EP0459486A2

EP0459486A2 - Pattern for manufacturing mold

Info

Publication number: EP0459486A2
Application number: EP91108870A
Authority: EP
Inventors: Hiroaki Nishio; Michitaka Sato; Akira Takase; Akira Kato
Original assignee: NKK Corp; Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1990-05-30
Filing date: 1991-05-29
Publication date: 1991-12-04
Also published as: EP0459486A3

Abstract

A pattern for manufacturing a mold consisting essentially of a flexible thin bag made of a material (1), which is impermeable with regard to the fluid contained therein and the material forming the mold (9), which is resistant to both of the fluid and the material forming the mold, and which has an elastic modulus in tension of 1 to 200 kg/mm², and the fluid contained therein (6), and a process for preparing a mold using the same, a process for preparing a collapsable pattern which comprises cooling to solidify an agglomerate of water-absorbable swellable organic polymer particles containing an aqueous solution into a prescribed form, and a process for preparing a water-soluble pattern which comprises filling a melted blend consisting essentialy of 5 to 95 wt. % of urea and 95 to 5 wt. % of a carbamate ester compatible with urea into a mold, and cooling to solidify it. According to the above means, the molds can be formed without broken at a pressure lower than 10 kg/cm², and the working environment problem caused by ammonia gas evolution is not present.

Description

BACKGROUND OF THE INVENTION

This invention relates to patterns for manufacturing a mold used in the filed of casting molten metal, casting of a slurry of metal powder or ceramic powder or filling and pressure molding of metal powder and ceramic powder, the patterns being collapsable in the mold, and processes for preparing the same.
The casting method using a ceramic mold is characterized by the excellence in the surface accuracy of the molded body. Moreover, since the ceramic mold is prepared, in general, by laminating a ceramic shell onto the surface of a pattern followed by collapsing to remove the pattern, it is not necessary to form a draft on the pattern. Therefore, the freedom of the form is remarkably increased, and the molding of a complex form is possible. This method is known as a precision casting, and a representative method is the lost wax method wherein the pattern is formed of wax which is removable by melting.
Another known method utilizes water-soluble urea as the material of the pattern (Japanese Patent KOKOKU No. 53-16362). In this method, urea is melted in the presence or absence of water, and polyvinyl alcohol is dissolved in the melted urea to obtain a uniform solution. The solution is rendered to a solid solution by cooling to a temperature in the range of 115 to 125°C resulting to crystallize a mixture composed of at least partially crystallized urea and an adduct of urea and polyvinyl alcohol. Then, the solid solution is formed into the pattern which is collapsable.
A forming method of the pattern disclosed comprises heating the powder of the solid solution at 90 to 115°C, and injecting it into a pattern-forming mold at a pressure of 300 to 1,500 kg/cm² to form the pattern.
In the lost wax method, the ceramic shell is occasionally broken by the expansion of the wax during melting the wax pattern for the removal. Therefore, various methods for preventing the ceramic shell from being broken were developed, such as, by melting the part being in contact with the shell prior to the other parts utilizing a thermal shock, by rendering the pattern hollow, and most commonly, by pressurizing the wax pattern from the outside with steam using an autoclave.
On the other hand, in the case of the urea pattern, the urea is dissolved in the water entering through the pores of the ceramic shell and the water being in contact with the part exposed to the outside by immersing the pattern into water at ordinary temperature. Therefore, the ceramic shell is not broken by the removal of the pattern at all, and in this regard, the urea pattern is superior to the wax pattern.
In the conventional process for molding the water-soluble urea pattern, a high injection pressure, i.e. 300 to 1,500 kg/cm², is necessary because of using a solid urea mixture as disclosed in Japanese Patent KOKOKU No. 53-16362. Therefore, a mold must be used, and the mold must be thick so as to resist the injection pressure. As a result, manufacturing cost of the mold is expensive, and an expensive injection molding machine is necessary according to the high injection pressure.
Although urea has a melting point of 133°C, when urea is heated from ordinary temperature, the decomposition rate of urea increases according to approaching the melting point. At the melting point, urea is melted and simultaneously decomposed with violence. According to the experiment tracing the method of Japanese Patent KOKOKU No. 53-16362 conducted by the inventors, urea was decomposed in the process of melting urea and dissolving polyvinyl alcohol in the melt, and a considerable amount of ammonia gas was evolved. Besides, urea was also decomposed in the process of cooling the above solution to a temperature range of 115 to 125°C to prepare a solid solution, and a considerable amount of ammonia gas was evolved. Since ammonia gas has a high toxicity, it was anticipated that there is a problem in the working environment without no corrective action. A considerable amount of plant and equipment investment is necessary for the improvement, and nevertheless, the working is inconvenient.

