WO2007066969A1

WO2007066969A1 - Power injection molding method for forming article comprising titanium and titanium coating method

Info

Publication number: WO2007066969A1
Application number: PCT/KR2006/005227
Authority: WO
Inventors: Young-Suk Park
Original assignee: Mtig Co., Ltd
Priority date: 2005-12-07
Filing date: 2006-12-06
Publication date: 2007-06-14
Also published as: KR100725209B1

Abstract

Disclosed herein are a method for producing a powder injection molded part and a method for coating an object with titanium. The method for producing the powder injection molded part comprises the steps of: mixing titanium hydride powder with a specific binder to provide a molding mixture; injection molding the molding mixture with a powder injection molding machine to form a molded part; debinding the molded part; and sintering the debound part. According to the disclosed method, a titanium molded part can be produced by using titanium hydride powder in an injection molding method.

Description

POWDER INJECTION MOLDING METHOD FOR FORMING ARTICLE COMPRISING TITANIUM AND TITANIUM

COATING METHOD

Technical Field

[1] The present invention relates to a method for producing a powder injection molded part and a method for coating an object with titanium, and more particularly to a method for producing a titanium powder injection molded part using titanium hydride powder and a method for coating an object with titanium.

Background Art

[2] Titanium has been used as a material for various tools or mechanical parts due to advantages such as excellent mechanical performance and harmlessness to the human body.

[3] Prior methods for producing tools, such as molded parts, using titanium include sintering methods using titanium powder, and methods for injection molding a mixture of titanium powder and a binder (see Korean Patent Registration No. 508471).

[4] However, titanium powder has a problem in that the surface of titanium powder reacts with atmospheric oxygen in a process of forming a molded particle, thus forming an oxide layer, which makes it difficult to bind pure titanium powder, thus reducing the mechanical performance of the resulting titanium molded parts.

[5] In addition to the injection molding methods as described above, methods disclosed in the art include a method of supplying hydrogen gas to titanium during sintering in order to prevent titanium powder from being oxidized upon the production of titanium molded parts, and a method of sintering titanium hydride in order to generate hydrogen gas during sintering (see Korean Patent Laid-Open Publication No. 2004-99477). Disclosure of Invention

Technical Problem

[6] It is an object of the present invention to provide a method for producing a titanium powder injection molded part having improved mechanical performance by using titanium hydride powder in an injection molding method.

[7] Another object of the present invention is to provide a titanium coating method for forming a titanium coating layer having excellent mechanical performance using titanium hydride as a coating material.

Technical Solution

[8] To achieve the above objects, in one aspect, the present invention provides a method for producing a powder injection molded part, comprising the steps of: mixing titanium hydride powder with a specific binder to provide a molding mixture; injection molding the molding mixture with a powder injection molding machine to form a molded part; debinding the molded part; and sintering the debound part.

[9] In the inventive method, the step of providing the molding mixture can further comprise uniformly mixing the titanium hydride powder with the binder at a temperature of 150-200 ⁰C for 2 hours.

[10] Also, the specific binder may comprise at least one selected from among low- density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene glycol (PEG) and paraffin wax (PW).

[11] Moreover, the molding mixture may consist of 40-60 vol% of the titanium hydride powder and the balance of the specific binder.

[12] Furthermore, the molding mixture may comprise 40-60 parts by volume of the titanium hydride powder, 10-20 parts by volume of LDPE, 10-20 parts by volume of HDPE, 5-10 parts by weight of PEG, and 1-10 parts by volume of PW.

[13] Also, the step of forming the molded part may comprise injecting the molding

mixture at a pressure of 2000-5000 psi to form the molded part.

[14] Moreover, the step of debinding the molded part may comprise a process of heating the molded part in a vacuum under an inert gas atmosphere from room temperature (20 0C) to 300 ⁰C at a heating rate of 1 °C/hr and then from 300 ⁰C to 700 ⁰C at a heating rate of 1 °C/hr.

[15] Also, the step of sintering the debound part may comprise a process of heating the molded part from 700 ⁰C to 1250 ⁰C at a heating rate of 5 °C/hr in a vacuum under an inert gas atmosphere.

[16] Also, the above production method may further comprise the steps of masking the sintered molded part and dipping the masked molded part in an acidic solution, in which an electric current is applied thereto to form an oxide layer having a specific thickness thereon, thus imparting a given color to the surface of the molded part.

