EP1600229A1

EP1600229A1 - Method of producing molds for lost wax precision casting

Info

Publication number: EP1600229A1
Application number: EP04711720A
Authority: EP
Inventors: Hidekazu Shonan Design Co. Ltd. SUZUKI; Taro Shonan Design Co. Ltd. KITA; Akio Shonan Design Co. Ltd. SAITO; Kazuki Shonan Design Co. Ltd. KAMATA; Masato Shonan Design Co. Ltd. KIKUCHIHARA
Original assignee: Shonan Design Co Ltd
Current assignee: Shonan Design Co Ltd
Priority date: 2003-02-21
Filing date: 2004-02-17
Publication date: 2005-11-30
Also published as: WO2004073905A1; US20060144555A1; JP4006696B2; JP2004261817A

Abstract

The problems of the invention are to establish a modeling resin composition excellent in shape retention and dewaxing properties in the skillful system which comprises a urethane resin easy if thermal decomposition, a plasticizer, and a wax or waxy component and a dewaxing method suitable for models made form the composition and to enhance the strength of a mold in the low temperature range wherein dewaxing is carried out. According to the invention, a mold is produced by applying a primary layer consisting of plural fireproof coating layers among which epoxy silicone layers are laid and a backup layer consisting of plural thick coating layers on the surface of a resin model made by curing a two-component reaction curing type urethane liquid resin (F) which consists of a multifunctional polyol component (A), a multifunctional polyisocyanate component (B), a plasticizer (C), a wax or waxy component (D), and hollow resin balloons (E), preheating the obtained composite in a low temperature range within 60 to 120 °C during 2 to 8 hours to conduct initial dewaxing of removing 5 to 50 % by weight of the resin model, raising the oven temperature gradually, and conducting the heating of the resulting composite in a medium temperature range of 150 to 500 °C during 2 to 5 hours and then the firing thereof in a high temperature range of 500 to 1000 °C during 2 to 5 hours.

Description

TECHNICAL FIELD

The present invention relates to a method for producing molds for a lost wax precision casting in which dewaxing properties of resin models is shown sufficiently and fracture of molds is prevented in the lost wax precision casting in which resin models are used as an alternative of wax models in order to improve retentivity of shapes that is a defect of the wax model used in a lost wax method.

BACKGROUND ART

A method of a lost wax precision casting as background arts as a whole is explained as follows. A method for a lost wax is to go through production processes such that a wax model which is the same shape as casting products is produced by injecting a melted wax component into a metal mold for mass production of wax models and removing it from the metal mold after cooling, a hollowed mold is produced by providing a refractory on the surface of the wax model and heating it to melt the wax model and make it flow out, and further burning out it completely at high temperature, and a product is produced by injecting a melted metal alloy into the mold and taking it out by breaking the mold after cooling and hardening.

In addition, when the wax model is produced by injecting a wax component into the metal mold, regular quantity of the wax models are produced by controlling injection temperature, injection pressure, holding time for injection pressure, and cooling temperature for removing the mold. The wax models produced in thus way are kept in a content temperature room to pay attention so as to maintain the dimensional accuracy as well as possible. In assembly of the wax model, sprue models produced separately are assembled to the wax model integrally in a state like a tree. A whole of the assembled model is called "Tree". Because a shape of the tree is a design for a sprue system by itself, it is designed by considering many factors such as a characteristic of melted metal, a size and a shape of a casting, casting conditions, cutting from the tree, etc.

The tree produced in thus way is coated in layers by soaking it in coating slurry and drying it repeatedly. A binder used in the coating slurry is colloidal silica, ethyl silicate, etc. The slurry is made by mixing refractory impalpable powder as filler into the binder. After the wax model is soaked in the slurry produced in thus way, stucco grains are sprinkled on the wax model, and the wax model is dried. Zircon sand and molochite grain are used as the stucco grains. The coating process is completed by repeating these steps several times.

Next, the wax model is melted in an autoclave at 120 to 150 °C to flow out of the mold. This is called "dewaxing". A shell mold resulting from the dewaxing step is burned in a high-temperature furnace at 700 to 1000 °C in order to remove residual wax and unburned carbon particles and increase strength of the mold. Melted alloy is cast into the casting mold produced in thus way, the casting mold is split away with a knockout machine after the alloy becomes cooled, Then, a casting is taken out, and runners and weirs and the like are cut away and removed from the casting. Newt, residual refractory material attached of the casting is eliminated by blasting. In addition, any repairable portions of it are repaired by welding, a surface of it is finished by NC machining system or a grinder, and a casting alloy product is completed by a heat treatment.

Various kinds of research and development have been made with regard to models used in the above mentioned lost wax method. First, the wax component constituting the wax models used as a model is normally a blend of paraffin, rosin, carnauba wax and terephthalic acid. Details of the wax component are described in Casting Guidebook (edited by the Japanese casters Association). Recently, JP 05-38549 A discloses an efficacy of the wax component containing melamine particles blended therein. It seems to be difficult that solid state properties for mechanical strength of the wax model is increased as long as the wax component holds a characteristic such that it is easy to be melted and dewaxed at high temperature. A lot of systems such as to combine and laminate synthetic resin to the wax model are reported. For instance, JP 5-23791 A discloses a model such that synthetic resin film is formed on a surface of the wax model. JP 5-329174 A discloses that a dental inlay casting model is produced with a heat meltable resin. JP 07-9084 A discloses a model which is formed by laminating a lost wax base on a photo-curing resin sheet. JP 7-29954 A discloses a model which is formed by applying a wax and plastic material onto an ornamental model comprising cotton yarn and a synthetic material. JP 7-47433 A discloses a model which is formed by injecting a lost wax into a metal mold into which a photo-curing resin model or heat meltable resin laminated model is inserted. JP 2000-263186 A discloses a model which is formed by laminating a lost wax base on an ultraviolet curing resin model. Thus, synthetic resin comes to be used in a part or the whole of the model, but this is to aim at increasing ability for holding a shape of the wax model and producing the model easily mainly.

Thus, it is known to use a model used with synthetic resin in a precision casting lost wax method as an alternative of the wax model. However, when the resin model is used as an alternative of the wax model, it is difficult for the resin model to flow out in a dewaxing process, and further a stress inside the mold is increased rapidly by expansion and large amount of combustion gas within an approximately range in which heat decomposition of the resin of the resin model is occurred, so that hair cracks or cracks are occurred in the mold, and the mold is broken down according to circumstances. Thus, in the present circumstance, the resin model is not used because flow-out, removing, heat decomposition or burning-out of the resin model is difficult in the dewaxing process or a first burning process.

In the case of producing a precision casting product in the lost wax method, the wax model comprising the wax component is used commonly. This is to make good use of characteristics such that the wax component can be melted, removed and flow out easily because the wax component can be liquefied over a melting point thereof, and that it can be burned out completely in the burning process. However, recently, precision casting products becomes complicated and serious dimensional accuracy is required to them. Accompanying with it, various problems are occurred, and especially any case that the wax model can not correspond is occurred.