SUMMARY OF THE INVENTION

An object of the invention is to provide a pattern which does not break a mold formed by using it and which is formable without or with pressurizing at a pressure lower than 10 kg/cm² and a process for preparing the same.
The above object has been achieved by a pattern for manufacturing a mold consisting essentially of a flexible thin bag made of a material, which is impermeable with regard to the fluid contained therein and the material forming the mold, which is resistant to both of the fluid and the material forming the mold, and which has an elastic modulus in tension of 1 to 200 kg/mm², and the fluid contained therein.
The above object has also been achieved by a process for preparing a collapsable pattern which comprises cooling to solidify an agglomerate of water-absorbable swellable organic polymer particles containing an aqueous solution into a prescribed form.
Another object of the invention is to provide a process for preparing a water-soluble urea pattern without the decomposition of urea.
The above object has been achieved by a process for preparing a water-soluble pattern which comprises filling a melted blend consisting essentially of 5 to 95 wt. % of urea and 95 to 5 wt. % of a carbamate ester compatible with urea into a mold, and cooing to solidify it.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a sectional view illustrating a state of rendering the thin bag into a stretched state by supplying a fluid into the bag and forming a mold.
Figure 2 is a sectional view illustrating a deflated state of the thin bag after forming the mold.
Figure 3 is a sectional view of a mold used in the examples of the invention.
Figure 4 is a sectional view illustrating a state of forming a mold for casting using the pattern formed by using the mold of Figure 3.

