FOR THE COATING

Technical Field

The present invention relates to a coating coated on casting moulds and cores used for metal casting processes, and a production method for the coating.

Background of the Invention

Along with forming processes which are machining, forging, welding, pressing while forming metals, another method is casting. Metal casting is described as a process of melting the metal or alloy selected, casting it to a mould cavity which is the negative of the desired form, and waiting it to harden in order to achieve a desired shape. Casting of a part is performed by casting the molten metal into a mould having a cavity at desired geometry and hardening the metal therein. After hardening, the part takes the shape of the mould, and volume of the part is slightly reduced with respect to the mould cavity. The casting process varies according to the technique used. Some of the applications that vary according to the technique used are sand casting, permanent mould casting, die casting, precision casting, Styrofoam model casting (lost foam) and continuous casting. In the sand casting, which is one of these applications, molten metal is cast into a mould prepared from sand, and a two-piece casting mould is prepared by compressing the sand around a model.

In the casting industry, sand moulds and cores into which molten metal is cast are coated with coatings, which contain refractory material, in order to be able to obtain a cleaner and smooth casting which is free of technical problems. According to the casting to be made, importance of the refractor selected is seen at the end of the casting. Not all refractory materials are suitable for all kinds of casting. Refractory casting coatings are basically divided into two as water-based and solvent-based coatings depending on the carrier liquids. Casting coatings which are vital in the casting industry enable casting quality to be increased and part machining costs to be decreased after the casting. In the casting industry, when a molten/liquid metal is filled into a mould, thermal, mechanical and physicochemical phenomena occur at the surface. As a result of such phenomena occurring at the surface, metal-mould reactions, reactions on the mould surface and metal-mould reactions occur. Realization possibility of a chemical reaction is based on free energy exchange of the system. This energy is Gibbs Free Energy (DQ) and is defined as:

DQ = DH - TAS

Gibbs Free Energy is calculated by a combination of enthalpy (H), temperature (T) and entropy (S), i.e. three different state functions. Change in free energy of the system measures a balance between two driving forces that determine whether the reaction is spontaneous.

AGO < 0 Reaction is spontaneous.

AGO = 0 System is in equilibrium.

AGO > 0 Reaction does not occur.

When the evaluation of said metal surface reactions is made in this context, an intermediate layer should be implemented to prevent possible reactions between the mould material and the molten metal, wherein the intermediate layer can increase Gibbs free energy and thus minimize entropy change depending on the temperature. The easiest way to implement this is using coatings for casting and core. Possible reactions between the mould material and the molten metal are the main casting defects that are common in the casting industry, wherein problems as metal penetration, mould/metal interactions/gas defects and sintering of sand-coating-metal mixture to the casting surface occur as a result of such reactions. For said gas defects, in case a serious gas emission occurs, the gas penetrates into the molten metal and the resulting gas bubbles lead to gas defects called as pin holes.

An exemplary coating containing refractory material is disclosed in CN101497105B. In this document, a water-based casting coating is detailed, wherein the water-based casting coating contains a refractory material, a suspending agent, a binding agent, a surfactant and preservatives. In the formulation of said coating material it is stated that the refractor can be selected as corundum, and the suspending agent can be selected as aluminium silicate which is an inorganic gel. Moreover, the process comprises the steps of: adding the suspension agent directly to the water and mixing the mixture in a high speed dispersion machine and distributing the same; adding auxiliary agents which comprise an antifoaming agent, a surfactant, a corrosion remover and the like, and mixing the auxiliary agents; adding the refractory powder to the mixture to be mixed; and adding the binder to the mixture to be mixed and adding water until the coating reaches a required thickness/density. Another example is disclosed in US2857285A. Refractory coatings containing aluminium silicate which are disclosed in this document are water-based and comprise sodium silicate and potassium silicate. These coatings are suitable for use at temperatures up to 1500°C. Another exemplary coating containing refractory material is disclosed in CN101817058A. Said document refers to a shield type coating and zircon is used as the refractory material. However, in these documents, only water or liquid mixtures containing high amounts of water are used as carrier liquid.