Namely, the problems regarding to the wax models are that edge portions are not formed accurately, that narrow ribs are formed difficultly, and that the narrow ribs are easy to be broken. Accordingly, thin portions must be removed more carefully at a demolding process, and there is technical limitation for manufacturing in the wax models with a thinner portion less than 1mm. The produced wax models have some problems such that they are fragile because hardness of surfaces thereof is lower, that dimensional accuracy thereof is not enough and that it is easy for them to be damaged by drop impact at carriage, and further have a problem such that they must be preserved in a constant temperature room because the produced wax models are deformed easily in a summer condition. Moreover, there is a problem such as to pay close attention when the wax models are carried in summer. These result from softening at approximately 80 °C because the wax component is a relative low-molecular organic matter. Thus, problems for the wax model are caused mainly by problems of the wax components. Though development work for changing composition of the wax component is carried out in order to solve the problems caused by thus wax component, the problems are not solved fundamentally because the wax component is a low melting point organic matter which is melted at temperature little higher than a normal temperature and it is in crystallization or solidification at the normal temperature, recently.

The inventors considered and examined resin components with characteristic of maintaining shapes, good remolding (heating / melting / flowing out characteristics) and superior high temperature flammability in resin models which is possible to solve or improve the problems of the wax models. An object of the invention is to set a resin composition for a model which is superior in a shape maintaining characteristic and dewaxing, to set a dewaxing method available to a resulting resin model, and to reinforce a casting mold within a low temperature range in which the dewaxing is carried out.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention is to comprise: a process for producing a resin model (H) in which a two-component reaction curing type urethane liquid resin (F) consisting of a multifunctional polyol component (A), a multifunctional polyisocyanate component (B), plasticizer (C), a wax component (D) and hollow resin balloon (E) is mixed, injected into a mold (G), hardened, and then demolded; a process producing a primary layer consisting of a plurality of coating layers laminated on a surface of the resin model (H) by repeating operation for forming the coating layer in which the resin model (H) is soaked in a solution for fireproof coating (I), picked up from it, dried, painted with epoxy silicon (N), dried and then hardened; a process for producing a backup layer consisting of a plurality of thick film coating layers laminated on the surface of the resin model (H) by repeating operation for forming the thick film coating layer in which the resin model (H) is soaked in the solution for fireproof coating (I), sprinkled with stucco (M), dried, painted with epoxy silicon (N), dried and then hardened; a process in which the resin model (H) coated with thick film fireproof coating multi-layer consisting of the primary layer and the backup layer is set in a furnace so as to turn down a sprue thereof, heated in a lower temperature range within 60 to 120 °C during 2 to 8 hours preliminarily, initially dewaxed within 5 to 50 wt% of the resin model; a process for completing dewaxing and burning-out of the resin model in which temperature in the furnace is increased gradually and heating is carried out in a middle temperature range within 150 to 500 °C during 2 to 5 hours; and a process for burning in a high temperature range within 500 to 1000 °C during 2 to 5 hours.

Besides, it is preferred that an average functional radix of the multifunctional polyol component (A) is 2.8 or larger, an average functional radix of the multifunctional polyisocyanate component (B) is 2.0 or larger, and a ratio NCO/OH is within 0.8 to 1.0.

Furthermore, the plasticizer (C) is preferably micro-dispersed through phase separation when the two-component reaction curing type urethane liquid resin (F) is hardened by reaction.

Moreover, the two-component reaction curing type urethane liquid resin (F) preferably contains polyether chains having a chemical structure indicated in the chemical structural formula as follows at 2- 20 wt% thereof. The chemical structural formula is:

In addition, the wax component (D) is in powder shape with a maximum diameter of 5 mm and has a melting point within 60 to 130 °C, and is contained within 3 to 30 wt% in the two-component reaction curing type urethane liquid resin (F).

Furthermore, it is preferred that the epoxy silicon (N) consists of bisphenol type epoxy (J), amino silane (K) and organic solvent (L).

Besides, it is preferred that particle diameters of the hollow resin balloons (E) are within 10 -100 µm, absolute specific gravities thereof are within 0.01 to 0.03, and contained within 0,001 - 0.5 wt % in the two-component reaction curing type urethane liquid resin (F).

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a process for producing a mold for a lost wax precision casting in which a resin model according to the present invention is used is explained.

In a first process for producing a mold for a lost wax precision casting in which a resin model is used, a two-component reaction curing type urethane liquid resin (F) consisting of a multifunctional polyol component (A), a multifunctional polyisocyanate component (B), plasticizer (C), a wax component (D) and hollow resin balloons (E) is mixed, injected into a mold (G), hardened and demolded from the mold (G) to produce the resin model (H).

In the first process, a binding material component of the two-component reaction curing type urethane liquid resin (F) comprises the multifunctional polyol component (A) and the multifunctional polyisocyanate component (B). Besides, the plasticizer (C), the wax component (D) and the hollow resin balloons (E) are additive components. The plasticizer (C) may be added into the multifunctional polyol component (A) or may be added as an ingredient when the multifunctional polyol component (A) is produced. Besides, the plasticizer (C) may be added into the multifunctional polyisocyanate component (B) or may be added as an ingredient when the multifunctional polyisocyanate component (B) is produced.

The wax component (D) may be added into the multifunctional polyol component (A), or may be added after the multifunctional polyol component (A) and the multifunctional polyisocyanate component (B) are mixed.

The wax component (D) may be added into the multifunctional polyisocyanate component (B), or may be added after the multifunctional polyol component (A) and the multifunctional polyisocyanate component (B) are mixed.

The hollow resin balloons (E) may be added into the multifunctional polyol component (A) or may be added after the multifunctional polyol component (A) and the multifunctional polyisocyanate component (B) are mixed.

The hollow resin balloons (E) may be added into the multifunctional polyisocyanate component (B).

Thus, the two-component reaction curing type urethane liquid resin (F) consisting of the multifunctional polyol component (A), the multifunctional polyisocyanate component (B), the plasticizer (C), the wax component (D) and the hollow resin balloons (E) is mixed and injected into the mold (G).

A metal mold, a simple mold, a resin mold, and a silicon rubber mold are available as the mold (G). The metal mold is available to mass production, the simple mold or the resin model is available to a middle quantity production, and the silicon rubber mold is available to a small quantity production. A mold design with draft is necessary in the metal mold, the simple mold or the resin model, but the mold design with draft is not an absolute condition in the silicon rubber mold. Namely, the model can be taken out by deforming the silicon rubber mold because of rubber elasticity and it can go back as before by canceling the deforming force. However, because the rubber elasticity becomes vanished by using many times, it is limited to form about 20 times. Accordingly, it is available to a small quantity production.

The two-component reaction curing type urethane liquid resin (F) injected into the mold (G) is reacted chemically at normal temperature and hardened in a set time. After maintaining a shape by hardening, the resin model (H) is taken out from the mold (G) by demolding. In a general way, mold releasing agent is painted on the mold (G) and dried in advance.

