KR100493949B1 - Binder for high density stainless steel sintering material and method for making high density stainless steel sintering material using the same - Google Patents

Abstract

25 to 30 wt% of polypropylene with a density of 0.89 to 0.91 g / cm ³ , 25 to 30 wt% of low density polyethylene with a density of 0.90 to 0.91 g / cm ³ , and a density of 0.88 to 0.90 g / cm ³ A binder for the production of a high density stainless sintered body, characterized in that consisting of paraffin wax 30 ~ 40wt%, stearic acid 5 ~ 10wt% of density 0.98 ~ 1.00g / cm ³ is disclosed.

Description

Binder for High Density Stainless Steel Sintering Material and Method for making High Density Stainless Steel Sintering Material using the same}

The present invention relates to a binder for a high density stainless sintered compact, and more particularly, to a binder suitable for producing a high density stainless sintered compact and excellent in mixing property and injection property with a metal powder.

In general, stainless steel, which is applied to watch bands, places the greatest emphasis on high brightness and high corrosion resistance. Therefore, in the case of the stainless sintered body by the conventional metal powder injection molding method, the sintered density is only 95-96% of the theoretical density, and due to the residual pores, it is impossible to realize high brightness through regular polishing, and the oxidizing substance remaining in the pores. This has the problem that corrosion resistance falls by.

Stainless steel sintered body is impossible to commercialize because the brightness is significantly reduced if the pores remain on the surface after polishing. Therefore, the sintered density is an important basis for the determination of brightness.

1 is a graph showing the yield of watch products according to the sintered density and brightness, as can be seen that the difference in the yield of the final product according to the difference in sintered density of 1 to 2%. Therefore, the stainless sintered body requires a sintered density of 98% or more relative to the theoretical density.

In addition, in view of corrosion resistance, it is considered that there is no problem in corrosion resistance if it is more than 97% of the theoretical density, but it is necessary to have a sintered density of 98% or more of the theoretical density in terms of quality control.

Based on these points, the sintered density of 95 to 96% of the theoretical density of the sintered body manufactured by the conventional metal powder injection molding method is not suitable for the stainless sintered body.

Metal powder injection molding technology is a technology that can mass-produce precision three-dimensional shaped precision parts without post-processing. As described above, the metal, the cemented carbide, the ceramic, and the like are mixed with a binder, which is a thermoplastic polymer, to produce a moldable mixture and to mold. In order to remove the binder, the molded injection molded product is removed by using a solvent extraction method or a pyrolysis method and then sintered at a specific temperature at high temperature to produce a product having precise dimensions and complex shapes.

In the conventional powder metallurgy, the disadvantages of shape constraint and relative sinter density of 85 ~ 90%, which can only be manufactured with relatively simple shape and axial symmetry by uniaxial pressure molding, can be solved by using the metal powder injection molding method. It is possible to manufacture products of dimensional shape and to have higher mechanical density with higher relative sinter density of 95%.

In addition, when manufacturing a stainless sintered body by a metal powder molding method by applying a stainless material that is difficult to process than ordinary carbon steel, it is possible to reduce the process cost and mass production according to the product. Since the mixture used for metal powder injection molding has a very high powder filling rate, the binder should give excellent flowability to the mixture to facilitate molding and have a strength sufficient to handle the molded body. In addition, since the remaining binder must be removed before the final sintering process, there should be no residual material after degreasing because of excellent degreasing properties. The manufacturing technology of the binder, which requires these characteristics, is the most important technology in the metal powder injection molding process and is a technical field that should be the first priority in the development of the manufacturing process.

Typical binder composition for injection molding of metal powder is plasticizer, powder and polymer wetting property which maintains shape and improves flexibility of sub binder and polymer which promote viscosity decrease of main binder and mixture and lubrication of mold. It is composed of a surfactant for improving the (wetting) and adhesion (adhesion).

The MPIF (Metal Powder Industry Federation) standard specifies the sintering characteristics of the 316L stainless sintered body by the metal powder injection molding method as follows. That is, the sintered density should be more than 7.6g / cm ³ (relative density> 95%), the tensile strength is 140 ~ 175Mpa, the elongation is 40 ~ 50%, the carbon should be less than 0.03wt%.