DETAILED DESCRIPTION OF THE INVENTION

The material composing the thin bag is substantially impermeable with regard to the fluid supplied thereinto and the material forming the mold supplied to the outside, and is resistant to them. Examples of the material composing the thin bag are regenerated cellulose, cellulose derivatives, polyolefins, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, polyester, polystyrene, hydrochlorinated rubber, polyamide, fluorocarbon resin and silicone rubber. The thin bag must be soft and flexible and must have an elastic modulus in tension (tensile modulus) of 1 to 200 kg/mm². When the tensile modulus in less than 1 kg/mm², the bag is expanded by a small internal pressure resulting in the degradation of the dimensional accuracy of the pattern. While, when the tensile modulus is beyond 200 kg/mm², the bag is stiff, and it is difficult to deflate sufficiently by discharging the fluid. A suitable thickness of the thin bag is 10 to 3,000 µm, preferably 200 to 1,000 µm. The thin bag repeats inflation and deflation by the supply and discharge of the fluid and repeats separation from the mold formed. Accordingly, when the thickness is less than 10 µm, the lifetime of the thin bag is short. While, when the thickness is beyond 3,000 µm, the thin bag resists deformation resulting in the difficulty of the separation from the mold and in the breakage of the mold during the separation.
The thin bag is formed into a desired form of the pattern, when it is stretched. Although the forming method is not limited, a most simple method comprises repeating pouring a liquid raw material into a mold for forming the pattern, discharging the liquid raw material and solidifying the liquid membrane formed on the inner surface of the mold. Another method comprises applying the liquid raw material onto the cavity of the mold for forming the pattern. As another method, the blow molding in the plastic field can be utilized. Another method comprises joining plural film pieces by welding or the like.
The kind of the fluid supplied to the thin bag is not limited, and it may be gas, such as air liquid such as water, slurry or the like. However, in the case that the material forming the mold is liquid, it is necessary to prevent the thin bag form deforming caused by the buoyancy acting thereon. Therefore, the fluid must have a density to the degree that the thin bag is not deformed by the buoyancy caused by the density difference. For that purpose, the density of the fluid may be adjusted by mixing a plurality of miscible fluids or by dissolving a solute or suspending particles in the case that the fluid is liquid.
When the mold is manufactured, the thin bag is rendered into a desired pattern form by supplying the fluid to stretch it. The pattern form may be kept by sealing the fluid in the bag without pressurizing or by the own weight of the fluid. In order to keep the pattern form, the fluid may be pressurized in the range not expanding the thin bag which is usually less than 10 kg/cm², preferably less than 5 kg/cm². In view of the workability, the material for forming the mold is added, in general, after the thin bag is stretched. However, it is also possible that the material for forming the mold is added around the thin bag in a deflated state, and then the thin bag is stretched by supplying the fluid.
The mold can be formed utilizing the solidification phenomenon, such as the solidification of melt wax or the solidification of gypsum slurry by hydration. It may be formed by fixing a powder of resin, ceramic, metal or the like by a binder. For example, silica sand or alumina powder is fixed around the pattern by using colloridal silica as the binder. Alternatively, polystyrene resin, epoxy resin, urea resin or the like may be foamed. When the adhesion force is great between the mold and the pattern, a mold releasing agent may be coated on the surface of the pattern. Suitable mold releasing agents include organic materials, such as stearic acid and silicone resin, and inorganic materials, such as talc.
After the mold is formed by solidifying the material forming the mold, the pattern is collapsed by discharging the fluid from the thin bag by suction, turning over or the like. At least, a temporary reduction of pressure is necessary for separating the thin bag from the wall of the mold. When an adhesion is present between the mold and the thin bag, the inner pressure of the thin bag is necessary to be reduced to a pressure capable of separating it. On the other hand, it is not necessary to discharge the whole amount of the fluid, but sufficient to discharge the fluid to the degree capable of taking out the thin bag without damaging the mold.
Even if the form of the molded article is complex, a great number of the article can be molded by using the above pattern repeatedly.
In the process for preparing a collapsable pattern, the water-absorbable swellable organic polymer particles have water-absorbing ability in a dry state and swell by absorbing an aqueous solution to obtain plasticity. The organic polymer includes synthetic polymers, such as cellulose graft polymers, starch graft polymers, acrylic acid-vinyl alcohol copolymer, sodium acrylate polymer and modified polyvinyl alcohols. Preferred organic polymers are sodium acrylate polymer and acrylic acid-vinyl alcohol copolymer. A suitable mean particle size before water absorption is 10 to 5,000 µm, and 50 to 500 µm is preferred. When the mean particle size is less than 10 µm, agglomerating ability of the particles is great. As a result, a part of the particles does not disperse, and remains in the aqueous solution without water absorption. The organic polymer particles are segregated to be exposed onto the surface of the pattern, and result in the degradation of the surface smoothness. While, when the mean particle size is beyond 5,000 µm, the pattern due to the big particles of the organic polymer appears on the surface, and results in the degradation of the surface smoothness. A suitable amount of the organic polymer is 0.1 to 20 parts by weight to 100 parts by weight of the aqueous solution, and 0.4 to 3 parts by weight is preferred. When the amount of the organic polymer is beyond 20 parts by weight, the swelling of the organic polymer particles is inhibited by meeting them each other. As a result, a part of the particles does not or insufficiently absorb water, and is exposed on the surface resulting in the degradation of the surface transferability. While, when the amount of the organic polymer is less than 0.1 part by weight, the pattern is liable to be broken because of binding a small amount of the organic polymer particles by a fragile thick layer of the deposited crystals.
Preferred solutes have a melting point higher than ordinary temperature and are easily dissolved in water. Examples of such a solute are chlorides, such as potassium chloride, sodium chloride, magnesium chloride and calcium chloride, alcohols, such as tertiary butanol, carboxylic acids, such as butyric acid, and carboxylic acid derivatives, such as urea, methyl carbamate and ethyl carbamate. It is preferred that the solute is composed of one or more of the above examples alone or mainly. The content of the solute is preferably higher within the range not to inhibit the absorption of the aqueous solution by the organic polymer particles, in view of increasing the strength of the collapsable pattern. The aqueous solution is prepared by dissolving an amount of the solute more than the solubility at ordinary temperature at a temperaure higher than the ordinary temperature. The amount of the solute is adjusted to a degree capable of depositing it from the solution which exceeds the saturation by cooling the agglomerate of the organic polymer particles containing the solution, and capable of obtaining the pattern having a sufficient strength. When the final cooling temperature is ordinary temperature, the amount of the solute must be more than the solubility at ordinary temperature at least by 2 wt. % in order to obtain the pattern having a sufficient strength, and more than about 10 wt. % is preferred. For example, in the case of urea, since the solubility at 20°C is 52 wt. %, at least the amount corresponding to 54 wt. % at 20°C must be dissolved. A preferable amount is more than about 60 wt. %. The upper limit of the urea content is about 96 wt. %. When the urea content is beyond, it is necessary to heat the urea solution higher than 120°C resulting in the decomposition of urea.
In order to improve the smoothness of the pattern surface, a lubricant insoluble or slightly soluble in water, such as methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, stearic acid or oleic acid, may be added to the aqueous solution.