In a second process, operation, such that a fireproof coating layer is formed by soaking the resin model (H) in a fireproof coating solution (I), picking it up and drying it, and further coating epoxy silicon (N) on it, drying it and hardening it, is repeated several times, for instance twice to five times, so that a primary layer consisting of a plurality of the fireproof coating layers is formed.

Namely, in the second process, when the resin model (H) is soaked in the fireproof coating solution (I) and taken out from it, the fireproof coating solution (I) is stuck on a surface of the resin model (H). The resin model on which the fireproof coating solution (I) is stuck is dried, and the epoxy silicon (N) is painted on the surface of the resin model (H), dried and hardened to form a fireproof coating layer on the surface of the resin model (H). Thus operation is repeated twice to five times, so that the primary layer consisting of two to five fireproof coating layers is formed. The epoxy silicon (N) is a liquid with low viscosity which is diluted by solvent, and is painted whole or partially by a spray. The fireproof coating solution (I) has to be dried during half a day or one day usually, but the epoxy silicon (N) is dried in one to three hours to be a film shape.

Because the primary layer is transcribed to a surface of a casting, relatively fine filler is combined, so that fine fireproof coating layer can be formed. It is preferred that the primary layer is formed by repeating the operation several times. Since cracks are occurred at drying in the case that a thick coating layer is formed once, it is necessary to form little by little.

Next, in a third process, a thick fireproof coating layer is formed on a surface of the resin model (H) by soaking the resin model in the fireproof coating solution (I), taking it out, sprinkling stucco (M) on it, drying it, painting epoxy silicon (N) on it, drying it and hardening it. Thus operation is repeated twice to five times, so that a thick backup layer consisting of two to five thick fireproof coating layers is formed.

In the third process, the resin model (H) is soaked in the fireproof coating solution (I) and taken out, and then is sprinkled with the stucco (M) thereon and dried. After that, by painting the epoxy silicon (N) and drying to be hardened, the thick fireproof coating layer is formed on the surface of the primary layer of the resin model (H). Thus operation is repeated twice to five times, so that the thick film backup layer consisting of two to five thick fireproof coating layers is formed. Namely, thick film can be gained by sprinkling the stucco (M) to be stuck.

In aforementioned second and third processes, the resin model (H) is coated with the primary layer laminated finely and the backup layer laminated thickly in a structure that the epoxy silicon layers are arranged between the fireproof coating layers respectively. Namely, layers formed with the fireproof coating solution (I) and layers formed with the epoxy silicon (N) are arranged alternately. The fireproof coating layers is that large strength thereof does not appear in a low temperature range within 60 and 120 °C, but appear by burning in a high temperature range within 500 to 1000 °C to be one kind of a ceramics layer.

On the other hand, because the layer formed with epoxy silicon (N) is filmed at normal temperature completely to be three-dimensional network structure with heat-resistance, it gives strength to a fireproof covering layer consisting of the primary layer and the backup layer in a low temperature range between 80 to 200 °C. It is supposed that it is burned out at a high temperature range within 500 to 1000 °C and the strength thereof is vanished. Thus, strength of the casting mold in the low temperature range is increased by arranging the layers formed with the epoxy silicon (N) between the layers.

Next, in a fourth process, the resin model (H) on which the primary layer and the backup layer to be a casting mold are formed is set in the furnace so as to turn the sprue gates thereof below, preliminary heating is carried out at 60 to 120 °C during two to eight hours, and 5 to 50 wt% of the resin model is dewaxed in an initial stage.

In the fourth process, the plasticizer (C) and the wax component (D) are softened or melted by the preliminary heating at 60 to 120 °C during two to eight hours, so that they are oozed from an inner part of the resin model (H) to the surface of the resin model (H). As combustion gas is not produced in the low temperature range, an inner stress in the casting mold is caused by an expansion of the resin model (H), so that no crack appear in the casting mold consisting of the fireproof covering layer because the casting mold is reinforced by the layer formed with the epoxy silicon (N) remarkably.

The oozed plasticizer (C) and the wax component (D) are stored in a gap between the resin model (H) and the casting mold and penetrated into a porous inner surface of the casting mold, so that increase of inner pressure of the casting mold is relieved. Furthermore, by increasing a stored quantity of the wax component (D), the wax component (D) goes down slowly and flows out from the sprue gates thereof, starting dewaxing. Once the dewaxing phenomenon is occurred, a dewaxing passage is formed, so that dewaxing is progressed in turn. When the temperature is increased in order to progress the dewaxing process in a short time, the temperature reaches in burning temperature range rapidly, so that the inner pressure of the casting mold is increased rapidly by accompanying pyrolysis gas, resulting that cracks appear on the casting mold even if it is reinforced by the layer formed with the epoxy silicon (N) remarkably.

In the fourth process, when the preliminary heating at 60 to 120 °C is carried out during more than eight hours, an economical problem is arisen because the process time becomes long. Besides, when the preliminary heating is carried out during less than two hours, oozing of the plasticizer (C) and the wax component (D) is not enough, so that the dewaxing is not progressed. Accordingly, the preliminary heating is more preferably carried out during within 2 to 4 hours at 60 to 120 °C. Furthermore, it is more preferred that 2 to 20 wt% of the resin model is dewaxed in the initial stage.

Next, in a fifth process, the temperature in the furnace is increased gradually and heating is carried out at 150 to 500 °C during 2 to 8 hours, so that the dewaxing and burning of the resin model are completed.

In the fifth process, a cured matter of the two-component reaction curing type urethane liquid resin (F) consisting of the multifunctional polyol component (A), the multifunctional polyisocyanate component (B), the plasticizer (C), the wax component (D) and the hollow resin balloons (E) is decomposed by heat and melted with pyrolysis gas being produced, and a resulting liquefied matter is flown out. At this point, an outflow passage has already been formed between the resin model (H) and the casting mold, the melted and liquefied matter is flown out through the outflow passage. At this time, because the inner pressure of the casting mold by the pyrolysis gas becomes force for pressing out the melted and liquefied matter, the dewaxing can be progressed smoothly and the pyrolysis gas can be discharged smoothly, so that appearance of cracks in the casting mold can be avoided.

Next, in a sixth process, the casting mold is calcined at 500 to 1000 °C.

In the sixth process, the resin model is burned out, and the casting mold is calcined to reach maximum strength thereof. Then, after cooling down gradually, an ash component remained inside the casting mold is removed, so that the casting mold without cracks or hair cracks is completed.

Hereinafter, component substances of the two-component reaction curing type urethane liquid resin (F) used in the present invention is explained.

A framework of the two-component reaction curing type liquid resin (B) is constituted of the multifunctional polyol component (A) and the multifunctional polyisocyanate component (B), chemical reaction is started by mixing two components, so that it is heated and hardened. The plasticizer (C) and the wax component (D) may be mixed into either the multifunctional polyol component (A) or the multifunctional isocyanate component (B), or may be mixed into both of them.