Brief Description of the Invention

With the present invention, there is developed a coating which is especially suitable for being coated on sand moulds and/or cores used for iron and steel casting processes, and a production method for the coating. The coating comprises at least one carrier liquid; at least one refractory material in the form of a powder; at least one rheology agent; and at least one suspension agent. The production method comprises the steps of: placing at least one carrier liquid and at least one rheology agent to a mixer; operating the mixer at a high speed to allow rheology agent to be dispersed homogenously in the mixer; and adding at least one refractory material in the form of a powder to the mixer.

Cores and moulds used in the casting industry emit too much gas during a casting process. While a very large surface is provided for the moulds so as to direct the gases outwards which move freely during the casting operation, core prints having a very small section are provided at the cores. These core prints correspond to the resting surfaces of the cores on the model. Since the coating according to the present invention is able to be used for both small and large casting parts regardless of geometry of the casting part, a high strength is provided on the resting surfaces of the cores coated with the coating according to the present invention. Cores and moulds having high strength prevent gas defects forming at the casting such that freely moving gas is removed out of the mould. With the present invention there is developed a coating which can increase Gibbs free energy and thus minimize entropy change depending on the temperature so as to form an intermediate layer between the materials used in casting industry, such as moulds and cores, and the liquid metal.

Object of the Invention

An object of the present invention is to develop a coating which is coated on casting moulds and cores used for metal casting processes, and a production method for the coating.

Another object of the present invention is to develop a coating which is coated on moulds and/or cores used for iron and steel casting processes.

A further object of the present invention is to develop a coating which decreases capillary dimensions of a lower layer.

Another object of the present invention is to develop a coating for improving smoothness of the casting surface by changing surface tension of the lower surface of the coating.

Yet another object of the present invention is to develop a coating for minimizing metal penetration defects at the lower surface of the coating.

Description of the Invention

In the metal casting processes, sand castings and cores into which molten metal is cast are coatinged in order to be able to obtain a cleaner and smooth casting which is free of technical problems. This term referred to as coating is a metallurgical process implemented for the surface processes to increase lifetime of a main material generally by protecting the material and achieve a smooth surface or appearance. Nature of a coating can vary according to the metal to be cast. Therefore, with the present invention there is developed a coating which is especially suitable for being coated on sand moulds and/or cores used for iron and steel casting processes.

The coating according to the present invention comprises at least one carrier liquid, at least one refractory material in the form of a powder, at least one rheology agent, and at least one suspension agent. The carrier liquid mentioned here is the main component since it makes other components a suspension and/or a solution. Said carrier liquid should be selected as a volatile liquid which can completely evaporate under circumstances of metal casting.

The production method of the coating according to the present invention comprises the steps of: placing at least one carrier liquid and at least one rheology agent to a mixer; operating the mixer at a high speed to allow rheology agent to be dispersed homogenously in the mixer; and adding at least one refractory material in the form of a powder to the mixer. The method further comprises the step of adding at least one colorant, at least one surface agent, and at least one binding agent into the mixture.

In a preferred embodiment of the invention, the carrier liquid has a boiling point of less than 115°C, preferably less than 95°C under normal pressure (under a pressure of 1 atm).

In an embodiment of the invention, the carrier liquid is selected from a group consisting of at least one alcohol, at least one volatile organic component and/or a mixture thereof. The at least one volatile organic component and/or at least one alcohol is selected to have a boiling point of less than 115°C, preferably less than 95°C. Here, the carrier liquid is preferably selected from a group consisting of alcohols, ethanol, methanol, isopropanol and/or combinations thereof that have 1 to 3 carbon atoms. Ratio of the carrier liquid weight to the total coating weight is selected within the range of 37% to 75%, preferably 50% to 70%. Here, the carrier liquid is preferably used in an anhydrous form.

Particle size of the refractory material is crucial in order for the coating according to the present invention to be in a steady state, to be suitable for use in applications with spraying, brush and/or washing equipment, and to be dispersed homogenously to the application surface. Accordingly, the refractory material is preferably selected to have a particle size within the range of 60 to 450pm, more specifically 75 to 150pm. The refractory material mentioned herein is selected to have a minimum porosity and reduce physicochemical reaction between the casting and the intermediate surface of the molten metal.