The aqueous solution may be prepared according to a conventional manner, such as heating a mixture of the solute and water with stirring.
In the process for preparing a collapsable pattern of the invention, the agglomerate of the water-absorbable swellable organic polymer particles containing the organic solution is filled in a mold for forming the pattern. The route to reach the above state is not limited, and for example, the aqueous soltion is prepared, and the organic polymer particles are put therein. After the organic polymer particles sufficiently absorb the aqueous solution to swell, the particles are filled in the mold. When burrs are formed by flowing into the mating surface of the mold, the viscosity may be raised up to a suitable value by increasing the rate of the organic polymer particles. Besides, when the forming speed is raised by pressurizing, the rate of the organic polymer particle may be further increased so as to become a viscosity corresponding to the forming pressure. In the case that the mold is made of a material having a problem of deformation caused by a high forming pressure, such as rubber, the agglomerate may be controlled to a viscosity not to induce deformation by the same method as above. In this case, the agglomerate is adjusted to a viscosity suitable for the forming pressure of less than 10 kg/cm², e.g. 1 to 2 kg/cm². If necessary, it is possible to form the pattern through the conventional injection molding using a mold by preparing the agglomerate having a viscosity corresponding to the high forming pressure of 300 to 1,500 kg/cm² as disclosed in Japanese Patent KOKOKU No. 53-16362 by increasing the rate of the organic polymer particles. The mold for forming the pattern may be collapsable. As another method, the agglomerate may be formed directly in the mold by putting the aqueous solution and the organic polymer particles thereinto separately.
After filling the agglomerate of the organic polymer particles containing the aqueous solution, the agglomerate is cooled to be solidified. The solidification occurs caused by the deposition of a solute of the aqueous solution due to the reduction of the solubility. That is, when the agglomerate is filled at a high temperature, the cooling may be stopped at a temperature higher than ordinary temperature. A simple method is gradual cooling to ordinary temperature. The demolding after solidified may be conducted according to a known method, and a suitable means is selected.
Using the pattern thus prepared, a mold for casting a molten metal or a mold for casting a slurry of a metal powder or a ceramic powder is formed. The latter case corresponds to both of the method of utilizing the solidification of a slurry and the method of absorbing a dispersion medium into a porous mold. The mold material may be applied onto the surface of the pattern. The pattern may be disposed in a vessel, and the mold material is filled into the space. The mold material may be selected from metal powders and ceramic powders previously blended with a binder. Gypsum is also applicable. In the case of casting a slurry of a metal powder or a ceramic powder, since the pattern is used at a low temperature from ordinary temperature to about 100°C, the mold material may be selected from a broader range. That is, resin powders are applicable. Moreover, it is also possible that a low melting metal or wax is melted and cast into a mold.
After forming the mold, the pattern is dissolved away by immersing into water. Since the collapsable pattern of the invention is made of mainly water-soluble substance, the pattern can be removed easily. A small amount of water-insoluble additives are removed by dispersing into water. The organic polymer particles further swell to increase fluiidity, and are removed easily together with water. The removal time can be shortened by warming water previously, because the dissolving speed is greater. When the agglomerate of the organic polymer particles is made in a low viscosity according to a aforementioned method, the pattern can also be removed by melting through heating from the outside of the mold.
In the above collapsable pattern, since the agglomerate of the organic polymer particles has plasticity, it can be filled into the cavity of the mold completely. During cooling a part of the solute deposits on the surface of the organic polymer particles, and the whole body is solidified by binding the organic polymer particles through the deposits. That is, the agglomerate of the organic polymer particles filled in the mold is cooled on the surface contacted with the mold to begin the discharge of the deposits, and this proceeds to the inside. As a result, the organic polymer particles are reduced, and the reduced particles containing the aqueous solution remian in the deposits. It was confirmed that contraction does not occur at the part contacted with the mold through this process. That is, dimensional shrinkage is little, and the pattern obtained has a very high dimensional accuracy. It is presumed that the reduced organic polymer particles with plasticity containing the aqueous solution loosen or lose the macro contraction stress over the whole pattern by their deformation.
In the process for preparing a water-soluble pattern, the carbamate ester has a compativility with urea, and lowers the melting point of the blend to not higher than 110 °C. Suitable carbamate esters are alkyl carbamate esters of which the number of the carbon atoms of the alkyl group is 1 to 6, such as methyl carbamate, ethyl carbamate, propyl carbamates and bytyl carbamates, halides thereof, such as trichloroethyl carbamates, benzyl carbamate, etc. Methyl carbamate and ethyl carbamate are particularly preferred.
The urea content of the melt is 5 to 95 wt. %, preferably 65 to 92 wt. %, and the carbamate ester content of the melt is 95 to 5 wt. %, preferably 35 to 8 wt. %. When the urea content or the carbamate ester content is out of the above range, the crystal grains on the surface of the pattern are great. As a result, the surface smoothness is degraded, and the crystal grains are liable to fall down. The urea content and the carbamate ester content are adjusted so that the melting point of the blend is not higher than 110°C within the above range according to the kind of the carbamate ester. The blend may contain a third component to the extent not to damage the characteristics of the blend.
The temperature of the melted blend is 50 to 110°C, and is set by considering the melting point, viscosity, etc. of the melted blend.
It is convenient that the melted blend is prepared on the outside of the mold and poured into the mold. In this case, the melted blend may be cast by mere pouring, i.e. wihtout pressurizing, or with pressurizing at a pressure lower than 50 kg/cm², particularly lower than 10 kg/cm². When the melted blend is cast without pressurizing, the urea content is preferably not higher than 80 wt. % and the casting temperature is not lower than 90°C. In the case of casting without pressurizing, the surface smoothness is excellent, although dimensional contraction occurs in the pattern. On the other hand, in the case of casting with pressurizing, the urea content is preferably not lower than 70 wt. %, and casting temperature is not higher than 80°C. When a higher pressure is employed, the urea content is preferably increased and the casting temperature is lowered. By raising the casting pressure, although, the mold cost increases due to the necessity of a high pressure resistance, the dimentional contraction of the pattern decreases resulting to improve the dimensional accuracy. As the pressure molding method, various known methods are applicable, such as injection molding, compression molding and transfer molding.
The solidification by cooling may be conducted naturally or forced through the mold.
After cooling, the mold is removed to take the pattern out.
In the above water-soluble pattern, the temperature of the melted blend can be lowered to the range not decomposing urea without the degradation of formability, smoothness, filling ability, solidification ability and the like by using a blend of urea and a carbamate ester in a prescribed rate as the material of the pattern. As a result, since the temperature of the blend containing urea can be lower than 110°C through the processes from the preparation of the blend to demolding, there is no problem of the contamination of the working environment by the evolution of ammonia gas. Moreover, since the pattern may be formed without pressurizing inexpensive rubber molds can be used. Since the pattern is water-soluble, it can easily be removed after the casting mold is formed.