The multifunctional polyol component (A) may be a low molecular polyol, a polyether polyol, an amine polyol, a polyester polyol, an acrylic polyol, or a butadiene polyol. Examples of the low molecular polyol include ethylene glycol, propylene glycol, 1 - 4 butanediol glycerine, trimethylol propane, pentaerythritol. Various kinds of polyether polyol with various molecular weights obtained by adding ethylene oxide or propylene oxide into the low molecular polyol are marketed. Terminal hydroxyl group becomes primary or secondary by various additional systems such as adding ethylene oxide by itself, adding propylene oxide by itself, adding a mixture of ethylene oxide and propylene oxide and adding ethylene oxide and propylene oxide separately in sequence. Thus, reactivities of the terminal hydroxyl groups are different one another, so that the polyether polyols different in hydrophilia according to which an additional chain is ethylene oxide or propylene oxide are marketed.

The amine polyol is a substance achieved by adding ethylene oxide or propylene oxide to a low molecular amine such as ammonia, ethylene diamine. Thus, as it contains tertiary nitrogen in the molecule thereof, it is a polyol having an effect such as to promote reactivity of the isocyanate. Accordingly, it is a necessary composition in the present invention in which rapid hardening is required.

The polyester polyol is that a molecular terminal thereof is moved to a hydroxyl group by esterifying a dibasic acid and a low molecular polyol. By selecting specific types of dibasic acid and low molecular diol, adjusting the molecular weights thereof and using a small quantity of a multifunctional low molecular polyol, diverse types of polyester polyol can be prepared. In addition, it may be a lactone polyester polyol gained by ε - caprolactam ring-opening polymerization. By adding alkylene oxide to them, various poly ester polyols with polyester chains and polyether chains are made.

The acrylic polyol is an acrylic oligomer having a plurality of hydroxyl groups in an acrylic chain, which is formed by polymerizing an acrylic monomer containing a hydroxyl group terminal with methyl acrylate or methyl meta-acrylate. Various types of acrylic polyols formed by selecting specific acrylic monomers and adjusting molecular weight thereof are marketed. A liquid resin dissolved in an organic solvent, with a high molecular weight achieved by raising the extent of polymerization to a level which film formation is enabled, constitutes paint with superior weather resistance due to slight cross-linking induced by aliphatic polyisocyanate. The butadiene polyol is a copolymer of butadiene containing a hydroxyl group at a terminal thereof and a compound having double bonds. It is a polyol with a relatively high level of hydrophobic property.

A urethane modified polyol with a hydroxyl group terminal obtained by jointing such multifunctional polyols via polyisocyanate may be used as well. In such a case, viscosity thereof tends to increase since the molecular weight increases slightly due to oligomerization resulting from the urethane modification. For this reason, it is desirable to form the urethane modified polyol by using only part of the multifunctional polyols.

The multifunctional polyisocyanate component (B) is a chemical compound containing two or more isocyanate groups in a single molecule thereof, and the polyol component is to contain two or more hydroxyl groups in a single molecule thereof. The isocyanate groups, which are functional groups with an extremely high level of reactivity, react with hydroxyl groups containing active hydrogen, amino groups and thiol groups. Since the isocyanate groups generally react with amino groups and thiol groups instantaneously, they are normally used only in combination with a less reactive isocyanate component or less reactive aromatic amines, but they still react fairly quickly and for this reason, such a combination is not commonly used.

The polyisocyanate component may be constituted with an aromatic polyisocyanate, an aliphatic polyisocyanate, or an alicyclic isocyanate. Typical examples of the aromatic polyisocyanate include tolylene diisocyanate and diphenyl methane diisocyanate. The tolylene diisocyanate is obtained as a mixture of various isomers in chemical reaction at production, and various industrial products with varying mixing ratios of 2,4-body and 2,6-body, e.g., TDI-100 (2,4-TDI 100%), TDI-80 (2,4-TDI 80%, 2,6-TDI 20%), TDI-65 (2,4-TDI 65%, 2,6-TDI 35%), are marketed. The diphenyl methane diisocyanate is obtained as a mixture of various isomers in chemical reaction at production, being pure MDI and polymeric MDI in industrial applications. The pure MDI is a dicaryonic, whereas polymeric MDI is a multicaryonic. While the pure MDI is insolated through distillation, the polymeric MDI is obtained as residue. Since the number of muticaryons in the polymeric MDI changes under different manufacturing conditions, various types of polymeric MDI are marketed from numerous manufacturers. In addition, other examples of the aromatic polyisocyanate include naphthalene diisocyanate that an isocyanate group is added to a naphthalene nucleus or tolidine diisocyanate. Examples of the aliphatic polyisocyanate include hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, and lysine diisocyanate. The alicyclic polyisocyanate may be hydrogenated xylylene diisocyanate obtained by hydrogenating xylylene diisocyanate, or hydrogenated MDI obtained by hydrogenating MDI.

Because the polyisocyanate is generally highly reactive and especially volatile polyisocyanate is highly toxic, they are normally used after undergoing various types of metamorphisms. Such a metamorphism may be urethane modification, dimerization, trimerization, polycarbonimidization, urea modification, prepolymerization, blocking and so on. These are to leave an isocyanate group at a terminal thereof by inducing self condensation by using higher reactivity of the isocyanate group or by joining it via an active component.

The two-component reaction curing type urethane liquid resin (F) containing the multifunctional polyol component (A) and the multifunctional polyisocyanate component as resin constituents contains 2 to 20 wt% of polyether chains, as indicated in the chemical structure formula presented below:

Polyether chains will be introduced by using polyether to constitute the multifunctional polyol component (A). When polyester polyol is used, if it is a polyether ester, polyether chains may be introduced. Otherwise, polyether chains may be introduced by way of a so-called quasi-prepolymer, with a terminal isocyanate joined with a polyether, which can be used as a multifunctional polyisocyanate component (B).

Because the polyether chains is a soft component and thermally decomposed, and liquefied, flown out and burned out by the thermal decomposition when it is heated at a high temperature during dewaxing and burning processes, it is suitable to the object of the present invention.

Loadings of the polyisocyanate component and the polyol component are designed so as to set NCO/OH ratio of the NCO radix and OH radix to a value close to 1.0 by calculating the NCO radix and the OH radix. When NCO/OH = 1.0, the loadings are designed so that the numbers of the isocyanate groups and hydroxyl groups are equal to each other, so that they are all used in the reaction. In other words, it is a setting at which maximum strength thereof is realized. According to the present invention, NCO/OH ratio is set 0.7 to 1.0 in a range not more than 1.0 by setting an average functional radix of the polyisocyanate component to 2.1 or more and setting an average functional radix to 3.0 or more. The radix is preferably within 0.8 to 0.9. When NCO/OH ratio is not more than 0.7, the isocyanate groups are in largely shortage condition, three-dimensional network structure can not be achieved after reaction curing, leading to a major reduction in the hardness, and ultimately the resin becomes too soft to retain the original shape. If, on the other hand, the NCO/OH ratio is not less than 1.0, the number of excess isocyanate groups becomes too large and too many isocyanate groups will be left unused in the reaction when the resin needs to be disengaged from the die. This may lead to undesirable results such as failure to achieve a specific level of hardness and inconsistent color at a surface of a resulting hardened matter, the surface of the hardened matter is foamed.