The raw material powder used for metal powder injection molding is generally manufactured by a high pressure moisture method or a gas spraying method. The powder produced by the high pressure moisture method uses water when the powder is granulated from the molten metal, so the manufacturing cost is low. However, the metal powder is oxidized, and the oxygen content of about 0.3 to 0.4 wt% is not only retained. Its disadvantage is that its properties are disadvantageous when sintering. In addition, the powder by the gas spray method has a low oxygen content of 0.1 ~ 0.15wt% and the spherical shape is spherical high filling density can easily achieve high density during sintering, but there is a disadvantage that is expensive.

In the kneading process of the metal powder and the binder, the metal powder and the binder are mixed, and the difference from the general mixer is that the resin is pressurized at a temperature of 10 to 20 ° C. above the melting point so as to melt raw materials such as paraffin wax, polyethylene, and polypropylene. To knead it.

The mixture prepared by this process is prepared in pellet form and injected into a mold to produce an injection molded body. The degreasing process is the removal process of the binder from the injection molded product formed into a specific shape, and the binder composed of about 10wt% of the injection molded product must be completely removed without causing any defects such as warping or cracking. It is a required process. As a removal method, a binder is removed only by heat by a circulating gas (US Pat. No. 4,404,166), and a solvent extraction method of selectively extracting only a low melting point material from a binder by a solvent and removing the remaining main binder as heat (US patent). 4,765,950) to remove the binder. In this case, as the solvent used in the solvent extraction process, hexane, heptane, acetone, benzene, decalin, alcohol, and the like are used.

In the final sintering process of the product, the sintering process is heated to a temperature range of 1300 ~ 1400 ℃ for a certain period of time in a vacuum atmosphere or a specific atmosphere to produce metal sintered body through diffusion of metal powder.

However, the conventional binder (US Pat. No. 4,404,166) is composed of polypropylene as the main binder and 20 wt% of polypropylene and low melting point waxes as the minor binder. When removing the wax, it takes a long time to remove the binder to prevent the shape from collapsing due to rapid degreasing, and when the mold composition is composed of low melting point waxes of 50 to 70wt%, the strength of the injection molding is low. There is a problem that the shape damage occurs when the molded body is ejected.

In addition, in order to manufacture a high-density stainless sintered body, a powder having a size of several μm, which is generally manufactured by a gas spraying method having an excellent filling rate, is used, but has a disadvantage of being expensive.

In addition, the conventional wax-based binder is a relatively inexpensive 20μm or more powder when the binder of the secondary binder is removed rapidly in the first removal process when the void is generated between the powder and the powder, if the particle size of the powder is large The porosity of the powder and the powder is also relatively large, resulting in deformation of the shape due to the weight of the powder.

For example, in the general process, when the raw powder is prepared by the gas spray method and the average particle diameter is 11.4㎛, the relative sintered density is 93.7%, which is less than the standard, so that the powder is manufactured by the gas spray method. It is recommended to use a powder having an average particle diameter of 10 μm or less. [G. R. White and R. M. German, Adv. in Powder Metal. & Particulate Materials, vol. 4, 1994, p. 185]

Accordingly, it is an object of the present invention to provide a binder which is suitable for producing a high density stainless sintered body for a watch band and which is excellent in mixing property, injection property and degreasing property with a metal powder.

Other objects and features of the present invention will be more clearly understood through the preferred embodiments described below.

According to an aspect of the present invention, a polypropylene having a density of 0.89 to 0.91 g / cm ³ of 25 to 30 wt%, a low density polyethylene of 25 to 30 wt% of a density of 0.90 to 0.91 g / cm ³ , and a density of 0.88 to 0.90 g / cm ³ of A binder for the production of a high density stainless sintered body, characterized in that consisting of paraffin wax 30 ~ 40wt%, stearic acid 5 ~ 10wt% of density 0.98 ~ 1.00g / cm ³ is disclosed.

Hereinafter, with reference to the accompanying drawings will be described in detail an embodiment of the present invention.

The binder used in the present invention is mixed with a dense powder such as stainless powder and used to produce a high quality molded product by preventing separation of the powder and the binder during mixing or injection molding in which a high shear stress is applied.