EXAMPLES

Example 1

An example of the thin bag pattern is illustrated in Figures 1 and 2. The thin bag 1 had an appearance of a combination of a column 20 mm in diameter, 120 mm in length and another column 10 mm in diameter, 15 mm in length in the stretched state, and was made of silicone rubber 1 mm in thickness having a tensile modulus of 40 kg/mm². The thin bag 1 was fixed to an upper vessel 2, and the upper vessel 2 was connected to a tank 3 by a tube 5 through a reversible tube pump 4. After water 6 at 20°C was fed to the tank 3, the water 6 was delivered to the thin bag 1 through the upper vessel 2 by starting the pump 4. When the pressure gauge 7 of the upper vessel 2 indicated 0.5 kg/cm², the pump was stopped. Subsequently, paraffin wax having a melting point of 65-67°C was melted by heating, and the melted paraffin wax 9 adjusted to 70°C was poured into the mold support 8. Immediately after, the thin bag was immersed in the melted paraffin wax 9, and naturally cooled. After 1 hour, as shown in Figure 2, the pump was started in the reverse direction, and the water 6 in the thin bag 1 was returned to the tank through the upper vessel 2. When 80 % of the water 6 in the thin bag 1 was discharged, the mold support 8 was lowered. As a result, the deflated thin bag 1 was taken out, and the solidified paraffin wax mold was obtained without damage.