[17] In another aspect, the present invention provides a coating method comprising the steps of: mixing titanium hydride powder with a binder to provide a coating material; applying the coating material on the surface of an object; debinding the object to which the coating material has been applied; and sintering the debound object.

[18] In the coating method, the step of applying the coating material may comprise a step of spraying and applying the coating material on the surface of the object.

[19] Also, the step of applying the coating material may comprise dipping the object in the coating solution to apply the coating material on the object.

[20] Furthermore, the step of debinding the object may comprise a process of heating the object in a vacuum under an inert gas atmosphere from room temperature (20 ⁰C) to 300 ⁰C at a heating rate of 1 °C/hr and then from 300 ⁰C to 700 ⁰C at a heating rate of 1

°C/hr.

[21] Also, the step of sintering the debound object may comprise a process of heating the object from 700 ⁰C to 1250 ⁰C at a heating rate of 5 °C/hr in a vacuum under an inert gas atmosphere.

[22] In addition, the coating method may further comprise the steps of: masking the sintered object; and dipping the masked object in an acidic solution, in which electric current is applied thereto to form an oxide layer having a specific thickness thereon, thus imparting a given color to the surface of the object.

Advantageous Effects

[23] As described above, according to the method for producing the powder injection molded particle according to the present invention, a mixture of titanium hydride powder and a specific binder is used as an injection molding material, and thus it is possible to prevent the formation of a titanium oxide layer, such that the binding between pure titanium powders can be easily achieved, leading to an improvement in the mechanical performance of the resulting titanium molded part.

[24] Also, according to the titanium coating method of the present invention, a mixture of titanium hydride powder and a specific binder is used as a material for coating an object, and thus it is possible to prevent the formation of an oxide layer when coating with titanium, such that the binding between pure titanium powders can be easily achieved, leading to an improvement in the mechanical performance of the resulting titanium coating layer. Meanwhile, a change in the colors of the titanium molded part and the titanium coating layer can be obtained by forming an oxide layer having a controlled thickness on the surfaces of the titanium molded part and the titanium coating layer.

Brief Description of the Drawings

[25] FIG. 1 is a flow chart explaining the method for producing a titanium powder

injection molded part according to an embodiment of the present invention.

[26] FIG. 2 is a schematic diagram explaining the construction of the system used in the production method of FIG. 1.

[27] FIG. 3 is a flow chart explaining the titanium coating method according to an

embodiment of the present invention.

[28] <Description of reference numerals>

[29] 10: molded part; 11: masking; and

[30] 20: glacial acetic acid solution

Mode for the Invention

[31] As used herein, the term "powder injection molding (PIM)" refers to a combination of advanced injection molding technology in the plastic field with advanced metal powder sintering technology in the powder metallurgy field, which is the latest processing technology enabling cost-effective mass production of highly functional complex precision parts, production of which is difficult or incurs high costs when using prior technologies, such as cutting, precision casting, die casting, and metal metallurgy.

[32] The powder injection molding comprises a mixing process, an injection molding process, a debinding process, a sintering process and the like, and is classified, according to molding material, into metal injection molding (MIM), ceramic injection molding (CIM) and the like.

[33] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[34] As shown in FIG. 1, a method for producing a powder injection molded part

according to an embodiment of the present invention comprises the steps of: (Sl 10) mixing titanium hydride powder with a binder to provide a molding mixture; (S 120) injection molding the molding mixture to form a molded part; (S 130) debinding the molded part; (S 140) sintering the molded part; and (S 150) masking the molded part and dipping the masked part in a glacial acetic acid solution to impart a specific color to the surface of the molded part.

[35] In the step (S 110) of providing the molding mixture, the titanium hydride powder and the binder are used as raw materials. The titanium hydride powder, used in this embodiment, is a commercial powder of more than 99.7% pure TiH (titanium hydride) produced using a hydride-dehydride (HDD) process, and has a relatively uniform particle size of 635 mesh.

[36] General titanium powder for injection molding is hexagonal in shape and has a mean particle size of about 20 D, whereas the titanium hydride powder, which is used in the embodiment of the present invention, is cubic in shape and has a mean particle size of about 8.5 D. Also, the titanium powder has a surface area per unit volume of 0.368 m /cc, whereas the titanium hydride powder used in the embodiment of the present invention has a surface area per unit volume of 0.997 m /cc.

[37] The use of the titanium hydride powder having the above-described characteristics can provide a significant increase in the density of a sintered body compared to the use of the prior titanium (Ti) powder. The titanium hydride powder will produce a pure titanium sintered body due to dehydrogenation during sintering.