In a preferred embodiment of the invention, the refractory material can be selected as corundum; as a mixture of corundum and alumina silicate; or as a mixture containing natural minerals and having a high corundum and alumina silicate percentage. The refractory material has a higher melting point than a casting temperature of the molten metal and/or does not undergo a physicochemical reaction with the molten metal. In case that the refractory material is selected as a mixture of corundum and alumina silicate, mass ratio of the refractory material in a mixture has been adjusted specifically by experimental studies, and the most suitable and effective mass composition ratio has been determined. Within this context, ratio of the weight of corundum selected as the refractory material to the total coating weight is preferably within the range of 15% to 45%. Ratio of weight of alumina silicate selected as the refractory material to the total coating weight is preferably within the range of 13% to 43%.

In another embodiment of the present invention, the coating preferably comprises a binding agent. Binding agents, after the coating implemented to the casting mould is dried, bind the content of the coating and enable the coating to adhere safely to the ground.

In another embodiment of the present invention, binding agents are preferably selected among cured binding agents. Through use of cured binding agents, a coating is achieved which has a high abrasion resistance. By this means, after the casting mould is prepared and coated with the coating according to the present invention, the coating is prevented from being corroded due to mechanical effects and spilling out of the mould or the core during transportation and enclosing operations. The binding agents are preferably selected from a group consisting of natural glass resins (cold box and/or hot box resins) and PVB which can be dissolved in alcohols. Ratio of the binder weight to the total coating weight is within the range of 0.1 % to 5%, preferably 0.5% to 2% in order for the coating according to the present invention to be adhered safely to the ground. Cores and moulds used in the casting industry are generally obtained from resin bonded mixtures. Therefore, when natural binding agents are used as the binding agent, cores and moulds have a high binding and strength properties. Thanks to the coating according to the present invention, a coating is formed which has a high resistance against thermal shocks, wherein high protection is provided for the surfaces that the molten metal primarily contacts during the casting process. By this way, once the metal in the environment starts to cool, refractory coating is prevented from shrinking.

In an alternative embodiment of the invention, the coating comprises at least one filler agent mixed with the rheology agent. Here, the mixture of rheology agent and the filler agent increases the viscosity of the coating. Thus, heavy components in the coating are prevented from being embedded on one hand, and the coating layer is enabled to comprise an equal compound at each point during implementation. In addition, the mixture of the rheology agent and the filler agent enables that the coating does not flow any further after it is implemented to the surface of the casting mould, and that an equal coating thickness is achieved at the vertical surfaces of the casting mould, as well. The mixture of the rheology agent and the filler agent preferably comprises silicates such as treated and modified attapulgite and organic clay. Here, ratio of weight of the mixture of rheology agent and filler agent to the total coating weight is within the range of 0.25% to 3.75%, more specifically 1.0% to 2.0%.

The coating of the present invention further comprises surface diffusing agents and antifoaming agents. Here, the surface diffusing agent preferably comprises a melamine formaldehyde based material. Ratio of the surface diffusing agent weight to the total coating weight is preferably within the range of 0.05% to 0.3%.

In another embodiment of the invention, the coating is ready-to-use in white-cream colour, while it may be red, green, yellow or dark grey when it contains metal oxides. In other words, the coating may contain colorants as metal oxide.

Viscosity of the coating according to the present invention is 11 to 25 s, in particular preferably 11 to 13 s which is determined according to DIN-4 cup. The coating has a value within the range of 1 to 2.2 g/cm ³ (0 - 120 °Be), preferably 1.18 to 1.35 g/cm ³ (30 to 45 °Be) by means of Baume buoyancy method.

Cores and moulds used in the casting industry emit too much gas during a casting process. While a very large surface is provided for the moulds so as to direct the gases outwards which move freely during the casting operation, core prints having a very small section are provided at the cores. These core prints correspond to the resting surfaces of the cores on the model. Since the coating according to the present invention is able to be used both for small and large casting parts regardless of geometry of the casting part, a high strength is provided on the resting surfaces of the cores coated with the coating according to the present invention. Cores and moulds having high strength prevent gas defects to be formed at the casting such that gas, which moves freely, is removed out of the mould. With the present invention there is developed a coating which can increase Gibbs free energy and thus minimize entropy change depending on the temperature so as to form an intermediate layer between the materials used in casting industry, such as moulds and cores, and the liquid metal.