Example 2

A cylindrical split mold 10 into two pieces shown in Figure 3 was prepared. The mold 10 was made of silicone rubber, and had a cylindrical cavity 11 having an inside diameter of 20 mm and a length of 120 mm of which the upper end was connected with a small size portion 12 having an inside diameter of 10 mm and a length of 20 mm for operation. The mold 10 had the same thickness of 10 mm at the side portion and the bottom portion, an outside diameter of 40 mm and a length of 150 mm. 20 g of water was put in a beaker in a bath of boiling water at 100°C, and 80 g in the sum of urea was divided into 10 times and dissolved in the water by putting therein with stirring. After stirring for 3 minutes, 0.3 g of acrylic acid-vinyl alcohol copolymer particles having a particle size of 200 µm was put in the aqueous urea solution with stirring. It was confirmed that the acrylic acid-vinyl alcohol copolymer particles sufficiently swelled, and the particles were cast into the mold. After cooling 1 hour naturally, the mold 10 was detached to obtain a collapsable pattern.
The pattern had a size of 20.03 mm in diameter and 120.08 mm in length which was almost the same as the cavity at ordinary temperature. The reason that the sie of the pattern was slightly greater was due to the thermal expansion of the silicone rubber mold by casting the agglomerate of the organic polymer particles at 100°C. The surface of the pattern was smooth.

Example 3

The aqueous solution at 100°C was prepared in the same manner as Eample 2. 0.3 g of acrylic acid-vinyl alcohol copolymer particles having a particle size of 200 µm was put in the same mold as Example 2. The aqueous solution was poured in the mold, and immediately stirred by a bar. After cooling for 1 hour naturally, the mold was detached, and the pattern was taken out.
The pattern had a size of 20.02 mm in diamter and 120.90 mm in length, and the contraction accompanied with the solidification did not occur. The surface of the pattern was smooth.

Example 4

85 parts by weight of urea, 10 parts by weight of water and 5 parts by weight of methyl cellulose were put in a vessel in a boiling water bath at 100°C, and dissolved with stirring to obtain an aqueous solution. 3 parts by weight of sodium acrylate polymer particles having a particle size of 200 µm was put in the vessel to obtain an agglomerate of organic polymer particles. The agglomerate was supplied to a low pressure injection molding machine, and injected into the same mold as Example 2 at an injection pressure of 3 kg/cm². After cooling for 1 hour naturally, the mold was detached to obtain a pattern.
The pattern had a size of 20.10 mm in diameter and 120.12 mm in length which was slightly greater than the cavity at ordinary temperature due to the expansion of the mold by the injection pressure. It was evaluated that the contraction due to the solidification did not occur. The surface of the pattern was smooth.
The results of Examples 2 to 4 are summarized in Table 1.

Example of Use 1

The pattern 13 prepared in Example 4 was hund in a soft steel mold support 14 by a hanger 15. Paraffin wax having a melting point of 65-67°C was melted by heating, and the melted paraffin wax adjusted to 70°C was poured into the mold support followed by natural cooling. After the paraffin wax was cooled to ordinary temperature, the mold support was immersed in a warm water at 40°C. As a result, most of the pattern was dissolved away after 16 minutes. The warm water was changed to fresh one at 40°C, the mold support was shaken therein, and the water was discharged. After the above extracting procedure was repeated three times, the pattern was removed completely. Thus, a paraffin wax for castin a high concentration slurry of a metal powder or a ceramic powder was obtained.

Example 5

The same mold as Example 2 was used. 20 g of water was put in a beaker in a boiling bath water at 100°C, and 40 g of methyl carbamate was put therein. Subsequently, 60 g in the sum of urea was added dividing into 10 times to obtain a melted blend wherein both were dissolved to each other. The melted blend was cast into the mold, and after cooling for 1 hour naturally, the mold was detached to obtain a water-soluble pattern.
The pattern has a size of 19.02 mm in diameter and 118.45 mm in length, and contraction occurred. However, the surface was very smooth. Ammonia gas was not evolved through the preparation of the aqueous solution to the molding, and the working environment was kept good.