As a catalyst that promotes the chemical reaction of the multifunctional polyol component (A) and the multifunctional polyisocyanate component (B), a metal catalyst or an amine catalyst may be used. Examples of a metal catalyst that may be used include octylic zinc, octylic lead, dibutyltin denatured, dibutyltin diacetate and the like. Examples of an amine catalyst that may be used include triethylene diamine, NN-dimethyl piperazine, N-methyl morpholine and the like. The catalyst is normally added into the polyol component. Under normal circumstances, the multifunctional polyol component (A) contains 1 to 1000 ppm of catalyst and a working life thereof is thus adjusted. According to the present invention, the catalyst is added in the multifunctional polyol component (A) so has to set the length of time over which work is enabled, i.e., the working life, to 5 minutes or less. If the working life is set to 5 minutes or more, the setting-disengaging time exceeds five hours, which may become problematic for resin model production. If the working life is less than 1 minute, the reaction viscosity rises quickly, making it difficult to secure a sufficient length of time for the double fluid mixing and casting processes. For these reasons, the working life should be set to 1 to 2 minutes.

The plasticizer (C) used in the present invention is an inactive chemical compound having no functional group that induces a chemical reaction with volatility insignificant enough to be disregarded. The plasticizer (C) may be an ester plasticizer, an ether plasticizer or an ester/ether plasticizer. More specifically, typical examples of the ester plasticizer are dioctyl adipate (DOA), dioctyl phthalate (DOP) and dibutyl phthalate (DBP). Alternatively, benzyl acetate, benzoic butyl, benzoic octyl, benzoic isopentyl, ethylene glycol benzoic diester, polyethylene glycol benzoic diester, propylene glycol benzoic diester, poly propylene glycol benzoic diester, ethylene glycol dioleate, polyethylene glycol dioleate, propylene glycol dioleate and polypropylene glycol dioleate. Examples of the ether plasticizer include ethylene glycol butyl ether, ethylene glycol diphenyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl butyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol dibutyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether and the like. Examples of the ethyl/ester plasticizer include ethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, diethylene glycol mono phenyl ether acetate and the like.

The plasticizer (C) is used in a quantity that amounts to 2 to 20 wt% relative to the entire weight of the two-component reaction curing type urethane liquid resin (F). If the content of the plasticizer (C) exceeds 20 wt%, the plasticizer (C) bleeds over the surface of the resin model readily to cause stickiness. If, on the other hand, the plasticizer (C) is used in a quantity amounting to less than 2 wt%, the thermally decomposed and melted resin does not flow out readily during the dewaxing and burning processes, since the plasticizer (C), which is a highly viscous liquid at room temperature but has a lower level of viscosity at higher temperatures, is not contained in sufficient quantity. In order to take full advantage of these characteristics of the plasticizer, it is desired to contain the plasticizer (C) as much as possible.

Accordingly, the present invention was conceived based upon the finding that a maximum content for the plasticizer (C) can be achieved by rapidly setting the resin within less than 5 minutes of working life and trapping the plasticizer (C) having undergone phase separation from the cured resin in the three-dimensional network structure of the cured resin in a state of micro dispersion.

Such phase separation micro dispersion can be regarded as a state in which the plasticizer (C) is enclosed by the cured resin assuming a honeycomb structure. The cured resin assuming the honeycomb structure has superior physical strength, and the honeycomb structure can also be regarded as a three-dimensional structure within which the plasticizer (C) is secured within the honeycomb and is not allowed to be released to the outside. The structure does not allow the plasticizer (C) to bleed over the surface of the hardened object to induce tackiness even when the plasticizer is contained at a relatively high ratio. If the phase separation micro dispersion structure is not adopted, the plasticizer is dissolved into the hardened resin, and once it reaches the saturation level, the plasticizer becomes bled over the surface of the hardened matter to result in tackiness. If the extent of bleeding is significant, the surface becomes sticky. The phase separation micro dispersion structure can be observed through an electron microscope. The formation of the phase separation micro dispersion structure needs to be aided by rapidly hardening the resin within a working life of 5 minutes or less. Preferably, the working life should be set to 3 minutes or less, and even more desirably to 1 to 2 minutes. If the working life is set to 5 minutes or more, the process of phase separation micro dispersion cannot be completed with ease, and since it will take a day or more to disengage the model during the model production, the model production will become an extremely slow process.

When the plasticizer (C) is contained in the two-component reaction curing type urethane liquid resin (F), it needs to be uniformly dissolved in the liquid resin, whereas the phase separation micro dispersion of the plasticizer from the cured resin is promoted during the reactive setting stage so that the micro dispersed plasticizer is trapped by the time the reactive setting process is completed, thereby preventing bleeding of the plasticizer onto the surface. The composition of the double fluid reactive setting liquid resin must be designed so as to strike an optimal balance by taking into consideration the factors discussed above. Namely, the composition must be designed within the range over which the hydrophilic and hydrophobic properties of the plasticizer (C) and the reactive setting resin are perfectly balanced. For this reason, it is effective to form the hydrophilic segment with an alkylene oxide chain and to form the hydrophobic segment with a hydrocarbon chain. The properties of the hydrophilic segment and the hydrophobic segment are determined by selecting a specific type of raw monomer. A certain degree of dissociation should be assured with regard to the balance between the hydrophilic property and the hydrophobic property. If the two-component reaction curing type urethane liquid resin (F) contains a large number of ethylene oxide chains, the hydrophilic property becomes more pronounced, whereas if the ethylene oxide chains are replaced with propylene oxide chains, the level of hydrophilic property is lowered. If ethylene oxide chains or propylene oxide chains are used in a smaller quantity, the hydrophobic property of the two-component reaction curing type urethane liquid resin (F) becomes more pronounced. By adjusting the types of hydrophobic and hydrophilic segments and their quantities the hydrophilic property and the hydrophobic property of the two-component reaction curing type urethane liquid resin (F) can be adjusted over a specific range. In addition, by adjusting the type and quantity of the plasticizer (C), the hydrophilic property and the hydrophobic property of the plasticizer (C) itself can be adjusted within a certain range. For instance, if the terminal of the plasticizer is constituted with alkyl ether, the level of the hydrophobic property increases as it changes to methyl ether, to ethyl ether, to butyl ether, and then to phenyl ether. By adjusting the chemical structure and the quantity of the plasticizer (C) and also by adjusting the chemical structure and the quantity of the two-component reaction curing type urethane liquid resin (F), the range over which the phase separation micro dispersion is achieved can be controlled.

The plasticizer (C) is in a liquid state at normal temperature. The plasticizer (C) is trapped in a condition of the micro dispersion inside the resin which is cured with the two-component reaction curing type urethane liquid resin. Thus, when heat expansion, thermal decomposition, and melting of the resin are started, the plasticizer (C) is soaked from the resin model as a high temperature and low-viscosity liquid. In other words, it contributes to increasing of dewaxing as a main component.

Next, the wax component (D) used in the present invention is explained.

