**4. Physical-chemical properties on the liquid metal-ceramic coating-expandable pattern-sand mould boundary**

Different to sand mould or metal mould casting, where liquid metal flows into the mould cavity, with expendable polymer pattern casting (EPC casting process), patterns and inflow systems made of polymers are retained in the mould until liquid metal flows in ("full mould" casting). In contact with liquid metal, polymer pattern degradation and expansion process is carried out violently, during a relatively short time, and is followed by moulding crystallization. In order to attain a high quality and cost effective production of castings by EPC casting process, it is necessary to achieve the balance of the expandable polymer patternliquid metal-ceramic coating-sand mould system at the stage of metal inflow, polymer pattern degradation and expansion and casting formation and solidification, Figure 4.a. It requires a systematic research of both complex notions and processes carried out in the pattern and the notions and processes carried out in the metal-pattern contact zone, as well as in the metalceramic coating-sand contact zone (Aćimović-Pavlović et al. 2007,2010,2011, Brome 1988).

a) System balance: liquid metal-ceramic coat-pattern-sand

c) minor permeability: higher coat layers' thickness and smaller sand grains

Fig. 4. EPC process: The role of ceramic coating

b) liquid products accumulation on casting's surface

Coat layer

Polystyrene patern which is been decomposing

accumulation

Liquid metal flow

d) major permeability: less coat layers' thickness and bigger sand grains

Ceramic Coating for Cast House Application 269

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

They must not contain materials subject to become softer or melted in contact with

 With EPC process, it is particularly important to have a suitable coating permeability for the products of degradation and evaporation of polymer pattern formed during

They must not form the compounds with low melting temperature with metal, its

To be uniformly distributed across the core or mould blend surface, pattern surface,

To get dried quickly, not to crack, not to peel off the mould surface during drying or

After drying, they must form a thin visible layer on the mould, core or pattern surface,

 Intermediate layer (made by metal oxides and coating ingredients) should have a space parameter grid closer to the coating material grid rather than metal oxides ; otherwise, this transitive layer tends to bind the metal oxides to form membranes on the alloy

 The coatings should form gases to repulse the metal oxides from the mould (core) walls, especially at sand mould casting with high vertical walls or upright core

 The coatings increase strength and abrasion resistance by polymer patterns at EPC process preventing their distortion and break during filling and compression of

 The coatings should make the separation of mould or core blend from castings easier, reducing the casting cleaning time (Aćimović et al. 2003, Svarika 1977, Tomović 1990,

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

their type:

liquid metal,

impurities or oxides,

firmly bound to this surface

Trumbulović et al. 2004).

positions

They must not stratification during utilization,

surface leading to blend immersion into castings;

unbound sand at the mould manufacture stage,

thickness on the pattern, Figure 5 (Brome 1988).

They must have suitable refractoriness,

To make a permeable layer for gases,

Refractory filler must have a low heat expansion coefficient,

They must be resistible to metal penetration into the mould or core wall,

contact with liquid metal while flowing into the "full mould",

mould surface, to adhere and to be firmly bound to the surface;

casting time and to be resistible to sudden temperature changes, That there is a possibility to control and set the coating layer gauge,

Not to create gases when in contact with liquid metal,

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-Pavlović et al. 2011).

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

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 mouldings with the properties set in advance.