Example 6

85 parts by weight of urea and 15 parts by weight of ethyl carbamate was put in a vessel in a boiling water bath at 100°C to obtain a melted blend wherein both were dissolved to each other. The vessel was cooled to 60°C by immersing in a water bath at 20°C to obtain a viscous liquid. The viscous liquid was supplied to a low pressure injection molding machine, and injected into the same mold as Example 2 at an injection pressure of 3 kg/cm². After cooling for 1 hour naturally, the mold was detached to obtain a pattern.
The pattern had a size of 20.08 mm in diameter and 120.15 mm in length which was slightly greater than the cavity at ordinary temperature due to the expansion of the mold by the injection pressure. It was evaluated that the contraction due to the solidification did not occur. The surface of the pattern was very smooth. A working environment problem caused by the evolution of ammonia gas did not occur through the preparation processes.

Example of Use 2

The pattern 13 prepared in Example 6 was hund in a soft steel mold support 14 by a hanger 15 as shown in Figure 4. Paraffin wax having a melting point of 65-67°C was melted by heating, and the melted paraffin wax adjusted to 70°C was poured into the mold support followed by natural cooling. After the paraffin wax was cooled to ordinary temperature, the mold support was immersed in a warm water at 40°C. As a result, most of the pattern was dissolved away after 16 minutes. The warm water was changed to fresh one at 40°C, the mold support was shaken therein, and the water was discharged. After the above extracting procedure was repeated three times, the pattern was removed completely. Thus, a paraffin wax for castin a high concentration slurry of a metal powder or a ceramic powder was obtained.

Claims

A pattern for manufacturing a mold consisting essentially of a flexible thin bag made of a material, which is impermeable with regard to the fluid contained therein and the material forming the mold, which is resistant to both of the fluid and the material forming the mold, and which has an elastic modulus in tension of 1 to 200 kg/mm², and the fluid contained therein.
The pattern of claim 1 wherein said material making the thin bag is a member selected from the group consisting of regenerated cellulose, cellulose derivatives, polyolefins, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, polyester, polystyrene, hydrochlorinated rubber, polyamide, fluorocarbon resin and silicone rubber.
The pattern of claim 1 wherein said thickness of the thin bag is 10 to 3,000 µm.
The pattern of claim 1 wherein said fluid has a density without deforming the pattern by the buoyancy caused by the density difference between the fluid and the material forming the mold.
A process for preparing a mold which comprises rendering a flexible thin bag into a stretched state by supplying a fluid, forming the mold using the thin bag in the stretched state as the pattern, deflating the thin bag by discharging the fluid, and taking the deflated thin bag out of the mold.
The process of claim 5 wherein said thin bag is made of a material, which is impermeable with regard to the fluid contained therein and the material forming the mold, which is resistant to both of the fluid and the material forming the mold, and which has an elastic modulus in tension of 1 to 200 kg/mm².
The process of claim 5 wherein said thin bag has a thickness of 10 to 3,000 µm.
The process of claim 5 wherein said fluid has a density without deforming the pattern by the buoyancy caused by the density difference between the fluid and the material forming the mold.
A process for preparing a collapsable pattern which comprises cooling to solidify an agglomerate of water-absorbable swellable organic polymer particles containing an aqueous solution into a prescribed form.
The process of claim 9 wherein said organic polymer is a member selected from the group consisting of cellulose graft polymers, starch graft polymers, acrylic acid-vinyl alcohol copolymer, sodium acrylate polymer and modified polyvinyl alcohols.
The process of claim 9 wherein said organic polymer is sodium acrylate polymer or acrylic acid polymer.
The process of claim 9 wherein at least the principal solute of said aqueous solution has a melting point higher than ordinary temperature and is easily soluble in water.
The process of claim 12 wherein said solute is a member selected from the group consisting of potassium chloride, sodium chloride, magnesium chloride, calcium chloride, tertiary butanol, lactic acid, urea, methyl carbamate, ethyl carbamate and blends thereof.
The process of claim 9 wherein the concentration of the solute in said aqueous solution is more than the solubility of the solute at ordinary temperature by more than 10 wt. % based on ordinary temperature.
The process of claim 9 wherein said agglomerate further contains a water insoluble or slightly soluble lubricant.
The process of claim 15 wherein said lubricant is a member selected from the group consisting of methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, stearic acid and oleic acid.
A process for preparing a water-soluble pattern which comprises filling a melted blend consisting essentialy of 5 to 95 wt. % of urea and 95 to 5 wt. % of a carbamate ester compatible with urea into a mold, and cooling to solidify it.
The process of claim 17 wherein a melting point of the blend is not higher than 110°C.