[38] The binder comprises at least one selected from among low-density polyethylene

(LDPE), high-density polyethylene (HDPE), polyethylene glycol (PEG) and paraffin wax (PW).

[39] With respect to the mixing of the titanium hydride powder with the binder, 40-60 vol% of the titanium hydride powder is mixed with the balance of the binder.

Preferably, 40-60 vol% of the titanium hydride powder is mixed with 10-20 vol% of LDPE, 10-20 vol% of HDPE, 5-10 vol% of PEG and 1-10 vol% of PW. This mixing process is carried out at a temperature of 150-200 ⁰C for about 2 hours. When the above mixing ratio is used, the molding mixture can have sufficient fluidity in an injection molding machine, and can maintain the strength thereof due to HDPE and LDPE just after injection, but before sintering. Also, in a debinding process, described below, PEG will be removed with hexane, so that pores will be formed in the molding mixture to remove PW through the pores, and then LDPE and HDPE will be sequentially removed, thus minimizing the change in shape of the resulting molded part.

[40] The mixing process can be carried out using, for example, a conventional double planetary mixer.

[41] If the titanium hydride powder is used in an amount of less than 40 vol%, the

molding mixture will have good fluidity in injection molding, but it will require a long time for debinding. On the other hand, if the titanium hydride powder is used in an amount of more than 60 vol%, the molding mixture will not have sufficient strength in injection molding. For this reason, the titanium hydride powder and the binder are preferably mixed at the above-specified mixing ratio.

[42] Also, if the mixing process is carried out at a temperature lower than the above- specified temperature range, or for a time shorter than the above-specified time, the binder will not have sufficient fluidity, making the mixing difficult. On the other hand, if the mixing process is carried out at a temperature higher than the above-specified temperature or for a time longer than the above- specified time, the binders for low- temperature applications can be debound during the mixing process. For this reason, the mixing process is preferably carried out under the above-specified conditions.

[43] This mixing process results in a mixture in which the binder covers each of the

titanium hydride powder particles. This molding mixture is readily broken into powder (feedstock) by the application of slight pressure, although it can also form a mass by virtue of the action of the binder.

[44] Thereafter, the obtained molding mixture is injected into a mold in a powder

injection molding system, thus obtaining a molded part having a given shape (S 120).

[45] The powder injection molding system, which is used in this embodiment, has a conventional construction comprising a hopper for feeding titanium powder, a plas- ticization cylinder, an injection screw, a mold, and the like.

[46] The injection molding process is preferably carried out at an injection pressure of

2000-5000 psi in a state in which the molding mixture was heated to 350 ⁰C.

[47] If the injection temperature is lower than 350 ⁰C, the molding mixture will have insufficient fluidity, and thus cannot be uniformly injection molded, and if it is higher than 350 ⁰C, the vaporization of the binder can occur. For this reason, the injection molding is preferably carried out in the above- specified temperature range.

[48] If the injection pressure is lower than 2000 psi, the molding mixture cannot be smoothly injected from a nozzle, and if it is higher than 5000 psi, the powder injection molding system can be overloaded. For this reason, the injection molding is preferably carried out in the above- specified pressure range.

[49] In the next step, the molded part is debound (S 130).

[50] The debinding process is conducted to remove the binder which has been added for the binding between the titanium hydride powders in the molded part. In this embodiment, the debinding is carried out by thermal decomposition in a vacuum furnace. Particularly, it is carried out by heating the molded part in a vacuum state (vacuum degree: 10 -10 atm) in an atmosphere of inert gas such as argon (Ar) from room temperature (20 ⁰C) to 300 ⁰C at a heating rate of 1 °C/hr, maintaining the molded part at 300 ⁰C for 3 hours, heating the molded part from 300 ⁰C to 700 ⁰C at a heating rate of 1 °C/hr, and then maintaining the molded part at 700 ⁰C for 3 hours.

[51] More specifically, in the initial temperature range in the debinding process,

channels for debinding the binder are formed in the injection molded part. In the intermediate temperature range, the binder for low-temperature applications is debound, and in the high-temperature range, the binder for high-temperature applications is then debound. For this reason, the debinding is carried out in the above- specified temperature range.