A wax exists naturally, and a typical example is a candle. A chemical composition of the natural wax is referred to as wax ester which is constituted with higher fatty acid and higher alcohol. The number of carbons in the higher fatty acid higher alcohol is 16 or higher in most instances. Since it is an ester compound, it has a small residual acid value. In other words, it contains residual free fatty acids. In addition, since numerous types of saturated and unsaturated higher fatty acids exist in the natural environment, certain types of wax contain unsaturated higher fatty acids or hydroxyl acid as well. These waxes have chemical structures close to that of paraffin and are crystallized or uncrystallized solid substances at room temperature. Their melting points are normally approximately 80°C. Typical examples of waxes include candela wax, carnauba wax, rice wax, bees wax, whale wax, montan wax. One of these wax substances may be used by itself, a plurality of the wax substances may be used in combination or a wax component containing a third constituent may be used. A main component thereof is an ester compound constituted with higher fatty aid and higher alcohol.

Examples of the wax include an oil wax, a synthetic wax and a natural wax.

The oil wax is a solid or a half-solid carbon hydride existing in crude oil at normal temperature, and it is classified to a paraffin wax, a microcrystalline wax and a petrolactam wax.

The paraffin wax is a solid wax at normal temperature whose main component is a straight-chain saturated hydrocarbon with carbon number of 20 to 36, which contains a small quantity of side-chain saturated hydrocarbon, naphthene and arene, and whose molecular weight is 300 to 500.

The microcrystalline wax is a little complicated compound having a side-chain in a main-chain thereof, whose molecular weight is 450 to 700 larger than that of the paraffin wax, and whose carbon number is within about 31 to 50. It is a solid wax at normal temperature containing a small quantity of a straight-chain saturated hydrocarbon, naphthene and arene except for the side-chain saturated hydrocarbon as a main component.

The petrolactam is a half solid wax at normal temperature containing a small quantity of a straight-chain saturated hydrocarbon, naphthene and arene except for the side-chain saturated hydrocarbon as a main component as well as the microcrystalline wax.

Examples of the synthetic waxes include a petroleum wax, a polyethylene wax, Fischer-Ttopsch wax, and a fatty synthetic wax.

The petroleum wax may be montan wax which is a black-brown wax which is gained by extracting from brown coal containing bitumen via organic solvent, and a melting point thereof is about 85 °C. Oxide of rough montan wax is marketed as an acid wax.

The polyethylene wax is produced by polymerization of ethylene or by thermal decomposition of general-purpose polyethylene. The polyethylene wax has 1000 to 10000 of the molecular weight as compared with that molecular weight of the natural wax is not more than 1000, so that differences of the solid state properties caused by the differences of the molecular weights and differences of the chemical structures such as a continuous structures of ethylene chains make various characteristics thereof different.

The wax component (D) of the present invention may be a single wax, but it is used preferably as a blended wax. Besides, the wax component (D) is a compound with strong paraffin's properties, namely it has a strong hydrophobic property and is solid at normal temperature. Accordingly, it has a nature that it can not be dissolved into the multifunctional polyol component (A), the multifunctional polyisocyanate component (B), or the plasticizer (C) easily. Thus, when it is mixed with the two-component reaction curing type urethane liquid resin (F), particles of the wax component (D) are in a condition that they are expanded and floated in a liquid system thereof because they are difficult to be dissolved. In thus condition, when the two-component reaction curing type urethane liquid resin (F) is hardened rapidly, they result to be buried and included in the hardened resin as solid state.

The wax component (D) is added so as to achieve a ratio of 1 to 20 wt%, preferably 5 to 10 wt% relative to the two-component reaction curing type urethane liquid resin (F). If the wax content is not more than 1 wt%, the desired effect of using the wax component (E) is vanished. If the wax content is not less than 20 wt%, fluidity of the two-component reaction curing type urethane liquid resin (F) becomes poor and operability of the resin model production is spoiled. In addition, strength of the resin model itself becomes low and possibility of cracking or breaking during demolding becomes high.

A melting point of the wax component (D) is set within 60 to 130 °C, preferably within 60 to 120 °C. The case that the melting point is not more than 60 °C is not available because the wax component (D) is melted by curing exotherm of the two-component reaction curing type urethane liquid resin (F) and the particles of the wax component (D) are merged and separated on an upper layer of the model. The case that the melting point exceeds 130 °C is not preferred because dewaxing is delayed due that molecular weight thereof becomes larger and melting viscosity thereof becomes higher.

The wax component (D) includes particles with diameters of not more than 5 mm. When they exceed 5 mm, uniform distribution of the wax component (D) in the model is spoiled because the wax component (D) is not flown into a portion with thickness not more than 5 mm. The particle of the wax component (D) is preferably not more than 1 mm. Thus particles of the wax component (D) are floated in the two-component reaction curing type urethane liquid resin (F) in a condition that surfaces thereof are swelled. Accordingly, the particles of the wax component (D) can not be surfaced rapidly. The particles are dispersed in the hardened resin model in a condition that they are covered with resin films respectively. Therefore, when thermal expansion, partially thermal decomposition and melting of the resin are started, the wax component (D) is soaked out of the resin model as a liquid with high temperature and low viscosity. Namely, it is contributed as a main composition for increasing dewaxing.

The hollow resin balloons (E) are resin beads that insides thereof are hollow. Phenolic resin balloons and acrylic resin are marketed. Particle diameters of these hollow resin balloon (E) are within approximately 10 to 100 µm and true specific gravities thereof are within 0.01 to 0.03. When thus hollow resin balloons (E) with light specific gravity are mixed with the two-component reaction curing type urethane liquid resin (F), they are surfacing on liquid level thereof. On the other hand, since the particles of the wax component (D) are mixed therein, floating of the hollow resin balloons (E) is delayed considerably because the floating of them is prevented by the particles of the wax component (D). Because the two-component reaction curing type urethane liquid resin (F) has a short pot life, chemical reaction thereof starts at a moment of mixing two components and the pot life thereof is a short time of 1 to 5 minutes, and increase of viscosity thereof is occurred at a termination time of an injection working, concentrative surfacing to the liquid level of the hollow resin balloons (E) is prevented.

When the particle diameters of the hollow resin balloons (E) are not more than 10 µm, flowability in a mixture system of the two-component reaction curing type urethane liquid resin (F) becomes poor, so that the injection working becomes difficult. When the particle diameters exceed 100 µm, the floating of the hollow resin balloons (E) in the mixture system of the two-component reaction curing type urethane liquid resin (F) becomes fast, so that it is not preferred because separation in the liquid occurs. The particle diameters are preferably within 20 to 80 µm.

The hollow resin balloons (E) are distributed uniformly in the resin model (H) in which the hollow resin balloons (E) are mixed with the two-component reaction curing type urethane liquid resin (F) to be hardened. Namely, this means that the hollow resin balloons (E) are distributed in the resin model in a condition that gas components are contained therein, and that components unnecessary for dewaxing and burning are distributed, effects such that they are largely contributed to dewaxing properties and burning properties appear.

