity of residue and it needs to be constantly controlled (Figure 9). When the temperature rises, coating layer mass drops until certain temperature is reached; afterwards,

**25 30 35**

**Temperature, 0 C**

**Time, min**

the mass increases with the temperature growth.

Fig. 10. Influence of drying time on humidity elimination from the coating (Brome 1988)

**Evaporated water, %**

**21**

Fig. 9. Temperature influence on deposit weight (Brome 1988)

**Deposit weight, g**

Coating layers applied must be completely dried either in an oven or in air, Figure10. If the coating is not dry enough, remaining humidity reacts with liquid metal causing porosity defects on castings.

To produce high quality coatings with the properties set in advance, it is necessary to attain a homogenous distribution of refractory filler in the coating suspension. It is also necessary to define the composition of the coating with rheology properties controlled and specially adjusted for a concrete application methods, Figure 11. To attain suspension sedimentation stability, filler particles should be up to 40·10-6 m in size, as it is expected that more tiny filler particles are precipitated slower and that a suspension may homogenize easier and faster. Furthermore, these particles more evenly and fully cover mould and pattern surfaces where the coating is applied. Homogenous coating suspension, Figure11.a, is obtained when the filler particles are rounded and even in size. When larger filler particles are present to a lower extent, with medium grain size of 45·10-6 m, Figure 11.b, it is estimated that the rounded particles with different grain size will also contribute to form a continuous coating layer, due to a better interaction of particles. In case the coating suspension is unevenly stirred during application, coating stratification happens, Figure 11.c (Clegg 1978, Trumbulović 2004).

a. homogenous coating suspension b. coating suspension with larger particles

c. non-homogenous coating suspension (drop-like)

Fig. 11. Microphotos of ceramic coating suspensions

One should bear in mind that homogenous distribution of refractory filler in a suspension depends on suspension preparation during coating application. Especially, an important process parameter is the coating suspension density, mostly within the limits from 1,8-2 kg/m3. In order to attain even coating layers on mould or pattern surfaces during immersion (into a coating suspension tank), the way of eliminating the excess of coating

Ceramic Coating for Cast House Application 281

a. sand casting b. EPC casting process

Quality control for foundry coatings and coating application process control as far as sand moulds and cores and expendable or meltable patterns are concerned is important for production of high quality castings. Establishing the quality of foundry coatings is defined by different standards for this type of refractory products. Standards are used to establish both, foundry classification and quality requirements, as well as technical conditions of application, coating sampling methods, coating test methods and marking of coatings and

coating layer must be easily taken off the castings surface when these are shaken or

coating penetration depth in the mould wall may be in the range from 0,5 to 2 ·10-3 m,

 the coating must be compatible with the type and density of polymer pattern, with the type and size of moulding sand grain and with other EPC process parameters (Svarika

the way of delivery. Most often, the following coating properties are tested:

Assessment of a coating is done according to the following criteria either:

 dry matter amount must not vary more than ±2% from the value declared, precipitated matters being precipitated for 24 h may amount to max 5%,

 the coating must evenly flow down when being applied, coating layer must not crack or bubble when being dried,

Fig. 13. Taking coating layers easily off the casting surface

**9. Control and faults** 

 application ability, drying behaviour, wipe-off resistance, dry matter amount, precipitation,

cleaned,

penetration (Svarika 1977, Tomović 1990).

coating layer must be wipe-off resistant

coating layer must be permeable to gases,

1977, Tomović 1990, Brome 1988).

depending on use, solvent and binding agent,

suspension is important. After immersion, "the cluster" (representing a number of patterns stacked at the common inflow system, thus prepared for EPC casting process) is taken out of suspension tank, it is kept in a vertical position for 5-10 s in order to decant the excess of suspension; then, it is inclined for 5s in order for the coating to be evenly distributed along the cluster surface, Figure 12.a-c. After being decanted, "the cluster" is ready for a drying process. When applying the coating on sand moulds by either a brush or rag, by either spraying or pouring, coating density has an important role and it should not exceed the value of 2-2.2 kg/m3, Figure 12.d. In this way, dried coating layers do not crack, peel or wipe off, and after casting they are easily taken off the castings surfaces, not requiring the castings to be subsequently cleaned, as shown in Figure 13. It has been established that adhesive forces among the filler particles increase with the filler concentration increment. Under the influence of rheology additives and binding agents, continuous and uniform coating layers might be formed on the treated surfaces. Such coating is easily adhered to the treated mould and pattern surfaces. Dried coating layer thicknesses influence gas permeability and they should be as low as possible; their range is from 0,5-1,5·10-3 m (Clegg 1978, Tomović 1990).

c. balancing the coating layer on the pattern d. coating application by rag

Fig. 12. Coating application stages

a. coating application by immersion b. taking off the excess of suspension

suspension is important. After immersion, "the cluster" (representing a number of patterns stacked at the common inflow system, thus prepared for EPC casting process) is taken out of suspension tank, it is kept in a vertical position for 5-10 s in order to decant the excess of suspension; then, it is inclined for 5s in order for the coating to be evenly distributed along the cluster surface, Figure 12.a-c. After being decanted, "the cluster" is ready for a drying process. When applying the coating on sand moulds by either a brush or rag, by either spraying or pouring, coating density has an important role and it should not exceed the value of 2-2.2 kg/m3, Figure 12.d. In this way, dried coating layers do not crack, peel or wipe off, and after casting they are easily taken off the castings surfaces, not requiring the castings to be subsequently cleaned, as shown in Figure 13. It has been established that adhesive forces among the filler particles increase with the filler concentration increment. Under the influence of rheology additives and binding agents, continuous and uniform coating layers might be formed on the treated surfaces. Such coating is easily adhered to the treated mould and pattern surfaces. Dried coating layer thicknesses influence gas permeability and they should be as low as possible; their range

a. coating application by immersion b. taking off the excess of suspension

c. balancing the coating layer on the pattern d. coating application by rag

is from 0,5-1,5·10-3 m (Clegg 1978, Tomović 1990).

Fig. 12. Coating application stages

a. sand casting b. EPC casting process

Fig. 13. Taking coating layers easily off the casting surface