According to the present invention, the binder is composed of 25 to 30 wt% of polypropylene having a density of 0.89 to 0.91 g / cm ³ , 25 to 30 wt% of low density polyethylene having a density of 0.90 to 0.91 g / cm ³ , and a density of 0.88 to 0.90 g / cm ³ . Paraffin wax 30-40 wt%, stearic acid 5-10 wt% with a density of 0.98-1.00 g / cm <^3> .

In the composition of the binder of the present invention, the polypropylene and the low density polyethylene serving as the main binder are composed of 55 wt% to 60 wt% of the total binder and have strength to facilitate injection molding.

In addition, in the final degreasing step of pyrolysis degreasing, thermal decomposition of low-density polyethylene having a melting point of 120 ° C. and polypropylene having a melting point of 170 ° C. occurs step by step, resulting in less deformation of the molded body and easy pyrolysis. Contamination of the sintered body can be minimized.

Furthermore, in the atmosphere degreasing process of the present invention, by using high purity hydrogen (H ₂ ) gas, residual carbon components remaining in the pyrolysis process can be removed, and the sintered density decrease due to surface oxidation of the powder prevents the average. High-density products can also be produced using powder having a particle diameter of 20 µm or less as a raw material.

Hereinafter, the binder used in the present invention will be described.

Figure 2 is a flow chart showing a metal powder injection molding method according to an embodiment of the present invention.

First, 85 to 90 wt% stainless raw powder and 10 to 15 wt% binder are charged into a dedicated kneader, and kneading is carried out by pressing and heating at 200 ° C. for 120 minutes (step S21).

The mixture thus prepared is formed into a pellet in a grinder to facilitate injection molding, and then injected into a mold in an injection molding machine to prepare an injection molded body (step S22).

Subsequently, the first degreasing process removes the low melting point binder and the surfactant, and the solvent is used as a solvent, for example, hexane (step S23). That is, the solvent temperature is raised to 35 to 40 ° C to dissolve and remove the binder in the solvent. In this case, if the injection molding degreasing time is about 5 to 8 hours based on the working temperature charging time, the debinding ratio of the sub-binder is 99% or more. However, the main binder remains, so that the wax-based sub-binder is excellent even after the removal of the wax-based binder, and the dimensions of the molded body are not changed even after the solvent is completely dried.

After the first degreasing step, the degreased molded body is dried (step S24), and the wet drying method in the solvent extraction apparatus and the dry drying method in the air are combined. The wet drying method uses the vaporized hexane to rapidly remove the solvent, hexane. This prevents cracking of the molded body due to volatilization. Conventionally, the treatment of organic solvents such as hexane, heptane, acetone in which the binder is dissolved has been a problem, but in the present invention, the solvent and the binder are separated through the regeneration apparatus, so that the solvent can be regenerated to prevent environmental problems occurring in the existing process. can do.

Subsequently, the main binder is degreased by hot air degreasing in a second degreasing process (step S25). That is, pyrolysis is carried out in a high purity hydrogen (H ₂ ) gas atmosphere to decompose and remove polypropylene and low density polyethylene, which are main binders, as heat, and to remove and remove the oxide film of the raw material powder generated in the manufacturing process by reacting with hydrogen and remaining binder. By removing the carbon component as a component to increase the sintering density of stainless products in the sintering process.

The molded body after the degreasing process is sintered in a vacuum and argon gas atmosphere (step S26). High vacuum sintering furnace is used to sinter at the maximum temperature of 1,340 ~ 1,380 ℃ in vacuum and argon (Ar) gas atmosphere. After maintaining for 120 minutes at the highest temperature to cool the stainless sintered body is produced.

Example

In the kneading of the stainless powder and the binder, the kneading machine, which is a kneader of pressure and heating type, is mixed with a stainless powder (particle size of 22 µm or more) manufactured by the gas spray method and a binder composed of [Table 1] in a mixer for 1 hour. The kneader is kneaded at a temperature of 200 ° C. for 2 hours. At this time, the rotation speed of the blade (Blade) is adjusted to 35 ~ 40 times / min.

ingredient Polypropylene Low density polyethylene Paraffin wax Stearic acid Ratio (wt%) 25 25 40 10 Density (g / cm ³ ) 0.91 0.90 0.89 0.99

The mixture is pulverized into pellets, charged into an injection machine hopper, and then injected by adjusting temperature and pressure. At this time, the mold temperature is maintained at about 40 ℃ and the injection machine nozzle temperature is set to 150 ~ 170 ℃ by adjusting the pressure suitable for the mold shape.