Loadings of the hollow resin balloons (E) are 0.001 to 1.0 wt% relative to the two-component reaction curing type urethane liquid resin (F). When the loadings thereof are not more than 0.001 wt%, the effects largely contributing to the dewaxing properties and the burning properties are decreased. When the loadings thereof is not less than 1.0 wt%, a mixture resin liquid system thereof becomes rough and the flowability becomes poor, so that a smooth injection working becomes difficult. The loadings of the hollow resin balloons (E) are 0.03 to 0.10 wt% relative to the two-component reaction curing type urethane liquid resin (F).

Inert organic compositions having no active hydrogen may be mixed to the two-component reaction curing type urethane liquid resin (F) comprising the multifunctional polyol component (A), the multifunctional polyisocyanate component (B), the plasticizer (C), the wax component (D) and the hollow resin balloons (E). For instance, examples of the inert organic compositions include organic solvents, anti-oxidizing agents, coloring matters, accelerators, emulsifying agents, thixotropic agents, etc.

Methyl acetate, ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, toluene, or xylene can be contained as an organic solvent in order to decrease viscosity thereof. Hindered phenols or hindered amine can be contained as an anti-oxidizing agent in order to prevent degradation by heat and by ultraviolet rays. Organic colors can be obtained as color matters in order to increase an aesthetic sense. Tertiary amine, a little bit of octyl acid zinc, or dibutyltin laurate can be contained as an accelerator. Amid composition or a surface active agent can be contained as an emulsifying agent. Note that it is not preferred to mix inorganic matters to be ashes after burning.

Next, the fireproof coating solution (I) is explained in detail.

The fireproof coating solution (I) is a solution like a slurry that impalpable powder of zirconia or alumina is mixed with colloidal silica or ethylene silicate. Because the colloidal silica or ethylene silicate as a binder used in the fireproof coating solution (I) has a pot life, PH control of the solution has to be done severely. The colloidal silica or ethylene silicate is hardened by dehydration reaction. In the case of burning it at high temperature, the dehydration is further occurred so that it becomes ceramics by combination of Si-O-Si to be a casting mold with high hardness. Since the ethylene silicate is used in an alcohol solvent system, alcohol component is scattered at drying. Since the colloidal silica is a water system, water component is scattered at drying. It shows a tendency to use the colloidal silica with no splash of alcohol extensively in the casting industry due to a problem of environmental pollution.

Next, epoxy silicon (N) used in the present invention is explained in detail.

The epoxy silicon (N) is a liquid with low viscosity which can be sprayed and which is formed by mixing amino silane (K) with bisphenol A type epoxy (J) and diluting a resulting mixture by organic solvent (L).

The bisphenol type epoxy (J) is an epoxy having a glycidyl group at a terminal of molecule shown as a following chemical structural formula, and there are a bisphenol A type epoxy and a bisphenol F type epoxy. The bisphenol A type epoxy is marketed as trademarks such as EPYCOAT 828 and EPYCOAT 1001. The bisphenol type epoxy (J) is shown by the following chemical structural formula.

The amino silane is marketed as a silane coupling agent and a composition that it has an alkoxysilyl group and an amino group at a terminal of molecule shown in the following chemical structural formula. H₂N(CH₂)_p[NH(CH₂)_p]_qSi(OR)_nR_m p:2-3
q: 0-1
n+m=3
R: C_aH_2a+1
(a=1 - 2)

Examples of the amino silane include γ-amino propyl triethoxy silane, N-(β amino ethyl)-γ- amino propyl triethoxy silane, N-(β amino ethyl)-γ- amino propyl methyl dimetoxy silane, etc.

The organic solvent (L) is preferably a diluted solvent for epoxy resin which does not spoil reaction between epoxy and amine. For instance, it is a keton system organic solvent or an ester system organic solvent as a true solvent dissolving the epoxy resin, or an ether system organic solvent, an aromatic system organic solvent or an alcohol system organic solvent to be a diluted solution which dilutes after dissolving the epoxy resin by the true solvent.

As the keton system organic solvent, methyl ethyl keton, methyl isobuthyl keton and the like are listed. As the ester system organic solvent, methyl acetate, ethyl acetate, propyl acetate and butyl acetate and the like are listed. As ether system organic solvent, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether and the like are listed. As the aromatic system organic solvent, toluene and xylene are listed typically. As the alcohol system organic solvent, methanol, ethanol, isopropyl alcohol, butyl alcohol, ethylene glycol mono methyl ether and the like are listed. Because these organic solvents include a very small quantity of water, it is preferred that they are dehydrated as much as possible to be used.

When the bisphenol type epoxy (J) and the amino silane (K) are mixed, the glycidyl group of the bisphenol type epoxy (J) and the amino group of the amino silane (K) are combined with each other by occurring addition reaction. The alkoxysilyl group of the amino silane (K) acts on dealcoholization reaction with moisture in the air or with OH groups on a surface of a fireproof coating layer as a foundation to form a film. The addition reaction between the glycidyl group and the amino group and the dealcoholization reaction of the alkoxysilyl group proceed smoothly at normal temperature.

Mixing ratio between the bisphenol type epoxy (J) and the amino silane (K) may be determined by weights such as to mix 1.5 to 2 moles of the amino silane with 1 mol of the bisphenol type epoxy.

A layer of the epoxy silicon (N) is glued strongly with a primary layer or a backup layer as a foundation. A mechanism for gluing is that: the alkoxysilyl group acts on the dealcoholization reaction with Si-OH group on the surface of the fireproof coating layer as a foundation; formation of Si-O-Si combination is occurred; and they join strongly by covalent bonding. Besides, it can be considered that an OH group created by the addition reaction between the epoxy group and the amino group acts on hydrogen bonding with Si-OH of the fireproof coating layer to be glued strongly.

The epoxy silicon (N) has a chemical structure with a relative rich heat performance in resins and is not thermally decomposed easily in a low temperature range (100 - 200 °C) to contribute to increasing strength of the casting mold. Since it is burned in a middle temperature range (300 - 500 °C), the strength thereof is vanished gradually. In a high temperature range (500 - 1000 °C), since the casting mold becomes ceramics by burning, the strength of the casting mold reaches the maximum thereof. Thus, the epoxy silicon (N) makes up with lack of the strength in the low temperature range (100 - 200 °C) where the strength of the casting mold is lacked and it is easy for cracks to occur.

When the resin model (H) according to the present invention is dewaxed, the best condition available to the dewaxing of the resin model (H) must be applied. Generally, in the lost wax precision casting, a temperature condition for dewaxing of a wax model is to increase temperature to 120 °C, to dewax at 120 to 150 °C during one to two hours, and to increase the temperature to 300 to 500 °C. When thus condition is applied to the resin model as it is, inner pressure of the casting mold is increased rapidly by thermal expansion and generation of combustion gas by thermal decomposition of the resin model, so that the cracks appear on the casting mold, as the case may be, the casting mold is broken. It is considered that: the reason why the cracks occurs on the casting mold is that an inner stress becomes larger at a low level of the strength of the casting mold and the strength of the casting mold can not be stood up to an inner stress.