[52] Also, in the debinding process, a lower heating rate is preferable only in technical terms. However, if the heating rate is too low, the vacuum furnace must be maintained at a high temperature for a long time, resulting in increases in production costs and operation time. Thus, it is preferable in terms of production to maintain the heating rate at the highest possible rate as long as the debinding process is smoothly conducted. On the other hand, if the heating rate is excessively high, undesirable phenomena, including the distortion of the molded part, internal cracking, pores and swelling, can occur during the debinding process. For this reason, the debinding process is preferably carried out at the above- specified heating rate.

[53] The heating rate in the vacuum furnace has a general meaning, and can also be the way of continuously elevating temperature at a constant rate during the debinding process. If necessary, a process of temporarily stopping the elevation of temperature to maintain the molded part at a fixed temperature can also be included in the debinding process.

[54] Also, when the temperature in the debinding process reaches a given temperature

(about 280 ⁰C), the titanium hydride in the molded part is decomposed to generate hydrogen gas. Due to this hydrogen gas, contaminants such as oxygen or carbon, which can exist in the vacuum furnace, will be prevented from coming close to or reacting with the titanium of the molded part.

[55] Meanwhile, the above debinding process may further comprise a debinding process, which is conducted in a solvent extraction process. The solvent extraction process is a process of dipping the injection molded part in a solvent to dissolve and remove the binder. The solvent, used in this case, may vary depending on the kind of binder, and it is possible to use methanol, butanol, hexane, dichloromethanol or the like. Particularly, if PEG is used as the binder, it can be extracted and removed from the molded part by dipping the injection molded part in hexane at 50-80 ⁰C for 3 hours.

[56] If this solvent extraction process is further included, it is preferably carried out as the step prior to the thermal decomposition debinding process.

[57] Thereafter, the debound molded part is sintered in a sintering furnace (S 140).

[58] The sintering process is carried out in a vacuum state (vacuum level: 10 to 10

atm) under an atmosphere of inert gas such as argon. It can be conducted in a separate sintering furnace and also be continuously conducted in the vacuum furnace in which the debinding process has been completed.

[59] The sintering of the molded part is carried out by heating the molded part from 700

⁰C to 1250 ⁰C at a heating rate of 5 °C/hr and then maintaining the molded part at 1250 0C for 1 hour. At temperatures lower than the specified temperature range, the titanium powder forming the molded part cannot have sufficient hardness even after sintering, and on the other hand, at temperatures higher than the above- specified temperature range, the sintered titanium powder can be thermally damaged. For this reason, the sintering process is preferably carried out under the above temperature conditions.

[60] Herein, the sintering process is conducted in the above- specified temperature

conditions for a specific time, which is determined depending on the properties of the molded part. For example, a sintering time of merely a few minutes can also be sufficient for the sintering process, and a sintering time of 2-3 hours can also be required for any molded part.

[61] According to one test example of a method for producing a powder injection

molded part according to one embodiment of the present invention, the titanium molded part formed according to the above production method showed properties of titanium density of 99% and an HRC hardness of 20. Such numerical values correspond to values similar to the hardness and density of a titanium molded part produced according to the prior casting method.

[62] Meanwhile, although the method for producing the titanium powder injection

molded part according to the embodiment of the present invention can end with the sintering step (S140), the method may also further comprise a step (S150) of imparting a color to the surface of the molded part. [63] In other words, as shown in FIG. 2, a masking 11 having a given shape is formed on a sintered molded part 10, and the molded part 10 is dipped in an acidic solution, for example, a glacial acetic acid solution, in which an electric current is applied thereto, so that an oxide layer having the shape of the masking 11 is formed on the surface of the molded part 10. The thickness of the surface oxide layer can be changed by adjusting the amount of the electric current that is applied to the molded part 10. Also, as the thickness of the surface oxide layer is changed, the surface color of the molded part 10 can be changed.

[64] The titanium molded part formed according to the above-described method can be used in a wide range of applications, because it shows excellent mechanical properties with respect to the corrosion resistance, strength, hardness, inherent gloss, etc. of titanium, and furthermore, titanium is harmless to the human body.

[65] Meanwhile, as shown in FIG. 3, the titanium coating method according to an

embodiment of the present invention comprises the steps of: (S210) mixing titanium hydride powder with a binder to provide a coating material; (S220) applying the coating material on the surface of an object; (S230) debinding the object; and (S240) sintering the object.

[66] The step of providing the coating material (S210) is the same as the step of

providing the molding mixture (Sl 10 of FIG. 1), described in the embodiment of the method of producing the powder injection molded part. However, the step SI lO uses the mixture of titanium hydride powder and the binder as the molding mixture for forming the molded part, but the step S210 uses the mixture as a coating material to be applied on the surface of the subject, as described below.