In the solvent degreasing process, which is a primary degreasing step, waxes as sub-binders are removed. As shown in FIG. 3A, the molded body 30 is mounted on the support 20 and charged in the hexane solvent in the degreasing bath 10. The solvent temperature is raised to 40 ° C. and then maintained for 8 hours before cooling.

When the solvent temperature is 20 ° C. or less, the molded body 30 is taken out of the solvent and then wet-dried in a state spanning the upper end of the degreasing bath 10. At this time, the atmosphere of the bathtub is rapidly volatilized from the molded body due to vaporized hexane. To prevent it. 3b shows a wet drying method. It is dried for 2 ~ 4 hours in wet drying and then for 1 hour in air.

In the secondary degreasing process, the main binder is degreased by hot air degreasing. Figure 4 is a graph showing an example of the process conditions of the secondary degreasing process of the present invention with reference to this, the degreasing conditions increase the temperature while injecting hydrogen gas 20L ~ 30L per minute. The hydrogen gas prevents the powder from being oxidized, thereby preventing the sintered density from deteriorating and quickly discharging the thermally decomposed binder to the outside, thereby remaining in the inside of the degreasing furnace to prevent re-reaction. The temperature increase rate in the process is heated to 0.3 ℃ / min ~ 1 ℃ / min for each section and cooled after maintaining for 8 hours at 400 ℃.

After the degreasing process, the molded product is sintered in a vacuum and argon gas atmosphere. Referring to FIG. 5, argon gas is sintered while injecting argon gas along a section to prevent chromium (Cr) from volatilizing in high vacuum. Sintering temperature range is 1,330 ~ 1,380 ℃ and it has a process of quenching using a cooling fan after maintaining for 2 hours at the maximum temperature range.

Through this manufacturing method, a high density stainless sintered body having a relative density of relative sintered density of 98% or more can be manufactured.

Table 2 shows the characteristics of the 316L stainless sintered body after the sintering process.

Sintering temperature section Sintered Density (g / cm ³ ) Relative Sintered Density (%)-Theoretical Density 8.0 Density g / cm ³ Residual carbon content (wt%) 1370 ℃, 2 hours 7.88 98.5 0.03

Although the above has been described based on the preferred embodiment of the present invention, it can be appropriately changed or modified by those skilled in the art without departing from the scope of the present invention. It is natural that such changes or modifications fall within the scope of the present invention without departing from the spirit of the present invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims described below.

As described in detail above, the binder according to the present invention has a strength capable of facilitating injection molding, and thermal decomposition of low-density propylene and propylene occurs step by step in the pyrolysis degreasing process, so that deformation of the molded body is small and easy. In addition, residual material such as carbon in the injection molded body can minimize contamination of the sintered body.

1 is a graph showing the yield of watch products according to the sintered density and brightness.

Figure 2 is a flow chart showing a metal powder injection molding method according to an embodiment of the present invention.

Figure 3a is a schematic diagram showing a primary degreasing process according to the present invention.

Figure 3b is a schematic diagram showing the wet degreasing process according to the present invention.

Figure 4 is a graph showing an example of the process conditions of the secondary degreasing process of the present invention.

5 is a graph showing an example of the process conditions of the sintering process of the present invention.

Claims (1)

Density of 0.89 ~ 0.91g / cm ³ of the polypropylene 25 ~ 30wt%, density of 0.90 ~ 0.91g / cm ³ low-density polyethylene, 25 ~ 30wt%, density of 0.88 ~ 0.90g / cm ³ of Binder for the production of high density stainless sintered body, characterized in that consisting of paraffin wax 30 ~ 40wt%, stearic acid 5 ~ 10wt% of density 0.98 ~ 1.00g / cm ³ .