As a result of considering conditions for dewaxing and burning available to the resin model of the present invention, the condition is that a process for primary dewaxing 5 to 50 wt% of the resin model by carrying out preliminary heating in a low temperature range within 60 to 120 °C during 2 to 8 hours is provided; a process for completing dewaxing and burning of the resin model by increasing a temperature in a furnace gradually and heating in a middle temperature range within 150 to 500 °C during 2 to 5 hours; and a process for burning in a high temperature range within 500 to 1000 °C. Namely, because the resin component difficult to dewax is a main component, rapid increasing temperature is avoided and the low temperature dewaxing condition is introduced.

The plasticizer (C) having undergone phase separation micro dispersion seems to be bled over an interface between the resin and the casting mold through a resin film by the preliminary heating at 60 to 120 °C during 2 to 8 hours. The plasticizer (C) is stored in the interface and starts flowing out from the sprue gates along an inner surface of the casting mold. Furthermore, the wax component (D) starts softening and liquefaction because temperature thereof is nearby or over a melting point thereof, and bleeds with the plasticizer (C) or flows out through the sprue gates via a passage through which the plasticizer (C) is flown out. In the low temperature range within 60 to 120 °C, the resin component is in a softening and degrading condition, but the thermal decomposition with gas has not occurred yet. Accordingly, inner pressure of the casting mold is caused by only thermal expansion of the resin component, it becomes a force which presses the plasticizer (C) and the wax component (D) to the sprue gates. Thus, because the pressure inside the casting mold is decreased by outflow of them, the casting mold can stand up to the inner pressure to avoid occurring of the cracks.

When the temperature is increasing over 300 °C, the resin component is decomposed thermally as decomposition gas is arisen and is a low molecule by fission of main chains and side chains, so that the resin itself is liquefied partially. The decomposition gas is arisen and confined inside the casting mold. Thus, though the inner pressure of the casting mold is increased, when the passage through which the plasticizer (C) and the wax component (D) are flown out is opened, the decomposition gas is vented out easily. Besides, because it becomes a force for pressing out a liquefaction residue to the sprue gates, when the liquefaction residue is flown out from the sprue gates, the decomposition gas is vented outside, so that the inner pressure of the casting mold is decreased. The thermal decomposition of the resin is advanced inside the casting mold, so that the decomposition gas is arisen. The dewaxing and the burning are happened one after another with thus various phenomena, the inner pressure of the casting mold seems to be increased slowly. When the dewaxing and the burning are activated at the same time, a large space is appeared between the casting mold and the resin model, so that the liquefaction residue is flown out with a flame.

The two-component reaction curing type urethane liquid resin (F) used in the present invention is constituted so as to harmonize workability of the injection, hardenability, maintenance of hardening matter's shape, dewaxing quality, thermal decomposition and combustion quality, and so as to be the resin model in which the defaults of the wax model are improved. It is an important point that the temperature condition for dewaxing slowly and gradually is set so as not to load an impossible stress to the casting mold when the resin model with thus components is dewaxed. Moreover, by being structure such as to put the epoxy silicon layer between the fireproof coating layers, the strength of the casting mold can be increased in the low temperature range.

Consideration and examination are carried out in three directions of the composition of the resin model, increasing the strength of the fireproof coating material, and application of the dewaxing condition available to the resin model, so that a method for producing the casting mold without cracks during the dewaxing and burning processes was found, coming to the present invention.

INDUSTRIAL APPLICABILITY

As explained above, several effects are appeared according to carrying out the precision casting by using the casting mold for the lost wax precision casting according to the present invention. The effects are as follows:

Casting products with a thin and complicated shape having a shape edges and ribs which are not achieved by the prior lost wax precision casting can be produced.

Titan casting products with a thin and complicated shape which are achieved to lightweight, high hardness, high heat resistance and high corrosive and whose dimensional accuracy is superior by applying to titan alloy can be produced.

Mechanical parts for racing cars, parts for airplanes and jet engines, parts for robots and parts for space exploitation rockets can be saved their weights in high accuracy.

Especially, casting products for a high-tech industry can be produced, large effects are expected.

Claims

A method for producing a casting mold for a lost wax precision casting characterized by comprising:

a process for producing a resin model (H) by mixing a two-component reaction curing type urethane liquid resin (F) comprising a multifunctional polyol component (A), a multifunctional isocyanate component (B), a plasticizer (C), a wax component (D) and hollow resin balloons (E), injecting a resulting mixture into a mold (G), hardening it and demolding it from said mold (G);

a process producing a primary layer consisting of a plurality of coating layers laminated on a surface of the resin model (H) by repeating operation for forming said coating layer in which said resin model (H) is soaked in a solution for fireproof coating (I), picked up from it, dried, painted with epoxy silicon (N), dried and then hardened;

a process for producing a backup layer consisting of a plurality of thick film coating layers laminated on the surface of the resin model (H) by repeating operation for forming the thick film coating layer in which the resin model (H) is soaked in the solution for fireproof coating (I), sprinkled with stucco (M), dried, painted with epoxy silicon (N), dried and then hardened;

a process in which the resin model (H) coated with thick film fireproof coating multi-layer consisting of the primary layer and the backup layer is set in a furnace so as to turn down a sprue thereof, heated in a lower temperature range within 60 to 120 °C during 2 to 8 hours preliminarily, initially dewaxed within 5 to 50 wt% of the resin model;

a process for completing dewaxing and burning-out of the resin model in which temperature in the furnace is increased gradually and heating is carried out in a middle temperature range within 150 to 500 °C during 2 to 5 hours; and

a process for burning in a high temperature range within 500 to 1000 °C during 2 to 5 hours.
A method for producing a casting mold for a lost wax precision casting according to claim 1, characterized in that an average functional radix of said multifunctional polyol component (A) is 2.8 or larger, an average functional radix of said multifunctional polyisocyanate component (B) is 2.0 or larger, and a ratio NCO/OH is within 0.8 to 1.0.
A method for producing a casting mold for a lost wax precision casting according to claim 1 or 2, characterized in that said plasticizer (C) is micro-dispersed through phase separation when the two-component reaction curing type urethane liquid resin (F) is hardened by reaction.
A method for producing a casting mold for a lost wax precision casting according to any one of claims 1 to 3, characterized in that said two-component reaction curing type urethane liquid resin (F) contains polyether chains indicated by a chemical structural formula as follows at 2 - 20 wt% thereof.
A method for producing a casting mold for a lost wax precision casting according to any one of claims 1 to 4, characterized in that said wax component (D) is in powder shape with a maximum diameter of 5 mm and has a melting point within 60 to 130 °C, and is contained within 3 to 30 wt% in said two-component reaction curing type urethane liquid resin (F).
A method for producing a casting mold for a lost wax precision casting according to any one of claims 1 to 5, characterized in that said epoxy silicon (N) consists of bisphenol type epoxy (J), amino silane (K) and organic solvent (L).
A method for producing a casting mold for a lost wax precision casting according to any one of claims 1 to 6, characterized in that particle diameters of the hollow resin balloons (E) are within 10 -100 µm, absolute specific gravities thereof are within 0.01 to 0.03, and contained within 0.001 - 0.5 wt % in the two-component reaction curing type urethane liquid resin (F).