[67] Thereafter, the coating material obtained in the step (S210) is applied on the surface of the objected to be coated (S220). The application of the coating material is carried out according to a method of spraying the coating material on the surface of the object using a conventional spray device, a method of dipping the object in a container containing the coating material, etc.

[68] After completion of the application of the coating material, the object is subjected to debinding treatment (S230) and sintering treatment (S240). The debinding treatment and the sintering treatment are the same as the step of debinding the molded part (S 130 of FIG. 1) and the step of sintering the molded part (S 140 of FIG. 1), described in the above embodiment of producing the molded part.

[69] Meanwhile, like the method of producing the titanium powder injection molded part, the titanium coating method may also comprise, in addition to the sintering step (S240), a step of imparting a color to the surface of the object (not shown).

[70] In the case of the object coated according to the above-described coating method, as described in the above embodiment of producing the molded part, the surface layer of titanium can be prevented from being oxidized due to hydrogen gas generated in the debinding process and/or the sintering process. For this reason, the binding between pure titanium particles can be easily achieved, and thus a titanium coating layer having mechanical properties compared to those in the prior titanium coating method can be obtained.

[71]

Claims

[1] A method for producing a powder injection molded part, comprising the steps of:

mixing titanium hydride powder with a specific binder to provide a molding mixture;

injection molding the molding mixture with a powder injection molding machine to form a molded part;

debinding the molded part; and

sintering the debound part.

[2] The method of Claim 1, wherein the step of providing the molding mixture

comprises uniformly mixing the titanium hydride powder with the binder at a temperature of 150-200 ⁰C for 2 hours.

[3] The method of Claim 1, wherein the specific binder comprises at least one

selected from among low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene glycol (PEG) and paraffin wax (PW).

[4] The method of Claim 1, wherein the molding mixture consists of 40-60 vol% of the titanium hydride powder and the balance of the specific binder.

[5] The method of Claim 1, wherein the molding mixture comprises 40-60 parts by volume of the titanium hydride powder, 10-20 parts by volume of LDPE, 10-20 parts by volume of HDPE, 5-10 parts by volume of PEG, and 1-10 parts by volume of PW.

[6] The method of Claim 1, wherein the step of forming the molded part comprises injecting the molding mixture at a pressure of 2000-5000 psi to form the molded part.

[7] The method of Claim 1, wherein the step of debinding the molded part comprises a process of heating the molded part in a vacuum under an inert gas atmosphere from room temperature (20 ⁰C) to 300 ⁰C at a heating rate of 1 °C/hr and then from 300 ⁰C to 700 ⁰C at a heating rate of 1 °C/hr.

[8] The method of Claim 1, wherein the step of sintering the molded part comprises a process of heating the molded part from 700 ⁰C to 1250 ⁰C at a heating rate of 5

°C/hr in a vacuum under an inert gas atmosphere.

[9] The method of Claim 1, further comprising the steps of masking the sintered molded part and dipping the masked molded part in an acidic solution, in which an electric current is applied thereto to form an oxide layer having a specific thickness thereon, to thereby impart a given color to the surface of the molded part.

[10] A coating method comprising the steps of:

mixing titanium hydride powder with a binder to provide a coating material; applying the coating material on the surface of an object;

debinding the object applied with the coating material; and

sintering the debound object.

[11] The method of Claim 10, wherein the step of applying the coating material

comprises a step of spraying and applying the coating material on the surface of the object.

[12] The method of Claim 10, wherein the step of applying the coating material

comprises dipping the object in the coating solution to apply the coating material on the object.

[13] The method of Claim 10, wherein the step of debinding the object comprises a process of heating the object in a vacuum under an inert gas atmosphere from room temperature (20 ⁰C) to 300 ⁰C at a heating rate of 1 °C/hr and then from 300

⁰C to 700 ⁰C at a heating rate of 1 °C/hr.

[14] The method of Claim 10, wherein the step of sintering the debound object

comprises a process of heating the object from 700 ⁰C to 1250 ⁰C at a heating rate of 5 °C/hr in a vacuum under an inert gas atmosphere.

[15] The method of Claim 10, further comprising the steps of masking the sintered object and dipping the masked object in an acidic solution, in which an electric current is applied thereto to form an oxide layer having a specific thickness thereon, to thereby impart a given color to the surface of the object.